The evolving landscape of PCSK9 inhibition in cancer

Abstract

Cancer is a disease with a significant global burden in terms of premature mortality, loss of productivity, healthcare expenditures, and impact on mental health. Recent decades have seen numerous advances in cancer research and treatment options. Recently, a new role of cholesterol-lowering PCSK9 inhibitor therapy has come to light in the context of cancer. PCSK9 is an enzyme that induces the degradation of low-density lipoprotein receptors (LDLRs), which are responsible for clearing cholesterol from the serum. Thus, PCSK9 inhibition is currently used to treat hypercholesterolemia, as it can upregulate LDLRs and enable cholesterol reduction through these receptors. The cholesterol-lowering effects of PCSK9 inhibitors have been suggested as a potential mechanism to combat cancer, as cancer cells have been found to increasingly rely on cholesterol for their growth needs. Additionally, PCSK9 inhibition has demonstrated the potential to induce cancer cell apoptosis through several pathways, increase the efficacy of a class of existing anticancer therapies, and boost the host immune response to cancer. A role in managing cancer- or cancer treatment- related development of dyslipidemia and life-threatening sepsis has also been suggested. This review examines the current evidence regarding the effects of PCSK9 inhibition in the context of different cancers and cancer-associated complications.

Graphical Abstract

1. Introduction

Cancer is the second leading cause of death worldwide, behind cardiovascular disease (Global Burden of Disease Cancer, 2022). In 2019 alone, there were 23.6 million new cancer cases and 10 million cancer deaths worldwide (Global Burden of Disease Cancer, 2022), surpassing a previous prediction of reaching an incidence of 22.2 million in 2030 (Bray et al., 2012). Global cancer incidence is expected to continue to grow due to an aging population, lifestyle factors, and population growth, with an estimate that by 2070 the incidence of all cancers combined will double with respect to 2020 (Soerjomataram and Bray, 2021). Cancer is a disease with a significant burden, with ramifications including premature mortality, major productivity losses (Bradley et al., 2008; May et al., 2020; Pearce et al., 2018), substantial healthcare expenditures (Ekwueme et al., 2019; Hofmarcher et al., 2020; Park and Look, 2019), and mental health disorders (Amiri and Behnezhad, 2020; Carreira et al., 2018; Sun et al., 2019).

The global burden of cancer highlights the need for treatment options with high levels of efficacy, tolerability, and affordability. Increased understanding of cancer biology, including identification of “driver” genetic mutations (Bailey et al., 2018; Sondka et al., 2018), pathways of escape from host immune surveillance (Hegde et al., 2020), and mechanisms of therapeutic resistance (Marine et al., 2020; Nazarian et al., 2010), have yielded tremendous advances in cancer treatments in the last few decades. Current treatment options include radiation therapy, surgery, chemotherapeutics, hormone therapy, immunotherapy, or any combination of these (Wang et al., 2018). Immunotherapy, in particular, is an innovative treatment approach that has been a breakthrough in oncology. Immunotherapy acts by reinstating the anticancer response of the host immune system (Tan et al., 2020). Immune checkpoint inhibitor (ICI) therapy is a subgroup of immunotherapies that include monoclonal antibodies to molecules that inhibit T cell activity, including programmed cell death 1 (PD-1), programmed cell death 1 ligand 1 (PD-L1), and cytotoxic T lymphocyte-associated protein-4 (CTLA-4) (Martins et al., 2019). These have shown promising results in several malignancies, including melanoma (Hodi et al., 2010), small cell lung cancer (Antonia et al., 2016) and non-small cell lung cancer (Hellmann et al., 2017), renal cell carcinoma (Motzer et al., 2018), non-Hodgkin’s lymphoma (Ansell et al., 2014), and others. However, despite the positive results compared to other therapies, only a fraction of patients are responsive to ICI treatment (Haslam and Prasad, 2019; Liu et al., 2020; Schadendorf et al., 2015). There have been many investigations into possible combinations with other therapies for optimization of ICI effects (Li et al., 2021; Liu et al., 2022b; Wang et al., 2022d; Yi et al., 2019; Yu et al., 2021), and recently, a synergistic effect of cholesterol-lowering proprotein convertase subtilisin kexin type 9 (PCSK9) inhibitor, evolocumab or alirocumab, and anti-PD-1 antibodies have shown good tumor suppressing efficacy (Liu et al., 2020; Wang et al., 2022c; Xie et al., 2022).

PCSK9 is an enzyme most known for its role in promoting the degradation of low-density lipoprotein receptors (LDLR) (Maxwell et al., 2005), which are responsible for clearing cholesterol from the bloodstream (Go and Mani, 2012); in this manner, PCSK9 increases serum cholesterol, and its upregulation is implicated in hypercholesterolemia (Guo et al., 2020). It follows that PCSK9 inhibition reduces serum cholesterol and is indicated in the management of hypercholesterolemia (Ge et al., 2021; Rosenson et al., 2019). Currently, there are two anti-PCSK9 monoclonal antibodies, alirocumab and evolocumab, that have been FDA-approved for their use in the management of hypercholesterolemia, and other PCSK9-targeting strategies, including anti-PCSK9 vaccination, genome editing, the use of siRNA, and more (Sun et al., 2022).

The cholesterol-lowering effects of PCSK9 inhibitor therapy have generated significant interest in their potential applications in the context of cancer. There is considerable evidence that increased cholesterol plays a role in cancer development and progression, suggesting that cholesterol depletion may be a way to combat cancer (Mayengbam et al., 2021). As such, PCSK9 inhibition is of considerable interest due to its cholesterol-lowering effects. In addition, PCSK9 inhibitors have also been found to increase the effects of ICI therapies, improve the host immune response against cancer cells, induce cancer cell apoptosis, and may have a role in the management of complications such as sepsis and dyslipidemia, which can also be seen in cancer patients and survivors; this review examines the evidence for the role of PCSK9 inhibitor therapy in the context of different cancers and cancer-related complications such as sepsis and dyslipidemia.

2. Physiological role of PCSK9

Proprotein convertases are a group of proteases responsible for the cleavage of precursor proteins, including growth factors, hormones, and surface glycoproteins, into their bioactive molecules (Seidah et al., 2014). PCSK9 was first described in 2003 as a neural apoptosis-regulated convertase 1, and is the ninth member of Kexin-like proprotein convertases (Seidah et al., 2003). It is a serine protease encoded by the PCSK9 gene on the small arm of chromosome 1 (Abifadel et al., 2003; Seidah et al., 2003). The enzyme is synthesized as a zymogen that undergoes intramolecular autocatalytic processing, and is secreted as a mature proteinase attached to its pro-segment (Naureckiene et al., 2003). Unlike other proprotein convertases, the effects of PCSK9 are exerted by binding and directing target proteins toward intracellular degradation rather than direct proteolysis (Cunningham et al., 2007; Li et al., 2007; Seidah et al., 2014). PCSK9 enzyme is mainly secreted by hepatocytes, although its expression has also been reported to a lesser extent in the kidneys, small intestine, central nervous system, adrenal glands, and reproductive system (Seidah et al., 2003; Zaid et al., 2008). The enzyme has roles in cholesterol metabolism, immune regulation, cancer, pancreatic function and diabetes, ferroptosis, and even in viral activation, as reviewed in (Alannan et al., 2022). In the following sections, we focus on the roles of PCSK9 in cholesterol metabolism and immune regulation in particular, for further discussion of possible applications in cancer.

2.1. PCSK9 and cholesterol metabolism

Transcriptional regulation of PCSK9 is exerted by the sterol-responsive element binding proteins (SREBP)s (Jeong et al., 2008) - mainly SREBP-2 and hepatocyte nuclear factor 1a (HNF1a) (Li et al., 2009) transcription factors. Both transcription factors are responsive to cholesterol depletion and regulate lipid metabolism (Liu et al., 2022a; Shimano and Sato, 2017), thereby intricately linking PCSK9 to lipid pathways. PCSK9 plays a key role in regulating cholesterol homeostasis by increasing the degradation of LDLR, which clears cholesterol from the bloodstream (Go and Mani, 2012) without impacting LDLR synthesis (Maxwell et al., 2005). There is also evidence that PCSK9 induces the degradation of other LDLR-related proteins, such as very low-density lipoprotein receptor (VLDLR),apolipoprotein E receptor 2 (ApoER2), (Poirier et al., 2008), and LDLR-related protein 1 (LRP-1) (Canuel et al., 2013), as well as the unrelated fatty acid receptor CD36 (Demers et al., 2015). The most well-characterized of these interactions is with LDLR.

PCSK9-mediated degradation of LDLR involves intracellular and extracellular pathways (Figure 1) (Poirier et al., 2009). Murine models have provided evidence that PCSK9 reduces hepatic LDLR levels and increases plasma LDL cholesterol through a mechanism involving LDLR and adaptor protein ARH-dependent endocytosis into the endosomal-lysosomal compartments (Lagace et al., 2006; Qian et al., 2007). In this pathway, secreted PCSK9 binds to the epidermal growth factor A (EGF-A) domain of LDLR, directing its trafficking from the plasma membrane to the endosomal and then lysosomal compartments (Zhang et al., 2007). Under normal circumstances, the acidification of the lysosomal compartment induces a conformational change in LDLR, prompting the release of LDL; LDLR is then recycled to the plasma membrane, and LDL is trafficked to the lysosome and degraded, releasing cholesterol to be redistributed within the cell (Seidah et al., 2014). However, PCSK9 alters this trajectory, binding more strongly to the EGF-A domain on LDLRs in acidic conditions (Fisher et al., 2007; Zhang et al., 2007), and targeting the LDLR-LDL complex to the lysosomes for destruction. Correspondingly, increasing the pH of acidic compartments was found to protect LDLR from PCSK9-mediated degradation (Holla et al., 2007).

Figure 1.

PCSK9 & LDLR pathway. Both LDLR and PCSK9 are intracellularly synthesized, the former being transported to the plasma membrane and the latter secreted into the plasma. In the absence of PCSK9 binding to LDLR (green arrows), the LDL-LDLR complex is endocytosed, followed by LDL degradation to cholesterol, which is distributed within the cell; simultaneously, LDLR is recycled back to the plasma membrane, where it can continue this cycle. The binding of PCSK9 to LDLR results in LDLR degradation through one of two pathways - an intracellular pathway (red arrows), in which newly synthesized PCSK9 binds to intracellular LDLR and targets its to the lysosome for degradation, and an extracellular pathway (orange arrows), in which secreted PCSK9 binds to the LDLRs on the plasma membrane, resulting in its endocytosis and eventual lysosomal degradation.

In addition to the endocytic, extracellular pathway of PCSK9-mediated LDLR destruction, there is evidence for an intracellular pathway in which PCSK9 secretion and LDLR-PCSK9 endocytosis are not required (Holla et al., 2007; Poirier et al., 2009). In vitro, the inhibition of LDLR transport out of the endoplasmic reticulum blocked PCSK9-mediated degradation of LDLR, suggesting that PCSK9 acts to degrade LDLR as it travels from the endoplasmic reticulum to the plasma membrane (Maxwell et al., 2005). A separate study demonstrated the presence of both pathways concurrently, finding that inhibition of intracellular trafficking increased LDLR levels without affecting extracellular PCSK9-mediated LDLR degradation (Poirier et al., 2009).

Through LDLR degradation, PCSK9 is responsible for increasing plasma cholesterol levels, which designates it as a potential therapeutic target in hypercholesterolemia (Banerjee et al., 2022; Pasta et al., 2020; Salaheldin et al., 2022). Indeed, gain-of-function mutations of the PCSK9 gene were linked to an autosomal dominant form of hypercholesterolemia soon after discovering the enzyme (Abifadel et al., 2003; Hopkins et al., 2015). Correspondingly, the therapeutic inhibition of PCSK9 has therefore been the object of much interest.

2.2. PCSK9 in inflammation and the immune response

In addition to its cholesterol-modulating activity, PCSK9 has also been found to play a role in inflammation and the immune response. Investigation of the functions of PCSK9 in inflammation has yielded seemingly contradictory results. Several studies provide evidence to suggest that PCSK9 increases the activation of inflammatory pathways in the context of atherogenesis. PCSK9 has been found to induce LDLR-independent pro-inflammatory macrophage activation (Katsuki et al., 2022; Wang et al., 2022a) and an LDLR-dependent increase in reactive oxygen species (ROS) and pro-inflammatory cytokines/chemokines such as interleukin (IL)-1β, IL-6, tumor necrosis factor α (TNF-α), chemokine C-X-C motif 2 (CXCL2), and monocyte chemoattractant protein 1 (MCP-1) mRNA (Jaén et al., 2022; Ricci et al., 2018). Increased serum PCSK9 was also associated with increased fractions of plaque with necrotic core tissue in acute coronary syndrome or stable angina, indicating higher levels of inflammation (Cheng et al., 2016). Accordingly, PCSK9 downregulation exerts anti-inflammatory effects by inhibiting the activation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), potentially increasing anti-inflammatory cytokines, and upregulating immunosuppressive T regulatory (Treg) cells in models of atherosclerosis (Liu and Frostegård, 2018; Tang et al., 2012; Tang et al., 2017). Furthermore, induction of PCSK9 was also reported to have an inflammation-promoting response in dendritic cells exposed to oxidized LDL; PCSK9 downregulation reduced their maturation and inhibited dendritic cell-mediated T cell activation and proliferation (Liu and Frostegård, 2018). Together, these findings suggest an inflammation-resolving benefit to PCSK9 inhibition.

On the other hand, multiple studies with cancer models have shown that PCSK9 inhibition can increase cytotoxic T cell activity (Lei et al., 2021; Liu et al., 2020; Wang et al., 2022c; Yuan et al., 2021), downregulate Treg cells (Wang et al., 2022c), and induce the polarization of macrophages into the pro-inflammatory M1 phenotype (Wang et al., 2022b). The reasons for these discrepancies are unclear; it is possible that PCSK9 can exert different effects in different disease models. Furthermore, considering reports that PCSK9 is upregulated in multiple different cancers (Marimuthu et al., 2013; Nagashima et al., 2016; Pitteri et al., 2011; Wong et al., 2022; Xu et al., 2021; Yuan et al., 2021; Zhou et al., 2020; Zia et al., 2020), discussed below. It is also possible that the flux of PCSK9 may determine its ultimate pro- or anti-inflammatory effects; in other words, a possibility of dose-dependent, bi-phasic effects. Even among different cancer models, conflicting trends are seen regarding the inflammatory effects of PCSK9; for instance, in hepatocellular carcinoma cells, PCSK9 suppressed tumor growth and the M2 phenotype (Hu et al., 2022), once again demonstrating pro-inflammatory effects that are opposite to what was reported in other cancer models. Interestingly, PCSK9 downregulation has been reported in liver tissue from hepatocellular carcinoma patients (Bhat et al., 2015), supporting the possibility of dose-dependent effects and explaining the contradiction between this and other cancer models. Immune applications of PCSK9 inhibition have received much interest in the context of cancer, warranting further investigation of this area.

3. PCSK9 in the setting of cancer

While the role of PCSK9 inhibition in managing hypercholesterolemia has been extensively studied, the effects of PCSK9 in the context of different cancers is relatively less explored. There is significant indication that PCSK9 levels may have a prognostic value in the context of many different cancers and may be altered to provide a growth advantage for the cancer. Additionally, the results of multiple in vitro and in vivo studies indicate potential direct antineoplastic effects of PCSK9 inhibition, and several clinical trials have been designed to examine this effect further [Table 1].

Table 1:

Clinical trials of PCSK9 inhibitors in cancer (https://clinicaltrials.gov).

NCT Number	Study Title	Status	Description
NCT05144529	A randomized pilot study to investigate the safety and immunologic impact of evolocumab when given in combination with ipilimumab and nivolumab in treatment-naїve patients with metastatic non-small cell lung cancer	Recruiting	Assessment of the safety, tolerability, and antitumor effects of combination therapy of evolocumab with standard immunotherapies nivolumab and ipilimumab in advanced non-small cell lung cancer
NCT04937413	PEskE: A phase 0/surgical window-of-opportunity study to evaluate the pharmacokinetics and pharmacodynamics of evolocumab in patients with recurrent high-grade glioma or glioblastoma	Recruiting	Assessment of potential immunotherapeutic effects of evolocumab in the treatment of grade IV malignant glioma and glioblastoma by testing its ability to cross the blood brain barrier
NCT04862260	A phase 1 feasibility study of cholesterol metabolism disruption (evolocumab, atorvastatin and ezetimibe) in combination with folfirinox in patients with metastatic pancreatic adenocarcinoma	Recruiting	Assessment of the safety and antitumor effects of cholesterol metabolism disruption using combination therapy with atorvastatin, ezetimibe, Repatha (evolocumab) in addition to standard FOLFIRINOX chemotherapy in newly diagnosed metastatic pancreatic adenocarcinoma
NCT03337698	A phase ib/ii, open-label, multicenter, randomized umbrella study evaluating the efficacy and safety of multiple immunotherapy-based treatment combinations in patients with metastatic non-small cell lung cancer (morpheus-lung)	Recruiting	Assessment of the antitumor effects of multiple immunotherapy-based treatments for non-small cell lung cancer, including one combination of chemotherapeutic atezolizumab with evolocumab
NCT05553834	A phase II study of PCSK9 inhibitor alirocumab and PD-1 inhibitor cemiplimab in patients with metastatic, refractory to prior anti PD-1 non-small cell lung cancer: TOP2201	Not yet recruiting	Assessment of the antitumor effects, safety, and tolerability of combination therapy with alirocumab and anti-PD1 therapy cemiplimab in patients with metastatic lung cancer that was refractory to previous anti-PD1 therapy
NCT05128539	A Phase I Clinical Study to Evaluate the Safety, Tolerability, Pharmacokinetics and Preliminary Efficacy of Recombinant Humanized Anti- PCSK9 Monoclonal Antibody (JS002) Combined With Toripalimab in Patients With Advanced Cancer	Recruiting	Assessment of the safety, tolerability, and efficacy of combination therapy with humanized anti-PCSK9 antibody JS002 and anti-PD1 therapy toripalimab in patients with advanced tumors

PCSK9 overexpression has been observed in multiple different cancer types, including colorectal cancer (Wong et al., 2022), breast cancer (Pitteri et al., 2011), lymphoblastic leukemia (Zia et al., 2020), a case of hepatitis B-associated hepatocellular carcinoma (Nagashima et al., 2016), and gastric cancer (Marimuthu et al., 2013; Xu et al., 2021; Zhou et al., 2020). Analysis of clinical samples of breast, lung, and colorectal cancers similarly displayed increased PCSK9 levels in cancerous regions compared to the adjacent normal areas (Yuan et al., 2021). Furthermore, increased PCSK9 levels have been associated with increased severity and worse prognosis of multiple cancers (Bhattacharya et al., 2021; Wong Chong et al., 2022; Wu et al., 2020), whereas lower levels of PCSK9 were associated with improved prognosis and reduced tumor growth (Bonaventura et al., 2019; Momtazi-Borojeni et al., 2019b, c; Xie et al., 2022).A recent Mendelian randomization analysis also revealed that genetically proxied PCSK9 inhibition was associated with a lower risk of prostate cancer (Fang et al., 2023). These findings suggest a strong possibility that PCSK9 downregulation may counter cancer progression. Interestingly, it is suggested that some compounds with promising anticancer properties in vitro, such as Eugenol, a phenolic nutraceutical, and Pseurotin A, an alkaloid derived from Aspergillus fumigatus that also demonstrated anticancer activity in vivo, may exert their effects by downregulating PCKS9 (Abdelwahed et al., 2020; Zia et al., 2020).

Inhibition of PCSK9 using siRNA, gene knockout, or anti-PCSK9 vaccination and antibodies promotes apoptosis in several cancer models in vitro, including human neuroglioma (Piao et al., 2015), lung adenocarcinoma (Xu et al., 2017), and pancreatic neuroendocrine neoplasms (Bai et al., 2017). Further in vitro investigation has revealed multiple pathways through which PCSK9 inhibition can increase apoptosis; these include activation of endoplasmic reticulum stress (Xu et al., 2017) and mitochondrial (Piao et al., 2015; Xu et al., 2017) signaling pathways (including by activation of pro-apoptotic caspase3, down-regulation of the anti-apoptotic proteins XIAP, survivin, and p-Akt, improved Bax/Bcl-2 ratio leading to cytochrome c release, increased GRP78, GRP94, phosphorylated protein kinase R-like ER kinase and phosphorylated eukaryotic initiation factor 2α), and inhibition of G₁-S cell cycle progression (Wong et al., 2022). On the other hand, PCSK9 has been found to upregulate heat shock protein 70 in vitro, leading to the upregulation of mitogen-activated protein kinase (MAPK) signaling (Xu et al., 2021), a pathway which has been implicated in the progression of human cancers (Drosten and Barbacid, 2020). PCSK9 was also found to activate the KRAS/MEK/ERK pathway in vitro in APC/KRAS mutant colorectal cancer, while PCSK9 inhibition suppressed cancer growth (Xu et al., 2021). Likewise, in a murine model of intestinal tumorigenesis, PCSK9 knock-in mice had increased adenoma formations and adenocarcinoma development, reduced tumor cell apoptosis, and decreased Bax/Bcl-2 ratio and cytokine signaling 3 protein (SOCS3) suppressor levels (Yang et al., 2021). This same study showed that PCSK9 knock-in was linked to activation of Janus kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) signaling (Yang et al., 2021), a pathway known to lead to increased tumorigenesis (Jin, 2020); and treatment with PCSK9 inhibitor evolocumab reversed these effects (Yang et al., 2021). These findings corroborate the previously described observations that PCSK9 downregulation is associated with reduced tumor growth. Interestingly, in studies using non-cancer models, including cells stimulated with oxidized LDL (Li et al., 2017; Liu et al., 2017; Wu et al., 2012b), vascular smooth muscle cells stimulated with LPS (Ding et al., 2016), and potassium-deprived cerebellar granular neurons (Kysenius et al., 2012), PCSK9 downregulation was protective against apoptosis. These findings completely oppose the pro-apoptotic effects of PCSK9 downregulation in various cancer models as described above, suggesting the possibility that PCSK9 inhibition may preferentially induce cancer cell apoptosis while sparing host cells. Further investigation into these effects is required.

PCSK9 inhibition may also not have the same effects across different cancers. Whereas cancer combatting effects were seen with PCSK9 inhibition in the studies discussed previously, no tumor growth reduction or survival benefit was seen with the use of anti-PCSK9 vaccination in mice bearing melanoma tumors (Momtazi-Borojeni et al., 2020), and in fact, PCSK9 siRNA protected prostate cancer cells against ionizing radiation-induced cell damage by its anti-apoptotic activity in vitro (Gan et al., 2017). It is thus possible that the effects of PCSK9 downregulation may vary depending on the cancer origin. Additionally, one group suggested that tumor-derived, rather than host PCSK9, is more important in facilitating melanoma growth based on in vivo findings, and further suggested this as a reason why anti-PCSK9 vaccination prior to tumor implantation, as in (Momtazi-Borojeni et al., 2020), may not be as effective in its anticancer effects (Gu et al., 2023). A protective effect of PCSK9 against cancer growth and metastasis in vitro and in vivo has also been reported for hepatocellular carcinoma (He et al., 2021), whereas increased development of hepatic cancer has been seen in PCSK9 knockout mice injected with hepatic carcinogen diethylnitrosamine than wild-type mice (Ioannou et al., 2022). PCSK9 downregulation has also been reported in this cancer based on liver tissue analysis of affected patients (Bhat et al., 2015), indicating that it may be a survival mechanism of the cancer cells. Interestingly, in one study, the in vitro antitumor effects of Actinidia Chinensis Planch root extract in the context of hepatocellular carcinoma were attributed to PCSK9 upregulation (He et al., 2017). These studies suggest that hepatocellular cancer may serve as an example of a cancer in which downregulation of PCSK9 may have deleterious effects, in opposition to the trends seen with the cancers previously discussed. Ioannou et al. suggest a possible explanation for this observation - that while the upregulation of LDLR on hepatocytes by PCSK9 reduces plasma cholesterol and may inhibit other cancers, the increased exposure of the liver to cholesterol may play a role in increasing hepatocarcinogenesis (Ioannou et al., 2022). This possibility highlights an important aspect of this discussion: while PCSK9 inhibition may reduce circulating LDL cholesterol and thus starve cancers developing in many tissues, tissues expressing PCSK9 and LDLR, such as the liver, kidneys, small intestine, etc, may paradoxically increase their local uptake of LDL and subsequently facilitate tumor growth there. Still, the results other studies demonstrate that drawing even a cancer-type-specific conclusion may not be so simple, as an association between elevated PCSK9 levels and poor prognosis in hepatocellular cancer has been described (Zhang et al., 2021). Similarly, an in vivo anticancer effect of PCSK9 inhibition has also been reported in this cancer setting, mechanistically attributed to metabolic exhaustion and cell death by ferroptosis (Alannan et al., 2023). Particularly in the context of liver cancers, PCSK9 is suggested to engage in complex interactions between antitumor immunity and lipid metabolism, and careful therapeutic approaches may enable effective use of PCSK9 inhibition strategies to curtail cancers of this organ (Alannan et al., 2022). It is possible that in addition to differences in the effects of PCSK9 inhibition based on tumor origin, other factors such as the stage of the cancer, rate of tumor growth, and cancer-cell-specific mutations may be essential to consider on an individual basis when considering the effects of PCSK9 inhibition.

3.1. Cholesterol-dependent anticancer effects of PCSK9 inhibition

Cancer cell metabolism is reprogrammed to provide the cells with a proliferative and survival advantage compared to normal cells (Faubert et al., 2020). Metabolic alterations in cancer cells include changes in metabolite uptake and preferential use of metabolic pathways differing from normal cell metabolism - for instance, preferential “aerobic glycolysis” known as the Warburg Effect, where glucose is converted to lactic acid even in the presence of abundant oxygen (Wu and Zhao, 2013). In addition to this well-known alteration in glucose metabolism, numerous modifications in lipid metabolism have also been described in the setting of cancer, including increased lipid uptake by modulating the levels of lipid transporters such as CD36 and heparin sulfate proteoglycan (Menard et al., 2016; Zhao et al., 2017), and expression of lipogenic and fatty acid oxidation enzymes such as acetyl CoA-carboxylase, fatty acid synthetase, carnitine palmitoyltransferase-I, and ATP citrate lyase; these changes are extensively reviewed in (Beloribi-Djefaflia et al., 2016). Cholesterol, in particular, is a member of the lipid family crucial for the growth and function of eukaryotic cells (Mouritsen and Zuckermann, 2004). It is a ubiquitous component of the cell membrane and has essential roles in regulating membrane structural integrity and transmembrane signaling. In addition, it is a precursor molecule for synthesizing steroid hormones (Aguilar-Ballester et al., 2020). Expectedly, cholesterol is an essential component of cells, and its importance has been described in various physiologic systems, including the central nervous system, immune system, reproductive system, and others (Cantuti-Castelvetri et al., 2018; Shahoei and Nelson, 2019; Shi et al., 2018).

The role of cholesterol in carcinogenesis has also been extensively described (Ding et al., 2019; Huang et al., 2020; Kuzu et al., 2016). In a prospective study in Korea, increased total cholesterol (≥ 240 mg/dL) was positively associated with prostate and colon cancer in males and breast cancer in females. In contrast, an inverse association was seen between liver, stomach, and lung cancer in males (Kitahara et al., 2011). Furthermore, increased LDL cholesterol and baseline plasma cholesterol have been linked to an increased risk of colorectal neoplasms (Tian et al., 2015) and high-grade prostate cancer (Shafique et al., 2012), respectively, and increased intracellular cholesterol was indeed found in patient-collected prostate cancer cells compared to non-cancerous prostate cells (Singh et al., 2017). On the other hand, increased total serum cholesterol was inversely associated with several cancers, including liver, pancreatic, hematopoietic, and non-melanoma of the skin (Strohmaier et al., 2013). As such, the prognostic value of cholesterol levels is currently unclear in the context of cancer. Interestingly, in a murine model of breast cancer, it was observed that plasma cholesterol levels were reduced during tumor development but not prior to its initiation (Llaverias et al., 2011). This suggests that cholesterol profiles may differ depending on the cancer stage and its origin, whereas low levels may indicate an increased cholesterol uptake or consumption by the cancer. In addition to serum cholesterol abnormalities, the upregulation of cholesterol-related genes has also been observed in several types of cancers, including sarcomas, acute myeloid leukemias, melanomas, and breast cancers (Cai et al., 2016; Kuzu et al., 2016). Cancer cells may be equipped to induce hyperlipidemia themselves, and in one in vivo model, this effect was seen to be through increasing VLDL production and decreasing LDL and VLDL turnover (Huang et al., 2016a). Cholesterol and its metabolites, such as 27-hydroxycholesterol and cholesteryl ester, have also been associated with increased tumor formation, proliferation, angiogenesis, and invasion (Antalis et al., 2010; Cruz et al., 2010; de Gonzalo-Calvo et al., 2015; Llaverias et al., 2011; Rodrigues dos Santos et al., 2014; Wang et al., 2017; Wu et al., 2013; Zhuang et al., 2005), suggesting that cancer cells are increasingly reliant on cholesterol for their growth needs. As such, it is a possibility that cholesterol depletion may interfere with the growth of cancer cells with much less impact on host cells, as the former have much higher requirements for cholesterol. Indeed, breast and prostate cancer cell lines were more sensitive to cholesterol depletion-induced apoptosis than their normal counterparts (Li et al., 2006).

Mechanistically, increased cholesterol in the mitochondrial membrane has been found to confer a protective effect against apoptosis by inhibiting pro-apoptotic Bax activation in vitro and in vivo (Lucken-Ardjomande et al., 2008) and contribute to resistance to chemotherapy in a model of hepatocellular carcinoma by increasing membrane order (Montero et al., 2008). Additionally, increased cholesterol may serve to activate signaling pathways that have been implicated in oncogenesis, including MAPK (Wang et al., 2017), the Hedgehog pathway via activation of the G-protein coupled receptor Smoothened (Huang et al., 2016b; Luchetti et al., 2016), KRAS/MEK/ERK (Xu et al., 2021), and JAK2/STAT3 pathways (Yang et al., 2021); these effects are reviewed in (Patel and Kashfi, 2022). Thus, cholesterol depletion has been proposed as an avenue for targeting cancer growth and metastasis (Figure 2) (Murai, 2015). Indeed, separate models showed that depletion of cholesterol and its derivatives reduced tumor cell proliferation (Cruz et al., 2010), increased cancer cell apoptosis (Zhang et al., 2018; Zhuang et al., 2005), impaired DNA damage repair and increased the efficacy of cisplatin treatment (Zhang et al., 2019b), and induced shedding of CD44 (Murai et al., 2011), a cell surface adhesion molecule implicated in tumor migration (Chen et al., 2018), thereby preventing tumor invasion. Additionally, inhibition of cholesterol esterification was found to increase CD8⁺ T cell proliferation and inhibit melanoma growth and metastasis by increasing plasma membrane cholesterol levels and T cell receptor clustering, signaling, and formation of the immunological synapse (Yang et al., 2016).

Figure 2.

PCSK9 inhibition benefits in cancer and cancer-related outcomes. PCSK9 inhibition has several effects that may be beneficial in the context of cancer. PCSK9 inhibition thwarts PCSK9-mediated degradation of LDLRs, allowing for increased cholesterol clearance and reduced serum cholesterol, which may have anticancer effects by starving cancer cells of cholesterol. PCSK9 inhibition has also been found to induce cancer cell apoptosis and increase the clearance of bacterial LPS and LTA through an LDLR-dependent mechanism, which may be beneficial in the setting of sepsis, to which cancer patients are particularly vulnerable. Lastly, inhibition of PCSK9 boosts the T cell response through a number of mechanisms; these include inhibiting the MHC I downregulation that cancer cells often induce to escape immune detection, demonstrating synergy with and increasing the efficacy of anti-PD1 ICI therapy, and allowing for increased LDLR-mediated TCR recycling. This ability to boost the T cell response may reinstate the host defense against cancer and be useful in protecting against sepsis in already immunosuppressed cancer patients.

Considering the promise of cholesterol depletion in combatting cancer, the known role of PCSK9 inhibition in lowering cholesterol, and the previously described effects of PCSK9 in facilitating cancer progression, PCSK9 inhibition therapy may be uniquely suited for cholesterol depletion-mediated anticancer effects (Gangloff et al., 2017).PCSK9 knockout was protective against melanoma metastasis in the liver of mice, whereas a high-cholesterol diet reversed this effect (Sun et al., 2012). These findings suggest that the anticancer effects of PCSK9 inhibition are at least partially attributable to cholesterol depletion. Furthermore, PCSK9 was seen to be induced in a tumor model of BCR-Abl-transformed B cells, and the experimental reduction in VLDL production reversed the tumor-induced PCSK9-mediated hepatic LDLR degradation and tumor growth (Huang et al., 2016a). In addition to its effects on LDLR as seen here, PCSK9 also downregulates the receptor CD36 which is involved in fatty acid transport and triglyceride storage (Demers et al., 2015), as well as LRP-1 (Canuel et al., 2013), however, the upregulation of both of these has been associated with the progression and metastasis of several different cancers in vitro and in vivo (Deng et al., 2019; Ladanyi et al., 2018; Langlois et al., 2010; Le et al., 2020; Zhao et al., 2017). Further investigation into these additional targets is required, as while PCSK9 inhibition and subsequent upregulation of LDLRs may be beneficial, its effects on other targets may perhaps not be as desirable.

It is also possible that the promise of cholesterol-lowering may extend to other cholesterol-lowering therapies, such as statins. A protective effect of cholesterol-lowering statin therapy against the development, progression, recurrence, and mortality from certain cancers has, indeed, been previously described (Huang et al., 2016c; Sakellakis et al., 2016; Wu et al., 2012a; Zhang et al., 2019a). Yet separate meta-analyses have found insufficient evidence for statin use in reducing cancer incidence or improving cancer survival (Cholesterol Treatment Trialists, 2012; Jeong et al., 2020). Moreover, statins have been found to upregulate circulating PCSK9 and PCSK9 gene expression (Careskey et al., 2008; Dubuc et al., 2004; Khera et al., 2015; Welder et al., 2010), limiting the therapy’s cholesterol-lowering effects. As previously described, PCSK9 upregulation may be associated with a worse prognosis in cancer, an additional drawback of statin therapy in this setting. This drawback may be addressed by combination therapy of a PCSK9 inhibitor with a statin for synergistic effects; for example, evolocumab was found to synergize with simvastatin to promote apoptosis in KRAS-mutant colorectal cancer cells in vitro and reduce tumor growth in vivo (Wong et al., 2022). This calls for robust studies that explore the promise of cholesterol-depleting anticancer effects of PCSK9 inhibitors alone and in combination with statins.

3.2. PCSK9 inhibition and anticancer immune response

There is also evidence to support the role of PCSK9 inhibition in increasing the antitumor immune response through its effects on CD8⁺ T cells (Figure 2). CD8⁺ T lymphocytes are adaptive immune cells at the center of the immune response to cancer (Raskov et al., 2021). Intratumoral CD8+ T cell infiltration is a major indicator of cancer growth, metastasis, and recurrence (Galon et al., 2006; Mlecnik et al., 2011; Pagès et al., 2005). Unfortunately, cancer cells can downregulate the antitumor activity of cytotoxic T cells and escape host immune surveillance through several mechanisms. Ultimately, the failure of the immune response is critical to cancer progression (Chow et al., 2012; Parcesepe et al., 2016).

One crucial mechanism through which cancer cells evade immune surveillance involves the downregulation of the major histocompatibility complex (MHC) I protein in the cancer cells (Garrido et al., 2018; Maeurer et al., 1996; Maleno et al., 2006; Ryschich et al., 2005). The MHC I protein is responsible for presenting endogenous peptide antigens to CD8+ T cells (Wieczorek et al., 2017), and its downregulation on cancer cells impairs the cytotoxic T cell anticancer response. Evidence suggests that PCSK9 contributes to MHC I downregulation in vitro and in vivo by its lysosomal degradation - similar to its action with LDLR (Liu et al., 2020). Accordingly, PCSK9 downregulation was found to increase MHC I expression on the surface of the tumor cells, increase intratumoral infiltration of CD8⁺ T cells, and suppress tumor growth in mice (Guo et al., 2022; Liu et al., 2020). These results suggest that increased PCSK9 levels may provide a survival advantage to cancer cells, and its inhibition may be an avenue to restore anti-cancer immunity.

Using a melanoma model, Gu et. al recently demonstrated that the effects of PCSK9 on cancer immunity extend beyond MHC I modulation, with widespread systemic effects that facilitate immune evasion in vitro and in vivo (Gu et al., 2023). In this study, PCSK9 gain-of-function tumors demonstrated an upregulation of granzymes and perforin 1 and correspondingly, CD8⁺ T cell, NK cell, and macrophage infiltration; however, this was accompanied by the upregulation of several immune checkpoints (Gu et al., 2023), possibly to counter the former immune-boosting effect. PCSK9 loss-of-function tumors, on the other hand, upregulated the granzymes and perforin 1 without any increase in immune checkpoint proteins, suggesting PCSK9 to be associated with a more immunosuppressive tumor microenvironment (Gu et al., 2023). The immune-dependent anticancer effects of PCSK9 downregulation are at least partially attributable to the LDLR-upregulating impact of PCSK9 inhibition, as demonstrated by the impaired cytotoxic T cell anticancer response among LDLR-depleted CD8⁺ T cells (Yuan et al., 2021). Expectedly, LDLR overexpression enhanced CD8⁺ T cell activity (Yuan et al., 2021). Mechanistically, LDLR was found to be responsible for recycling the T cell receptor complex, which is crucial for CD8⁺ T cell function, and LDLR deficiency yielded reduced surface T cell receptor levels in CD8⁺ T cells (Yuan et al., 2021). Further investigation with clinical colorectal cancer samples revealed that increased PCSK9 levels were inversely correlated with cell surface T cell receptor complex levels and CD3⁺ T cell infiltration of the tumor tissue, and were associated with a worse prognosis (Yuan et al., 2021). Thus, by inhibiting the degradation of LDLRs, PCSK9 inhibition can directly increase the T cell response to cancer. In addition, the cholesterol-lowering effect of PCSK9 inhibitors that arise directly from their inhibition of LDLR degradation (Lagace, 2014) is also implicated in boosting the T cell response. Increased cholesterol in the tumor microenvironment has been found to induce CD8⁺ T cell exhaustion, reduce antitumor activity, and reduce the effects of T cell adoptive therapy using IL-9 secreting CD8⁺ T cells (Ma et al., 2019; Ma et al., 2021). In contrast, cholesterol depletion has stiffened cancer cell membranes, increased cytotoxic T-cell anticancer activity, and improved the therapeutic efficacy of adoptive T-cell cells in vitro and in vivo (Lei et al., 2021). Thus, PCSK9 has two separate LDLR-dependent mechanisms that may be promising in the context of immune activity against cancer.

There have also been reports of synergistic effects between ICIs and PCSK9 inhibitors in cancer setting. ICIs, as described previously, are a promising advance in cancer treatments as they increase T cell activity against cancer cells by blocking inhibitory signals (Martins et al., 2019); however, limited response rates are a persisting issue with these treatments (Haslam and Prasad, 2019; Liu et al., 2020; Schadendorf et al., 2015). Interestingly, increased efficacy of anti-PD1 ICI therapy has been reported in vitro and in vivo when combined with PCSK9 downregulation (Liu et al., 2020; Wang et al., 2022c), which is a promising finding. However, one effect of anti-PD1 treatment that limits its efficacy is the induction of Treg cells (Kamada et al., 2019; Kumagai et al., 2020; Wang et al., 2022c), an immunosuppressive cell type (Ohue and Nishikawa, 2019) which has been previously identified to promote immune evasion by cancer cells (Tanaka and Sakaguchi, 2019; Vinay et al., 2015). In addition, one study revealed that anti-PD1 therapy induced increased PCSK9 expression alongside the induction of Treg cells in vivo. In contrast, the combination of PCSK9 neutralizing antibodies and anti-PD1 therapy enhanced CD8+ T cell infiltration and reversed the rise in Treg levels seen with sole anti-PD1 treatment (Wang et al., 2022c). Interestingly, while the anti-PCSK9 antibodies could reduce the growth of colon cancer resistant to anti-PD1 ICI therapy, this effect was absent in PCSK9-deficient tumors, leading to the suggestion that tumor-derived PCSK9 rather than host PCSK9 is the target of the antibodies (Liu et al., 2020). In this manner, anti-PCSK9 antibodies may preferentially target cancer cells without disrupting host cells and increase the effects of anti-PD1 therapy when both are used together.

The enhanced cytotoxic T-cell function is of significant interest in combatting cancer. The preceding evidence suggests that PCSK9 inhibition may play a role in this aspect through multiple LDLR -dependent and -independent mechanisms. In addition to the effects of sole PCSK9 inhibition on the anticancer immune response, the synergy that has been demonstrated between anti-PD1 ICI therapy and PCSK9 inhibition makes it all the more promising in the context of cancer. This combination’s synergistic effects are being more closely examined in clinical trials (NCT05144529, NCT03337698).

3.3. PCSK9 and cancer angiogenesi

In relatively few studies, PCSK9 has also been found to play a role in angiogenesis, which is vital in supplying cancer cells with required metabolites, removing waste products, and enabling metastasis (Nishida et al., 2006). Recently, the overexpression of PCSK9 has been associated with the suppression of angiogenesis, whereas downregulating PCSK9 reversed this effect (Zhan et al., 2023). PCSK9 inhibitor evolocumab similarly increased the release of vascular endothelial growth factor (VEGF), a key mediator of angiogenesis in cancer (Carmeliet, 2005), in human umbilical vein endothelial cells (Safaeian et al., 2019). These findings were corroborated by a study in which PCSK9 inhibition in patients with coronary artery disease resulted in the upregulation of vascular endothelial growth factor A (Hrovat et al., 2022). A pro-angiogenic effect of PCSK9 inhibition may be a drawback of using PCSK9 inhibitors in cancer and necessitate its combination with other therapies such as anti-angiogenic agents in this setting. Yet there is also evidence from a pre-print publication to suggest that PCSK9 inhibition in cancer may still prove beneficial in this context by interrupting tumor co-option (Rada et al., 2022). Co-option is a non-angiogenic mechanism through which cancer cells hijack the existing host vasculature for growth and cell migration, and this phenomenon has even been suggested as a potential mechanism for anti-angiogenic drug resistance (Donnem et al., 2013). Thus, co-option may be as important a tumor blood supply as angiogenesis and merits a similar level of investigation in cancer management. Rada et. al demonstrated that increased serum cholesterol in patients with colorectal cancer liver metastases was associated with increased vessel co-option, and further in vivo investigation revealed both atorvastatin and evolocumab lipid-lowering therapies to attenuate the establishment of vessel co-opting liver metastases (Rada et al., 2022). Thus, while PCSK9 inhibition may further angiogenesis, which may hinder its proposed anticancer benefits, it may also reduce co-option through cholesterol-lowering, which is of benefit in this context. Further studies are required to extensively examine the effects of PCSK9 inhibition, on angiogenesis and vessel co-option in the context of cancer, and elucidate the overall effect of PCSK9 on tumor blood supply through these separate mechanisms.

4. PCSK9 inhibition in cancer-related complications

Beyond its effects on combatting cancer growth and metastasis, another matter of interest is the role of PCSK9 inhibition in the management of potential long-term complications related to cancer and its treatment. The various cholesterol- and immune-dependent mechanisms previously discussed in the context of inhibiting cancer growth and metastasis may also hold promise for other conditions associated with cancer, such as treatment-related dyslipidemia and sepsis. These implications are discussed in the following sections.

4.1. PCSK9 inhibition for dyslipidemia in cancer

A 1996 study first reported an increased risk of metabolic syndrome, including dyslipidemia, in long-term survivors of childhood cancer (Talvensaari et al., 1996). Since then, metabolic syndrome and dyslipidemic profiles have been described in acute lymphoblastic leukemia survivors (Chow et al., 2010; Malhotra et al., 2012; Morel et al., 2017), nephroblastoma, and neuroblastoma (van Waas et al., 2012). Likewise, hypercholesterolemia has been described as a paraneoplastic syndrome in hepatocellular carcinoma (Sohda et al., 2008). The development of dyslipidemia is a widely reported complication of several anticancer therapies, and studies have reported that it may either resolve after treatment (Willemse et al., 2014) or continue long-term (Morel et al., 2017). Many cancer-combatting therapies, including localized radiotherapy, surgery, hormonal therapy, and chemotherapy, have been implicated in developing dyslipidemia (Chueh and Yoo, 2017) with mechanisms including gonadal failure, modulation of apolipoprotein levels, and inhibition of lipoprotein lipase activity (de Jesus et al., 2022). In addition, the development of dyslipidemia is significantly increased in association with abdominal irradiation (van Waas et al., 2012) and treatments consisting of total body irradiation-based hematopoietic stem cell transplants (Chow et al., 2010).

Cholesterol-lowering therapies may thus be of immense importance in cancer patients, both for those with pre-existing dyslipidemia and those in whom cancer treatments may induce dyslipidemia. With its role in controlling cholesterol metabolism, the combination of PCSK9 inhibition with standard cancer therapies may lead to improved anticancer effects, as discussed previously, with the additional benefit of controlling the dyslipidemic effects of those standard therapies. Studies for this specific implication of PCSK9 inhibitors in cancer are limited; however, Tsakiridou et al. reported a case of resistant hypercholesterolemia induced by mitotane treatment for adrenocortical cancer, in which the PCSK9 inhibitor evolocumab allowed successful management after the failure of combination therapy with rosuvastatin and ezetimibe (Tsakiridou et al., 2018). The results of this case suggest that PCSK9 may have a unique role in managing cancer treatment-related dyslipidemia compared to other lipid-lowering agents. Additionally, as previously discussed, there is a possibility that the combination of a statin in addition to PCSK9 inhibitors may have enhanced potential in the management of dyslipidemia compared to either therapy alone. In one randomized control trial, adding alirocumab to atorvastatin produced the greatest lipid-lowering effect compared to alternative options such as increasing the dosage of atorvastatin, adding ezetimibe, or switching to rosuvastatin (Bays et al., 2015). To our knowledge, no prior studies have examined this effect in dyslipidemia in the context of current or past cancer diagnosis, and additional robust studies are warranted to explore this avenue further.

4.2. PCSK9 inhibition for elevated sepsis risk in cancer

Sepsis is an additional cancer-related complication. It is a condition of life-threatening multi-organ failure that arises from a dysregulation of the immune response to a pathogen. There is increasing evidence that sepsis is an immunosuppressive disorder that increases the population of Treg cells (Leng et al., 2013), induces extensive lymphocyte apoptosis (Hotchkiss et al., 1999), impairs the recovery of naïve CD8⁺ T cells (Condotta et al., 2013), and suppresses the CD8⁺ T cell response in an MHC I-dependent manner (Guo et al., 2021). In this regard, sepsis shares similarities with the immune dysregulation seen in cancer (Venet and Monneret, 2018). The same ICI therapies adopted in the context of cancer are being investigated with promising results in sepsis (Patil et al., 2017).

Immunosuppression during sepsis leaves patients unable to clear the offending pathogen and vulnerable to secondary infections (Patil et al., 2017). Thus, the immunosuppressive effects deployed by cancer cells to evade immune surveillance, combined with the immunosuppressive side effects of several cancer therapies (García Muñoz et al., 2014; van Meir et al., 2017), leave cancer patients particularly vulnerable to sepsis development and poor prognosis. The risk of sepsis is increased almost 10-fold among cancer patients compared to noncancer patients (Danai et al., 2006), and an increased risk of in-hospital mortality has also been reported (Hensley et al., 2019). Interestingly, not only is there an increased risk of sepsis among cancer patients, but sepsis has also been associated with the development of several cancers, including the lung, colon, rectum, liver, and of myeloid leukemia among elderly patients (Liu et al., 2019). The reason behind these findings is unknown, but potential explanations include the carcinogenic role of pathogens in the development of those cancers, inflammation-induced cancer predisposition, and negative effects of certain antibiotics (Liu et al., 2019). In addition, several studies have demonstrated the long-term consequences of sepsis, including the persistence of increased risk of all-cause mortality and major cardiovascular events for five years after discharge (Ou et al., 2016) and associations with increased cognitive impairment (Iwashyna et al., 2010). These findings highlight the importance of sepsis prevention and management, especially in the setting of malignancy.

PCSK9 inhibition has provided promising results in this area as well. Interestingly, plasma PCSK9 levels were significantly increased in sepsis and were associated with developing multiple organ failure (Boyd et al., 2016), suggesting a benefit to PCSK9-downregulation in this context. Indeed, reduced inflammatory cytokines reduced, mitochondrial DNA damage, and improved survival were noted with PCSK9 downregulation in murine and human responses to LPS (Ding et al., 2016; Walley et al., 2014). Additionally, the presence of loss of function variants of the PCSK9 gene was associated with higher 28-day survival, lower one-year death rate, and lower infection-related readmission in sepsis patients (Genga et al., 2018; Leung et al., 2019).

Mechanistically, the degradation of LDLRs by PCSK9 may play a role in interfering with the hepatic clearance of LPS released by gram-negative bacteria and lipoteichoic acid released by gram-positive bacteria (Figure 2). Hepatocyte LDLRs are responsible for the uptake of pathogenic lipids such as bacterial LPS and lipoteichoic acid from the bloodstream and subsequent hepatic clearance, an important function hindering the development of sepsis (Grin et al., 2018; Leung et al., 2019; Topchiy et al., 2016). In accordance with the LDLR-downregulating effects of PCSK9, incubation of hepatocytes with PCSK9 was found to reduce LPS and lipoteichoic acid uptake (Leung et al., 2019; Topchiy et al., 2016; Walley et al., 2014). Additionally, LPS itself, along with other inflammation-inducing agents zymosan and turpentine, were found to increase hepatic PCSK9 mRNA levels (Feingold et al., 2008), suggesting that these pathogens utilize increased PCSK9 levels to promote their virulence and may be combatted by PCSK9 downregulation. In addition to the clearance of these offending agents, there is a possibility that PCSK9 may be linked to viral infectivity through LDLR-dependent effects as well, from the small amount of evidence with Hepatitis C virus, Dengue virus, and perhaps SARS-CoV-2; this is reviewed further in (Seidah and Prat, 2022). Thus, potential antiviral activity may serve as a second mechanism through which PCSK9 inhibition is beneficial in curtailing sepsis risk in cancer patients especially (Seidah and Prat, 2022) the immune-boosting effects of PCSK9 inhibition also may have potential benefits in sepsis. Specifically, the synergistic effects of PCSK9 inhibition with anti-PD1 therapy (Liu et al., 2020; Wang et al., 2022c) might be of major therapeutic interest in sepsis, considering the promising results of anti-PD1 treatment in septic states (Brahmamdam et al., 2010; Patera et al., 2016). The role of alirocumab and evolocumab in sepsis is currently being investigated in three clinical trials (NCT03869073, NCT03634293, NCT05469347).

5. PCSK9-targeting therapeutic strategies

Recognition of the potential of PCSK9 inhibition in hypercholesterolemia and, more recently, other conditions, such as cancer, has led to the developing of several PCSK9-targeting therapeutic strategies. As mentioned in the previous discussion, PCSK9 is intracellularly synthesized and transported out of the cell; it can bind LDLR inside the cell during its transport or, after being secreted, bind to plasma membrane LDLR and induce its degradation through either pathway (Figure 1). Disrupting these events at any stage can be a strategy to counter the effects of PCSK9. The main methods for targeting PCSK9 include inhibition of its binding to LDLRs, downregulation of PCSK9 expression, and interference with the secretion of PCSK9.

Humanized monoclonal anti-PCSK9 antibodies are currently the most widely used PCSK9-targeting therapy. There are currently two anti-PCSK9 antibodies: alirocumab (trade name Praluent) and evolocumab (trade name Repatha) that are approved for use in hypercholesterolemia A third anti-PCSK9 monoclonal antibody, bococizumab, was in the process of development but was discontinued after reports of drawbacks including the formation of anti-drug antibodies, injection-site reactions, and significant variability in cholesterol-lowering effects (Ridker et al., 2017). Monoclonal antibodies fall into the first category of PCSK9-targeting therapies, as they bind to the catalytic domain of PCSK9 and inhibit its binding to LDLR, thus preserving LDLR from degradation (Chan et al., 2009; Ni et al., 2011). Upregulated LDLR the enables increased LDL uptake and reduces serum cholesterol. A favorable safety profile and high level of evidence supporting the use of these therapies in the management of dyslipidemia and prevention of cardiovascular events have been reported in several trials and meta-analyses (Cao et al., 2019; Jones et al., 2016; Kaddoura et al., 2020; Raal Frederick et al., 2014; Sabatine et al., 2017; Schmidt et al., 2020; Schwartz et al., 2018). However, despite their promising results, anti-PCSK9 monoclonal antibodies come with some limitations that may affect their widespread use, including cost (Arrieta et al., 2017; Kazi et al., 2016), in which statins have demonstrated superiority (Kazi et al., 2016), and the requirement for subcutaneous injections every 2–4 weeks (Schmidli, 2016), which may raise concerns about access and adherence.

Anti-PCSK9 vaccines have been proposed to address some of these limitations by providing longer-lasting effects and reducing costs. Anti-PCSK9 vaccination serves to induce host antibodies to PCSK9 and thus, in the same manner as monoclonal antibodies interfere with PCSK9-LDLR binding. Several anti-PCSK9 vaccines and delivery systems have demonstrated safe and effective induction of anti-PCSK9 antibodies in animal models, including an anti-PCSK9 peptide-based vaccine (Galabova et al., 2014), virus-like particle-based vaccine (Crossey et al., 2015), AT04 vaccine (Landlinger et al., 2017), and a peptide vaccine using a nanoliposome-based delivery system (Momtazi-Borojeni et al., 2019a). In addition, a “Liposomal Immunogenic Fused PCSK9-Tetanus plus Alum adjuvant” anti-PCSK9 vaccine was also found to be safe and immunogenic in a pre-clinical trial in non-human primates (Momtazi-Borojeni et al., 2021). However, optimizing the safety and efficacy of anti-PCSK9 vaccination is still in progress.

Adnectins are another alternative similar to monoclonal antibodies. These are small synthetic proteins derived from the 10^th type III domain of fibronectin (Lipovsek, 2011), and can be designed to bind with high affinity to specific target proteins. An anti-PCSK9 adnectin, BMS-962476, and shown effective lowering of LDL and PCSK9 in various animal models (Mitchell et al., 2014). A human study has validated the promise of this adnectin, demonstrating a strong reduction in LDL and PCSK9 levels with the use of BMS-962476 with minimal adverse effects reported (Stein Evan et al., 2014). Further investigation into this avenue has also led to the development of a recombinant fusion protein, LIB003, linking a PCSK9 adnectin to human serum albumin for longer half-life; in a placebo-controlled, double-blind, single ascending dose study, this protein achieved a ≥90% reduction in free PCSK9 (Stein Evan et al., 2019). A follow-up phase 2 study reported minimal adverse effects associated with this therapy (Stein et al., 2019), and there are several clinical trials further investigating this protein (NCT05234775, NCT04797247, NCT05004675, NCT03545438, NCT04034485, NCT04790513, NCT04806893, NCT04797104, NCT04798430, NCT03549260).

Along with anti-PCSK9 monoclonal antibodies, anti-PCSK9 vaccination, and anti-PCSK9 adnectins, several other less explored strategies have also been identified for their potential in disrupting PCSK9-LDLR binding and restoring LDLR recycling.One of these strategies includes the use of mimetic peptides, which, as according to their name, can mimic specific target proteins and thus inhibit their interaction with other endogenous targets. Several of these proteins have been investigated for their potential to inhibit PCSK9, including two separate analogs of the EGF-A domains of LDLR (Schroeder et al., 2014; Zhang et al., 2014). Additionally, there is significant interest in development of (1) PCSK9 C-terminal domain analogs, as this protein fragment was found to compete with PCSK9 in vitro for LDLR binding (Du et al., 2011), and (2) analogs of Annexin A2, which is an endogenous PCSK9 inhibitor (Mayer et al., 2008; Seidah et al., 2012) through a translational mechanism (Ly et al., 2014). The development of heparin sulfate mimetics has also been suggested as a potential strategy to inhibit PCSK9; for further reading, refer to (Catapano et al., 2020). Recently, a small molecule, ₁₃PCSK9i, has even demonstrated the ability to inhibit PCSK9-LDLR interaction through an allosteric mechanism in vivo (Brousseau et al., 2022). Certain natural compounds are also promising in this regard, such as Pseurotin A - an alkaloid derived from the fungus Aspergillus fumigatus; this compound has demonstrated the ability to inhibit the interaction between LDLR and PCSK9 at the EGF-A binding pocket (Abdelwahed et al., 2020). Thus, there may many more strategies to inhibit PCSK9-LDLR interaction beyondthe well-known anti-PCSK9 monoclonal antibodies, and further investigation into these avenues may yield more fine-tuned options.

In addition to inhibiting the binding of PCSK9 to LDLR, there are multiple therapeutic strategies that act by downregulating PCSK9 expression. These can act at various events involved in the ultimate expression of the protein, starting from gene expression to RNA transcription to protein translation. Targeting the first of these steps, CRISPR-Cas9 genome editing is suggested as a permanent way to downregulate PCSK9 (Ding et al., 2014). This strategy has already been applied and demonstrated promise in vivo in mice (Ding et al., 2014; Wang et al., 2016). In vivo base editing of PCSK9 is a related strategy that has also been explored in non-human primates, with promising findings of PCSK9 downregulation (Musunuru et al., 2021; Rothgangl et al., 2021). The drawbacks to this therapy, are the ethical concerns with gene editing, as well as the potential for causing off-target mutations (Nishikido and Ray, 2019).

At the level of RNA, there are several strategies to downregulate PCSK9 expression. The use of siRNA to induce gene knockdown by targeting mRNA for degradation has received the most interest in this category, and the chemically synthesized siRNA Inclisiran has already been approved for use in hypercholesterolemia. The use of PCSK9 siRNA has demonstrated favorable results in murine models (Frank-Kamenetsky et al., 2008) and in a phase 1 clinical trial (Fitzgerald et al., 2014), with reports of PCSK9 downregulation up to 70%. Inclisiran has also demonstrated promising results (Fitzgerald et al., 2016; Leiter et al., 2018; Raal et al., 2020), which is the subject of several current clinical trials. Interestingly, to our knowledge, no clinical data is available for Inclisiran in the specific context of cancer, and investigation of this subject is necessary as per the previous discussion of the potential benefits to PCSK9 inhibition in this setting. The use of antisense oligonucleotides (ASO), which can bind to mRNA and prevent its translation, is an additional strategy of interest in this regard, leading to the development of several ASO agents that were found to significantly downregulate PCSK9 in vitro, in vivo, and in a randomized placebo-controlled trial (Graham et al., 2007; Gupta et al., 2010; Lindholm et al., 2012; van Poelgeest et al., 2015; Yamamoto et al., 2012). The development of these agents has hit roadblocks such as reports of associated nephrotoxicity (van Poelgeest et al., 2015) and limited binding affinity, however, this avenue should continue to be further explored.

The last method through which PCSK9 expression can be is at the step of translation., Pharmacological agents such as (R)-N-(isoquinolin-1-yl)-3-(4-methoxyphenyl)-N-(piperidin-3-yl)propanamide (R-IMPP) (Petersen et al., 2016), and the small molecule PF-06446846 (Lintner et al., 2017) have demonstrated the ability to inhibit PCSK9 translation; mechanistically, the former inhibits PCSK9 translation by binding to the 80S ribosome, while the latter does so through inhibition during the elongation phase (Li et al., 2019). This strategy is also highly promising, and should be further investigated.

Lastly, interference with PCSK9 secretion into the circulation through targeting of the proteins sortilin and sec24a has also been proposed as a strategy to interfere with PCSK9 function (Alannan et al., 2022). Both of these have an essential function in transportation of PCSK9 after its intracellular production to the circulation, thus, inhibiting either of these may be an interesting strategy to explore further. The vast number of potential PCSK9-targeting therapies indicates that further investigations may yield even more optimal treatments than currently available.

6. Summary and Perspectives

There appears to be some controversy and inconsistencies regarding the role of PCSK9 in cancer risk. For example, PCSK9 levels are elevated and associated with worse prognosis in some cancers, while the opposite is true in others. Thus, additional preclinical and clinical investigations are needed to evaluate the potential role of PCSK9 in cancer. How PCSK9 acts in cancer pathophysiology, and the risks or benefits associated with its inhibition are questions that need more investigation. In addition, the safety and efficacy of anti-PCSK9 therapy in cancer need to be better evaluated. Although PCSK9 inhibition combatted many cancers, it increased cancer progression for some cancers. So, it is unclear if PCSK9 inhibition alone may be suitable or apply to all cancer management modalities. Importantly, anti-PCSK9 therapy will decrease circulating LDL cholesterol while increasing the exposure of PCSK9- and LDLR-expressing tissues to LDL cholesterol, perhaps providing a partial explanation for the contradictory anti- and pro- cancer effects of PCSK9 inhibition seen in different tumor models. It is also important to note that there are polymorphisms in PCSK9 (Cai et al., 2015), another factor that needs further investigation regarding its inhibition in the cancer arena. Furthermore, there are differences in the levels and activity of PCSK9 among different ethnicities, with Black individuals having higher circulating PCSK9 levels than White individuals (Enkhmaa et al., 2020).

Current PCSK9 inhibitors in clinical practice are quite expensive, which opens another pandora’s box as to who can afford them and for whom they are prescribed. A study using a national outpatient clinic registry linked to zip-code level on household income from the US Census to assess characteristics of patients with the atherosclerotic cardiovascular disease between September 1, 2015, and September 30, 2019, found that receiving a PCSK9 prescription was associated with White race and high estimated household income. Hispanics had lower odds of receiving PCSK9 prescriptions (Blumenthal et al., 2021). This study highlighted racial and socioeconomic factors underscoring inherent biases, which is quite alarming.

The prevalence of dyslipidemia and cardiovascular disease (CVD) is higher in Black individuals compared with non‐Hispanic White individuals (Jackson et al., 2020); however, as a cohort, they are less likely to be prescribed lipid‐lowering medications than White individuals (Kalra, 2021). Furthermore, minorities are underrepresented in many large cardiovascular trials, the focus being on older White men (Kalra, 2021). The effects of PCSK9 inhibitors in different races addressing CVD management and cancer need further studies. We need concrete recommendations to bridge the racial divide in CVD and cancer prevention and control.

Highlights.

• PCSK9 inhibition is a cholesterol-lowering therapy that may have promising anticancer effects.

• PCSK9 inhibition may combat cancer by cholesterol depletion, induction of cancer cell apoptosis, and boosting the host immune response.

• Cancer and its treatments can create a susceptibility to dyslipidemia and sepsis.

• PCSK9 inhibitor treatment may be useful in managing cancer-related sepsis and dyslipidemia in addition to its anticancer effects.

Abbreviations:

ApoER2

apolipoprotein E receptor 2

CTLA-4

cytotoxic T lymphocyte-associated protein-4

CXCL2

chemokine C-X-C motif 2

EGF-A

epidermal growth factor A

HNF1a

hepatocyte nuclear factor 1a

ICI

immune checkpoint inhibitor

IL

interleukin

JAK2/STAT3

Janus kinase 2/signal transducer and activator of transcription 3

LDLR

low-density lipoprotein receptors

MAPK

mitogen-activated protein kinase

MCP-1

monocyte chemoattractant protein 1

MHC

major histocompatibility complex

NF-κB

nuclear factor kappa-light-chain-enhancer of activated B cells

PCSK9

proprotein convertase subtilisin kexin type 9

PD-1

programmed cell death 1

PD-L1

programmed cell death 1 ligand 1

ROS

reactive oxygen species

SOCS3

cytokine signaling 3 protein

SREBP

sterol-responsive element binding protein

TNF-α

tumor necrosis factor α

Treg

T regulatory

VEGF

vascular endothelial growth factor

VLDLR

very low density lipoprotein receptor