Molecular targets of bempedoic acid and related decoy fatty acids

Abstract

Disorders of lipid metabolism, including hyperlipidemia, atherosclerosis, and metabolic dysfunction-associated steatotic liver disease, are increasing across the globe. Bempedoic acid (BPA) is a first-in-class drug for the treatment of hypercholesterolemia and cardiac risk reduction, which may particularly benefit those who do not tolerate statins. Inhibition of hepatic ATP-citrate lyase (ACLY) is widely accepted as the main mediator of its observed clinical effects. However, BPA treatment also has ACLY-independent effects on lipid metabolism, as the structural similarity of BPA to endogenous fatty acids allows it to trigger multiple lipid-signaling pathways. Here, we review the molecular targets of BPA and related ‘decoy fatty acid’ drugs and identify areas where further study is warranted as these molecules are evaluated for clinical indications.

‘Decoy fatty acid’ drugs such as BPA leverage metabolic regulation and signaling of endogenous fatty acids to treat lipid disorders

Metabolism is a highly regulated process, and key intermediates or end products of metabolic pathways often participate in negative feedback loops and/or modulate signal transduction [1]. These mechanisms ensure that when demand for a metabolite is met, its synthesis is controlled to conserve resources and maintain homeostasis. Conversely, dysregulation or imbalances in metabolism can lead to or exacerbate pathologies. For example, disordered lipid metabolism underpins conditions such as hyperlipidemia (see Glossary) [2], metabolic dysfunction-associated steatotic liver disease (MASLD) [3], and even cancer [4]. Three major nodes of fatty acid biology that have been studied to treat disordered lipid metabolism include: (i) controlling the balance of lipid synthesis and oxidation; (ii) transcriptional regulation of metabolic enzymes by peroxisome proliferation-activated receptors (PPARs); and (iii) signaling by free fatty acid-activated G-protein coupled receptors (GPCRs), also called free fatty acid receptors (FFARs) (Box 1 and Figure 1).

Box 1. Regulatory nodes of lipid metabolism.

Here, we briefly review three major regulatory nodes of lipid metabolism and point the reader to additional reviews that go into more depth on each topic:

Lipid synthesis and oxidation

The first step of fatty acid synthesis is initiated by cytosolic ACC, which produces malonyl-CoA from acetyl-CoA. When ACC is active, the accumulation of malonyl-CoA inhibits carnitine palmitoyltransferase 1 (CPT-I), which is responsible for bringing fatty acids into the mitochondria for oxidation as a component of the carnitine shuttle. Consequently, fatty acid oxidation is inhibited when fatty acids are being synthesized, preventing a futile cycle. Acetyl-CoA and malonyl-CoA are then used by FASN to make palmitate, which is readily converted to palmitoyl-CoA by long-chain acyl-CoA synthetases (ACSLs). Palmitoyl-CoA directly binds and inhibits ACC, thus slowing fatty acid synthesis during its accumulation [96], and additionally activates AMPK [40]. AMPK is a central fuel-sensing protein; activated AMPK in turn phosphorylates several targets including ACC, which decreases its activity. Palmitoyl-CoA is a more effective inhibitor of phosphorylated ACC compared to unphosphorylated, thus compounding the suppression of ACC activity [96]. When ACC is inhibited, malonyl-CoA levels drop, and the now-uninhibited CPT-I can transport lipids into the mitochondria for fatty acid oxidation. Thus, excess fatty acids not only slow fatty acid synthesis, but also promote fatty acid oxidation (see Figure 1A in main text). More information on the regulation of lipid synthesis and oxidation as well as therapeutic targeting of these enzymes can be found in other recent reviews [97,98].

Peroxisome proliferation-activated receptors (PPARs)

Lipids can also regulate metabolism transcriptionally by acting as ligands for PPARs, which belong to the nuclear hormone receptor family of transcription factors. Metabolic regulation and therapeutic targeting of each PPAR isoform is extensively reviewed elsewhere [99,100]. PPARα is expressed in several tissues and is well-known for its role in the liver where it regulates fatty acid transport, fatty acid oxidation, and ketogenesis in response to diet; PPARβ/δ is broadly expressed, including in skeletal muscle where it is involved in regulating glucose and lipid metabolism in response to exercise; and PPARγ is prominently expressed in adipocytes where it acts as a master regulator of adipogenesis (see Figure 1B in main text). PPARs are involved in many disease areas including inflammation, type 2 diabetes, cardiovascular disease, cancer, and neurodegeneration. A wide array of molecules have been found to bind PPARs, and fatty acids are among their several natural ligands. Consequently, there is a longstanding interest in developing synthetic PPAR modulators.

G-protein coupled receptors (GPCRs)/free fatty acid receptors (FFARs)

GPCRs are a large family of cell surface receptors that play crucial roles in many physiological processes. FFARs are a subset of GPCRs that are involved in lipid-sensing pathways. FFAR1/GPR40 and FFAR4/GPR120 are expressed in different cell types but can both bind long-chain fatty acids (LCFAs). FFAR1/GPR40 activation mediates insulin and gut hormone secretion, making it an attractive drug target for diabetes and obesity (see Figure 1C in main text). FFAR biology and pharmacology are extensively reviewed elsewhere [74,101].

Figure 1. Fatty acids in metabolic regulation and signaling.

(A) Overview of lipid synthesis, which occurs in the cytosol, and fatty acid oxidation, which occurs in mitochondria and peroxisomes. Major nodes of regulation and feedback inhibition are highlighted. (B) PPAR isoforms are expressed across multiple tissues and implicated in many metabolic processes. (C) Free fatty acid receptor 1 is a G-protein coupled receptor with multiple natural and synthetic ligands, including long-chain fatty acids, which produce antidiabetic and anti-obesogenic effects when activated. Abbreviations: ACC, acetyl-CoA carboxylase; ACLY, ATP-citrate lyase; ACSS2, short chain acyl-CoA synthetase 2; AMPK, AMP-activated protein kinase; CPT, carnitine palmitoyltransferase; FASN, fatty acid synthetase; FFAR, free fatty acid receptor; HMGCR, 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase; HMGCS, 3-hydroxy-3-methylglutaryl-CoA synthase; LCFA, long-chain fatty acid; PPAR, peroxisome proliferator-activated receptor.

Given that fatty acids regulate several aspects of lipid metabolism, fatty acid analogs, occasionally referred to as ‘fraudulent’ [5] or decoy fatty acid drugs, have been developed and tested since the 1970s [6,7]. These drugs ideally mimic fatty acids enough to trigger desirable regulation of lipid metabolism, but not enough for the drugs themselves to be extensively metabolized (Table 1). One such drug, BPA, was approved by the US FDA in 2020 to treat hypercholesterolemia in conjunction with maximally tolerated statins in individuals with heterozygous familial hypercholesterolemia or with established atherosclerosis; the first molecule in this class to be approved for clinical use. Within cells, BPA is esterified to its CoA form (bempedoyl-CoA, BP-CoA) by the enzyme ACSVL1 (alias FATP2) (Figure 2A), which is primarily expressed in the liver and, to a lesser extent, the kidney (Figure 2B). BP-CoA, but not BPA, inhibits ATP-citrate lyase (ACLY), one of the major sources of acetyl-CoA in the cytosol and nucleus, which contributes to fatty acid and cholesterol synthesis (Figure 1A) [8,9]. Two other decoy fatty acids (MEDICA 16 and TTA, Table 2) have also been shown to inhibit ACLY [10,11]. Other related decoy fatty acids, including K-111, gemcabene, LDT409, ESP 15228, and 326E, are described in Table 1.

Table 1.

Chemical structures, sources, and current usage of decoy fatty acids

Name (aliases)	Chemical structure	Source	(Pre)clinical progress
MEDICA 16		β,β’-methyl-substituted α/ω-dicarboxylic acids library [11], academic laboratory	Rodents show reduced lipid synthesis in liver [102], lower plasma lipids [103], and attenuated signs of atherosclerosis [75]; not tested in humans.
Tetradecylthio-acetic acid (TTA)		3-thia fatty acids library [57,104], academic laboratory	Phase 2 trial in males with type 2 diabetes showed favorable effects on dyslipidemia [82]; clinical testing discontinued due to reduced cardiac efficiency observed in mice [105].
K-111 (BM 17.0744)		ω-Substituted alkyl carboxylic acid library [79], Boehringer Mannheim/Roche Pharmaceuticals	Obese rhesus monkeys show favorable effects on body weight, insulin sensitivity, and high-density lipoprotein–cholesterol (HDL-C) levels [80]; not tested in humans.
Gemcabene (PD 72953)		Gemfibrozil diacid derivatives [62], Parke-Davis Pharmaceutical	Phase 2 trials show gemcabene alone raises HDL-C in patients [106] and improves circulating lipids in patients already receiving statins [107] with acceptable safety profiles.
LDT409		Saturated anacardic acid derivatives sourced from cashew nut shell liquid [64], academic laboratory	Mice on a high-fat diet show weight loss, improved glucose and lipid homeostasis in plasma, and lowered hepatic lipid accumulation [65].
ESP 15228 (10g)		Ketone-derived long-chain hydrocarbon library [47], Esperion Therapeutics	Rats showed reduced circulating lipids [47]; not tested in humans.
Bempedoic acid (BPA, ESP 55016, ETC-1002, Nexletol)		ESP 15228 derivative [41], Esperion Therapeutics	FDA approved for the treatment of hypercholesterolemia and cardiac risk reduction [14,16,17].
326E		BPA derivative [108], academic laboratory	Rodents and rhesus monkeys showed similar effects to BPA with increased bioavailability [108]; human trials have been initiated (chictr.org.cn, ChiCTR2200057793).

Figure 2. Bempedoic acid (BPA) is converted to bempedoyl-CoA (BP-CoA) by the enzyme ACSVL1.

(A) Conversion of BPA to BP-CoA by ACSVL1, also called FATP2. (B) Tissue distribution of ACSVL1 mRNA (gene name SLC27A2), sourced from The Human Protein Atlas (https://www.proteinatlas.org/ENSG00000140284-SLC27A2).

Table 2.

Molecular targets of decoy fatty acids or their -CoA derivatives

Target	Known decoy fatty acids	Free or -CoA conjugated	Refs
ACLY	BPA	−CoA	[8,9]
	MEDICA 16	−CoA	[11]
	TTA	Unknown	[10]
AMPK	BPA	−CoA	[8,9]
	MEDICA 16	−CoA	[36–38]
Fatty acid synthesis	BPA (ACC)	−CoA	[41]
	MEDICA 16 (ACC)	−CoA >> Free	[50]
	TTA (FASN)	Unknown	[10]
PPAR	MEDICA 16 (PPARα)	Unknown	[54,55]
	BPAa (PPARα)	Unknown	[23,66,67]
	TTAa (pan-PPAR)	Unknown	[56–58]
	K-111a (PPARα)	Unknown	[59]
	LDT409 (pan-PPAR)	Free	[64,65]
FFAR1/GPR40	MEDICA 16	Free	[71,73]
Mitochondrial membrane	MEDICA 16	Free	[86,88,89]
	TTA	Free	[87]
C/EBP	Gemcabenea (C/EBPδ and β)	Unknown	[95]
	MEDICA 16a (C/EBPβ)	Unknown	[37]

^aDirect binding not confirmed

ACLY is upstream of 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMGCR), the primary target of statins (Figure 1A). While statins are currently widely used to treat hypercholesterolemia, some patients experience intolerable myotoxicity [12]. Thus, BPA was predicted to benefit statin-intolerant patients due to its tissue specificity. To this end, the Cholesterol Lowering via Bempedoic acid, an ACL-Inhibiting Regimen (CLEAR) Outcomes clinical trial was designed to evaluate the effectiveness of BPA as a single agent for cholesterol management [13]. Patients on BPA had lower low-density lipoprotein–cholesterol (LDL-C) and experienced fewer major adverse cardiovascular events than those on placebo [14,15], which led to expansion of approval of BPA to include primary prevention of cardiovascular disease and secondary prevention in patients who have already had a cardiovascular event, as well as treatment of hypercholesterolemia without the requirement for concurrent statin use. Encouragingly, BPA did not appear to trigger new-onset diabetes; another known drawback of statins [14,16,17]. A more detailed comparison of the clinical effects of BPA, statins, and other lipid-lowering treatments and how BPA is currently being integrated into patient care can be found elsewhere [18,19].

Given that BPA is well tolerated by patients, interest has increased in expanding its use to treat other diseases where hepatic ACLY is a major player, including MASLD and liver cancer [20–28]. The BPA-activating enzyme ACSVL1 is also expressed in the kidneys (Figure 2B), and BPA is tolerated in patients with mild or moderate renal impairment [29], opening up the possibility that BPA might additionally benefit patients with certain kidney diseases. For example, BPA treatment improved renal function in a mouse model of polycystic kidney disease [30], and targeting lipid synthesis in clear cell renal cell carcinoma may be a promising therapeutic avenue given that this tumor type is characterized by aberrant lipid metabolism and uptake leading to accumulation of large lipid droplets [31].

However, substantial evidence indicates that BPA has targets in addition to ACLY that may contribute to its beneficial effects in vivo. BPA treatment suppresses hepatic lipid synthesis in mice consuming high amounts of fructose [23], and BPA consistently suppresses accumulation of liver triglycerides and cholesterol esters in mice on high-fat diets [21,23,27,32]. In contrast, liver-specific ACLY deletion has no effect on hepatic lipid synthesis in mice on a high-fructose diet (due to compensatory upregulation of ACSS2, see Figure 1A) [33], and ACLY perturbation has variable effects on hepatic lipid accumulation in mice depending on the models used and timing of ACLY targeting [21,23,28,34,35]. Importantly, BPA lowers hepatic lipid levels in both wild type mice and in mice lacking hepatic ACLY with diet-induced MASLD [23]. Thus, BPA reduces steatosis independently of ACLY and appears to have pleiotropic effects on lipid metabolism that may underpin its benefits.

While clinical use of BPA continues to expand, the recent evidence of ACLY-independent effects prompted us to reexamine the literature on BPA and similar fatty acid mimics in terms of the molecular mechanisms that contribute to their beneficial effects. In this review, we summarize the decoy fatty acids that have been developed, including BPA, and highlight published data pointing to established and putative molecular targets beyond ACLY. We hope that exploring the additional potential targets of BPA, BP-CoA, and related molecules will help guide the rational use of decoy fatty acids in the discovery of new lipid biology as well as in the clinic.

Pathways targeted by BPA and other decoy fatty acids

AMP-activated protein kinase

An additional mechanism through which BPA may regulate fatty acid synthesis is by activating AMP-activated protein kinase (AMPK), which is the most well-established target of BPA other than ACLY [8], and has been also shown to be activated by MEDICA 16 [36–38]. AMPK, a heterotrimer consisting of a catalytic α subunit and regulatory β and γ subunits, is a sensor of cellular energy charge that is activated when nutrients are scarce to suppress biosynthesis and stimulate catabolism [39]. For example, it inhibits acetyl-CoA carboxylase (ACC) activity (and therefore fatty acid synthesis) by phosphorylation at serine 79 (Figure 1A) [39]. AMPK β1-containing isoforms can be directly activated by long-chain fatty acyl-CoAs (LCFA-CoAs) [40]. Changes in AMPK or ACC phosphorylation states were not initially observed in primary rat hepatocytes after one hour treatment with BPA [41]. However, it was later reported that AMPK phosphorylation by LKB1 and ACC phosphorylation by AMPK is transiently increased about 10 min after BPA treatment [8]. The same study reported a marked increase in AMPK and ACC phosphorylation in rat livers after 14 days of daily BPA administration. In a follow up study, BP-CoA, but not BPA, was shown to activate human purified AMPK isoform α1β1γ1, but not α1β2γ1 [ 9]. To address if AMPK activation was necessary for LDL-C lowering, Ampkβ1^−/− mice were crossed with Apoe^−/− mice; a model of atherosclerosis. BPA controlled plasma lipids and improved signs of atherosclerosis independently of AMPK in these mice [9].

Despite AMPK activation likely being dispensable for BPA-mediated LDL-C lowering, AMPK may be relevant to other effects of BPA. Liver-specific genetic and pharmacological activation of AMPK mitigates MASLD in animal models [42–45]. Accordingly, BPA treatment lowers hepatic lipid levels in mouse models of MASLD [21,23,27,32]. Notably, mouse livers predominantly express β1-containing AMPK isoforms, which are activated by BP-CoA, while human livers mainly express the α1β2γ1 complex, which is not activated by BP-CoA [9,46]. Thus, AMPK-mediated effects of BPA treatment may be blunted in humans and must be evaluated empirically.

Fatty acid synthesis pathway

ACC converts acetyl-CoA to malonyl-CoA as the first committed step of fatty acid synthesis, followed by palmitate synthesis by fatty acid synthase (FASN). In addition to ACLY, acetyl-CoA for fatty acid and sterol synthesis can be generated by short chain acyl-CoA synthetase 2 (ACSS2), from acetate (Figure 1A). Leading up to the development of BPA, Esperion Therapeutics initially synthesized a library of ketone-derived long-chain fatty acid derivatives, which were tested for their ability to inhibit lipid synthesis using an assay that measured incorporation of ¹⁴C-labeled acetate into newly synthesized lipids [47]. Similarly, the first characterization of BPA showed dose-dependent decreases of ¹⁴C acetate incorporation into fatty acids and sterols both in cells and in vivo [41]. Since acetate metabolism is independent of ACLY, the observed reduction in lipid labeling from acetate suggests that BPA must also directly or indirectly target other enzymes in the de novo lipogenesis pathway. MEDICA 16 and TTA similarly inhibit acetate-dependent lipid synthesis in cultured cells [11,48], and an abstract summarizing unpublished data suggests the same is true for gemcabene [49]. Moreover, BP-CoA, but not BPA, dose-dependently inhibited activity of partially purified ACC [41]. Treatment of primary rat hepatocytes in this study also led to increased CPT-I-dependent β-oxidation of fatty acids, which is consistent with the downstream effects of ACC inhibition (i.e., reduced malonyl-CoA abundance) (Figure 1A) [41]. In line with this, serum β-hydroxybutyrate, a product of fatty acid oxidation and ketogenesis, was elevated in BPA-treated rats over controls [41].

CoA-activated MEDICA 16 (MEDICA 16 CoA), and to a lesser extent free MEDICA 16, also directly inhibit ACC [50]. MEDICA 16 CoA inhibits the noncarboxylated form of ACC which binds ATP with a K_i of 58 μM, and it also inhibits the carboxylated intermediate form of ACC which binds acetyl-CoA with a K_i of 2 μM [50]. Free MEDICA 16 was found to inhibit ACC with a K_i of 70 μM by competing with citrate, an allosteric activator [1,50]. MEDICA 16 decreased ACC activity in rat livers, but not skeletal muscle [51]; since MEDICA 16 has been shown to be converted to its CoA form in liver [52], these in vivo data are consistent with the CoA form of MEDICA 16 acting as a more potent ACC inhibitor than the free form. Importantly, the K_i of MEDICA 16 CoA for ACLY is 16 μM [11], suggesting that ACLY and ACC inhibition occur at a similar dosage. Interestingly, TTA was found to inhibit FASN, the enzyme downstream of ACC, but not ACC itself [10].

Mevalonate pathway

Acetyl-CoA can also be used for sterol synthesis via the mevalonate pathway (Figure 1A). While ACC inhibition can explain why BPA treatment lowers labeled acetate incorporation into fatty acids, it does not explain the similar trend observed in sterols. Incorporation of ¹⁴C mevalonolactone (which is readily converted to mevalonate) into sterols is unchanged by BPA treatment [41], suggesting that an enzyme earlier in the mevalonate pathway (ACSS2, ACAT2, HMGCS, or HMGCR) is directly or indirectly affected by BPA or BP-CoA. A later study on BPA showed that treatment of primary rat hepatocytes resulted in a transient increase in HMGCR phosphorylation at serine 872 [8], which is known to inhibit enzymatic activity [53]. HMGCR phosphorylation can occur downstream of AMPK activation [8] (Figure 1A; see discussion on AMPK above), providing a possible mechanism of BPA-dependent inhibition of sterol synthesis. Of note, BPA treatment was also shown to reduce levels of the acetyl-CoA acetyltransferase (ACAT) 2 product acetoacetyl-CoA in mouse livers independently of ACLY [23], although no direct evidence for ACAT2 inhibition has been reported. Thus, BPA and BP-CoA may inhibit both fatty acid and sterol synthesis by multiple mechanisms.

Peroxisome proliferator-activated receptors

MEDICA 16, TTA, K-111, and gemcabene were all hypothesized to act as PPAR agonists early in their development. MEDICA 16 induces peroxisomal proliferation and increases expression of PPARα target genes in hepatocytes [54,55], TTA treatment results in pan-PPAR activation [56–58], and K-111 treatment results in PPARα activation [59]. Free fatty acids have been shown to bind to PPARs w ith n anomolar K_d values similar to synthetic PPAR activators, and even cocrystalize with PPARs [60,61]. However, cell-free studies of PPAR activation with MEDICA 16, TTA, and K-111 have not been reported, precluding claims of direct engagement in the cell. Gemcabene was originally thought to directly activate both PPARα and PPARγ [62], but the observed upregulation of PPAR target genes upon treatment was later shown to be through an indirect, unknown mechanism [63].

Recently, LDT409 and similar chemical derivatives were developed as novel PPAR agonists [64]. These compounds were tested for PPAR activation using a luciferase reporter gene assay and direct PPAR binding using a thermal shift assay with purified proteins. Both assays demonstrated that LDT409 has strong activity for PPARα and PPARγ and weaker activity for PPARβ. In addition, PPAR activation was confirmed in vitro using cultured adipocytes and in vivo using a transgenic zebrafish model [64]. In mice, LDT409 stimulated PPAR-dependent gene expression to a similar extent as WY14643 and Rosiglitazone, known activators of PPARα and PPARγ, respectively [65]. LDT409 lowered food intake and hyperlipidemia while increasing energy expenditure, leading to weight loss and improved glycemic control in mice on a high-fat diet. These effects were largely attributed to the PPARα target FGF21 [65].

Multiple recent studies have shown that BPA promotes hepatic expression of PPARα target genes [23,66,67], with one study using a luciferase-based reporter cell line to demonstrate that BPA treatment activates human PPARα (EC₅₀ = 168 μM) and PPARγ (EC₅₀ = 332 μM), although less potently than the agonists GW7647 (PPARα EC₅₀ = 5.3 nM), and rosiglitazone (PPARγ EC₅₀ = 76 nM) [67]. However, it remains unclear if BPA or BP-CoA are direct ligands, or if they activate PPARα/γ via an alternative mechanism. For example, BPA treatment in mice increases levels of hepatic PPARα itself at both the mRNA and protein levels [66]. Moreover, BPA may impact the availability of endogenous PPARα lipid ligands. The phospholipid PC-16:0/18:1 is an established endogenous PPARα ligand with a lower affinity for PPARα than a synthetic ligand (K_i= 33.2 μM vs. 11.06 μM for WY14643 as measured by displacement of ³H-WY14643) [ 68]. Induction of target gene expression by PC-16:0/18:1 was found to be comparable to that by the synthetic PPARα activators GW7647 and WY14643 [68,69]. Interestingly, PC-16:0/18:1 abundance has been shown to be regulated in a circadian manner in mice fed a high-fat diet [70]. Hepatic ACLY knockout reduces the abundance of PC-16:0/18:1 and expression of PPARα target genes in livers of mice fed a Western diet at the circadian peak, while BPA treatment rescues PC-16:0/18:1 abundance in ACLY-deficient animals [23]. Importantly, although ACSVL1 is a peroxisomal enzyme, the formation of BP-CoA is comparable in WT mice and mice with hepatic ACLY deficiency [23]. Cumulatively, the data suggest that PPARα may be an important contributor to the effects of BPA on lipid metabolism, though more mechanistic studies are needed.

Additional cellular targets of decoy fatty acids

Since decoy fatty acids tend to engage similar targets, the extensive body of research on these compounds can provide insight into additional pathways that might be impacted by BPA.

Free fatty acid receptors

FFARs are a family of G-protein-coupled receptors that trigger signaling cascades in response to extracellular lipid sensing (Box 1). FFAR1/GPR40 can be activated by medium- to long-chain fatty acids, and FFAR4/GPR120 can be activated by long-chain fatty acids [71,72]. Although BPA itself has not reportedly been tested against FFARs, MEDICA 16 specifically activates FFAR1/GPR40 but not FFAR4/GPR120 [71,73]. FFAR1/GPR40 activation of pancreatic β-cells has been shown to have antidiabetic effects [74]. BPA [41], MEDICA 16 [75–77], BM 17.0744/K-111 [78–80], and TTA [81] are all reported to improve insulin sensitivity in insulin-resistant animal models, with TTA also increasing insulin levels in patients with type 2 diabetes mellitus [82]. Early clinical trials suggested that BPA might reduce hemoglobin A1C (HbA1C) in patients with diabetes or prediabetes [83] and reduce the risk of new-onset or worsening diabetes [84]. However, the more recent CLEAR Outcomes trial, which had a larger patient cohort and longer treatment duration, showed no effect on HbA1C compared to placebo [17]. Notably, BPA does not appear to increase new-onset diabetes, which is a known risk factor of statin therapy [17]. It remains to be explored if FFARs are activated by BPA at clinically relevant dosages.

Mitochondrial uncoupling

Excess saturated LCFAs can directly interfere with the mitochondrial membrane, disrupting the proton gradient and thus uncoupling ATP-production from the electron transport chain [85]. Similarly, MEDICA 16 and TTA can cause mitochondrial uncoupling by ‘flip-flopping,’ or rapid transbilayer movement in the inner mitochondrial membrane [86–89]. However, these effects are seen using concentrations of MEDICA 16 of at least 100 μM, compared to its K_i for ACLY of 16 μM, so it is unclear if these effects would be physiologically relevant at normal dosages in vivo.

Inflammatory pathways

Although BPA and other decoy fatty acids were developed to treat lipid disorders, metabolism and inflammation often go hand in hand. In animal models, BPA and MEDICA 16 both improve markers of atherosclerosis, a vascular disorder compounded by both dyslipidemia and inflammation [9,75]. In a preclinical model of hepatic steatosis, BPA reduces inflammation, activation of hepatic stellate cells, a nd fibrosis, effects not observed with hepatic ACLY knockout [21]. This study also showed that hepatic stellate cells, like hepatocytes, express ACSVL1, suggesting that BPA can also be activated in these cells (Figure 3A).

BPA added to statin therapy significantly lowered C-reactive protein (CRP) in patients with known atherosclerotic disease and/or heterozygous familial hypercholesterolemia who had residual inflammatory risk when taking statins alone [90]. CRP is produced in large amounts by the liver in response to inflammation, infection, or tissue damage, and is associated with an increased risk of heart disease [91]. The mechanistic basis for the clinical finding that BPA lowers CRP is unknown. Possible relevant transcriptional mediators that decoy fatty acids may impact include C/EBP family transcription factors, NF-κB, and STAT3. Proinflammatory molecules, such as lipo-polysaccharides (LPS), IL-6, and IL-1β, induce binding of all these transcription factors to the promoter of CRP and activate CRP expression [92–94]. Gemcabene treatment decreases the binding of C/EBPδ (and to a lesser extent C/EBPβ) to the CRP promoter in IL-6 and IL-1β stimulated human hepatoma cells [95]. This study also suggests that NF-κB-mediated transcription of CRP may be disrupted by gemcabene, but this finding is inconclusive since the mutation introduced into the NF-κB response element to test this hypothesis overlaps the C/EBP binding site. Additionally, MEDICA 16 or related analogs decrease CRP production in human-CRP transgenic mice after LPS or IL-6 induction [37], although the proposed mechanism differs from that of gemcabene. In this study, MEDICA 16 increased expression of C/EBPβ mouse homologs, which bound to and inhibited the activity of the transcription factor FoxO1, along with FoxO1-dependent STAT3 activity. In parallel, MEDICA 16 treatment increased AMPK-mediated export of FoxO1 out of the nucleus [37]. BPA treatment reduces FoxO1 protein levels in rat liver and primary rat hepatocytes [8], but the effect of BPA on FoxO1 activity and nuclear/cytosolic distribution was not investigated. More research is needed to decipher how decoy fatty acids affect inflammatory pathways and if these molecules can be leveraged to treat inflammatory conditions.

Concluding remarks and future perspectives

The recent clinical success of BPA shows that decoy fatty acids are promising tools to help the increasing number of patients suffering from metabolic disorders. Specifically, BPA improves markers of disordered lipid metabolism and inflammation in patients while reducing the risk of adverse cardiac events. Decades of research on decoy fatty acids have demonstrated effects on multiple nodes of lipid metabolism and regulation (Table 2). However, the molecular mechanisms of decoy fatty acids, namely on lipid synthesis enzymes, PPARs, FFARs, and inflammatory pathways, are incompletely understood (Outstanding questions).

Outstanding questions.

How does BPA inhibit sterol and fatty acid synthesis independently of ACLY? What are the direct targets of BPA within these pathways, and do these contribute to its clinical benefits?

Do decoy fatty acids or their CoA derivatives directly activate PPARs?

What are the downstream effects of decoy fatty acid-mediated FFAR1 activation and mitochondrial uncoupling? Does BPA impact these pathways at pharmacological doses?

How do decoy fatty acids lower CRP, and can they be leveraged to treat inflammatory disorders?

Preclinical studies suggest that BPA or related molecules may additionally have potential to treat diseases such as MASLD, liver cancer, and polycystic kidney disease. An additional hurdle for researchers will be to untangle the pleiotropic effects of BPA on lipid metabolism to determine which pathways predominantly mediate specific effects in patients, which could aid in further refining therapeutic strategies. This is complicated by the fact that preclinical models do not perfectly recapitulate lipid metabolism and regulation in humans. Ultimately, more studies to better understand the mechanisms of decoy fatty acids will be key to strategically leveraging BPA and similar molecules for therapeutic benefit.

Highlights.

Decoy fatty acids, including bempedoic acid (BPA), structurally resemble endogenous fatty acids and trigger many of the same regulatory and signaling pathways.

Hepatic ATP-citrate lyase (ACLY) inhibition is considered the primary mechanism of low-density lipoprotein-cholesterol lowering with BPA treatment.

In addition to targeting ACLY, BPA has been shown to activate AMP-activated protein kinase, inhibit acetyl-CoA carboxylase, and promote peroxisome proliferator-activated receptor α activation.

Other decoy fatty acids are known to increase mitochondrial uncoupling, activate free fatty acid receptor 1, and lower C-reactive protein.

Glossary

Atherosclerosis

inflammatory condition associated with the buildup of lipid plaques in the arteries, causing them to narrow and harden

Decoy fatty acid

synthetic molecules that chemically resemble natural fatty acids and may trigger fatty acid signaling pathways without being extensively metabolized

Hemoglobin A1C (HbA1C)

a measure of glucose-conjugated hemoglobin which is commonly used as a proxy for average blood sugar levels. High HbA1C can be an indicator of poor glycemic control and diabetes

Hepatic stellate cells

minor population of livers cells which are activated upon liver injury and contribute to fibrosis

Heterozygous familial hypercholesterolemia

an autosomal dominant genetic disorder, often occurring within the gene encoding for the LDL receptor, which leads to abnormally high LDL-C and increased risk of cardiovascular disease

High-density lipoprotein–cholesterol (HDL-C)

cholesterol associated with HDL, a molecular complex that transports lipids from cells and arterial walls to the liver. Higher HDL-C tends to be protective against atherosclerosis

Hypercholesterolemia

a pathological imbalance of LDL-C and HDL-C.

Hyperlipidemia

pathologically high levels of circulating cholesterol and/or triglycerides

Low-density lipoprotein–cholesterol (LDL-C)

cholesterol associated with LDL, a molecular complex that delivers lipids to cells and is associated with increased risk of cardiovascular disease

Metabolic dysfunction-associated steatotic liver disease (MASLD)

chronic buildup of lipids in hepatocytes resulting from metabolic dysfunction which can progress to metabolic dysfunction-associated steatohepatitis and lead to liver failure

Mevalonolactone

cell-permeable metabolic precursor of mevalonate

Pancreatic β-cell

a specialized cell type that is responsible for synthesizing and secreting insulin in response to increased circulating glucose

Statins

drugs that lower circulating cholesterol primarily by inhibiting HMG-CoA reductase, the rate-limiting enzyme in cholesterol synthesis

Steatosis

buildup of excess fat in the liver, also known as fatty liver disease

Type 2 diabetes mellitus

chronic condition characterized by insulin resistance and poor glycemic control