The Multifunctional Protein p62 and Its Mechanistic Roles in Cancers

Abstract

The multifunctional signaling hub p62 is well recognized as a ubiquitin sensor and a selective autophagy receptor. As a ubiquitin sensor, p62 promotes NFκB activation by facilitating TRAF6 ubiquitination and aggregation. As a selective autophagy receptor, p62 sorts ubiquitinated substrates including p62 itself for lysosome-mediated degradation. p62 plays crucial roles in myriad cellular processes including DNA damage response, aging/senescence, infection and immunity, chronic inflammation, and cancerogenesis, dependent on or independent of autophagy. Targeting p62-mediated autophagy may represent a promising strategy for clinical interventions of different cancers. In this review, we summarize the transcriptional and post-translational regulation of p62, and its mechanistic roles in cancers, with the emphasis on its roles in regulation of DNA damage response and its connection to the cGAS-STING-mediated antitumor immune response, which is promising for cancer vaccine design.

1. INTRODUCTION

p62 (also named EBIAP, ZIP3, SQSTM1/Sequestosome-1), a human homolog of mouse ZIPs (Zeta PKC-interacting proteins), functions as a multifunctional signaling hub that controls myraid cellular processes, including DNA damage response (DDR), cancer development, aging, osteoclastogenesis, inflammation and immunity, cell differentiation, neurotrophin properties and obesity, with or without the involvement of autophagy [1–5]. It is crucial for cellular homeostasis and detoxification of reactive oxygen species (ROS) during these processes.

p62’s functions are well defined by its interacting motifs (Fig. 1), with its PB1 domain and ubiquitin-associated (UBA) domain involved in protein aggregation, LC3-interacting region (LIR) targeting ubiquitinated proteins to autophagy, and Keap1 interacting region (KIR) motif regulating ROS [1–6]. The PB1 domain interacts with a panel of proteins including MEKK3 that regulates NFκB activity [7], and is also responsible for p62 oligomerization [8]. In addition, p62 was shown to interact with the adaptor protein LIMD1 in the LIMD1-p62-TRAF6-PKCζ multi-protein complex [9, 10].

Fig. (1). Scheme of the protein domains of human p62.

PB1: Phox/Bem1p protein–protein binding domain. AID: atypical PKC interacting domain [170]. ZNF: Zinc finger. LB: LIM protein binding. TB: TRAF6 binding. LIR: LC3-interacting region that mediates interaction with ATG8 family. UBA: Ubiquitin-binding region that binds specifically to K63-linked polyubiquitin chains of polyubiquitinated substrates [4, 88]. NES: nuclear export signal. NLS: nuclear localization signal [65].

2. REGULATION OF P62

2.1. Transcriptional Regulation of p62

p62 is induced by NFκB in myriad biological processes, by NRF2 in response to oxidative stress [11], and by AP1 in the context of Ras-induced oncogenesis [12]. Expression of p62 is strongly induced during LPS priming of macrophages in an NFκB-dependent manner [13]. p62 is also induced by the Ets factor PDEF and by Ras that accounts for at least 25% of human cancers [12, 14]. The p62 gene promoter contains binding sites for NFκB, AP1, ARE (responsive to NRF2), Ets, ATF4, Myc, Sp1 and Pu.1, and other potential transcription factors [11–17], which account for its transcriptional regulation in diverse contexts.

The antioxidant transcription factor NRF2 is constitutively ubiquitinated for proteasomal degradation under normoxia by the Ub E3 ligase complex Keap1/Cul3/RBX1. ROS/oxidative stress triggers autophagic degradation of Keap1, resulting in the accumulation and activation of NRF2, which induces expression of p62 by binding to the ARE in the p62 gene promoter [18–25]. In turn, p62 binds to Keap1 via p62 KIR domain, disrupting Keap1-NRF2 interaction, resulting in NRF2 stabilization and activation [26].

Another main transcription factor for p62 induction is NFκB, which is activated in response to myriad cellular insults including pathogenic infections and genotoxic and oxidative stresses. Consequently, p62 plays a crucial role in immunity and inflammation, aging/senescence, and cancer initiation and development.

Interestingly, our ongoing investigation has generated an increasing pool of evidence showing that p62 is also induced by HIV persistent infection, and by Epstein-Barr Virus (EBV) in its latent infection through both LMP1 and ROS signaling pathways. Moreover, our evidence shows that p62 is a key component of the LMP1 signalosome for NFκB activation and regulates oxidative stress and DNA damage response in EBV latency through a LIMD1-dependent autophagy mechanism (to be published). ROS is produced separately by the EBV products LMP1, EBNA1/2, and EBERs [27–31].

2.2. Regulation of p62 Protein Stability and Activity

As the first identified selective autophagy receptor, p62 is well known to mediate selective autophagy. Interestingly, p62 itself is also a selective autophagy substrate that is targeted by autophagy for degradation [26, 32–42]. We have also shown that excess oxidative stress induces p62 degradation in EBV latency [43], through a LIMD1-dependent autophagy mechanism that is under further investigation. p62 protein level is increased with age due to the decreases of autophagic activity in many tissues and organs [44], and is associated with DNA damage foci in the nucleus [45]. However, transcriptional expression of p62 decreased with age due to aging-associated oxidative DNA damage of p62 gene promoter [46, 47].

p62-mediated autophagic degradation activity is enhanced through Keap1/Cullin3-mediated ubiquitination of K420 located in the UBA domain [48, 49], and the E2 conjugating enzymes UBE2D2 or UBE2D3 are likely involved [35]. Ubiquitination of p62 by RNF26 or RNF166 also promotes p62-mediated autophagic activity [49]. However, TRIM21-mediated K63-ubiquitination of p62 K7 in the PB1 domain abrogates p62 oligomerization and sequestration of Keap1 in antioxidant response [50], and PARKIN-mediated ubiquitination of K13 in the PB1 domain promotes p62 degradation [50a].

Phosphorylation of human p62 at Ser349 (equivalent to mouse Ser351) by mTOR, VPS34, PKCδ, or TAK1, in turn, regulates the Keap1-NRF2 pathway by competing with NRF2 for Keap1 binding in a positive feedback loop [11, 22, 24, 51, 52]. Ser349 of p62 is constitutively phosphorylated in HCC cell lines such as Huh-1 and contributes to cell proliferation [24]. Phosphorylation of mouse p62 at S405 and S409 (equivalent to human S403 and S407) located at the UBA potentiates p62-Ub binding in different mechanisms: phosphorylation of S405 promotes p62-Ub binding probably via charged residue interactions; however, phosphorylation of S409 destabilizes the UBA dimer interface and is essential for the autophagic degradation of ubiquitinated proteins, and promotes consequent S405 phosphorylation [53, 54]. Phosphorylation of human p62 at T269 and S272 by CDK1 is critical for the cell to properly enter and exit mitosis by maintaining the levels of cyclin B1 and CDK1 activity; deficient phosphorylation of these sites accelerates tumorigenic transformation in response to Ras-mediated oncogenesis [55].

Ubiquitination of p62 mediated by the E2 conjugating enzymes UBE2D2 or UBE2D3 is required for p62 activation of selective autophagy, and likely K420 in the C-terminal UBA is a crucial site for this event, but phosphorylation of S403 is not required [35]. Thus, phosphorylation and ubiquitination may be two independent events both required for p62 activation of selective autophagy.

3. P62 MEDIATES SELECTIVE AUTOPHAGY AS AN AUTOPHAGY RECEPTOR

Autophagy, with either non-selective (random) or selective mechanism, is a unique intracellular process to digest cellular constituents by lysosomes in the cytosol, for turning over damaged and even functional components or organelles under diverse stresses, such as nutrient deprivation, viral replication, cancer hypoxia, and genotoxic stress [23, 57–59]. Autophagy has pleiotropic functions in DDR, cell-cycle regulation, starvation adaptation, aging, neurodegeneration, infection and inflammation, and cancer development; functions as either tumor suppressor at early stage or promoter at late stage [60–62]. Autophagy is also required for maintaining homeostasis even in normal cells without stress by controlling protein and organelle quality and quantity [63]. However, excessive autophagy can lead to autophagic cell death (autosis), which, different from apoptosis, is characterized by the formation of many autophagic structures in morphology [62, 64–66].

Distinct from non-selective autophagy, selective autophagy targets specific “selective” substrates in a signal-dependent manner, and is mediated by the autophagy cargo receptors including p62, NBR1, TAX1BP1, NDP52, OPTN, TRIMs and TOLLIP [34, 42, 67–71], which are all involved in K63-linked ubiquitination-mediated autophagy. In contrast to ubiquitin-proteasome system (UPS) that specifically targets K48-linked ubiquitinated proteins in both the cytoplasm and nucleus, selective autophagy is primarily responsible for degradation of ubiquitinated long-living damaged or unfolded proteins including p62 itself, with any types of ubiquitin chains (K63 chain preferred) and also of monoubiquitin, and is restricted to the cytoplasm [36]. Thus, ubiquitination of the substrates is a prerequisite for their recognition by selective autophagy in eukaryotes [34–36, 41, 58, 72–76]. However, recent studies have disclosed ubiquitination-independent selective autophagy mechanisms, which involve an increasing pool of mitochondrion-located LC3 receptors such as FKBP8, CHDH, FUNDC1, PHB2, Cardiolipin, BNIP3, and NIX [71, 77]. For K48-linked ubiquitinated proteins, p62 targets them to either proteasomal or lysosomal degradation, and thus serves as a bridge that links ubiquitin-mediated proteasomal and autophagolysosomal processes [45, 53, 73, 78–86].

Phosphorylation of mouse p62 S405 and S409 (equivalent to human S403 and S407) in the UBA by ULK1, CK2, or TBK1 is required for its activation of autophagy [53, 54, 56, 87]. In addition to p62 phosphorylation and p62-Ub interaction, oligomerization of p62 is also required for the formation and functions of p62-mediated selective autophagy; oligomeric p62 senses Ub stress by linking ubiquitinated proteins or intracellular bacterial/viral particles to ATG8 (known as LC3 in mammalians) presented on autophagosome membrane, resulting in the degradation of the targets. p62 C-terminus has a LC3-interacting region (LIR) (Fig. 1), which is responsible for its interaction with ATG8 family. p62 also binds to ubiquitinated mitochondrial membrane, resulting in mitophagy [64].

4. P62 MEDIATES SIGNAL TRANSDUCTION AS A UBIQUITIN SENSOR

The UBA of p62 enables p62 function as a “ubiquitin sensor” (Fig. 1), which binds to K63 ubiquitin chains not only to target autophagic degradation, but also to facilitate NFκB activation downstream of Ras, TNFR, IL1R, TRANCE-R, nerve growth factor (NGF), and Toll-like receptor (TLR) signaling pathways [1–3, 12, 88–92]. p62 has a TRAF6-binding domain (Fig. 1), and specifically interacts with TRAF6, but not with TRAF5 or −2, to facilitate TRAF6 K63-linked ubiquitination, in which both the N-terminal dimerization domain and the UBA domain are also required [88, 90, 93]. Thus, p62 has at least two roles in activating NFκB. First, p62 probably favors the phosphorylation of IKKβ by functioning as a ubiquitin receptor that facilitates recruitment of ubiquitinated signal intermediators. Second, p62 facilitates TRAF6 K63-linked polyubiquitination by interacting with TRAF6. The interaction of p62 with TRAF6 promotes its oligomerization and subsequent activation, leading to K63 polyubiquitination of TRAF6 for NFκB activation [92]. TRAF6-mediated ubiquitination of NEMO also requires p62 [94]. In addition, p62 functions as a signal adaptor for Keap1 and the mTORC1 complex; binding to RIP1 facilitates NFκB activation in human ovarian cancer cells [95].

5. AUTOPHAGY-DEPENDENT AND -INDEPENDENT ROLES OF P62 IN CANCERS

p62 promotes tumor survival in different contexts, through the mechanisms involving its roles as both signaling adaptor and autophagy receptor [63]. p62 is upregulated at considerable levels in different cancer cells, including breast and prostate cancers, where it is required for induction of selective autophagy to support cancer cell survival [14, 96–98], and is induced by Ras that accounts for at least 25% of human cancers [12]. p62 overexpression in hepatocellular carcinoma (HCC) predicts poor prognosis [97]. In mouse models with defective autophagy, p62 ablation decreases tumorigenesis [12]. However, massive autophagy induction in turn promotes p62 autophagic degradation and higher levels of p62 may result from defective autophagy [32].

Autophagy plays a dual role in cancers; as either tumor suppressor at early stage or promoter at late stage [60, 62, 99–102]. Autophagy plays a pro-tumorigenic role in a p62-dependent manner at later stages when ROS and DNA damage are deleterious; inhibition of autophagy attenuates DNA damage-triggered chromatin ubiquitination, resulting in genomic instability [103]. Most cancers, if not all, are vulnerable to DNA damages and have high levels of genomic instability due to harboring deficient DNA repair mechanisms; the induction of autophagy serves as an oncogenic mechanism that endows these cancer cells with ability to resist DNA damage to support their survival [60, 62, 104–109]. p62-mediated autophagy also confers resistance to conventional chemotherapeutic agents in cancers with deficient DDR [64, 103, 106, 108, 110]. In some other cancer settings, autophagy may play anti-cancer roles, depending on the microenvironment stress, the status of the immune system, and other factors [63].

5.1. p62 Inhibits DNA Damage Response

DNA damage is directly linked to many human diseases including cancer. Massive DNA damage, especially double-strand breaks (DSBs), usually has severe effects on the cell, such as senescence and cell death [111]. Unrepaired DSBs also cause genomic instability that promotes malignant transformation under certain conditions, and is the most common feature of oncogenic viral infection [111–113]. Thus, eukaryotic organisms have developed complicated mechanisms to repair DNA damage to ensure genomic integrity. Homologous recombination (HR) and non-homologous end joining (NHEJ) are two major mechanisms involved in DSBs’ repair [114–119]. The choice between these two mechanisms (HR and NHEJ) depends on the cell cycle phase and is carefully regulated by integrated signaling networks [116–119]. 53BP1 and BRCA1 play a central role in determining these two pathways [116, 117].

Reactive oxygen/nitrogen species (ROS and RNS) are the major causes of endogenous DNA damage [61, 120, 121]. They can directly modify DNA and generate different levels of lesions, including DSBs (Fig. 2) [111, 122]. In general, ROS incites inflammation, and excess inflammation in turn causes oxidative stress, ultimately resulting in tissue damage and chronic inflammation [120]. Mitochonsdria produce the majority of ROS/RNS and play a centrol role in ROS-caused DNA damage [123]. The mitochondria in malignant cells are functionally and structurally deregulated and are able to overproduce ROS [124]. Viruses can also manipulate host DDR machinery in that infected cells recognize viral replication as DNA damage stress [125–128]. In the absence of active viral replication, ROS produced in viral persistence is the cause of DNA damage. Nevertheless, the contributions of ROS and chronic inflammation to carcinogenesis are controversial; they either promote cell-autonomous apoptosis and anticancer immunosurveillance, or stimulate autocrine or paracrine processes that favor carcinogenesis [129].

Fig. (2). The interplay between autophagy and DDR.

ROS production is induced mainly by mitochondria in response to genotoxic stress, chronical viral infections, tumor hypoxia, or cancer chemotherapeutic drugs. ROS may cause different biological consequences: (1) ROS induce DNA double strand breaks (DSB) directly. DSBs activate DNA repair mechanisms and autophagy mediated by ATM, PARP1, and/or p53. Autophagy promotes DNA repair and alleviates oxidative stress, so as to favor cancer cell survival, by degrading p62 that inhibits DNA repair through interacting with RNF168 and promoting proteasome-mediated degradation of DNA repair proteins such as RAD51 and CHK1. (2) ROS promote p62 expression through the Keap1-NRF2 pathway, and consequently induction of p62-mediated autophagy. In turn, phosphorylated p62 (Ser349) by mTOR positively regulates NRF2 stability and activity through competing with Keap1 for NRF2 binding. (3) ROS produced in tumor hypoxia stabilize the transcription factor HIF1α, leading to the transcriptional activation of BNIP3, consequently promotes autophagy induction by repressing the Beclin1-Bcl2 interaction; (4) ROS can activate NFκB that inhibits BNIP3 transcription or induces production of pro-inflammatory cytokines.

ROS also induce autophagy, which consequently participates in repair of ROS-triggered DNA damage. Defective autophagy confers a pro-inflammatory phenotype in response to various cellular stresses, whereas germline variants of the autophagy-related genes Atg16l1 and Atg5l are associated with chronic inflammatory disease [130]. There are a few mechanisms accounting for ROS induction of autophagy (Fig. 2): (1) ROS upregulates transcription factor activity including NFκB and NRF2, leading to autophagy gene expression (Beclin1 and/or p62) in cancer cells [18, 19]. (2) an increasing body of evidence suggests that it may involve mTOR (S2448) phosphorylation and inhibition by the AMPK cascade that is activated by oxidative DNA damage [64, 131]. AMPK is activated through phosphorylation at Thr172 by ATM-, PARP1- or p53-mediated signaling [64]. mTOR is also inhibited by the PI3K/Akt and DRAM pathways. AMPK, pTEN, and DRAM, as well as Bak and Bax involved in mitochondrial ROS response, are all transcriptionally induced by p53 that is activated upon DNA damage [64, 132]. Then the mTOR target ULK1 (Ser757) phosphorylation is impaired and ULK1 activity is released, which phosphorylates FIP200 and ATG13 in the complex ULK1-ATG13-FIP200, further leading to formation of the complex containing vps34 (classic III PI3K)-Beclin 1-UVRAG and consequent formation of autophagosome; (3) ROS induced by hypoxia stabilizes HIF1α, which induces expression of BH3-only Bcl2 family members such as Bcl2-interacting protein 3 (BNIP3) and Noxa that disrupt Beclin1-Bcl2 interaction, consequently leading to the initiation of autophagy [18, 133]. (4) ROS causes oxidization of ATG4, resulting in the accumulation of autophagosomes.

An emerging body of evidence indicates that autophagy and DNA damage response (DDR) closely crosstalk, in which p62 plays a key role [45]. Autophagy promotes degradation of p62, which has been reported to inhibit HR, but promote NHEJ, at least through following mechanisms: (1) p62 represses chromatin ubiquitination through direct interaction with RNF168, and therefore represses the recruitment of RNF168, BRCA1, RAP80, and RAD51 to the DSB (double-strand breaks) foci [103, 134]; The E3 ligase RNF168 catalyzes at least two different signaling entities in response to DSBs: fine tuning of HR through K63-linked ubiquitin chains, and promoting NHEJ by K15 monoubiquitination that facilitates recruitment of 53BP1 [117, 135]. Equally important, RNF168-mediated H2A K15 ubiquitination, but not RAP80 complex, is also required for 53BP1 recruitment to DNA damage foci for NHEJ repair [116]. Similar to RNF168, RNF169 also promotes HR-mediated DNA repair, by preventing the accumulation of 53BP1 and RAP80, proteins involved in NHEJ [136]. (2) Nuclear p62 promotes proteasomal degradation of RAD51 and CHK1 in the nucleus. In this case, phosphorylated CHK1 at Ser317 is preferred substrate for proteasome-mediated degradation [45, 137]; (3) p62 induces expression of NHEJ-specific repair proteins such as 53BP1 via the NRF2-Keap1 pathway [18–24]. In this case, phosphorylation of p62 at Ser349 is required [24].

In addition to above mechanisms, p62 may inhibit DNA repair through other mechanisms that have not been fully elucidated. For example, nuclear p62 interacts with and inhibits PML nuclear bodies, which are involved in DNA repair [65, 138]. Moreover, other selective and non-selective autophagy mechanisms, such as chaperone-mediated autophagy, also participate in DDR, by regulating stability of DDR-related proteins such as HP1α and CHK1 [139, 140], and by regulating p62-dependent and independent cellular functions required for DDR [131].

5.2. p62-dependent Autophagy Attenuates cGAS-STING-mediated Antitumor Immunity

Autophagy is long recognized as an innate immune mechanism that eliminates invading pathogens at least by targeting them for autophagic digestion. In this regard, p62 and related receptors serve as a special category of pathogen-recognition receptors called SLRs (sequestosome 1/p62-like receptors) [57, 141, 142].

Recent research has shown that accumulation of damaged self-DNA fragments in the cytoplasm in response to metastatic stress, radiation therapy, chemotherapeutic agents, or carcinogens can trigger cGAS-STING-mediated type I IFN antitumor immune response [143–145]. The STING ligand cGAMP triggers phosphorylation and activation of the key autophagy-related kinase ULK1 [146], which, in addition to being required for autophagy induction, phosphorylates STING S366 that is believed to be required for STING activation (Fig. 3) [147]. Moreover, TBK1, a key kinase for STING and IRF3 phosphorylation in the cGAS-STING pathway, also phosphorylates p62 for induction of p62-mediated autophagy [56, 87]. In turn, p62-mediated selective autophagy impairs cGAS-STING signaling by selectively promoting degradation of p62 and STING [56], and by interacting of Beclin1 with cGAS leading to blockage of cGAMP synthesis [148] (Fig. 3). The antioxidant transcription factor NRF2, which transactivates the p62 gene promoter and induces p62 expression in response to oxidative stress, decreases STING mRNA stability and negatively regulates STING expression, and consequently impairs STING-mediated antiviral or anti-tumor immunity [149]. Mitochondrial damage can stabilize the mitochondrial kinase PINK1, resulting in phosphorylation of ubiquitin, which further induces mitophagy by activating and recruiting parkin. A recent report shows that parkin and PINK1 mitigate STING-mediated inflammation by promoting mitophagy [150].

Fig. (3). Interplay between p62-mediated autophagy and the cGAS-STING-mediated DNA sensing pathway.

Oxidative stress triggers DNA damage in the nucleus and mitochondria. DNA fragments are then released into the cytoplasm, where they induce p62-mediated selective autophagy through different mechanisms and the cGAS-STING-mediated DNA sensing innate immune pathway. The kinases ULK1 and TBK1 play dual roles in these two pathways. ULK1 phosphorylates both mTOR and p62 for autophagy induction as well as STING for downstream IFN-I immune response. TBK1 phosphorylates both STING and IRF3, as well as p62. In turn, p62-mediated selective autophagy impairs cGAS-STING signaling by selectively promoting degradation of STING and p62 (p62 promotes production of DNA fragments by inhibiting DNA repair), and by interacting of Beclin1 with cGAS leading to blockage of cGAMP synthesis.

On the other hand, failure of resolving acute antitumor immune response or chronic viral infections can trigger low levels of oxidative DNA damage, which elicits cGAS-STING-mediated chronic immune activation leading to T cell exhaustion, immunosenescence, and chronic inflammation (inflammaging), which promote cancer initiation rather than antitumor immunity [151–153].

5.3. Interplay between Autophagy and Apoptosis

Autophagy either inhibits or promotes apoptosis, and many stimuli can induce both events. Autophagy promotes cancer cell survival by inhibiting DNA damage-induced apoptosis, and contributes to chemotherapy-induced resistance to apoptosis, in a p62-dependent manner [92]. Autophagy inhibition results in the accumulation of dysfunctional mitochondria and damaged mitochondrial DNA, which consequently causes apoptotic cell death [154]. However, autophagy precedes apoptosis when the protective ability of autophagy is overcome by the stimulus such as ionizing radiation or chemotherapeutic agents; in turn, apoptosis can inhibit autophagy [58, 133, 155–157].

The Bcl2 family plays key roles in linking autophagy and apoptosis [132, 133, 158]. The pro-apoptotic member Bim and anti-apoptotic members Bcl2 and Bcl-Xl inhibit autophagy through direct interaction with Beclin1 [159, 160], and disruption of the Bcl2-Beclin1 complex increases autophagy activity that promotes longevity in mice [161]. However, the other two pro-apoptotic members, BNIP3 and BNIP3L/NIX, function as mitophagy receptors that interact directly with processed LC3 through their LIR domains to promote mitophagy [77].

The tumor suppressor p53 is another important protein linking autophagy with apoptosis. Autophagy can be induced by DNA damage via PARP1- or p53-mediated pathways [162, 163]. DNA damage induces p53-mediated repair mechanisms. p53 promotes autophagy in the nucleus and mitochondria by inducing multiple pro-autophagic and pro-apoptotic genes; autophagy induced by nuclear p53 promotes apoptosis rather than survival [155]. However, cytoplasmic p53 functions as the “master” autophagy suppressor that inhibits TIGAR-mediated pathway rather than mTOR-dependent pathway [164]. A recent report disclosed a new role of nuclear p53 that inhibits autophagy by repressing transcription of PINK1, which plays a key role in mitophagy [165].

PARP1 activation, a hallmark of the cellular response to DNA damage and required for various DNA repair mechanisms, stimulates autophagy induction by activating the AMPK cascades [166]. However, excessive DNA damage can cause PARP1 hyperactivation, resulting in the exhaustion of the cellular NAD+ energy pools and mitochondrial defects through inactivation of SIRT1, and eventually leading to cell death [64].

In addition to the Bcl2 family, p53 and PARP1, autophagy is regulated by other tumor suppressors and oncogenes. Tumor suppressors (such as Rb, mTOR, Beclin-1) promote, whereas oncogenes (such as Ras) repress autophagy induction [61]. The microphthalmia/TFE (MiTF/TFE) subfamily of basic helix–loop–helix leucine zipper transcription factors, which promote autophagy and lysosomal biogenesis, is encoded by oncogenes [167].

CLOSING REMARKS

The autophagy-p62 pathway has definite roles in tumor initiation, development and metastasis in varying settings [62, 101, 102, 168]. Thus, the autophagy machinery and autophagy-related genes are protected from “loss-of-function” alterations in most cancers [63]. The heavy reliance of these cancer cells on autophagy for survival suggests targeting autophagy in these cancers may be a viable therapeutic strategy [62, 100, 169].

Although inhibition of autophagolysosomal activity alone is not a specific strategy for cancer therapy, jointly targeting related oncogene-specific responders such as BRAF, p53, KRAS, and the MiT/TFE family of transcription factors may overcome this shortage [63]. As such, further identification of context-specific molecular mechanisms coupling with the autophagy event, such as the cGAS-STING pathway, in a given setting will advance this field and help design more specific strategies for clinical applications.

LIST OF ABBREVIATIONS

AMPK

5-AMP-Activated Protein Kinase Catalytic Subunit Alpha

ARE

Antioxidant Response Element

ATG

Autophagy-related proteins (S. Cerevisiae)

ATM

The protein Serine/Threonine kinase Ataxia Telangiectasia Mutated

cGAMP

2’3’-GMP-AMP

cGAS

cGMP-AMP Synthase

DDR

DNA Damage Response

HR

Homologous recombination

mTOR

mammalian Target Of Rapamycin

NHEJ

non-homologous end joining

NRF2

Nuclear Factor Erythroid 2-Related Factor 2

ROS

Reactive Oxygen Species

STING

STimulator of Interferon Genes

TIGAR

P53 Induced Glycolysis Regulatory Phosphatase

ULK1

Unc-51-Like autophagy-activating Kinase 1, an ortholog of yeast ATG1