ARC serves a protective role against cellular stress and is involved in homeostasis and development (24,25). In a rabbit model of cardiac regional ischemia following reperfusion, rabbits overexpressing ARC exhibited decreased apoptosis in the left ventricle (24). In a murine pulmonary hypertension model, the vascular remodeling capacity of ARC-deficient mice was diminished and accompanied by enhanced apoptosis and decreased proliferation in response to chronic hypoxia-induced pulmonary hypertension (25). In chick embryo myocardial cardiomyocytes, ARC overexpression protected cells from oxidative injury and promoted their survival (26). H9c2 cells overexpressing ARC were significantly more resistant to hypoxia-induced apoptosis (27). In addition, enforced ARC overexpression significantly prevented doxorubicin (DOX)-induced cardiotoxicity (28). Overexpression of ARC remarkably suppressed whole cell K_v currents (I_K(V)) in transfected H9c2 cells following treatment with staurosporine, an apoptosis inducer that increased I_K(V) in wild-type cells and induced apoptosis (29). These data suggest that ARC protects myocardial cells from stress-induced apoptosis as well as chemical toxicity.

In the liver, ARC overexpression completely blocked the hepatocyte apoptosis or necrosis regulated via the Fas cell surface death receptor (Fas)/tumor necrosis factor (TNF) signaling pathway (30,31). Furthermore, ARC overexpression protected muscle fibers from apoptosis induced by mechanical stress or oxidative damage (32). Inhibition of ARC promoted cell death by increasing the production of pro-apoptotic factors, decreasing the stability of mitochondrial membranes and increasing the level of reactive oxygen species (ROS) following neomycin injury (23).

ARC prevented β-cell apoptosis induced by several cell death inducers, including endoplasmic reticulum (ER) stress and palmitate, while depletion of ARC in isolated islets significantly increased apoptosis in palmitate-treated cells (22). ARC expression was previously documented in testicular tissue, regulating apoptosis of primary spermatocytes (20). It was reported that ARC antagonized the adverse effect of zoldronate in osteoblasts, promoted osteogenic growth and differentiation of osteoblasts, and decreased apoptosis (33).

In addition to its antiapoptotic role, ARC participates in the regulation of cell differentiation and proliferation. Overexpression of ARC suppressed myogenic differentiation via caspase inhibition (34), whereas the absence of ARC altered fiber-type distribution, resulting in muscle atrophy and decreased force-generating capacity (35). Russell et al (36) demonstrated that mutations, which alter the post-translational modification of ARC, may lead to myoclonus.

ARC expression in cancer cells has been associated with increased chemoresistance. High expression levels of ARC in breast cancer cells promoted tumor growth, invasion and metastasis, and augmented chemoresistance (9,43). A high expression level of ARC in colorectal cancer liver metastasis enhanced the anti-apoptotic ability of colorectal cancer (37). ARC is highly expressed in newly diagnosed acute myeloid leukemia (AML) samples and has been demonstrated to decrease apoptosis induced by cytarabine and other agents. ARC may therefore contribute to drug resistance and increase survival time of leukemia cells (44–46). The high expression of cytoplasmic and nuclear ARC in nasopharyngeal carcinoma tissues are correlated with advanced local invasion. Overexpression of ARC in NPC 6-10B cells plays an important role in X-radiation and cisplatin resistance, and prolongs NPC cells survival by blocking the activation of casapse-8 and casapse-2 (47).

Chen et al (48) revealed that Me1007 cells overexpressing ARC inhibited caspase-8 activation, while another study indicated that ER stress-induced apoptosis (49). Carter et al (50) transplanted ARC-knockdown AML cells into mice and demonstrated a significantly lower leukemia burden, increased survival rate and enhanced sensitivity to chemotherapy compared with controls. Conversely, ARC upregulated prostaglandin E2/β-catenin production in the tumor microenvironment and augmented chemoresistance in AML cells (50). Huang et al (51) reported that increased ARC expression levels inhibited apoptosis in colon cancer. The aforementioned studies demonstrated that ARC serves an important role in the survival of cancer cells and chemotherapy resistance.

Rat sarcoma (Ras) controls ARC levels by transcriptional regulation and maintaining protein stability. Ras activates the Nol3 promoter in a MEK/ERK-dependent manner and significantly increases the production of ARC mRNA. Furthermore, Ras prevents ARC degradation via the ubiquitin-proteasome pathway (43). The transcription factor fork head box O3 (FOXO3A) expressed in cardiomyocytes and skeletal muscle cells activates ARC expression by directly binding and trans-activating its promoter (52). ARC protein levels in muscle are increased as a result of endurance training; exercise resulted in a 37.5% increase in ARC protein levels (53) (Fig. 2).

Death signals induced by hypoxia (27,54), ischemia-reperfusion (I/R) (14,55), and other forms of stress (14,55–57) have been revealed to significantly decrease ARC levels. ARC contains lysine residues at positions 17, 68 and 163. The decrease in ARC levels during apoptosis are mediated by the decreased stability of the ARC protein, an effect indicated to be mediated by ubiquitin-proteasomal-mediated degradation (55,57). Previous studies have revealed that specific microRNAs (miRNAs/miRs) the decreased the expression level of ARC directly or indirectly (39,58,59). ARC mRNA 3′-untranslated regions contain two miR-185 binding sites. miR-185 modulates ARC expression at the post-transcriptional level and negatively regulates ARC expression (39). miR-30d downregulates FOXO3A and inhibits ARC expression (58). miR-155 directly decreases the expression of FOXO3A and its downstream protein ARC, and may cause renal proptosis under ischemia/reperfusion injury conditions (59). p53 serves an important role in decreasing the expression and transcription of ARC as well as promoting ARC degradation via p53-induced ubiquitin E3 ligase (57,60). A previous study revealed that oxidative stress enhanced post-transcriptional degradation of ARC via the ubiquitin-proteasome pathway (61). Sorting nexin-13 (SNX13), a potent mediator of heart failure, was revealed to directly modulate ARC stability and promote ARC degradation, thus increasing the apoptotic death of cardiomyocytes (62) (Fig. 2).

A previous study revealed that the ARC protein had a molecular weight of ~34 kDa in human skeletal muscle and various adherent human cancer cell lines, and 38 kDa in human lymphoma cell lines (63). McMillan et al (64) revealed that ARC extracted from normotensive rats had a molecular weight of ~30 kDa, compared with 32 kDa in a spontaneously hypertensive rat. This shift in molecular weight suggested that the ARC protein may undergo post-translational modifications in different and cell types. Li et al (62) reported that ARC mRNA levels did not differ between cardiac tissues extracted from rats with heart failure and controls or between SNX13-deficient neonatal rat ventricular myocytes and normal cardiomyocytes, suggesting that the regulation of ARC expression begins at the posttranscriptional level. Dowds et al (65) revealed that during the serum withdrawal stage in an in vitro culture, post-translational regulation increased ARC stability and lead to its accumulation in the nucleus, which promotes cell survival (Fig. 2). Phosphorylation is a functional post-transcriptional modification and Li et al (66) revealed that casein kinase 2 (CK2), is a serine/threonine protein kinase that phosphorylates ARC at threonine 149. T149 phosphorylation directs ARC to the mitochondria and allows ARC to exert its anti-apoptotic effects (Fig. 2).

ARC inhibits the extrinsic pathway by interacting with Fas and Fas associated via the death domain. This subsequently prevents the homotypic interactions required for the assembly of the death-inducing signaling complex, which is important for the activation of caspases and further apoptotic signaling (14). A previous study revealed that ARC selectively interacts with the apoptosis initiator caspase-2 and caspase-8 and attenuates death receptor-induced apoptosis, involving the Fas cell surface death receptor, TNF receptor superfamily member 1A and TNF receptor superfamily member 25, or adaptor-induced apoptosis, involving TNFRSF1A associated via death domain and CASP8 and FADD like apoptosis regulator (10). Ekhterae et al (29) suggested that upregulation of ARC decreases I_K(V) density and inhibits the staurosporine-induced I_K(V) increase in cardiomyocytes, thereby enhancing survival and attenuating apoptosis by inhibiting K_v channel activity (29) (Fig. 3, Table I).

ARC antagonizes the intrinsic pathway by interacting with apoptosis-associated proteins (11). The binding of ARC and Bax, which can inhibit Bax activation and accumulation in the mitochondria (15), increases the Bcl2 apoptosis regulator/Bax ratio (71,72), stabilizes the mitochondrial membrane and prevents cytochrome c release (27,73,74). Furthermore, ARC is also involved in the regulation of mitochondrial dynamics. For instance, ARC interacts with PUMA via its N terminus, and suppresses PUMA-mediated translocation of density regulated re-initiation and release factor in mitochondria, preventing mitochondrial fission and subsequent apoptosis (11).

miR-532-3p negatively regulates ARC expression and sensitizes cardiomyocytes to DOX-induced mitochondrial fission and apoptosis (75). It has been reported that ARC participates in cardioprotection and suppresses apoptosis caused by hypoxia and reoxygenation by preventing cytochrome c release from the mitochondria in a caspase-independent pattern (27,76). Furthermore, ARC overexpression prevented oxidative stress-induced cell apoptosis by protecting mitochondrial function independently of caspase inhibition, suggesting that ARC may act on a mitochondrial level (56).

ARC interacts with protein kinase RNA-like ER kinase and binds inositol-requiring protein 1α, which prevents C/EBP homologous protein induction, diminishes the ER stress response and blocks Ca²⁺ release from ER (22). Furthermore, ARC not only blocks Ca²⁺ release but also binds Ca²⁺ through its C-terminal P/E-rich domain. Therefore, ARC suppresses intracellular Ca²⁺ increase and further prevents Ca²⁺ mediated apoptosis (16).

The proline-glutamic acid-rich region of ARC could bind with and negatively regulate p53 by inhibiting its transcriptional function, tetramerization and causing its cytoplasmic localization, thereby preventing p53 transfer into the nucleus and abrogate p53-induced apoptosis (17).

ARC binds with and prevents the activation of JUN N-terminal kinase (JNK), inhibiting cell death dependent on the JNK signaling pathway (30,77,78). ARC blocks acetaminophen- induced hepatic damage by antagonizing the JNK signaling pathway and preventing ROS production (77). ARC decreases amyloid-induced JNK phosphorylation, and ARC upregulation decreases β-cell apoptosis induced by activation of the JNK signaling pathway (78). Furthermore, ARC blocks Fas- and TNF-regulated cell death by JNK-dependent or independent pathways (30) (Fig. 3, Table I).

ARC inhibits apoptosis induced by death receptor activation (10), oxidative stress (14), serum deprivation (27), ischemia-reperfusion (56), doxorubicin and γ-radiation (79). ARC is a potent anti-apoptotic protein that prevents permeabilization of the mitochondrial outer membrane and decreases pro-apoptotic protein release and DNA fragmentation (64).