p73 regulates DRAM-independent autophagy that does not contribute to programmed cell death

Evading programmed cell death is a common event in tumour development. The p53 family member, p73, is a potent inducer of death and a determinant of chemotherapeutic response, but different to p53, is rarely mutated in cancer. Understanding cell death pathways downstream of p53 and p73 is therefore pivotal to understand both the development and treatment of malignant disease. Recently, p53 has been shown to modulate autophagy--a membrane trafficking process, which degrades long-lived proteins and organelles. This requires a p53 target gene, DRAM, and both DRAM and autophagy are critical for p53-mediated death. We report here that TA-p73 also regulates DRAM and autophagy, with different TA-p73 isoforms regulating DRAM and autophagy to varying extents. RNAi knockdown of DRAM, however, revealed that p73's modulation of autophagy is DRAM-independent. Also, p73's ability to induce death, again different to p53, is neither dependent on DRAM nor autophagy. In contrast to TA-p73, deltaN-p73 is a negative regulator of p53-induced and p73-induced autophagy, but does not affect autophagy induced by amino-acid starvation. These studies, therefore, represent not only the first report that p73 modulates autophagy but also highlight important differences in the mechanism by which starvation, p53 and p73 regulate autophagy and how this contributes to programmed cell death.     

DRAM, a p53-induced modulator of autophagy, is critical for apoptosis

Inactivation of cell death is a major step in tumor development, and p53, a tumor suppressor frequently mutated in cancer, is a critical mediator of cell death. While a role for p53 in apoptosis is well established, direct links to other pathways controlling cell death are unknown. Here we describe DRAM (damage-regulated autophagy modulator), a p53 target gene encoding a lysosomal protein that induces macroautophagy, as an effector of p53-mediated death. We show that p53 induces autophagy in a DRAM-dependent manner and, while overexpression of DRAM alone causes minimal cell death, DRAM is essential for p53-mediated apoptosis. Moreover, analysis of DRAM in primary tumors revealed frequent decreased expression often accompanied by retention of wild-type p53. Collectively therefore, these studies not only report a stress-induced regulator of autophagy but also highlight the relationship of DRAM and autophagy to p53 function and damage-induced programmed cell death.     

Reduced expression of DRAM2/TMEM77 in tumor cells interferes with cell death

Although the role of autophagy in tumorigenesis remains controversial, recent reports support the notion that inhibition of autophagy promotes tumor formation. Damage-regulated autophagy regulator (DRAM) has been identified as an effector molecule that is critical for p53-mediated apoptosis, and we investigated whether there might be other DRAM-like molecules linking autophagy and apoptosis. In this study, we cloned a novel DRAM-homologous protein, DRAM2, and showed that the expression of DRAM2 is down-regulated in ovarian tumors. DRAM2 is mainly localized in the lysosome, and co-localizes with DRAM. While expression of DRAM or DRAM2 individually did not induce cell death, co-expression of DRAM2 with DRAM significantly induced cell death, while the silencing of endogenous DRAM2 attenuated cell death, suggesting that DRAM2 is involved in cell death. Thus, we propose that reduced expression of DRAM2 may contribute to enhanced cell survival in tumor cells.     

ASPP2过表达可增强DRAM介导的自噬对HepG2细胞的促调亡作用

p53凋亡刺激蛋白2（apoptosis stimulating protein 2 of p53, ASPP2）能特异性地与p53蛋白结合并增强其促凋亡功能,进而发挥抗肿瘤作用.最近文献提示,自噬对肿瘤发生、发展及肿瘤细胞对抗肿瘤药物的反应都具有重要作用.在本研究中,甲基磺酸(MMS)处理HepG2细胞24 h后,用calcein AM/PI和M30染色检测细胞凋亡,可引起早期（M30免疫组化阳性）和晚期细胞凋亡（PI染色阳性）. 给HepG2细胞转染GFP-LC3质粒后,发现MMS处理24 h可引起自噬的发生. ASPP2腺病毒（rAd-ASPP2）感染HepG2细胞引起ASPP2过表达后,再用MMS处理24 h,能引起更明显的早期、晚期细胞凋亡和自噬. 荧光定量PCR检测发现,rAd-ASPP2诱导了更高的BCL-2相关X蛋白基因（BAX）和p53蛋白的目的基因p53诱导的自噬调节蛋白（p53-induced modulator of autophagy,DRAM）的表达. 但仅用rAd-ASPP2处理HepG2细胞不能引起自噬和凋亡.利用2条DRAM特异性的siRNA下调DRAM的表达,发现rAd-ASPP2引起的自噬被完全抑制, 早期和晚期凋亡均部分被抑制,同时BAX 的mRNA水平也明显下降. 以上结果说明,ASPP2可通过上调BAX和DRAM基因的转录而促进MMS引起的HepG2细胞凋亡; 另外,DRAM介导的自噬是ASPP2促进MMS引起的肿瘤细胞凋亡的机制之一. 该研究可为肝癌的基因治疗提供新的思路.     

Evidence for the interplay between JNK and p53-DRAM signaling pathways in the regulation of autophagy

p53 and JNK are two apoptosis-regulatory factors frequently deregulated in cancer cells and also involved in the modulation of autophagy. We have recently investigated the links between these two signalling pathways in terms of the regulation of autophagy. We showed that 2-methoxyestradiol (2-ME), an antitumoral compound, enhances autophagy and apoptosis in Ewing sarcoma cells through the activation of both p53 and JNK pathways. In this context, p53 regulates, at least partially, JNK activation which in turn modulates autophagy through two distinct mechanisms: on the one hand it promotes Bcl-2 phosphorylation resulting in the dissociation of the Beclin 1-Bcl-2 complex and on the other hand it leads to the upregulation of DRAM (Damage-Regulated Autophagy Modulator), a p53 target gene. The critical role of DRAM in 2-ME–mediated autophagy and apoptosis is underlined by the fact that its silencing efficiently prevents the induction of both processes. These findings not only report the interplay between JNK and p53 in the regulation of autophagy but also uncover the role of JNK activation in the regulation of DRAM, a pro-autophagic and pro-apoptotic protein.     

Analysis of DRAM-related proteins reveals evolutionarily conserved and divergent roles in the control of autophagy

Autophagy is a membrane-trafficking process that serves to deliver cytoplasmic proteins and organelles to the lysosome for degradation. The process is genetically defined and many of the factors involved are conserved from yeast to man. Recently, a number of new autophagy regulators have been defined, including the Damage-Regulated Autophagy Modulator (DRAM), which is a lysosomal protein that links autophagy and the tumor suppressor, p53. We describe here analysis of DRAM-related proteins which reveals evolutionary conservation and divergence of DRAM’s role in autophagy. We report that humans have 5 other proteins that show significant homology to DRAM. The closest of these, which we have termed DRAM2, displays 45% identity and 67% conservation when compared to DRAM. Interestingly, although similar to DRAM in terms of homology, DRAM2 is different from DRAM as it not induced by p53 or p73. DRAM2 is also a lysosomal protein, but again unlike DRAM its over-expression does not modulate autophagy. In contrast to humans, the Drosophila genome only encodes one DRAM-like protein, which is approximately equal in similarity to human DRAM and DRAM2. This questions, therefore, whether DRAM function is conserved from fly to man or whether DRAM’s capacity to regulate autophagy has evolved in higher eukaryotes. Expression of DmDRAM, however, clearly revealed an ability to modulate autophagy. This points, therefore, to a conserved role of DRAM in this process and that additional human proteins have more recently evolved which, while potentially sharing some similarities to DRAM function, may not be as intrinsically connected to autophagy regulation.     

The expression of damage-regulated autophagy modulator 2 (DRAM2) contributes to autophagy induction

Autophagy is a membrane trafficking process involved in intracellular degradation and recycling in eukaryotic cells. DRAM2 (damage-regulated autophagy modulator 2) is a homologue of DRAM that regulates p53-mediated cell death. As its name implies, DRAM expression induces autophagy in a p53-dependent manner; however, the role of DRAM2 in autophagy is not clear. In this study, we report that DRAM2 expression contributes to autophagy induction. Overexpression of DRAM2 induces cytoplasmic GFP-LC3 punctuates, and increases the level of endogenous LC3-II. Moreover, the silencing of endogenous DRAM2 interferes with starvation-induced autophagy. Thus, we propose that DRAM2 as well as DRAM are involved in autophagy.     

Increased autophagy in placentas of intrauterine growth-restricted pregnancies

Background

Unexplained intrauterine growth restriction (IUGR) may be a consequence of placental insufficiency; however, its etiology is not fully understood. We surmised that defective placentation in IUGR dysregulates cellular bioenergic homeostasis, leading to increased autophagy in the villous trophoblast. The aims of this work were (1) to compare the differences in autophagy, p53 expression, and apoptosis between placentas of women with normal or IUGR pregnancies; (2) to study the effects of hypoxia and the role of p53 in regulating trophoblast autophagy; and (3) to investigate the relationship between autophagy and apoptosis in hypoxic trophoblasts.

Methodology/Principal Findings

Compared with normal pregnant women, women with IUGR had higher placental levels of autophagy-related proteins LC3B-II, beclin-1, and damage-regulated autophagy modulator (DRAM), with increased p53 and caspase-cleaved cytokeratin 18 (M30). Furthermore, cytotrophoblasts cultured under hypoxia (2% oxygen) in the presence or absence of nutlin-3 (a p53 activity stimulator) had higher levels of LC3B-II, DRAM, and M30 proteins and increased Bax mRNA expression compared with controls cultured under standard conditions. In contrast, administration of pifithrin-α (a p53 activity inhibitor) during hypoxia resulted in protein levels that were similar to those of the control groups. Moreover, cytotrophoblasts transfected with LC3B, beclin-1, or DRAM siRNA had higher levels of M30 compared with the controls under hypoxia. However, transfection with Bcl-2 or Bax siRNA did not cause any significant change in the levels of LC3B-II in hypoxic cytotrophoblasts.

Conclusions/Significance

Together, these results suggest that there is a crosstalk between autophagy and apoptosis in IUGR and that p53 plays a pivotal and complex role in regulating trophoblast cell turnover in response to hypoxic stress.     

ASPP2以p53非依赖形式促进自噬引起Hep3B细胞凋亡

p53凋亡刺激蛋白2（apoptosis stimulating protein 2 of p53, ASPP2）能特异性地与p53蛋白结合并增强其促凋亡的功能,进而发挥抗肿瘤作用. 本室前期研究发现,ASPP2可以通过p53-DRAM自噬途径诱导细胞凋亡. 在本研究中,利用ASPP2 腺病毒感染Hep3B细胞(p53缺陷型肝癌细胞系)并用甲基磺酸(MMS)处理后; Calcein AM/PI和M30染色检测细胞凋亡;GFP-LC3质粒转染细胞后检测自噬; 荧光定量PCR和免疫印迹检测自噬基因表达. 结果表明,ASPP2在p53缺陷的Hep3B细胞内可诱导发生凋亡;在MMS存在和缺失条件下, Adr-ASPP2均引起自噬体水平升高及自噬基因的表达增 加,且MMS协同Adr-ASPP2能使自噬水平增加; 进一步用VPS34 siRNA和DRAM siRNA抑 制自噬发现,细胞凋亡水平下降, 说明由Adr-ASPP2诱发经损伤相关自噬调节蛋白（ DRAM）介导的自噬参与了肝癌细胞系凋亡的发生. 综上结果表明,ASPP2可以通过非p53依赖的DRAM介导自噬,并促进肝癌细胞凋亡. 该研究可为肝癌的基因治疗提供新的思路.     

p53 mediates mitochondria dysfunction-triggered autophagy activation and cell death in rat striatum

In vivo administration of the mitochondrial inhibitor 3-nitropropionic acid (3-NP) produces striatal pathology mimicking Huntington disease (HD). However, the mechanisms of cell death induced by metabolic impairment are not fully understood. The present study investigated contributions of p53 signaling pathway to autophagy activation and cell death induced by 3-NP. Rat striatum was intoxicated with 3-NP by stereotaxic injection. Morphological and biochemical analyses demonstrated activation of autophagy in striatal cells as evidenced by increased the formation of autophagosomes, the expression of active lysosomal cathepsin B and D, microtubule associate protein light chain 3 (LC3) and conversion of LC3-I to LC3-II. 3-NP upregulated the expression of tumor suppressor protein 53 (p53) and its target genes including Bax, p53-upregulated modulator of apoptosis (PUMA) and damage-regulated autophagy modulator (DRAM). 3-NP-induced elevations in pro-apoptotic proteins Bax and PUMA, autophagic proteins LC3-II and DRAM were significantly reduced by the p53 specific inhibitor pifithrin-α (PFT). PFT also significantly inhibited 3-NP-induced striatal damage. Similarly, 3-NP-induced DNA fragmentation and striatal cell death were robustly attenuated by the autophagy inhibitor 3-methyladenine (3-MA) and bafilomycin A1 (BFA). These results suggest that p53 plays roles in signaling both autophagy and apoptosis. Autophagy, at least partially, contributes to neurodegeneration induced by mitochondria dysfunction.     

p53, Autophagy and Tumor Suppression

Autophagy was recently established as a novel tumor suppression mechanism, which stimulated a wave of investigations that were aimed at understanding exactly how autophagy prevents tumorigenesis, as well as to determine to what extent autophagy is implicated in human cancers. Autophagy might exert its tumor suppression function at the subcellular level by removing defective cytoplasmic components, such as damaged mitochondria. In addition, it might function at the cellular level by helping in the orderly removal of damaged cells. Previous studies indicated that autophagy is compromised in human breast, ovarian and prostate cancers. Recent research revealed that autophagy is activated by p53, a critical tumor suppressor that is involved in most, if not all, tumorigenesis. This study places autophagy in a broader context of human cancers. Future work elucidating the role of autophagy in the p53 circuit and p53 function might provide more insight into tumorigenesis and targeted cancer chemotherapy.     

p53, Autophagy and tumor suppression

Autophagy was recently established as a novel tumor suppression mechanism, which stimulated a wave of investigations that were aimed at understanding exactly how autophagy prevents tumorigenesis, as well as to determine to what extent autophagy is implicated in human cancers. Autophagy might exert its tumor suppression function at the subcellular level by removing defective cytoplasmic components, such as damaged mitochondria. In addition, it might function at the cellular level by helping in the orderly removal of damaged cells. Previous studies indicated that autophagy is compromised in human breast, ovarian and prostate cancers. Recent research revealed that autophagy is activated by p53, a critical tumor suppressor that is involved in most, if not all, tumorigenesis. This study places autophagy in a broader context of human cancers. Future work elucidating the role of autophagy in the p53 circuit and p53 function might provide more insight into tumorigenesis and targeted cancer chemotherapy.     

Role of the newer p53 family proteins in malignancy

The most recently identified members of the p53 family, p63 and p73, share certain structural and functional similarities with p53. Both p63 and p73 can bind to canonical p53-DNA-binding sites, transactivate the promoters of known p53 target genes and induce apoptosis. Despite these similarities there are many important differences. In contrast to p53, p63 and p73 give rise to multiple distinct protein isoforms that have different functional properties. Upstream signaling pathways involved in the activation of p63 and p73 differ from those involved in p53 activation. Only a subset of the DNA damaging agents that induce p53 can induce p73. Cellular and viral oncoproteins can discriminate between p53 and the newer family members. In addition, the levels of p63 and p73 are affected by certain states of cellular differentiation. Finally, it is becoming clear that the newest members of the p53 family are not classical tumor suppressor genes. In contrast to the high prevalence of p53 mutations in human cancers, p63 and p73 mutations are rare. Indeed, levels of p73 increase during malignant progression. In addition, unlike p53-/- mice, mice lacking p63 and p73 do not develop tumors, but instead have significant developmental abnormalities. Mutations in p63 have also been detected in humans with the ectodermal dysplastic syndrome EEC. Further studies are required to determine whether qualitative or quantitative differences in the expression of p63 and p73 isoforms are important in the development of human cancers.     

TP53INP1, a tumor suppressor, interacts with LC3 and ATG8-family proteins through the LC3-interacting region (LIR) and promotes autophagy-dependent cell death

TP53INP1 (tumor protein 53-induced nuclear protein 1) is a tumor suppressor, whose expression is downregulated in cancers from different organs. It was described as a p53 target gene involved in cell death, cell-cycle arrest and cellular migration. In this work, we show that TP53INP1 is also able to interact with ATG8-family proteins and to induce autophagy-dependent cell death. In agreement with this finding, we observe that TP53INP1, which is mainly nuclear, relocalizes in autophagosomes during autophagy where it is eventually degraded. TP53INP1-LC3 interaction occurs via a functional LC3-interacting region (LIR). Inactivating mutations of this sequence abolish TP53INP1-LC3 interaction, relocalize TP53INP1 in autophagosomes and decrease TP53INP1 ability to trigger cell death. Interestingly, TP53INP1 binds to ATG8-family proteins with higher affinity than p62, suggesting that it could partially displace p62 from autophagosomes, modifying thereby their composition. Moreover, silencing the expression of autophagy related genes (ATG5 or Beclin-1) or inhibiting caspase activity significantly decreases cell death induced by TP53INP1. These data indicate that cell death observed after TP53INP1-LC3 interaction depends on both autophagy and caspase activity. We conclude that TP53INP1 could act as a tumor suppressor by inducing cell death by caspase-dependent autophagy.     

Phosphorylated AKT inhibits the apoptosis induced by DRAM-mediated mitophagy in hepatocellular carcinoma by preventing the translocation of DRAM to mitochondria

Increasing autophagy is beneficial for curing hepatocellular carcinoma (HCC). Damage-regulated autophagy modulator (DRAM) was recently reported to induce apoptosis by mediating autophagy. However, the effects of DRAM-mediated autophagy on apoptosis in HCC cells remain unclear. In this study, normal hepatocytes (7702) and HCC cell lines (HepG2, Hep3B and Huh7) were starved for 48 h. Starvation induced apoptosis and autophagy in all cell lines. We determined that starvation also induced DRAM expression and DRAM-mediated autophagy in both normal hepatocytes and HCC cells. However, DRAM-mediated autophagy was involved in apoptosis in normal hepatocytes but not in HCC cells, suggesting that DRAM-mediated autophagy fails to induce apoptosis in hepatoma in response to starvation. Immunoblot and immunofluorescence assays demonstrated that DRAM translocated to mitochondria and induced mitophagy, which led to apoptosis in 7702 cells. In HCC cells, starvation also activated the phosphatidylinositol 3-kinase (PI3K)/AKT pathway, which blocks the translocation of DRAM to mitochondria through the binding of p-AKT to DRAM in the cytoplasm. Inactivation of the PI3K/AKT pathway rescued DRAM translocation to mitochondria; subsequently, mitochondrial DRAM induced apoptosis in HCC cells by mediating mitophagy. Our findings open new avenues for the investigation of the mechanisms of DRAM-mediated autophagy and suggest that promoting DRAM-mediated autophagy together with PI3K/AKT inhibition might be more effective for autophagy-based therapy in hepatoma.