Inhibition of p53 protein aggregation as a cancer treatment strategy

The p53 protein plays a critical role in the prevention of genome mutations in the body, however, this protein is frequently mutated in cancer and almost all cancers exhibit malfunction along the p53 pathway. In addition to a loss of activity, mutant p53 protein is prone to unfolding and aggregation, eventually forming amyloid aggregates. There continues to be a considerable effort to develop strategies to restore normal p53 expression and activity and this review details recent advances in small-molecule stabilization of mutant p53 protein and the design of p53 aggregation inhibitors.     

Docosahexaenoic acid induces autophagy through p53/AMPK/mTOR signaling and promotes apoptosis in human cancer cells harboring wild-type p53

Docosahexaenoic acid (DHA) has been reported to induce tumor cell death by apoptosis. However, little is known about the effects of DHA on autophagy, another complex well-programmed process characterized by the sequestration of cytoplasmic material within autophagosomes. Here, we show that DHA increased both the level of microtubule-associated protein light-chain 3 and the number of autophagic vacuoles without impairing autophagic vesicle turnover, indicating that DHA induces not only apoptosis but also autophagy. We also observed that DHA-induced autophagy was accompanied by p53 loss. Inhibition of p53 increased DHA-induced autophagy and prevention of p53 degradation significantly led to the attenuation of DHA-induced autophagy, suggesting that DHA-induced autophagy is mediated by p53. Further experiments showed that the mechanism of DHA-induced autophagy associated with p53 attenuation involved an increase in the active form of AMP-activated protein kinase and a decrease in the activity of mammalian target of rapamycin. In addition, compelling evidence for the interplay between autophagy and apoptosis induced by DHA is supported by the findings that autophagy inhibition suppressed apoptosis and further autophagy induction enhanced apoptosis in response to DHA treatment. Overall, our results demonstrate that autophagy contributes to the cytotoxicity of DHA in cancer cells harboring wild-type p53.     

Docosahexaenoic acid induces autophagy through p53/AMPK/mTOR signaling and promotes apoptosis in human cancer cells harboring wild-type p53

Docosahexaenoic acid (DHA) has been reported to induce tumor cell death by apoptosis. However, little is known about the effects of DHA on autophagy, another complex well-programmed process characterized by the sequestration of cytoplasmic material within autophagosomes. Here, we show that DHA increased both the level of microtubule-associated protein light-chain 3 and the number of autophagic vacuoles without impairing autophagic vesicle turnover, indicating that DHA induces not only apoptosis but also autophagy. We also observed that DHA-induced autophagy was accompanied by p53 loss. Inhibition of p53 increased DHA-induced autophagy and prevention of p53 degradation significantly led to the attenuation of DHA-induced autophagy, suggesting that DHA-induced autophagy is mediated by p53. Further experiments showed that the mechanism of DHA-induced autophagy associated with p53 attenuation involved an increase in the active form of AMP-activated protein kinase and a decrease in the activity of mammalian target of rapamycin. In addition, compelling evidence for the interplay between autophagy and apoptosis induced by DHA is supported by the findings that autophagy inhibition suppressed apoptosis and further autophagy induction enhanced apoptosis in response to DHA treatment. Overall, our results demonstrate that autophagy contributes to the cytotoxicity of DHA in cancer cells harboring wild-type p53.     

p53 represses autophagy in a cell cycle-dependent fashion

Autophagy is one of the principal mechanisms of cellular defense against nutrient depletion and damage to cytoplasmic organelles. When p53 is inhibited by a pharmacological antagonist (cyclic pifithrin-?), depleted by a specific small interfering RNA (siRNA) or deleted by homologous recombination, multiple signs of autophagy are induced. Here, we show by epistatic analysis that p53 inhibition results in a maximum level of autophagy that cannot be further enhanced by a variety of different autophagy inducers including lithium, tunicamycin-induced stress of the endoplasmic reticulum (ER) or inhibition of Bcl-2 and Bcl-XL with the BH3 mimetic ABT737. Chemical inducers of autophagy (including rapamycin, lithium, tunicamycin and ABT737) induced rapid depletion of the p53 protein. The absence or the inhibition of p53 caused autophagy mostly in the G1 phase, less so in the S phase and spares the G2/M phase of the cell cycle. The possible pathophysiological implications of these findings are discussed.     

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.     

Polymorphisms in p53 and the p53 pathway: roles in cancer susceptibility and response to treatment

The p53 tumour suppressor protein lies at the crossroads of multiple cellular response pathways that control the fate of the cell in response to endogenous or exogenous stresses and inactivation of the p53 tumour suppressor signalling pathway is seen in most human cancers. Such aberrant p53 activity may be caused by mutations in the TP53 gene sequence producing truncated or inactive mutant proteins, or by aberrant production of other proteins that regulate p53 activity, such as gene amplification and overexpression of MDM2 or viral proteins that inhibit or degrade p53. Recent studies have also suggested that inherited genetic polymorphisms in the p53 pathway influence tumour formation, progression and/or response to therapy. In some cases, these variants are clearly associated with clinico-pathological variables or prognosis of cancer, whereas in other cases the evidence is less conclusive. Here, we review the evidence that common polymorphisms in various aspects of p53 biology have important consequences for overall tumour susceptibility, clinico-pathology and prognosis. We also suggest reasons for some of the reported discrepancies in the effects of common polymorphisms on tumourigenesis, which relate to the complexity of effects on tumour formation in combination with other oncogenic changes and other polymorphisms. It is likely that future studies of combinations of polymorphisms in the p53 pathway will be useful for predicting tumour susceptibility in the human population and may serve as predictive biomarkers of tumour response to standard therapies.     

Inflammatory signaling cascades and autophagy in cancer

Tumor-associated inflammation is predictive of poor prognosis and drives a variety of tumorigenic phenotypes, including tumor proliferation and survival, angiogenesis, invasiveness, and metastasis. Here, we review mammalian data addressing the interaction of macroautophagy/autophagy with key signaling cascades associated with tumor inflammation. Although our understanding of this area remains incomplete, certain inflammatory pathways have emerged as important mediators of the crosstalk between autophagy and inflammation in tumors. Consistent with the multifaceted roles for autophagy in tumor cells, results to date support the hypothesis that inflammatory pathways can suppress or induce autophagy in a context-dependent manner; in turn, autophagy suppresses or promotes inflammation in cancers. Furthermore, emerging data suggest that autophagy may influence cytokine production and secretion via diverse mechanisms, which has implications for the immune and inflammatory microenvironment in tumors.     

p53 translational control: a new facet of p53 regulation and its implication for tumorigenesis and cancer therapeutics

While posttranslational regulation of p53 levels by its interaction with the ubiquitin ligase MDM2 is widely accepted, it has recently become clear that regulation of p53 translation also contributes to p53 induction following DNA damage. However, the mechanisms underlying the translational control of p53 are still poorly understood. In this review, we will focus on the translational regulation of p53 through the 5'- and 3'-untranslated regions of its mRNA. We will also discuss in detail the recent discovery of the p53 internal ribosome entry site (IRES), its role in p53 translation in response to DNA damage, and how it might lead to a better understanding of the process of oncogenesis and provide new avenues for cancer therapeutics.     

Mevalonate cascade regulation of airway mesenchymal cell autophagy and apoptosis: a dual role for p53

Statins inhibit the proximal steps of cholesterol biosynthesis, and are linked to health benefits in various conditions, including cancer and lung disease. We have previously investigated apoptotic pathways triggered by statins in airway mesenchymal cells, and identified reduced prenylation of small GTPases as a primary effector mechanism leading to p53-mediated cell death. Here, we extend our studies of statin-induced cell death by assessing endpoints of both apoptosis and autophagy, and investigating their interplay and coincident regulation. Using primary cultured human airway smooth muscle (HASM) and human airway fibroblasts (HAF), autophagy, and autophagosome formation and flux were assessed by transmission electron microscopy, cytochemistry (lysosome number and co-localization with LC3) and immunoblotting (LC3 lipidation and Atg12-5 complex formation). Chemical inhibition of autophagy increased simvastatin-induced caspase activation and cell death. Similarly, Atg5 silencing with shRNA, thus preventing Atg5-12 complex formation, increased pro-apoptotic effects of simvastatin. Simvastatin concomitantly increased p53-dependent expression of p53 up-regulated modulator of apoptosis (PUMA), NOXA, and damage-regulated autophagy modulator (DRAM). Notably both mevalonate cascade inhibition-induced autophagy and apoptosis were p53 dependent: simvastatin increased nuclear p53 accumulation, and both cyclic pifithrin-α and p53 shRNAi partially inhibited NOXA, PUMA expression and caspase-3/7 cleavage (apoptosis) and DRAM expression, Atg5-12 complex formation, LC3 lipidation, and autophagosome formation (autophagy). Furthermore, the autophagy response is induced rapidly, significantly delaying apoptosis, suggesting the existence of a temporally coordinated p53 regulation network. These findings are relevant for the development of statin-based therapeutic approaches in obstructive airway disease.     

抑癌基因p53与细胞自噬

细胞自噬(autophagy)是一种在进化上高度保守的代谢通路,它发生的分子机制和信号调控途径相当复杂,其中mTOR信号通路和Beclin1复合物发挥了最重要的调控作用,p53也是细胞自噬重要的调节因子。研究发现,p53可通过多种途径调节细胞自噬水平,这主要决定于它的亚细胞定位。在细胞核中,p53可通过多种方式上调细胞自噬;而在细胞质中,p53对细胞自噬具有负性调节作用,可抑制细胞自噬的发生。探究清楚p53与细胞自噬之间的调控关系将有助于人类正确认识由于细胞自噬功能异常所诱导的肿瘤的发生发展过程,从而最终攻克各种肿瘤性疾病。     

Mitochondrial dynamics: a strategy for avoiding autophagy

Cells normally respond to a lack of nutrients by activating autophagy, a prominent pro-survival pathway that involves the catabolism and recycling of cytoplasmic material. Recent results indicate that mitochondria actively elongate during autophagy, thereby avoiding their degradation and sustaining cell viability.