p53: The Janus of autophagy?

The autophagy pathway functions in adaptation to nutrient stress and tumour suppression. The p53 tumour suppressor, previously thought to positively regulate autophagy, may also inhibit it. This dual interplay between p53 and autophagy regulation is enigmatic, but may underlie key aspects of metabolism and cancer biology.     

Autophagy augments the self-renewal of lung cancer stem cells by the degradation of ubiquitinated p53

It has been postulated that cancer stem cells (CSCs) are involved in all aspects of human cancer, although the mechanisms governing the regulation of CSC self-renewal in the cancer state remain poorly defined. In the literature, both the pro- and anti-oncogenic activities of autophagy have been demonstrated and are context-dependent. Mounting evidence has shown augmentation of CSC stemness by autophagy, yet mechanistic characterization and understanding are lacking. In the present study, by generating stable human lung CSC cell lines with the wild-type TP53 (A549), as well as cell lines in which TP53 was deleted (H1229), we show, for the first time, that autophagy augments the stemness of lung CSCs by degrading ubiquitinated p53. Furthermore, Zeb1 is required for TP53 regulation of CSC self-renewal. Moreover, TCGA data mining and analysis show that Atg5 and Zeb1 are poor prognostic markers of lung cancer. In summary, this study has elucidated a new CSC-based mechanism underlying the oncogenic activity of autophagy and the tumor suppressor activity of p53 in cancer, i.e., CSCs can exploit the autophagy-p53-Zeb1 axis for self-renewal, oncogenesis, and progression.Subject terms: Cancer stem cells, Cancer stem cells     

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.     

A PINK1-mediated mitophagy pathway decides the fate of tumors—to be benign or malignant?

Macroautophagy/autophagy plays a dual role in cancer depending on the stage of tumorigenesis. Autophagy prevents tumor initiation by suppressing chronic tissue damage, inflammation, accumulation of damaged organelles and genome instability. Autophagy can also sustain tumor metabolism and provide nutrients for tumor growth and survival via nutrient recycling. Moreover, autophagy is required for benign tumors to progress to malignant tumors. Emerging evidence indicates that autophagy or mitophagy can inactivate tumor suppressors such as TP53/TRP53/p53 to promote tumor progression once carcinogenesis has been initiated.     

Autophagy and cancer therapy

Autophagy is a dynamic process of protein degradation, which is typically observed during nutrient deprivation. Recently, interest in autophagy has been renewed among oncologists, because different types of cancer cells undergo autophagy after various anticancer therapies. This type of nonapoptotic cell death has been documented mainly by observing morphological changes, e.g., numerous autophagic vacuoles in the cytoplasm of dying cells. Thus, autophagic cell death is considered programmed cell death type II, whereas apoptosis is programmed cell death type I. These two types of cell death are predominantly distinctive, but many studies demonstrate cross-talk between them. Whether autophagy in cancer cells causes death or protects cells is controversial. In multiple studies, autophagy has been inhibited pharmacologically or genetically, resulting in contrasting outcomes--survival or death--depending on the specific context. Interestingly, the regulatory pathways of autophagy share several molecules with the oncogenic pathways activated by tyrosine kinase receptors. Tumor suppressors such as Beclin 1, PTEN and p53 also play an important role in autophagy induction. Taken together, these accumulating data may lead to development of new cancer therapies that manipulate autophagy.     

Role and regulation of autophagy in cancer

Autophagy is an intracellular self-degradative mechanism which responds to cellular conditions like stress or starvation and plays a key role in regulating cell metabolism, energy homeostasis, starvation adaptation, development and cell death. Numerous studies have stipulated the participation of autophagy in cancer, but the role of autophagy either as tumor suppressor or tumor promoter is not clearly understood. However, mechanisms by which autophagy promotes cancer involves a diverse range of modifications of autophagy associated proteins such as ATGs, Beclin-1, mTOR, p53, KRAS etc. and autophagy pathways like mTOR, PI3K, MAPK, EGFR, HIF and NFκB. Furthermore, several researches have highlighted a context-dependent, cell type and stage-dependent regulation of autophagy in cancer. Alongside this, the interaction between tumor cells and their microenvironment including hypoxia has a great potential in modulating autophagy response in favour to substantiate cancer cell metabolism, self-proliferation and metastasis. In this review article, we highlight the mechanism of autophagy and their contribution to cancer cell proliferation and development. In addition, we discuss about tumor microenvironment interaction and their consequence on selective autophagy pathways and the involvement of autophagy in various tumor types and their therapeutic interventions concentrated on exploiting autophagy as a potential target to improve cancer therapy.     

Molecular mechanisms of Polygonatum cyrtonema lectin-induced apoptosis and autophagy in cancer cells

Polygonatum cyrtonema lectin (PCL), a mannose/sialic acid-binding lectin, has been reported to display remarkable inhibitory and cytotoxic activity toward cancer cells. However, the precise mechanism by which PCL induces tumor cell death is still only rudimentarily understood. In the present study, PCL was shown to markedly inhibit the growth of human melanoma A375 cells with concomitant low toxicity to the normal melanocytes. Subsequently, PCL was found to simultaneously induce A375 cell apoptosis and autophagy. The mechanism of apoptosis following treatment with PCL involved regulation of Bax, Bcl-xL and Bcl-2 proteins, which then caused collapse of the mitochondrial membrane potential, leading to cytochrome c release and caspase activation. The treatment with PCL also abrogated the glutathione antioxidant system, and induced mitochondria to generate massive ROS accumulation, which subsequently resulted in p38 and p53 activation. Further experimental data confirmed that the ROS-p38-p53 pathway could be involved in the stimulation of autophagy, suggesting that autophagy may play a death-promoting role via the above-mentioned apoptotic pathway. In conclusion, these findings indicate that PCL induces both apoptosis and autophagy in cancer cells through a mitochondria-mediated ROS-p38-p53 pathway.     

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.     

The fine tuning of metabolism,autophagy and differentiation during in vitro myogenesis

Although the mechanisms controlling skeletal muscle homeostasis have been identified, there is a lack of knowledge of the integrated dynamic processes occurring during myogenesis and their regulation. Here, metabolism, autophagy and differentiation were concomitantly analyzed in mouse muscle satellite cell (MSC)-derived myoblasts and their cross-talk addressed by drug and genetic manipulation. We show that increased mitochondrial biogenesis and activation of mammalian target of rapamycin complex 1 inactivation-independent basal autophagy characterize the conversion of myoblasts into myotubes. Notably, inhibition of autophagic flux halts cell fusion in the latest stages of differentiation and, conversely, when the fusion step of myocytes is impaired the biogenesis of autophagosomes is also impaired. By using myoblasts derived from p53 null mice, we show that in the absence of p53 glycolysis prevails and mitochondrial biogenesis is strongly impaired. P53 null myoblasts show defective terminal differentiation and attenuated basal autophagy when switched into differentiating culture conditions. In conclusion, we demonstrate that basal autophagy contributes to a correct execution of myogenesis and that physiological p53 activity is required for muscle homeostasis by regulating metabolism and by affecting autophagy and differentiation.Muscle satellite cells (MSCs) in adult muscle remain quiescent until external stimuli (such as injury or even exercise) trigger their re-entry into the cell cycle. Their progeny, myoblasts, fuse to form new multinucleated myofibers. In this study, the ability of MSC to give rise to muscle progenitor cells (that is, myoblasts) that could differentiate and fuse in vitro has been exploited to analyze the integrated network of signaling pathways that operate during myogenesis.Autophagy undergoes a fine tuning during cell and tissue differentiation in order to adapt to the dynamic changes occurring in the tissue microenvironment.¹ Using the stable C2C12 cell line, it was shown that autophagy is induced during muscle differentiation despite the concomitant activation of mammalian target of rapamycin (mTOR).² Interestingly, inhibition of autophagy was found to impair the differentiation and fusion of C2C12 myoblasts, while favoring their apoptosis.³ Autophagy is increased in muscle in several physiological and pathological conditions, including fasting, atrophy and exercise.⁴ Much less is known about the link between cell metabolism and autophagy during muscle differentiation under physiological conditions.p53 has been shown to promote myoblast differentiation by regulating the function of pRb,^{5, 6} and to have a pleiotropic role in muscle metabolism by promoting exercise-induced mitochondrial biogenesis in skeletal muscle.^{7, 8} How the physiological level of p53 impacts on these changes during differentiation has not been explored.The role of p53 in the regulation of autophagy is multi-facets.⁹ Nuclear p53 positively regulates autophagy following exogenous stress, resulting in a pro-death or pro-survival outcomes.¹⁰ Conversely, cytoplasmic p53 inhibits autophagy under starvation or endoplasmic reticulum (ER) stress.¹¹ What is the role of p53 in the regulation of basal autophagy during myogenesis and its physiological implications are still unknown.We address these issues using mouse skeletal MSC-derived myoblasts that when differentiate in vitro well mimic the dynamic processes occurring in vivo when a myoblast is asked to differentiate and fuse into a fully differentiated myotube. The findings here reported unravel a clear role for basal autophagy in muscle differentiation and identify a role for p53 in muscle metabolism and basal autophagy.     

Core signaling pathways of survival/death in autophagy-related cancer networks

Autophagy (macroautophagy), an evolutionarily conserved lysosomal degradation process, is implicated in a wide variety of pathological processes including cancer. Autophagy plays the Janus role in regulating several survival or death signaling pathways that may decide the fate of cancer cell. Accumulating evidence has revealed the core molecular machinery of autophagy in tumor initiation and progression; however, the intricate relationships between autophagy and cancer are still in its infancy. In this review, we summarize several key survival/death pathways such as mTOR subnetwork, Beclin 1 interactome, and p53 signaling that may play the crucial roles for the regulation of the autophagy-related cancer networks. Therefore, a better understanding of the relationships between autophagy and cancer may ultimately allow cancer biologists and clinicians to harness core autophagic pathways for the discovery of potential novel drug targets.