p53-Dependent and p53-independent activation of autophagy by ARF

SIRT7 promotes lung cancer progression by destabilizing the tumor suppressor ARF

Sirtuin 7 (SIRT7) is a member of the mammalian family of nicotinamide adenine dinucleotide (NAD+)-dependent histone/protein deacetylases, known as sirtuins. It acts as a potent oncogene in numerous malignancies, but the molecular mechanisms employed by SIRT7 to sustain lung cancer progression remain largely uncharacterized. We demonstrate that SIRT7 exerts oncogenic functions in lung cancer cells by destabilizing the tumor suppressor alternative reading frame (ARF). SIRT7 directly interacts with ARF and prevents binding of ARF to nucleophosmin, thereby promoting proteasomal-dependent degradation of ARF. We show that SIRT7-mediated degradation of ARF increases expression of protumorigenic genes and stimulates proliferation of non-small-cell lung cancer (NSCLC) cells both in vitro and in vivo in a mouse xenograft model. Bioinformatics analysis of transcriptome data from human lung adenocarcinomas revealed a correlation between SIRT7 expression and increased activity of genes normally repressed by ARF. We propose that disruption of SIRT7-ARF signaling stabilizes ARF and thus attenuates cancer cell proliferation, offering a strategy to mitigate NSCLC progression.

Expanding Roles of the E2F-RB-p53 Pathway in Tumor Suppression

The transcription factor E2F links the RB pathway to the p53 pathway upon loss of function of pRB, thereby playing a pivotal role in the suppression of tumorigenesis. E2F fulfills a major role in cell proliferation by controlling a variety of growth-associated genes. The activity of E2F is controlled by the tumor suppressor pRB, which binds to E2F and actively suppresses target gene expression, thereby restraining cell proliferation. Signaling pathways originating from growth stimulative and growth suppressive signals converge on pRB (the RB pathway) to regulate E2F activity. In most cancers, the function of pRB is compromised by oncogenic mutations, and E2F activity is enhanced, thereby facilitating cell proliferation to promote tumorigenesis. Upon such events, E2F activates the Arf tumor suppressor gene, leading to activation of the tumor suppressor p53 to protect cells from tumorigenesis. ARF inactivates MDM2, which facilitates degradation of p53 through proteasome by ubiquitination (the p53 pathway). P53 suppresses tumorigenesis by inducing cellular senescence or apoptosis. Hence, in almost all cancers, the p53 pathway is also disabled. Here we will introduce the canonical functions of the RB-E2F-p53 pathway first and then the non-classical functions of each component, which may be relevant to cancer biology.

Growth or death? Control of cell destiny by mTOR and autophagy pathways

One of the central regulators of cell growth, proliferation, and metabolism is the mammalian target of rapamycin, mTOR, which exists in two structurally and functionally different complexes: mTORC1 and mTORC2; unlike m TORC2, mTORC1 is activated in response to the sufficiency of nutrients and is inhibited by rapamycin. mTOR complexes have critical roles not only in protein synthesis, gene transcription regulation, proliferation, tumor metabolism, but also in the regulation of the programmed cell death mechanisms such as autophagy and apoptosis. Autophagy is a conserved catabolic mechanism in which damaged molecules are recycled in response to nutrient starvation. Emerging evidence indicates that the mTOR signaling pathway is frequently activated in tumors. In addition, dysregulation of autophagy was associated with the development of a variety of human diseases, such as cancer and aging. Since mTOR can inhibit the induction of the autophagic process from the early stages of autophagosome formation to the late stage of lysosome degradation, the use of mTOR inhibitors to regulate autophagy could be considered a potential therapeutic option. The present review sheds light on the mTOR and autophagy signaling pathways and the mechanisms of regulation of mTOR-autophagy.

Autophagy: A newly discovered protective mechanism in the marine rotifer Brachionus plicatilis in response to BDE-47 exposure

2,2',4,4'-Tetrabromodiphenyl ether (BDE-47) is a persistent organic pollutant that spreads widely in the marine environment. Our previous studies found that it had adverse effects on the marine rotifer Brachionus plicatilis and caused a series of stress responses. The present study was performed to verify the occurrence of autophagy and explore its role in B. plicatilis' coping with BDE-47 exposure. Rotifers were exposed to 0.05, 0.2, 0.8, and 3.2 mg/L BDE-47 for 24 h, respectively. Detections of the autophagy marker protein LC3 by western blot and autophagosomes by MDC staining demonstrated the occurrence of autophagy. The levels of autophagy were significantly increased in BDE-47-treated groups with a peak in 0.8 mg/L group. A series of indicators responded to BDE-47 exposure, including reactive oxygen species (ROS), GSH/GSSG ratio, superoxide dismutase (SOD) activity, and malonaldehyde (MDA), collectively indicating the occurrence of oxidative stress. The potential interplay between autophagy and oxidative stress in B. plicatilis was explored in the 0.8 mg/L group through a series of additions. The ROS level was significantly decreased by the addition of the ROS generation inhibitor diphenyleneiodonium chloride, to a level even lower than that in the blank control, and concomitantly, autophagosome was almost undetectable, indicating that a certain level of ROS was essential for the occurrence of autophagy. Autophagy was weakened by the addition of the autophagy inhibitor 3-methyladenine coincident with the great elevation of ROS, indicating that activated autophagy contributed to reducing the ROS level. Additional proof of this relation was obtained from the direct opposite effects of the autophagy inhibitor bafilomycin A1 and the autophagy activator rapamycin: the former increased the MDA content significantly, whereas the latter decreased it significantly. The combined results suggested that autophagy alleviated oxidative stress and might be a newly discovered protective mechanism in B. plicatilis coping with BDE-47 exposure.

Targeting p53 pathways: mechanisms, structures, and advances in therapy

The TP53 tumor suppressor is the most frequently altered gene in human cancers, and has been a major focus of oncology research. The p53 protein is a transcription factor that can activate the expression of multiple target genes and plays critical roles in regulating cell cycle, apoptosis, and genomic stability, and is widely regarded as the "guardian of the genome". Accumulating evidence has shown that p53 also regulates cell metabolism, ferroptosis, tumor microenvironment, autophagy and so on, all of which contribute to tumor suppression. Mutations in TP53 not only impair its tumor suppressor function, but also confer oncogenic properties to p53 mutants. Since p53 is mutated and inactivated in most malignant tumors, it has been a very attractive target for developing new anti-cancer drugs. However, until recently, p53 was considered an "undruggable" target and little progress has been made with p53-targeted therapies. Here, we provide a systematic review of the diverse molecular mechanisms of the p53 signaling pathway and how TP53 mutations impact tumor progression. We also discuss key structural features of the p53 protein and its inactivation by oncogenic mutations. In addition, we review the efforts that have been made in p53-targeted therapies, and discuss the challenges that have been encountered in clinical development.

Correlation of Ferroptosis and Other Types of Cell Death in Neurodegenerative Diseases

Ferroptosis is defined as an iron-dependent, non-apoptotic cell death pathway, with specific morphological phenotypes and biochemical changes. There is a growing realization that ferroptosis has significant implications for several neurodegenerative diseases. Even though ferroptosis is different from other forms of programmed death such as apoptosis and autophagic death, they involve a number of common protein molecules. This review focuses on current research on ferroptosis and summarizes the cross-talk among ferroptosis, apoptosis, and autophagy that are implicated in neurodegenerative diseases. We hope that this information provides new ideas for understanding the mechanisms and searching for potential therapeutic approaches and prevention of neurodegenerative diseases.

It’s Getting Complicated—A Fresh Look at p53-MDM2-ARF Triangle in Tumorigenesis and Cancer Therapy

Anti-tumorigenic mechanisms mediated by the tumor suppressor p53, upon oncogenic stresses, are our bodies’ greatest weapons to battle against cancer onset and development. Consequently, factors that possess significant p53-regulating activities have been subjects of serious interest from the cancer research community. Among them, MDM2 and ARF are considered the most influential p53 regulators due to their abilities to inhibit and activate p53 functions, respectively. MDM2 inhibits p53 by promoting ubiquitination and proteasome-mediated degradation of p53, while ARF activates p53 by physically interacting with MDM2 to block its access to p53. This conventional understanding of p53-MDM2-ARF functional triangle have guided the direction of p53 research, as well as the development of p53-based therapeutic strategies for the last 30 years. Our increasing knowledge of this triangle during this time, especially through identification of p53-independent functions of MDM2 and ARF, have uncovered many under-appreciated molecular mechanisms connecting these three proteins. Through recognizing both antagonizing and synergizing relationships among them, our consideration for harnessing these relationships to develop effective cancer therapies needs an update accordingly. In this review, we will re-visit the conventional wisdom regarding p53-MDM2-ARF tumor-regulating mechanisms, highlight impactful studies contributing to the modern look of their relationships, and summarize ongoing efforts to target this pathway for effective cancer treatments. A refreshed appreciation of p53-MDM2-ARF network can bring innovative approaches to develop new generations of genetically-informed and clinically-effective cancer therapies.

Mutation of the Conserved Threonine 8 within the Human ARF Tumour Suppressor Protein Regulates Autophagy

Background: The ARF tumour suppressor plays a well-established role as a tumour suppressor, halting cell growth by both p53-dependent and independent pathways in several cellular stress response circuits. However, data collected in recent years challenged the traditional role of this protein as a tumour suppressor. Cancer cells expressing high ARF levels showed that its expression, far from being dispensable, is required to guarantee tumour cell survival. In particular, ARF can promote autophagy, a self-digestion pathway that helps cells cope with stressful growth conditions arising during both physiological and pathological processes. Methods: We previously showed that ARF is regulated through the activation of the protein kinase C (PKC)-dependent pathway and that an ARF phospho-mimetic mutant on the threonine residue 8, ARF-T8D, sustains cell proliferation in HeLa cells. We now explored the role of ARF phosphorylation in both basal and starvation-induced autophagy by analysing autophagic flux in cells transfected with either WT and ARF phosphorylation mutants by immunoblot and immunofluorescence. Results: Here, we show that endogenous ARF expression in HeLa cells is required for starvation-induced autophagy. Further, we provide evidence that the hyper-expression of ARF-T8D appears to inhibit autophagy in both HeLa and lung cancer cells H1299. This effect is due to the cells’ inability to elicit autophagosomes formation upon T8D expression. Conclusions: Our results lead to the hypothesis that ARF phosphorylation could be a mechanism through which the protein promotes or counteracts autophagy. Several observations underline how autophagy could serve a dual role in cancer progression, either protecting healthy cells from damage or aiding cancerous cells to survive. Our results indicate that ARF phosphorylation controls protein’s ability to promote or counteract autophagy, providing evidence of the dual role played by ARF in cancer progression.

The Post-Translational Regulation of Epithelial-Mesenchymal Transition-Inducing Transcription Factors in Cancer Metastasis

Epithelial-mesenchymal transition (EMT) is generally observed in normal embryogenesis and wound healing. However, this process can occur in cancer cells and lead to metastasis. The contribution of EMT in both development and pathology has been studied widely. This transition requires the up- and down-regulation of specific proteins, both of which are regulated by EMT-inducing transcription factors (EMT-TFs), mainly represented by the families of Snail, Twist, and ZEB proteins. This review highlights the roles of key EMT-TFs and their post-translational regulation in cancer metastasis.

The p53 Pathway and Metabolism: The Tree That Hides the Forest

Simple Summary

The p53 pathway is a major tumor suppressor pathway that prevents the propagation of abnormal cells by regulating DNA repair, cell cycle progression, cell death, or senescence. The multiple cellular processes regulated by p53 were more recently extended to the control of metabolism, and many studies support the notion that perturbations of p53-associated metabolic activities are linked to cancer development. Converging lines of evidence support the notion that, in addition to p53, other key components of this molecular cascade are also important regulators of metabolism. Here, we illustrate the underestimated complexity of the metabolic network controlled by the p53 pathway and show how its perturbation contributes to human diseases including cancer, aging, and metabolic diseases.

Abstract

The p53 pathway is functionally inactivated in most, if not all, human cancers. The p53 protein is a central effector of numerous stress-related molecular cascades. p53 controls a safeguard mechanism that prevents accumulation of abnormal cells and their transformation by regulating DNA repair, cell cycle progression, cell death, or senescence. The multiple cellular processes regulated by p53 were more recently extended to the control of metabolism and many studies support the notion that perturbations of p53-associated metabolic activities are linked to cancer development, as well as to other pathophysiological conditions including aging, type II diabetes, and liver disease. Although much less documented than p53 metabolic activities, converging lines of evidence indicate that other key components of this tumor suppressor pathway are also involved in cellular metabolism through p53-dependent as well as p53-independent mechanisms. Thus, at least from a metabolic standpoint, the p53 pathway must be considered as a non-linear pathway, but the complex metabolic network controlled by these p53 regulators and the mechanisms by which their activities are coordinated with p53 metabolic functions remain poorly understood. In this review, we highlight some of the metabolic pathways controlled by several central components of the p53 pathway and their role in tissue homeostasis, metabolic diseases, and cancer.

Roles of ARF tumour suppressor protein in lung cancer: time to hit the nail on the head!

Owing to its poor prognosis, the World Health Organization (WHO) lists lung cancer on top of the list when it comes to growing mortality rates and incidence. Usually, there are two types of lung cancer, small-cell lung cancer (SCLC) and non-small-cell lung cancer (NSCLC), which also includes adenocarcinoma, squamous cell carcinoma and large cell carcinomas. ARF, also known in humans as p14ARF and in the mouse as p19ARF, is a nucleolar protein and a member of INK4, a family of cyclin-independent kinase inhibitors (CKI). These genes are clustered on chromosome number 9p21 within the locus of CDKN2A. NSCLC has reported the role of p14ARF as a potential target. p14ARF has a basic mechanism to inhibit mouse double minute 2 protein that exhibits inhibitory action on p53, a phosphoprotein tumour suppressor, thus playing a role in various tumour-related activities such as growth inhibition, DNA damage, autophagy, apoptosis, cell cycle arrest and others. Extensive cancer research is ongoing and updated reports regarding the role of ARF in lung cancer are available. This article summarizes the available lung cancer ARF data, its molecular mechanisms and its associated signalling pathways. Attempts have been made to show how p14ARF functions in different types of lung cancer providing a thought to look upon ARF as a new target for treating the debilitating condition of lung cancer.

Identification and Validation of Six Autophagy-related Long Non-coding RNAs as Prognostic Signature in Colorectal Cancer

Colorectal cancer (CRC) is a commonly occurring tumour with poor prognosis. Autophagy-related long non-coding RNAs (lncRNAs) have received much attention as biomarkers for cancer prognosis and diagnosis. However, few studies have focused on their prognostic predictive value specifically in CRC. This research aimed to construct a robust autophagy-related lncRNA prognostic signature for CRC. Autophagy-related lncRNAs from The Cancer Genome Atlas database were screened using univariate Cox, LASSO, and multivariate Cox regression analyses, and the resulting key lncRNAs were used to establish a prognostic risk score model. Furthermore, quantitative real-time polymerase chain reaction (qRT-PCR) analysis was performed to detect the expression of several lncRNAs in cancer tissues from CRC patients and in normal tissues adjacent to the cancer tissues. A prognostic signature comprising lncRNAs AC125603.2, LINC00909, AC016876.1, MIR210HG, AC009237.14, and LINC01063 was identified in patients with CRC. A graphical nomogram based on the autophagy-related lncRNA signature was developed to predict CRC patients' 1-, 3-, and 5-year survival. Overall survival in patients with low risk scores was significantly better than in those with high risk scores (P < 0.0001); a similar result was obtained in an internal validation sample. The nomogram was shown to be suitable for clinical use and gave correct predictions. The 1- and 3-year values of the area under the receiver operating characteristic curve were 0.797 and 0.771 in the model sample, and 0.656 and 0.642 in the internal validation sample, respectively. The C-index values for the verification samples and training samples were 0.756 (95% CI = 0.668-0.762) and 0.715 (95% CI = 0.683-0.829), respectively. Gene set enrichment analysis showed that the six autophagy-related lncRNAs were greatly enriched in CRC-related signalling pathways, including p53 and VEGF signalling. The qRT-PCR results showed that the expression of lncRNAs in CRC was higher than that in adjacent tissues, consistent with the expression trends of lncRNAs in the CRC data set. In summary, we established a signature of six autophagy-related lncRNAs that could effectively guide clinical prediction of prognosis in patients with CRC. This lncRNA signature has significant clinical implications for improving the prediction of outcomes and, with further prospective validation, could be used to guide tailored therapy for CRC patients.

A research agenda for ageing in China in the 21st century (2nd edition): Focusing on basic and translational research, long-term care, policy and social networks

One of the key issues facing public healthcare is the global trend of an increasingly ageing society which continues to present policy makers and caregivers with formidable healthcare and socio-economic challenges. Ageing is the primary contributor to a broad spectrum of chronic disorders all associated with a lower quality of life in the elderly. In 2019, the Chinese population constituted 18 % of the world population, with 164.5 million Chinese citizens aged 65 and above (65+), and 26 million aged 80 or above (80+). China has become an ageing society, and as it continues to age it will continue to exacerbate the burden borne by current family and public healthcare systems. Major healthcare challenges involved with caring for the elderly in China include the management of chronic non-communicable diseases (CNCDs), physical frailty, neurodegenerative diseases, cardiovascular diseases, with emerging challenges such as providing sufficient dental care, combating the rising prevalence of sexually transmitted diseases among nursing home communities, providing support for increased incidences of immune diseases, and the growing necessity to provide palliative care for the elderly. At the governmental level, it is necessary to make long-term strategic plans to respond to the pressures of an ageing society, especially to establish a nationwide, affordable, annual health check system to facilitate early diagnosis and provide access to affordable treatments. China has begun work on several activities to address these issues including the recent completion of the of the Ten-year Health-Care Reform project, the implementation of the Healthy China 2030 Action Plan, and the opening of the National Clinical Research Center for Geriatric Disorders. There are also societal challenges, namely the shift from an extended family system in which the younger provide home care for their elderly family members, to the current trend in which young people are increasingly migrating towards major cities for work, increasing reliance on nursing homes to compensate, especially following the outcomes of the 'one child policy' and the 'empty-nest elderly' phenomenon. At the individual level, it is important to provide avenues for people to seek and improve their own knowledge of health and disease, to encourage them to seek medical check-ups to prevent/manage illness, and to find ways to promote modifiable health-related behaviors (social activity, exercise, healthy diets, reasonable diet supplements) to enable healthier, happier, longer, and more productive lives in the elderly. Finally, at the technological or treatment level, there is a focus on modern technologies to counteract the negative effects of ageing. Researchers are striving to produce drugs that can mimic the effects of 'exercising more, eating less', while other anti-ageing molecules from molecular gerontologists could help to improve 'healthspan' in the elderly. Machine learning, 'Big Data', and other novel technologies can also be used to monitor disease patterns at the population level and may be used to inform policy design in the future. Collectively, synergies across disciplines on policies, geriatric care, drug development, personal awareness, the use of big data, machine learning and personalized medicine will transform China into a country that enables the most for its elderly, maximizing and celebrating their longevity in the coming decades. This is the 2nd edition of the review paper (Fang EF et al., Ageing Re. Rev. 2015).

Impact of c-MYC expression on proliferation, differentiation, and risk of neoplastic transformation of human mesenchymal stromal cells

BACKGROUND

Mesenchymal stromal cells isolated from bone marrow (MSC) represent an attractive source of adult stem cells for regenerative medicine. However, thorough research is required into their clinical application safety issues concerning a risk of potential neoplastic degeneration in a process of MSC propagation in cell culture for therapeutic applications. Expansion protocols could preselect MSC with elevated levels of growth-promoting transcription factors with oncogenic potential, such as c-MYC. We addressed the question whether c-MYC expression affects the growth and differentiation potential of human MSC upon extensive passaging in cell culture and assessed a risk of tumorigenic transformation caused by MSC overexpressing c-MYC in vivo.

METHODS

MSC were subjected to retroviral transduction to induce expression of c-MYC, or GFP, as a control. Cells were expanded, and effects of c-MYC overexpression on osteogenesis, adipogenesis, and chondrogenesis were monitored. Ectopic bone formation properties were tested in SCID mice. A potential risk of tumorigenesis imposed by MSC with c-MYC overexpression was evaluated.

RESULTS

C-MYC levels accumulated during ex vivo passaging, and overexpression enabled the transformed MSC to significantly overgrow competing control cells in culture. C-MYC-MSC acquired enhanced biological functions of c-MYC: its increased DNA-binding activity, elevated expression of the c-MYC-binding partner MAX, and induction of antagonists P19ARF/P16INK4A. Overexpression of c-MYC stimulated MSC proliferation and reduced osteogenic, adipogenic, and chondrogenic differentiation. Surprisingly, c-MYC overexpression also caused an increased COL10A1/COL2A1 expression ratio upon chondrogenesis, suggesting a role in hypertrophic degeneration. However, the in vivo ectopic bone formation ability of c-MYC-transduced MSC remained comparable to control GFP-MSC. There was no indication of tumor growth in any tissue after transplantation of c-MYC-MSC in mice.

CONCLUSIONS

C-MYC expression promoted high proliferation rates of MSC, attenuated but not abrogated their differentiation capacity, and did not immediately lead to tumor formation in the tested in vivo mouse model. However, upregulation of MYC antagonists P19ARF/P16INK4A promoting apoptosis and senescence, as well as an observed shift towards a hypertrophic collagen phenotype and cartilage degeneration, point to lack of safety for clinical application of MSC that were manipulated to overexpress c-MYC for their better expansion.

Dual Role of the Alternative Reading Frame ARF Protein in Cancer

The CDKN2a/ARF locus expresses two partially overlapping transcripts that encode two distinct proteins, namely p14ARF (p19Arf in mouse) and p16INK4a, which present no sequence identity. Initial data obtained in mice showed that both proteins are potent tumor suppressors. In line with a tumor-suppressive role, ARF-deficient mice develop lymphomas, sarcomas, and adenocarcinomas, with a median survival rate of one year of age. In humans, the importance of ARF inactivation in cancer is less clear whereas a more obvious role has been documented for p16INK4a. Indeed, many alterations in human tumors result in the elimination of the entire locus, while the majority of point mutations affect p16INK4a. Nevertheless, specific mutations of p14ARF have been described in different types of human cancers such as colorectal and gastric carcinomas, melanoma and glioblastoma. The activity of the tumor suppressor ARF has been shown to rely on both p53-dependent and independent functions. However, novel data collected in the last years has challenged the traditional and established role of this protein as a tumor suppressor. In particular, tumors retaining ARF expression evolve to metastatic and invasive phenotypes and in humans are associated with a poor prognosis. In this review, the recent evidence and the molecular mechanisms of a novel role played by ARF will be presented and discussed, both in pathological and physiological contexts.

Semaphorin 4A Stabilizes Human Regulatory T Cell Phenotype via Plexin B1

We previously reported that neuroimmune semaphorin (Sema) 4A regulates the severity of experimental allergic asthma and increases regulatory T (Treg) cell numbers in vivo; however, the mechanisms of Sema4A action remain unknown. It was also reported that Sema4A controls murine Treg cell function and survival acting through neuropilin 1 (NRP-1) receptor. To clarify Sema4A action on human T cells, we employed T cell lines (HuT78 and HuT102), human PBMCs, and CD4+ T cells in phenotypic and functional assays. We found that HuT78 demonstrated a T effector-like phenotype (CD4+CD25lowFoxp3-), whereas HuT102 expressed a Treg-like phenotype (CD4+CD25hi Foxp3+). Neither cell line expressed NRP-1. HuT102 cells expressed Sema4A counter receptor Plexin B1, whereas HuT78 cells were Sema4A+. All human peripheral blood CD4+ T cells, including Treg cells, expressed PlexinB1 and lacked both NRP-1 and -2. However, NRP-1 and Sema4A were detected on CD3negativeCD4intermediate human monocytes. Culture of HuT cells with soluble Sema4A led to an upregulation of CD25 and Foxp3 markers on HuT102 cells. Addition of Sema4A increased the relative numbers of CD4+CD25+Foxp3+ cells in PBMCs and CD4+ T cells, which were NRP-1negative but PlexinB1+, suggesting the role of this receptor in Treg cell stability. The inclusion of anti-PlexinB1 blocking Ab in cultures before recombinant Sema4A addition significantly decreased Treg cell numbers as compared with cultures with recombinant Sema4A alone. Sema4A was as effective as TGF-β in inducible Treg cell induction from CD4+CD25depleted cells but did not enhance Treg cell suppressive activity in vitro. These results suggest strategies for the development of new Sema4A-based therapeutic measures to combat allergic inflammatory diseases. ImmunoHorizons, 2019, 3: 71-87.

17β-estradiol inhibits human umbilical vascular endothelial cell senescence by regulating autophagy via p53

Vascular endothelial cell (VEC) senescence is an initiating factor in numerous cardiovascular diseases. Recent studies showed that 17β-estradiol (17β-E2), an estrogen with numerous biological activities such as inhibition of atherosclerosis, protects VECs from senescence. However, the effects of 17β-E2 on human umbilical VECs (HUVECs) remain unknown. This study investigated the anti-senescent effect of 17β-E2 on HUVECs and explored the underlying mechanism with respect to autophagy and p53 activity. First, rapamycin and 3-methyladenine were used to clarify the relationship between autophagy and senescence in HUVECs, and an inverse relationship was demonstrated. Next, the effect of 17β-E2 on H₂O₂-induced senescence of HUVECs was examined. Increased autophagy induced by 17β-E2 inhibited H₂O₂-induced senescence of HUVECs, increased cell viability, and maintained HUVEC morphology. 17β-E2 pre-treatment also decreased cell cycle arrest, decreased the dephosphorylation of Rb, decreased the production of ET-1, and increased the production of NO. Most importantly, 17β-E2 pre-treatment increased autophagy by activating p53 and its downstream effector p53-upregulated modulator of apoptosis (PUMA). Overall, our data indicate the critical role of autophagy in the anti-senescent effect of 17β-E2 on HUVECs.

Helicobacter pylori pathogen regulates p14ARF tumor suppressor and autophagy in gastric epithelial cells

Infection with Helicobacter pylori is one of the strongest risk factors for development of gastric cancer. Although these bacteria infect approximately half of the world's population, only a small fraction of infected individuals develops gastric malignancies. Interactions between host and bacterial virulence factors are complex and interrelated, making it difficult to elucidate specific processes associated with H. pylori-induced tumorigenesis. In this study, we found that H. pylori inhibits p14ARF tumor suppressor by inducing its degradation. This effect was found to be strain-specific. Downregulation of p14ARF induced by H. pylori leads to inhibition of autophagy in a p53-independent manner in infected cells. We identified TRIP12 protein as E3 ubiquitin ligase that is upregulated by H. pylori, inducing ubiquitination and subsequent degradation of p14ARF protein. Using isogenic H. pylori mutants, we found that induction of TRIP12 is mediated by bacterial virulence factor CagA. Increased expression of TRIP12 protein was found in infected gastric epithelial cells in vitro and human gastric mucosa of H. pylori-infected individuals. In conclusion, our data demonstrate a new mechanism of ARF inhibition that may affect host-bacteria interactions and facilitate tumorigenic transformation in the stomach.

Dynamics of p14ARF and Focal Adhesion Kinase-Mediated Autophagy in Cancer

It has been widely shown that the focal adhesion kinase (FAK) is involved in nearly every aspect of cancer, from invasion to metastasis to epithelial–mesenchymal transition and maintenance of cancer stem cells. FAK has been shown to interact with p14ARF (alternative reading frame)—a well-established tumor suppressor—and functions in the negative regulation of cancer through both p53-dependent and -independent pathways. Interestingly, both FAK and ARF (human and mouse counterpart) proteins, as well as p53, are involved in autophagy—a process of “self-digestion”—whose main function is the recycling of cellular components and quality control of proteins and organelles. In the last years, an unexpected role of p14ARF in the survival of cancer cells has been underlined in different cellular contexts, suggesting a novel pro-oncogenic function of this protein. In this review, the mechanisms whereby ARF and FAK control autophagy are presented, as well as the role of autophagy in cell migration and spreading. Integrated investigation of these cell functions is extremely important to understand the mechanism of the basis of cell transformation and migration and thus cancer development.

p53-Mediated Molecular Control of Autophagy in Tumor Cells

Autophagy is an indispensable mechanism of the eukaryotic cell, facilitating the removal and renewal of cellular components and thereby balancing the cell's energy consumption and homeostasis. Deregulation of autophagy is now regarded as one of the characteristic key features contributing to the development of tumors. In recent years, the suppression of autophagy in combination with chemotherapeutic treatment has been approached as a novel therapy in cancer treatment. However, depending on the type of cancer and context, interference with the autophagic machinery can either promote or disrupt tumorigenesis. Therefore, disclosure of the major signaling pathways that regulate autophagy and control tumorigenesis is crucial. To date, several tumor suppressor proteins and oncogenes have emerged as eminent regulators of autophagy whose depletion or mutation favor tumor formation. The mammalian cell "janitor" p53 belongs to one of these tumor suppressors that are most commonly mutated in human tumors. Experimental evidence over the last decade convincingly reports that p53 can act as either an activator or an inhibitor of autophagy depending on its subcellular localization and its mode of action. This finding gains particular significance as p53 deficiency or mutant variants of p53 that accumulate in the cytoplasm of tumor cells enable activation of autophagy. Accordingly, we recently identified p53 as a molecular hub that regulates autophagy and apoptosis in histone deacetylase inhibitor-treated uterine sarcoma cells. In light of this novel experimental evidence, in this review, we focus on p53 signaling as a mediator of the autophagic pathway in tumor cells.

Emerging Roles of p53 Family Members in Glucose Metabolism

Glucose is the key source for most organisms to provide energy, as well as the key source for metabolites to generate building blocks in cells. The deregulation of glucose homeostasis occurs in various diseases, including the enhanced aerobic glycolysis that is observed in cancers, and insulin resistance in diabetes. Although p53 is thought to suppress tumorigenesis primarily by inducing cell cycle arrest, apoptosis, and senescence in response to stress, the non-canonical functions of p53 in cellular energy homeostasis and metabolism are also emerging as critical factors for tumor suppression. Increasing evidence suggests that p53 plays a significant role in regulating glucose homeostasis. Furthermore, the p53 family members p63 and p73, as well as gain-of-function p53 mutants, are also involved in glucose metabolism. Indeed, how this protein family regulates cellular energy levels is complicated and difficult to disentangle. This review discusses the roles of the p53 family in multiple metabolic processes, such as glycolysis, gluconeogenesis, aerobic respiration, and autophagy. We also discuss how the dysregulation of the p53 family in these processes leads to diseases such as cancer and diabetes. Elucidating the complexities of the p53 family members in glucose homeostasis will improve our understanding of these diseases.

Dysregulation of YAP by ARF Stimulated with Tea-derived Carbon Nanodots

YAP is a downstream nuclear transcription factor of Hippo pathway which plays an essential role in development, cell growth, organ size and homeostasis. It was previously identified that elevation of YAP in genomics of genetic engineered mouse (GEM) model of prostate cancer is associated with Pten/Trp53 inactivation and ARF elevation hypothesizing the essential crosstalk of AKT/mTOR/YAP with ARF in prostate cancer. However, the detailed function and trafficking of YAP in cancer cells remains unclear. Using GEM microarray model, we found ARF dysregulates Hippo and Wnt pathways. In particular, ARF knockdown reduced non-nuclear localization of YAP which led to an increase in F-actin. Mechanistically, ARF knockdown suppressed protein turnover of β-catenin/YAP, and therefore enhanced the activity of AKT and phosphorylation of YAP. Moreover, we found tea-derived carbon dots can interact with ARF in nucleus that may further lead to the non-nuclear localization of YAP. Thus, we reported a novel crosstalk of ARF/β-catenin dysregulated YAP in Hippo pathway and a new approach to stimulate ARF-mediated signaling to inhibit nuclear YAP using nanomaterials implicating an innovative avenue for treatment of cancer.

NRF2 Is a Major Target of ARF in p53-Independent Tumor Suppression

Although ARF can suppress tumor growth by activating p53 function, the mechanisms by which it suppresses tumor growth independently of p53 are not well understood. Here, we identified ARF as a key regulator of nuclear factor E2-related factor 2 (NRF2) through complex purification. ARF inhibits the ability of NRF2 to transcriptionally activate its target genes, including SLC7A11, a component of the cystine/glutamate antiporter that regulates reactive oxygen species (ROS)-induced ferroptosis. As a consequence, ARF expression sensitizes cells to ferroptosis in a p53-independent manner while ARF depletion induces NRF2 activation and promotes cancer cell survival in response to oxidative stress. Moreover, the ability of ARF to induce p53-independent tumor growth suppression in mouse xenograft models is significantly abrogated upon NRF2 overexpression. These results demonstrate that NRF2 is a major target of p53-independent tumor suppression by ARF and also suggest that the ARF-NRF2 interaction acts as a new checkpoint for oxidative stress responses.

Immunomodulatory and antitumor effects of type I interferons and their application in cancer therapy

During the last decades, the pleiotropic antitumor functions exerted by type I interferons (IFNs) have become universally acknowledged, especially their role in mediating interactions between the tumor and the immune system. Indeed, type I IFNs are now appreciated as a critical component of dendritic cell (DC) driven T cell responses to cancer. Here we focus on IFN-α and IFN-β, and their antitumor effects, impact on immune responses and their use as therapeutic agents. IFN-α/β share many properties, including activation of the JAK-STAT signaling pathway and induction of a variety of cellular phenotypes. For example, type I IFNs drive not only the high maturation status of DCs, but also have a direct impact in cytotoxic T lymphocytes, NK cell activation, induction of tumor cell death and inhibition of angiogenesis. A variety of stimuli, including some standard cancer treatments, promote the expression of endogenous IFN-α/β, which then participates as a fundamental component of immunogenic cell death. Systemic treatment with recombinant protein has been used for the treatment of melanoma. The induction of endogenous IFN-α/β has been tested, including stimulation through pattern recognition receptors. Gene therapies involving IFN-α/β have also been described. Thus, harnessing type I IFNs as an effective tool for cancer therapy continues to be studied.

GADD45A inhibits autophagy by regulating the interaction between BECN1 and PIK3C3

GADD45A is a TP53-regulated and DNA damage-inducible tumor suppressor protein, which regulates cell cycle arrest, apoptosis, and DNA repair, and inhibits tumor growth and angiogenesis. However, the function of GADD45A in autophagy remains unknown. In this report, we demonstrate that GADD45A plays an important role in regulating the process of autophagy. GADD45A is able to decrease LC3-II expression and numbers of autophagosomes in mouse tissues and different cancer cell lines. Using bafilomycin A1 treatment, we have observed that GADD45A regulates autophagosome initiation. Likely, GADD45A inhibition of autophagy is through its influence on the interaction between BECN1 and PIK3C3. Immunoprecipitation and GST affinity isolation assays exhibit that GADD45A directly interacts with BECN1, and in turn dissociates the BECN1-PIK3C3 complex. Furthermore, we have mapped the 71 to 81 amino acids of the GADD45A protein that are necessary for the GADD45A interaction with BECN1. Knockdown of BECN1 can abolish autophagy alterations induced by GADD45A. Taken together, these findings provide the novel evidence that GADD45A inhibits autophagy via impairing the BECN1-PIK3C3 complex formation.

Peptide-based tumor inhibitor encoding mitochondrial p14(ARF) is highly efficacious to diverse tumors

p14^ARF is one of the major tumor suppressors conventionally identified both as the mdm2‐binding molecule restoring p53 function in the nucleus, and as a nucleophosmin‐binding partner inside the nucleolous to stabilize ribosomal RNA. However, its recently reported mitochondrial localization has pointed to novel properties as a tumor suppressor. At the same time, functional peptides are gaining much attention in nanomedicine for their in vivo utility as non‐invasive biologics. We previously reported the p14^ARF‐specific peptide that restored the sensitivity to gefitinib on the gefitinib‐resistant lung cancer cells. Based on the information of this prototype peptide, here we generated the more powerful anti‐tumor peptide “r9‐CatB‐p14 MIS,” which comprises the minimal inhibitory sequence of the mitochondrial targeting p14^ARF protein in combination with the proteolytic cleavage site for cathepsin B, which is activated in various tumor cells, fused with the nine‐polyarginine‐domain for cell penetration, and demonstrated its novel action of regulating mitochondrial function in accordance with localization of endogenous p14^ARF. The p14 MIS peptide showed a potent tumor inhibiton in vitro and in vivo against not only lung cancer cells but also tumor cells of diverse lineages, via modulating mitochondrial membrane potential, with minimal cytotoxicity to non‐neoplastic cells and tissues. Hence, this mitochondrially targeted p14 peptide agent provides a novel basis for non‐invasive peptide‐based antitumor therapeutics.

Transduction of Recombinant M3-p53-R12 Protein Enhances Human Leukemia Cell Apoptosis

Tumor suppressor protein p53 plays important roles in initiating cell cycle arrest and promoting tumor cell apoptosis. Previous studies have shown that p53 is either mutated or defective in approximately 50% of human cancers; therefore restoring normal p53 activity in cancer cells might be an effective anticancer therapeutic approach. Herein, we designed a chimeric p53 protein flanked with the MyoD N-terminal transcriptional activation domain (amino acids 1-62, called M3) and a poly-arginine (R12) cell penetrating signal in its N-and C-termini respectively. This chimeric protein, M3-p53-R12, can be expressed in E. coli and purified using immobilized metal ion chromatography followed by serial refolding dialysis. The purified M3-p53-R12 protein retains DNA-binding activity and gains of cell penetrating ability. Using MTT assay, we demonstrated that M3-p53-R12 inhibited the growth of K562, Jurkat as well as HL-60 leukemia cells carrying mutant p53 genes. Results from FACS analysis also demonstrated that transduction of M3-p53-R12 protein induced cell cycle arrest of these leukemia cells. Of special note, M3-p53-R12 has no apoptotic effect on normal mesenchymal stem cells (MSC) and leukocytes, highlighting its differential effects on normal and tumor cells. To sum up, our results reveal that purified recombinant M3-p53-R12 protein has functions of suppressing the leukemia cell lines' proliferation and launching cell apoptosis, suggesting the feasibility of using M3-p53-R12 protein as an anticancer drug. In the future we will test whether this chimeric protein can preferentially trigger the death of malignant cancer cells without affecting normal cells in animals carrying endogenous or xenographic tumors.

Nucleolus-derived mediators in oncogenic stress response and activation of p53-dependent pathways

Rapid growth and division of cells, including tumor ones, is correlated with intensive protein biosynthesis. The output of nucleoli, organelles where translational machineries are formed, depends on a rate of particular stages of ribosome production and on accessibility of elements crucial for their effective functioning, including substrates, enzymes as well as energy resources. Different factors that induce cellular stress also often lead to nucleolar dysfunction which results in ribosome biogenesis impairment. Such nucleolar disorders, called nucleolar or ribosomal stress, usually affect cellular functioning which in fact is a result of p53-dependent pathway activation, elicited as a response to stress. These pathways direct cells to new destinations such as cell cycle arrest, damage repair, differentiation, autophagy, programmed cell death or aging. In the case of impaired nucleolar functioning, nucleolar and ribosomal proteins mediate activation of the p53 pathways. They are also triggered as a response to oncogenic factor overexpression to protect tissues and organs against extensive proliferation of abnormal cells. Intentional impairment of any step of ribosome biosynthesis which would direct the cells to these destinations could be a strategy used in anticancer therapy. This review presents current knowledge on a nucleolus, mainly in relation to cancer biology, which is an important and extremely sensitive element of the mechanism participating in cellular stress reaction mediating activation of the p53 pathways in order to counteract stress effects, especially cancer development.

Resveratrol overcomes gefitinib resistance by increasing the intracellular gefitinib concentration and triggering apoptosis, autophagy and senescence in PC9/G NSCLC cells

Gefitinib (Gef) provides clinical benefits to non-small cell lung cancer (NSCLC) patients with activating EGFR mutations. However, acquired resistance (AR) is a major obstacle to effective Gef therapy. This study demonstrated that resveratrol (Res) could synergize with Gef to inhibit the proliferation of Gef-resistant NSCLC cells. The underlying mechanisms of synergism were investigated, and the results showed that cotreatment with Gef and Res could inhibit EGFR phosphorylation by increasing intracellular Gef accumulation through the impairment of Gef elimination from PC9/G cells. Consistently, CYP1A1 and ABCG2 expression were inhibited. Meanwhile, the cotreatment significantly induced cell apoptosis, autophagy, cell cycle arrest and senescence accompanied by increased expression of cleaved caspase-3, LC3B-II, p53 and p21. Further studies revealed that autophagy inhibition enhanced apoptosis and abrogated senescence while apoptosis inhibition had no notable effect on cell autophagy and senescence during cotreatment with Gef and Res. These results indicated that in addition to apoptosis, senescence promoted by autophagy contributes to the antiproliferation effect of combined Gef and Res on PC9/G cells. In conclusion, combined treatment with Gef and Res may represent a rational strategy to overcome AR in NSCLC cells.

Syk-mediated tyrosine phosphorylation of mule promotes TNF-induced JNK activation and cell death

The transcription factor Miz1 negatively regulates TNF-induced JNK activation and cell death by suppressing TRAF2 K63-polyubiquitination; upon TNF stimulation, the suppression is relieved by Mule/ARF-BP1-mediated Miz1 ubiquitination and subsequent degradation. It is not known how Mule is activated by TNF. Here we report that TNF activates Mule by inducing the dissociation of Mule from its inhibitor ARF. ARF binds to and thereby inhibits the E3 ligase activity of Mule in the steady state. TNF induces tyrosine phosphorylation of Mule, which subsequently dissociates from ARF and becomes activated. Inhibition of Mule phosphorylation by silencing of the Spleen Tyrosine Kinase (Syk) prevents its dissociation from ARF, thereby inhibiting Mule E3 ligase activity and TNF-induced JNK activation and cell death. Our data provides a missing link in TNF signaling pathway that leads to JNK activation and cell death.

Conversion of cell-survival activity of Akt into apoptotic death of cancer cells by two mutations on the BIM BH3 domain

Survival and proliferation of cancer cells are often associated with hyperactivity of the serine/threonine kinase, Akt. Herein, we show that prosurvival activity of Akt can be converted into prodeath activity by embedding an Akt recognition sequence in the apoptogenic BH3 domain of human BIM. The recognition sequence was created by introducing two mutations, I155R and E158S, into the core region of the BIM BH3 domain. Although a 21-mer BIM BH3 peptide containing these two mutations bound weakly to BCL-X_L and BCL-2, this peptide with phosphorylation of Ser158 bound to these proteins with a dissociation constant of <10 nM. The crystal structure of the phosphorylated peptide bound to BCL-X_L revealed that the phospho-Ser158 makes favorable interactions with two BCL-X_L residues, which cannot be formed with unphosphorylated Ser158. Remarkably, the designed peptide showed a cytotoxic effect on PTEN-null PC3 tumor cells whose Akt activity is aberrantly high. The cell-killing activity disappeared when the cellular Akt activity was lowered by ectopic PTEN expression. Thus, these results lay a foundation for developing a peptide or protein agent that is dormant in normal cells but is transformed into a potent apoptogenic molecule upon phosphorylation by hyperactivity of Akt in cancer cells.

p53-mediated autophagic regulation: A prospective strategy for cancer therapy

Autophagy is a major catabolic process that degrades and recycles cytosolic components in autophagosomes, which fuse with lysosomes. This process enables starving cells to sustain their energy requirements and metabolic states, thus facilitating their survival, especially in cancer pathogenesis. The regulation of autophagy is quite intricate. It involves a series of signaling cascades including p53, known as the best-characterized tumor suppressor protein. Recent reports have indicated that p53 plays dual roles in regulating autophagy depending on its subcellular localization. Nuclear p53 facilitates autophagy by transactivating its target genes, whereas cytoplasmic p53 mainly inhibits autophagy through extranuclear, transcription-independent mechanisms. The relationship between autophagy and neoplasia is complicated. It may be intrinsically associated with the functional status of p53, but this is not clearly elucidated. This review focuses on the role of p53 as a master regulator of autophagy. We conclude that the contextual role of autophagy in cancer, which could be switched by p53 status, is expected to be developed into a new anticancer therapeutic approach.

Autophagy in malignant transformation and cancer progression

Autophagy plays a key role in the maintenance of cellular homeostasis. In healthy cells, such a homeostatic activity constitutes a robust barrier against malignant transformation. Accordingly, many oncoproteins inhibit, and several oncosuppressor proteins promote, autophagy. Moreover, autophagy is required for optimal anticancer immunosurveillance. In neoplastic cells, however, autophagic responses constitute a means to cope with intracellular and environmental stress, thus favoring tumor progression. This implies that at least in some cases, oncogenesis proceeds along with a temporary inhibition of autophagy or a gain of molecular functions that antagonize its oncosuppressive activity. Here, we discuss the differential impact of autophagy on distinct phases of tumorigenesis and the implications of this concept for the use of autophagy modulators in cancer therapy.

The kinase Jnk2 promotes stress-induced mitophagy by targeting the small mitochondrial form of the tumor suppressor ARF for degradation

Mitophagy is essential for cellular homeostasis but the regulatory mechanism is largely unknown. Here we report that the kinase Jnk2 is required for stress-induced mitophagy. Jnk2 promoted ubiquitination and proteasomal degradation of small mitochondrial form of ARF (smARF). Loss of Jnk2 led to accumulation of smARF, which in turn induced excessive autophagic activity, resulting in lysosomal degradation of the mitophagy adaptor p62 in the steady state. The depletion of p62 prevented Jnk2-deficient cells from mounting mitophagy upon stress. Jnk2-deficient mice displayed defective mitophagy, resulting in tissue damage under hypoxic stress, as well as hyperactivation of inflammasome and increased mortality in sepsis. Our finding defines a unique mechanism of maintaining immune homeostasis that protects the host from tissue damage and mortality.

AMPK is required for PM2.5-induced autophagy in human lung epithelial A549 cells

The aim is to investigate the molecular mechanisms underlying the PM2.5-induced autophagy in human lung cancer epithelial cells (A549). The effects of the PM2.5 on morphological and biochemical markers of autophagy in A549 were analyzed by electron microscopy, GFP-LC3 puncta was observed by confocal fluorescence microscope. The effects of phosphorylation of AMPK, mTOR, AKT, ERK, JNK, and p53 on LC3II in A549 were observed following PM2.5 exposure; the role of autophagy in PM2.5-induced apoptosis was examined using 3-methyladenine and rapamycin. PM2.5 induced morphological and biochemical markers of autophagy in A549. Phosphorylation of AMPK and dephosphorylation of mTOR were observed following PM2.5 treatment, and AMPK inhibitor blocked LC3B-II expression. In addition, we demonstrated that PM2.5-induced autophagy confers a pro-survival role in host defense.

A short acidic motif in ARF guards against mitochondrial dysfunction and melanoma susceptibility

ARF is a small, highly basic protein that can be induced by oncogenic stimuli and exerts growth-inhibitory and tumour-suppressive activities through the activation of p53. Here we show that, in human melanocytes, ARF is cytoplasmic, constitutively expressed, and required for maintaining low steady-state levels of superoxide under conditions of mitochondrial dysfunction. This mitochondrial activity of ARF is independent of its known autophagic and p53-dependent functions, and involves the evolutionarily conserved acidic motif GHDDGQ, which exhibits weak homology to BCL-2 homology 3 (BH3) domains and mediates interaction with BCL-xL--an important regulator of mitochondrial redox homeostasis. Melanoma-predisposing CDKN2A germline mutations, which affect conserved glycine and aspartate residues within the GHDDGQ motif, impair the ability of ARF to control superoxide production and suppress growth of melanoma cells in vivo. These results reveal an important cell-protective function of ARF that links mitochondrial dysfunction and susceptibility to melanoma.

p53 suppresses stress-induced cellular senescence via regulation of autophagy under the deprivation of serum

The tumor suppressor p53 is widely known for its ability to induce cell cycle arrest or cell death, therefore preventing neoplastic progression. Previous studies have demonstrated novel roles for p53 in the regulation of autophagy and senescence. p53 can not only exert cell cycle‑arresting and senescence‑promoting or suppressing functions, but can also induce autophagic flux, particularly under conditions of nutrient deprivation. The present study demonstrated that p53 was capable of activating autophagy, which permits cell survival under conditions of serum starvation, and suppresses cellular senescence through inhibition of the mammalian target of rapamycin pathway. These results suggest that active autophagy may be a potential mechanism by which p53 suppresses cellular senescence, in response to serum starvation. The findings of the present study provide a potential mechanism for suppression of senescence by p53.

The role of autophagy in the placenta as a regulator of cell death

The placenta is a temporary fetomaternal organ capable of supporting fetal growth and development during pregnancy. In particular, abnormal development and dysfunction of the placenta due to cha nges in the proliferation, differentiation, cell death, and invasion of trophoblasts induce several gynecological diseases as well as abnormal fetal development. Autophagy is a catalytic process that maintains cellular structures by recycling building blocks derived from damaged microorganelles or proteins resulting from digestion in lysosomes. Additionally, autophagy is necessary to maintain homeostasis during cellular growth, development, and differentiation, and to protect cells from nutritional deficiencies or factors related to metabolism inhibition. Induced autophagy by various environmental factors has a dual role: it facilitates cellular survival in normal conditions, but the cascade of cellular death is accelerated by over-activated autophagy. Therefore, cellular death by autophagy has been known as programmed cell death type II. Autophagy causes or inhibits cellular death via the other mechanism, apoptosis, which is programmed cell death type I. Recently, it has been reported that autophagy increases in placenta-related obstetrical diseases such as preeclampsia and intrauterine growth retardation, although the mechanisms are still unclear. In particular, abnormal autophagic mechanisms prevent trophoblast invasion and inhibit trophoblast functions. Therefore, the objectives of this review are to examine the characteristics and functions of autophagy and to investigate the role of autophagy in the placenta and the trophoblast as a regulator of cell death.

Short mitochondrial ARF triggers Parkin/PINK1-dependent mitophagy

Parkinson disease (PD) is a complex neurodegenerative disease characterized by the loss of dopaminergic neurons in the substantia nigra. Multiple genes have been associated with PD, including Parkin and PINK1. Recent studies have established that the Parkin and PINK1 proteins function in a common mitochondrial quality control pathway, whereby disruption of the mitochondrial membrane potential leads to PINK1 stabilization at the mitochondrial outer surface. PINK1 accumulation leads to Parkin recruitment from the cytosol, which in turn promotes the degradation of the damaged mitochondria by autophagy (mitophagy). Most studies characterizing PINK1/Parkin mitophagy have relied on high concentrations of chemical uncouplers to trigger mitochondrial depolarization, a stimulus that has been difficult to adapt to neuronal systems and one unlikely to faithfully model the mitochondrial damage that occurs in PD. Here, we report that the short mitochondrial isoform of ARF (smARF), previously identified as an alternate translation product of the tumor suppressor p19ARF, depolarizes mitochondria and promotes mitophagy in a Parkin/PINK1-dependent manner, both in cell lines and in neurons. The work positions smARF upstream of PINK1 and Parkin and demonstrates that mitophagy can be triggered by intrinsic signaling cascades.

Slug regulates E-cadherin repression via p19Arf in prostate tumorigenesis

SLUG represses E-cadherin to promote epithelial-mesenchymal transition (EMT) in various cancers. Mechanisms that regulate SLUG/E-cadherin pathway remain poorly understood, especially during tumorigenesis in vivo. Here we report that p19(Arf) (p14(ARF) in human) stabilizes Slug to inhibit E-cadherin in prostate cancer mouse models. Inactivation of p19(Arf) reduces Slug levels, resulting in increased E-cadherin expression and delaying the onset and progression of prostate cancer in Pten/Trp53 double null mice. Mechanistically, p14(ARF) stabilizes SLUG through increased sumoylation at lysine residue 192. Importantly, levels of SLUG and p14(ARF) are positively correlated in human prostate cancer specimens. These data demonstrated that ARF modulates the SLUG/E-cadherin signaling axis for augmenting prostate tumorigenesis in vivo, revealing a novel paradigm where the oncogenic functions of SLUG require ARF to target E-cadherin in prostate cancer. Collectively, our findings further support that ARF has dual tumor suppressive/oncogenic roles in cancers in a context-dependent manner.

The Mdm network and its regulation of p53 activities: a rheostat of cancer risk

The potent transcriptional activity of p53 (Trp53, TP53) must be kept in check for normal cell growth and survival. Tumors, which drastically deviate from these parameters, have evolved multiple mechanisms to inactivate TP53, the most prevalent of which is the emergence of TP53 missense mutations, some of which have gain-of-function activities. Another important mechanism by which tumors bypass TP53 functions is via increased levels of two TP53 inhibitors, MDM2, and MDM4. Studies in humans and in mice reveal the complexity of TP53 regulation and the exquisite sensitivity of this pathway to small changes in regulation. Here, we summarize the factors that impinge on TP53 activity and thus cell death/arrest or tumor development.

H3K9 histone methyltransferase G9a-mediated transcriptional activation of p21

We report that H3K9 HMTase G9a activates transcription of the cell cycle regulatory gene, p21, in p53-null H1299 cells. Positive regulation of p21 by G9a is independent of its HMTase activity. We demonstrate that G9a upregulates p21 via interaction with PCAF, and provide evidence that the activating complex is recruited to the p21 promoter upon DNA damage-inducing agent etoposide treatment. Our study suggests that G9a decreases proliferation and cell viability by increasing the level of p21-mediated apoptosis. Our results suggest that G9a functions as a coactivator for p21 transcription, and directs cells to undergo apoptosis.

Metabolic regulation by p53 family members

The function of p53 is best understood in response to genotoxic stress, but increasing evidence suggests that p53 also plays a key role in the regulation of metabolic homeostasis. p53 and its family members directly influence various metabolic pathways, enabling cells to respond to metabolic stress. These functions are likely to be important for restraining the development of cancer but could also have a profound effect on the development of metabolic diseases, including diabetes. A better understanding of the metabolic functions of p53 family members may aid in the identification of therapeutic targets and reveal novel uses for p53-modulating drugs.

A conserved domain in exon 2 coding for the human and murine ARF tumor suppressor protein is required for autophagy induction

The ARF tumor suppressor, encoded by the CDKN2A gene, has a well-defined role regulating TP53 stability; this activity maps to exon 1β of CDKN2A. In contrast, little is known about the function(s) of exon 2 of ARF, which contains the majority of mutations in human cancer. In addition to controlling TP53 stability, ARF also has a role in the induction of autophagy. However, whether the principal molecule involved is full-length ARF, or a small molecular weight variant called smARF, has been controversial. Additionally, whether tumor-derived mutations in exon 2 of CDKN2A affect ARF's autophagy function is unknown. Finally, whereas it is known that silencing or inhibiting TP53 induces autophagy, the contribution of ARF to this induction is unknown. In this report we used multiple autophagy assays to map a region located in the highly conserved 5' end of exon 2 of CDKN2A that is necessary for autophagy induction by both human and murine ARF. We showed that mutations in exon 2 of CDKN2A that affect the coding potential of ARF, but not p16INK4a, all impair the ability of ARF to induce autophagy. We showed that whereas full-length ARF can induce autophagy, our combined data suggest that smARF instead induces mitophagy (selective autophagy of mitochondria), thus potentially resolving some confusion regarding the role of these variants. Finally, we showed that silencing Tp53 induces autophagy in an ARF-dependent manner. Our data indicated that a conserved domain in ARF mediates autophagy, and for the first time they implicate autophagy in ARF's tumor suppressor function.

Regulation of autophagy by stress-responsive transcription factors

Autophagy is an evolutionarily conserved process that promotes the lysosomal degradation of intracellular components including organelles and portions of the cytoplasm. Besides operating as a quality control mechanism in steady-state conditions, autophagy is upregulated in response to a variety of homeostatic perturbations. In this setting, autophagy mediates prominent cytoprotective effects as it sustains energetic homeostasis and contributes to the removal of cytotoxic stimuli, thus orchestrating a cell-wide, multipronged adaptive response to stress. In line with the critical role of autophagy in health and disease, defects in the autophagic machinery as well as in autophagy-regulatory signaling pathways have been associated with multiple human pathologies, including neurodegenerative disorders, autoimmune conditions and cancer. Accumulating evidence indicates that the autophagic response to stress may proceed in two phases. Thus, a rapid increase in the autophagic flux, which occurs within minutes or hours of exposure to stressful conditions and is entirely mediated by post-translational protein modifications, is generally followed by a delayed and protracted autophagic response that relies on the activation of specific transcriptional programs. Stress-responsive transcription factors including p53, NF-κB and STAT3 have recently been shown to play a major role in the regulation of both these phases of the autophagic response. Here, we will discuss the molecular mechanisms whereby autophagy is orchestrated by stress-responsive transcription factors.

CDK inhibitors (p16/p19/p21) induce senescence and autophagy in cancer-associated fibroblasts, "fueling" tumor growth via paracrine interactions, without an increase in neo-angiogenesis

Here, we investigated the compartment-specific role of cell cycle arrest and senescence in breast cancer tumor growth. For this purpose, we generated a number of hTERT-immortalized senescent fibroblast cell lines overexpressing CDK inhibitors, such as p16(INK4A), p19(ARF) or p21(WAF1/CIP1). Interestingly, all these senescent fibroblast cell lines showed evidence of increased susceptibility toward the induction of autophagy (either at baseline or after starvation), as well as significant mitochondrial dysfunction. Most importantly, these senescent fibroblasts also dramatically promoted tumor growth (up to ~2-fold), without any comparable increases in tumor angiogenesis. Conversely, we generated human breast cancer cells (MDA-MB-231 cells) overexpressing CDK inhibitors, namely p16(INK4A) or p21(WAF1/CIP1). Senescent MDA-MB-231 cells also showed increased expression of markers of cell cycle arrest and autophagy, including β-galactosidase, as predicted. Senescent MDA-MB-231 cells had retarded tumor growth, with up to a near 2-fold reduction in tumor volume. Thus, the effects of CDK inhibitors are compartment-specific and are related to their metabolic effects, which results in the induction of autophagy and mitochondrial dysfunction. Finally, induction of cell cycle arrest with specific inhibitors (PD0332991) or cellular stressors [hydrogen peroxide (H₂O₂) or starvation] indicated that the onset of autophagy and senescence are inextricably linked biological processes. The compartment-specific induction of senescence (and hence autophagy) may be a new therapeutic target that could be exploited for the successful treatment of human breast cancer patients.

Regulation of the autophagic bcl-2/beclin 1 interaction

Autophagy is an intracellular degradation process responsible for the delivery of cellular material to the lysosomes. One of the key mechanisms for control of autophagy is the modulation of the interaction between the autophagic protein Beclin 1 and the members of the anti-apoptotic Bcl-2 family (e.g., Bcl-2, Bcl-X_L and Mcl-1). This binding is regulated by a variety of proteins and compounds that are able to enhance or inhibit the Bcl-2/Beclin 1 interaction in order to repress or activate autophagy, respectively. In this review we will focus on this interaction and discuss its characteristics, relevance and regulation.