Perturbation of endoplasmic reticulum (ER) homeostasis results in a stress condition termed "ER stress" determining the activation of a finely regulated program defined as unfolded protein response ...(UPR) and whose primary aim is to restore this organelle's physiological activity. Several physiological and pathological stimuli deregulate normal ER activity causing UPR activation, such as hypoxia, glucose shortage, genome instability, and cytotoxic compounds administration. Some of these stimuli are frequently observed during uncontrolled proliferation of transformed cells, resulting in tumor core formation and stage progression. Therefore, it is not surprising that ER stress is usually induced during solid tumor development and stage progression, becoming an hallmark of such malignancies. Several UPR components are in fact deregulated in different tumor types, and accumulating data indicate their active involvement in tumor development/progression. However, although the UPR program is primarily a pro-survival process, sustained and/or prolonged stress may result in cell death induction. Therefore, understanding the mechanism(s) regulating the cell survival/death decision under ER stress condition may be crucial in order to specifically target tumor cells and possibly circumvent or overcome tumor resistance to therapies. In this review, we discuss the role played by the UPR program in tumor initiation, progression and resistance to therapy, highlighting the recent advances that have improved our understanding of the molecular mechanisms that regulate the survival/death switch.
Autophagy, a main intracellular catabolic process, is induced in response to a variety of cellular stresses to promptly degrade harmful agents and to coordinate the activity of prosurvival and ...prodeath processes in order to determine the fate of the injured cells. While the main components of the autophagy machinery are well characterized, the molecular mechanisms that confer selectivity to this process both in terms of stress detection and cargo engulfment have only been partly elucidated. Here, we discuss the emerging role played by the E3 ubiquitin ligases of the TRIM family in regulating autophagy in physiological and pathological conditions, such as inflammation, infection, tumorigenesis, and muscle atrophy. TRIM proteins employ different strategies to regulate the activity of the core autophagy machinery, acting either as scaffold proteins or via ubiquitin-mediated mechanisms. Moreover, they confer high selectivity to the autophagy-mediated degradation as described for the innate immune response, where TRIM proteins mediate both the engulfment of pathogens within autophagosomes and modulate the immune response by controlling the stability of signaling regulators. Importantly, the elucidation of the molecular mechanisms underlying the regulation of autophagy by TRIMs is providing important insights into how selective types of autophagy are altered under pathological conditions, as recently shown in cancer and muscular dystrophy.
Autophagy is important in the basal or stress-induced clearance of bulk cytosol, damaged organelles, pathogens and selected proteins by specific vesicles, the autophagosomes. Following mTOR ...(mammalian target of rapamycin) inhibition, autophagosome formation is primed by the ULK1 and the beclin-1-Vps34-AMBRA1 complexes, which are linked together by a scaffold platform, the exocyst. Although several regulative steps have been described along this pathway, few targets of mTOR are known, and the cross-talk between ULK1 and beclin 1 complexes is still not fully understood. We show that under non-autophagic conditions, mTOR inhibits AMBRA1 by phosphorylation, whereas on autophagy induction, AMBRA1 is dephosphorylated. In this condition, AMBRA1, interacting with the E3-ligase TRAF6, supports ULK1 ubiquitylation by LYS-63-linked chains, and its subsequent stabilization, self-association and function. As ULK1 has been shown to activate AMBRA1 by phosphorylation, the proposed pathway may act as a positive regulation loop, which may be targeted in human disorders linked to impaired autophagy.
The incidence of melanoma is increasing over the years with a still poor prognosis and the lack of a cure able to guarantee an adequate survival of patients. Although the new immuno-based coupled to ...target therapeutic strategy is encouraging, the appearance of targeted/cross-resistance and/or side effects such as autoimmune disorders could limit its clinical use. Alternative therapeutic strategies are therefore urgently needed to efficiently kill melanoma cells. Ferroptosis induction and execution were evaluated in metastasis-derived wild-type and oncogenic BRAF melanoma cells, and the process responsible for the resistance has been dissected at molecular level. Although efficiently induced in all cells, in an oncogenic BRAF- and ER stress-independent way, most cells were resistant to ferroptosis execution. At molecular level we found that: resistant cells efficiently activate NRF2 which in turn upregulates the early ferroptotic marker CHAC1, in an ER stress-independent manner, and the aldo-keto reductases AKR1C1 ÷ 3 which degrades the 12/15-LOX-generated lipid peroxides thus resulting in ferroptotic cell death resistance. However, inhibiting AKRs activity/expression completely resensitizes resistant melanoma cells to ferroptosis execution. Finally, we found that the ferroptotic susceptibility associated with the differentiation of melanoma cells cannot be applied to metastatic-derived cells, due to the EMT-associated gene expression reprogramming process. However, we identified SCL7A11 as a valuable marker to predict the susceptibility of metastatic melanoma cells to ferroptosis. Our results identify the use of pro-ferroptotic drugs coupled to AKRs inhibitors as a new valuable strategy to efficiently kill human skin melanoma cells.
The Ser/Thr protein kinase ULK1 is an upstream macroautophagy/autophagy regulator that is rapidly activated to ensure a proper adaptive response to stress conditions. Signaling pathways modulating ...ULK1 activity have been extensively characterized in response to nutrient/energy shortage, which mainly act by mediating ULK1 post-translational modifications, such as phosphorylation, acetylation and ubiquitination. Less characterized is how tissue-specific stress signals are able to activate ULK1 to induce autophagy. Our recent study has uncovered the E3 ubiquitin ligase TRIM32 as a novel ULK1 activator that regulates autophagy in muscle cells upon atrophy induction. TRIM32 is conveyed to ULK1 by the autophagy cofactor AMBRA1 to stimulate its kinase activity through unanchored K63-linked polyubiquitin chains. Notably, mutations in TRIM32 responsible for limb-girdle muscular dystrophy 2H disrupt its ability to bind ULK1 and to induce autophagy in muscle cells, resulting in a dysregulated activation of the atrophic process. In conclusion, we have identified a novel molecular mechanism by which autophagy is regulated in muscles, whose alteration is associated with the development of muscular dystrophy.
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.
Autophagy has been described to have tumor‐suppressive as well as tumor‐promoting functions. This review discusses how stage and context alters the role for autophagy in cancer, and argues for further research prior to targeting autophagy in cancer therapy.
Transglutaminase type 2 (TG2) is a multifunctional enzyme that plays a key role in mitochondria homeostasis under stressful cellular conditions. TG2 interactome analysis reveals an enzyme interaction ...with GRP75 (glucose-regulated protein 75). GRP75 localizes in mitochondria-associated membranes (MAMs) and acts as a bridging molecule between the two organelles by assembling the IP3R-GRP75-VDAC complex, which is involved in the transport of Ca2+ from the endoplasmic reticulum (ER) to mitochondria. We demonstrate that the TG2 and GRP75 interaction occurs in MAMs. The absence of the TG2-GRP75 interaction leads to an increase of the interaction between IP3R-3 and GRP75; a decrease of the number of ER-mitochondria contact sites; an impairment of the ER-mitochondrial Ca2+ flux; and an altered profile of the MAM proteome. These findings indicate TG2 is a key regulatory element of the MAMs.
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•TG2 interacts with GRP75 in mitochondria-associated membranes (MAMs)•TG2 influences the number of ER-mitochondria contact sites•TG2 regulates the interaction between IP3R3 and GRP75•TG2 controls ER-mitochondrial Ca2+ flux and protein expression in MAMs
TG2 is an enzyme that plays a key role in mitochondria homeostasis. D’eletto et al. found that TG2 interacts with GRP75, a protein localized in mitochondria-associated membranes (MAMs). TG2 regulates the number of ER-mitochondria contact sites and Ca2+ flux, suggesting a key regulatory role in MAMs.
The pathogenesis of SARS-CoV-2 remains to be completely understood, and detailed SARS-CoV-2 cellular cytopathic effects requires definition. We performed a comparative ultrastructural study of ...SARS-CoV-1 and SARS-CoV-2 infection in Vero E6 cells and in lungs from deceased COVID-19 patients. SARS-CoV-2 induces rapid death associated with profound ultrastructural changes in Vero cells. Type II pneumocytes in lung tissue showed prominent altered features with numerous vacuoles and swollen mitochondria with presence of abundant lipid droplets. The accumulation of lipids was the most striking finding we observed in SARS-CoV-2 infected cells, both in vitro and in the lungs of patients, suggesting that lipids can be involved in SARS-CoV-2 pathogenesis. Considering that in most cases, COVID-19 patients show alteration of blood cholesterol and lipoprotein homeostasis, our findings highlight a peculiar important topic that can suggest new approaches for pharmacological treatment to contrast the pathogenicity of SARS-CoV-2.
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