The three ‘P’s of mitophagy: PARKIN,PINK1, and post-translational modifications

Two Parkinson''s disease (PD)-associated proteins, the mitochondrial kinase PINK1 and the E3-ubiquitin (Ub) ligase PARKIN, are central to mitochondrial quality control. In this pathway, PINK1 accumulates on defective mitochondria, eliciting the translocation of PARKIN from the cytosol to mediate the clearance of damaged mitochondria via autophagy (mitophagy). Throughout the different stages of mitophagy, post-translational modifications (PTMs) are critical for the regulation of PINK1 and PARKIN activity and function. Indeed, activation and recruitment of PARKIN onto damaged mitochondria involves PINK1-mediated phosphorylation of both PARKIN and Ub. Through a stepwise cascade, PARKIN is converted from an autoinhibited enzyme into an active phospho-Ub-dependent E3 ligase. Upon activation, PARKIN ubiquitinates itself in concert with many different mitochondrial substrates. The Ub conjugates attached to these substrates can in turn be phosphorylated by PINK1, which triggers further cycles of PARKIN recruitment and activation. This feed-forward amplification loop regulates both PARKIN activity and mitophagy. However, the precise steps and sequence of PTMs in this cascade are only now being uncovered. For instance, the Ub conjugates assembled by PARKIN consist predominantly of noncanonical K6-linked Ub chains. Moreover, these modifications are reversible and can be disassembled by deubiquitinating enzymes (DUBs), including Ub-specific protease 8 (USP8), USP15, and USP30. However, PINK1-mediated phosphorylation of Ub can impede the activity of these DUBs, adding a new layer of complexity to the regulation of PARKIN-mediated mitophagy by PTMs. It is therefore evident that further insight into how PTMs regulate the PINK1–PARKIN pathway will be critical for our understanding of mitochondrial quality control.     

USP21 promotes cell proliferation by maintaining the EZH2 level in diffuse large B‐cell lymphoma

BackgroundDiffuse large B‐cell lymphoma (DLBCL) is the most common category of non‐Hodgkin lymphoma (NHL). However, the underlying molecular mechanism of DLBCL remains unclear.MethodsReal‐time PCR and Western blot analysis were performed to assess the expression of ubiquitin‐specific peptidase 21 (USP21) or enhancer of zeste 2 polycomb repressive complex 2 subunit (EZH2). CCK8 assay and cell death staining were carried out to examine the role of USP21 in cell proliferation and cell death, respectively.ResultsWe found that the deubiquitinase USP21 was highly expressed in the DLBCL lymphoid tissue. The expression of USP21 promoted DLBCL cell proliferation, while it had no obvious effect on cell death. In addition, we found that USP21 regulated cell proliferation via cysteine 221, the catalytic site of USP21. Furthermore, we identified that USP21 could stabilize EZH2, a protein required for germinal center formation and lymphoma formation.ConclusionThe deubiquitinase USP21 promotes cell proliferation by maintaining the EZH2 protein level in DLBCL.     

SRC-3 coactivator regulates cell resistance to cytotoxic stress via TRAF4-mediated p53 destabilization

Steroid receptor coactivator 3 (SRC-3) is an oncogenic nuclear receptor coactivator that plays a significant role in drug resistance. Using a lentiviral cDNA library rescue screening approach, we identified a SRC-3 downstream gene—TRAF4 (tumor necrosis factor [TNF] receptor associated-factor 4)—that functions in cell resistance to cytotoxic stress. TRAF4 expression is positively correlated with SRC-3 expression in human breast cancers. Similar to that observed for SRC-3 overexpression, breast cancer cells overexpressing TRAF4 are more resistant to stress-induced death. Here, we further dissected the underlying molecular mechanism for SRC-3 and TRAF4-mediated resistance to cytotoxic agents. We observed that SRC-3 expression is inversely correlated with the expression of p53-regulated proapoptotic genes in breast cancers and further found that SRC-3 and TRAF4 overexpression diminished cytotoxic stress-induced up-regulation of the tumor suppressor p53 protein. To determine the mechanism, we showed that the TRAF domain of TRAF4 bound to the N-terminal TRAF-like region of the deubiquitinase HAUSP (herpesvirus-associated ubiquitin-specific protease; also named USP7) and blocked the access of p53 to the same region of HAUSP. This TRAF4-mediated inhibition of HAUSP then led to the loss of p53 deubiquitination and its stabilization in response to cellular stress. Consistent with this cellular function, we also found that TRAF4 overexpression in breast cancer patients was associated significantly with poor prognosis. Because of SRC-3''s ability to abrogate p53 function, our results suggest that SRC-3 overexpression may be especially important in tumors in which p53 is not mutated.     

Emerging Role of Ubiquitination in the Regulation of PD-1/PD-L1 in Cancer Immunotherapy

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去泛素化酶与肿瘤相关性的研究进展

泛素-蛋白酶体途径是大多数细胞内蛋白质降解的主要方式,去泛素化酶（Deubiquitinating enzymes,DUBs）可以逆转这一过程,对其进行反向调节.此外,去泛素化酶可以稳定抑癌基因,从而抑制了肿瘤的发生.对去泛素化酶及其抑制剂的深入研究,为抗肿瘤药物的靶向治疗提供理论依据.本文将重点对去泛素化酶及其在肿瘤中的作用进行综述.     

USP9X deubiquitinates and stabilizes CDC123 to promote breast carcinogenesis through regulating cell cycle

Cell division cycle 123 (CDC123) has been implicated in a variety of human diseases. However, it remains unclear whether CDC123 plays a role in tumorigenesis and how its abundance is regulated. In this study, we found that CDC123 was highly expressed in breast cancer cells, and its high expression was positively correlated with a poor prognosis. Knowndown of CDC123 impaired the proliferation of breast cancer cells. Mechanistically, we identified a deubiquitinase, ubiquitin-specific peptidase 9, X-linked (USP9X), that could physically interact with and deubiquitinate K48-linked ubiquitinated CDC123 at the K308 site. Therefore, the expression of CDC123 was positively correlated with USP9X in breast cancer cells. In addition, we found that deletion of either USP9X or CDC123 led to altered expression of cell cycle-related genes and resulted in the accumulation of cells population in the G0/G1 phase, thereby suppressing cell proliferation. Treatment with the deubiquitinase inhibitor of USP9X, WP1130 (Degrasyn, a small molecule compound that USP9X deubiquitinase inhibitor), also led to the accumulation of breast cancer cells in the G0/G1 phase, but this effect could be rescued by overexpression of CDC123. Furthermore, our study revealed that the USP9X/CDC123 axis promotes the occurrence and development of breast cancer through regulating the cell cycle, and suggests that it may be a potential target for breast cancer intervention. In conclusion, our study demonstrates that USP9X is a key regulator of CDC123, providing a novel pathway for the maintenance of CDC123 abundance in cells, and supports USP9X/CDC123 as a potential target for breast cancer intervention through regulating the cell cycle.