The p33ING1b tumor suppressor cooperates with p53 to induce apoptosis in response to etoposide in human osteosarcoma cells

p33ING1b induces cell cycle arrest and stimulates DNA repair, apoptosis and chemosensitivity. The magnitude of some p33ING1b effects may be due to activation of the tumor suppressor p53. To investigate if the p33ING1b protein affected chemosensitivity of osteosarcoma cells, we overexpressed p33ING1b in p53+/+ U2OS cells or in p53-mutant MG63 cells, and then assessed for growth arrest and apoptosis after treatment with etoposide. p33ING1b increased etoposide-induced growth inhibition and apoptosis to a much greater degree in p53+/+ U2OS cells than in p53-mutant MG63 cells. Moreover, ectopic expression of p33ING1b markedly upregulated p53, p21WAF1 and bax protein levels and activated caspase-3 protein kinase in etoposide-treated U2OS cells. Together, our data indicate that p33ING1b prominently enhances etoposide-induced apoptosis through p53-dependent pathways in human osteosarcoma cells. p33ING1b may be an important marker and/or therapeutic target in the prevention and treatment of metastatic osteosarcoma.     

Wig1 prevents cellular senescence by regulating p21 mRNA decay through control of RISC recruitment

Premature senescence, a key strategy used to suppress carcinogenesis, can be driven by p53/p21 proteins in response to various stresses. Here, we demonstrate that Wig1 plays a critical role in this process through regulation of p21 mRNA stability. Wig1 controls the association of Argonaute2 (Ago2), a central component of the RNA‐induced silencing complex (RISC), with target p21 mRNA via binding of the stem‐loop structure near the microRNA (miRNA) target site. Depletion of Wig1 prohibited miRNA‐mediated p21 mRNA decay and resulted in premature senescence. Wig1 plays an essential role in cell proliferation, as demonstrated in tumour xenografts in mice, and Wig1 and p21 mRNA levels are inversely correlated in human normal and cancer tissues. Together, our data indicate a novel role of Wig1 in RISC target accessibility, which is a key step in RNA‐mediated gene silencing. In addition, these findings indicate that fine‐tuning of p21 levels by Wig1 is essential for the prevention of cellular senescence.     

The ING1b tumor suppressor facilitates nucleotide excision repair by promoting chromatin accessibility to XPA

ING1b is the most studied ING family protein and perhaps the most ubiquitously and abundantly expressed. This protein is involved in the regulation of various biological functions ranging from senescence, cell cycle arrest, apoptosis, to DNA repair. ING1b is upregulated by UV irradiation and enhances the removal of bulky nucleic acid photoproducts. In this study, we provide evidence that ING1b mediates nucleotide excision repair by facilitating the access to damaged nucleosomal DNA. We demonstrate that ING1b is not recruited to UV-induced DNA lesions but enhances nucleotide excision repair only in XPC-proficient cells, implying an essential role in early steps of the 'access, repair, restore' model. We also find that ING1b alters histone acetylation dynamics upon exposure to UV radiation and induces chromatin relaxation in microccocal nuclease digestion assay, revealing that ING1b may allow better access to nucleotide excision repair machinery. More importantly, ING1b associates with chromatin in a UV-inducible manner and facilitates DNA access to nucleotide excision repair factor XPA. Furthermore, depletion of the endogenous ING1b results to the sensitization of cells at S-phase to UV irradiation. Taken together, these observations establish a role of ING1b acting as a chromatin accessibility factor for DNA damage recognition proteins upon genotoxic injury.     

The PCNA-associated factor KIAA0101/p15(PAF) binds the potential tumor suppressor product p33ING1b

The KIAA0101/p15(PAF)/OEATC-1 protein was initially isolated in a yeast two-hybrid screen for proliferating cell nuclear antigen (PCNA) binding partners, and was shown to bind PCNA competitively with the cell cycle regulator p21(WAF). PCNA is involved in DNA replication and damage repair. Using polyclonal antisera raised against a p15(PAF) fusion protein, we have shown that in a range of mammalian tumor and non-tumor cell lines the endogenous p15(PAF) protein localises to the nucleus and the mitochondria. Under normal conditions no co-localisation with PCNA could be detected, however following exposure to UV it was possible to co-immunoprecipitate p15(PAF) and PCNA from a number of cell lines, suggesting a UV-enhanced association of the two proteins. Overexpression of p15(PAF) in mammalian cells was also found to protect cells from UV-induced cell death. Based on similarities between the behaviour of p15(PAF) and the potential tumor suppressor product p33ING1b, we have further shown that these two proteins interact in the same complex in cell cultures. This suggests that p15(PAF) forms part of a larger protein complex potentially involved in the regulation of DNA repair, apoptosis and cell cycle progression.     

SnoN functions as a tumour suppressor by inducing premature senescence

SnoN represses TGF‐β signalling to promote cell proliferation and has been defined as a proto‐oncogene partly due to its elevated expression in many human cancer cells. Although the anti‐tumourigenic activity of SnoN has been suggested, the molecular basis for this has not been defined. We showed here that high levels of SnoN exert anti‐oncogenic activity by inducing senescence. SnoN interacts with the promyelocytic leukaemia (PML) protein and is recruited to the PML nuclear bodies where it stabilizes p53, leading to premature senescence. Furthermore, overexpression of SnoN inhibits oncogenic transformation induced by Ras and Myc in vitro and significantly blocks papilloma development in vivo in a carcinogen‐induced skin tumourigenesis model. The few papillomas that were developed displayed high levels of senescence and spontaneously regressed. Our study has revealed a novel Smad‐independent pathway of SnoN function that mediates its anti‐oncogenic activity.     

mTORC1 and p53

A balance must be struck between cell growth and stress responses to ensure that cells proliferate without accumulating damaged DNA. This balance means that optimal cell proliferation requires the integration of pro-growth and stress-response pathways. mTOR (mechanistic target of rapamycin) is a pleiotropic kinase found in complex 1 (mTORC1). The mTORC1 pathway governs a response to mitogenic signals with high energy levels to promote protein synthesis and cell growth. In contrast, the p53 DNA damage response pathway is the arbiter of cell proliferation, restraining mTORC1 under conditions of genotoxic stress. Recent studies suggest a complicated integration of these pathways to ensure successful cell growth and proliferation without compromising genome maintenance. Deciphering this integration could be key to understanding the potential clinical usefulness of mTORC1 inhibitors like rapamycin. Here we discuss how these p53-mTORC1 interactions might play a role in the suppression of cancer and perhaps the development of cellular senescence and organismal aging.     

NQO1 Stabilizes p53 in Response to Oncogene-Induced Senescence

Cellular senescence is a state of permanent cellular arrest that provides an initial barrier to cell transformation and tumorigenesis. In this study, we report that expression of NAD(P)H:quinone oxidoreductase 1 (NQO1), a cytoplasmic 2-electron reductase, is induced during oncogene-induced senescence (OIS). Depletion of NQO1 resulted in the delayed onset of senescence. In contrast, ectopic expression of NQO1 enhanced the senescence phenotype. Analysis of the mechanism underlying the up-regulation of NQO1 expression during senescence identified that NQO1 promotes p53 accumulation in an MDM2 and ubiquitin independent manner, which reinforces the cellular senescence phenotype. Specifically, we demonstrated that NRF2/KEAP1 signaling regulates NQO1 expression during OIS. More importantly, we confirmed that depletion of NQO1 facilitates cell transformation and tumorigenesis, which indicates that NQO1 takes part in the senescence barrier and has anti-oncogenic properties in cell transformation.     

NORE1A is a double barreled Ras senescence effector that activates p53 and Rb

Although Ras is a potent oncogene in human tumors it has the paradoxical ability to promote Oncogene Induced Senescence (OIS). This appears to serve as a major barrier to Ras driven transformation in vivo. The signaling pathways used by Ras to promote senescence remain relatively poorly understood, but appear to invoke both the p53 and the Rb master tumor suppressors. Exactly how Ras communicates with p53 and Rb has remained something of a puzzle. NORE1A is a direct Ras effector that is frequently downregulated in human tumors. We have now found that it serves as a powerful Ras senescence effector. Moreover, we have defined signaling mechanisms that allows Ras to control both p53 and Rb post-translational modifications via the NORE1A scaffolding molecule. Indeed, NORE1A can be detected in complex with both p53 and Rb. Thus, by coupling Ras to both tumor suppressors, NORE1A forms a major component of the Ras senescence machinery and serves as the missing link between Ras and p53/Rb.     

细胞衰老与细胞自噬的生物学关联及其意义

细胞衰老是指细胞生理功能的衰减,包括增殖能力下降、细胞周期停滞、对促凋亡应激不敏感、衰老相关基因和蛋白表达增加,伴有形态学衰老改变,渐趋死亡的现象,其至少可分为复制性衰老和应激诱导的衰老。细胞自噬属于细胞＂自食＂现象,是细胞依赖溶酶体的分解代谢过程,能降解受损蛋白、衰老或损伤的细胞器等细胞结构,可被多种应激所触发。细胞自噬的典型特征是形成自噬体并呈递给溶酶体,该过程在蛋白质和细胞器质量控制中起基础作用并维持了细胞能量的稳态。最新研究表明,自噬与细胞衰老密切相关,参与蛋白酶和自噬相关调节的BAG蛋白家族中BAG3/BAG1比值在复制性衰老时增高,且BAG3在细胞衰老时能介导自噬的激活。在Ras诱导的细胞衰老进程中亦可观察到较高的自噬活性。再者,自噬作为生物机体抗衰老的效应因子的遗传学证据已在低等真核生物中发现。还有研究证实,作为人类精液主要组分的亚精胺能够触发组蛋白H3脱乙酰基作用,此改变上调了自噬相关转录物的表达,继而引发自噬活性增强,从而延缓了多种细胞的衰老进程。另有研究显示,在P53/Arf的正常调节下,小鼠的衰老进程得以延缓,而Arf在细胞自噬过程的调节中亦是不可或缺的。总之,自噬活性的改变影响细胞衰老进程并可作为细胞衰老新的效应机理。