p53与抗肿瘤免疫

p53是迄今发现的与肿瘤发生、发展关系最为密切的抑癌基因,正常的p53蛋白通过诱导细胞周期阻滞和细胞凋亡等途径对细胞增殖发挥负调控作用,而在肿瘤细胞中异常表达的p53,尤其是突变型p53蛋白可以诱导机体产生细胞和体液免疫反应,导致对肿瘤细胞的免疫耐受或排斥。本文中对p53的功能、p53在抗肿瘤等免疫反应中的作用,及其在肿瘤治疗中的应用做一综述。     

CDKN2A基因变异与肿瘤

细胞周期依赖性激酶抑制基因（CDKN2A）为抑癌基因，编码两种周期抑制蛋白p16INK4a和p14ARF；进而通过p16INK4a—CDK4（和CDK6）-pRb途径和p14ARF—mdm2-p53途径发挥细胞周期调控作用。研究发现在多种肿瘤中均存在CDKN2A的基因变异，现就近年来的研究状况做一简要概述。     

PTEN与p53基因功能的相互作用

PTEN和p53是两个最重要的抑癌基因,他们在细胞周期调控、细胞凋亡以及肿瘤发生发展过程中均发挥重要作用。尽管二者功能及机制各异,p53主要是阻滞细胞周期或诱导细胞凋亡,PTEN主要是阻断细胞接受的外界生存增殖信号,但二者在功能上有交互作用。     

p53的两种结合蛋白:53BP1和53BP2

p53基因是一种广谱的肿瘤抑制基因,其产物p53为一多功能的转录调节因子,可以发挥调节细胞生长、细胞凋亡和DNA修复的作用。在人类肿瘤已发现多种p53基因的点突变,但突变并不是p53蛋白质丧失功能的唯一途径。胞内蛋白可能影响其活性和功能。53BP1和53BP2是p53在胞浆中的两种结合蛋白。本文阐述了有关这两种蛋白质的研究进展。     

p53功能调节的新进展

p53的结构和功能异常与肿瘤的发生密切相关。研究表明 ,p53的磷酸化和 或乙酰化可直接影响p53的稳定性及活性 ,多种蛋白包括癌基因蛋白 (如Mdm2 ,p3 3 ING1,E1a ,c Myc等 )亦通过不同途径影响p53的功能。了解p53功能的调节将助于进一步认识其在肿瘤发生发展中的作用。     

ASPP2对p53家族成员诱导凋亡功能的调控作用及其在肿瘤治疗中的意义

ASPP2（p53凋亡激活蛋白2，Apoptosis Stimulating Proteins of p53 2）作为p53家族共同转录激活辅因子能和p53野生型、p63和p73结合，并促进其对下游促凋亡靶基因的转录，促进细胞凋亡。研究表明ASPP2参与细胞生长，凋亡以及损伤应激等一系列的生理反应，对研究肿瘤发生和治疗具有重要意义。     

P53翻译后修饰研究进展

肿瘤抑制基因p53在肿瘤发生中发挥着重要作用。P53蛋白的翻译后修饰及其与多种细胞蛋白间相互作用使P53蛋白呈现功能多样性。P53的翻译后修饰不是单个位点的修饰而是包括磷酸化、乙酰化、泛素化及SUMO化作用的多位点修饰。翻译后修饰对P53功能至关重要,更可能与某些肿瘤的发生密切相关。发生在蛋白质水平上的P53的功能性灭活是没有发生p53基因突变肿瘤发生的重要机制之一。     

ASPP2对P53家族的调控及其在肿瘤治疗中的意义

P53凋亡激活蛋白2(apoptosis stimulating proteins of P53 2,ASPP2)作为P53家族共同转录激活辅因子能和p53野生型、p63和p73结合,并促进其对下游促凋亡靶基因的转录,促进细胞凋亡.研究表明ASPP2参与细胞生长,凋亡以及损伤应激等一系列的生理反应,对研究肿瘤发生和治疗具有重要意义.     

神经突起导向因子netrin-1及其受体与肿瘤发生

近年来的研究发现,netrin1和其受体基因在多种肿瘤中表达下调,未与配体netrin1结合的DCC和UNC5H能够诱导细胞凋亡,而在结合配体后则抑制细胞凋亡。在肿瘤细胞中凋亡通路通常被抑制,而且在50%以上的肿瘤中p53是失活的。netrin1与其受体结合后能完全抑制p53诱导的细胞凋亡;同时p53也直接调节netrin1及其受体基因的表达。因此,netrin1及其受体可能在肿瘤发生过程中发挥重要作用。     

p73基因与细胞凋亡

p73是经典抑癌基因p53的类似物。经研究证实，p73具有两面性。它可表达为两种相互独立却又密切相关的蛋白质即TApT3和DNp73。它们在细胞凋亡与肿瘤发生的过程中起着彼此对立以相互影响的作用。同时在细胞凋亡的过程中，p73的功能又受到E2F-1、C-Abl、C-Myc、P300、PKCδ等各种因素的影响。本文现就p73在细胞凋亡中的作用以及其影响因素作一综述。     

Combined analysis of p53 and RB pathways in epithelial ovarian cancer

Disruptions of the p16-CDK4/cyclin D1-pRb pathway (RB pathway) and the p14ARF-MDM2-p53 pathway (p53 pathway) are important mechanisms in the development of human malignancies. In this study, we investigated RB and p53 pathways in 46 epithelial ovarian cancers (EOCs). In the RB pathway, 16 (34.8%) of 46 cases had p16 gene alterations or loss of expression. The deletion of the p16 gene was a rare event. In 7 cases, we observed methylation in the 5'CpG island in the promoter region of the p16 gene. Abnormal expressions of pRb and CDK4/cyclin D1 were 10.9% and 30.4%, respectively. In the p53 pathway, 10 (21.7%) of 46 cases had p14ARF gene alterations or abnormal expression. In 4 cases, methylation in the 5'CpG island in the promoter region of the p14ARF gene was present. MDM2 overexpression was a rare event. Thirty-six (78.3%) of 46 patients had p53 gene alterations or expression. In our studied cases, p14ARF abnormalities were independent of p16 abnormalities. Abnormal RB and p53 pathways were present in 60.9% and 80.4% of cases, respectively. In conclusion, disruptions of p53 and RB pathways are frequent events and the inverse correlations were present between the abnormality of p16 and p14ARF in EOCs.     

Polycomb complexes regulate cellular senescence by repression of ARF in cooperation with E2F3

Cellular senescence is a program in normal cells triggered in response to various types of stress that cells experience when they are explanted into culture. In this study, functional analyses on the role of the class II polycomb complex in cellular senescence were performed using mouse embryo fibroblasts (MEFs) with a genetically deleted member of the complex, Mel18. Mel18-null MEFs undergo typical premature senescence accompanied by the up-regulation of ARF/p53/p16(INK4a) and decrease of Ring1b/Bmi1. Our results demonstrated that ARF or p53 deletion cancels the senescence in Mel18-null MEFs, and the fact that p16(INK4a) is up-regulated in double-null MEFs suggests that the ARF/p53 pathway plays a central role in stress-induced senescence. The in vivo binding of Ring1b and E2F3b to the ARF promoter decreased progressively in senescence, and Mel18 inactivation accelerated the exfoliation of Ring1b/E2F3b from the promoter sequence, indicating the cooperation of polycombs/E2F3b on ARF expression and cellular senescence. Taken together, it seems that class II polycomb proteins and E2F3b dually control cellular senescence via the ARF/p53 pathway.     

Novel ARF/p53-independent senescence pathways in cancer repression

Cellular senescence, which can be induced by various stimuli, is a stress response that manifests as irreversible cell cycle arrest. Recent studies have revealed that cellular senescence can serve as a critical barrier for cancer development. Induction of cellular senescence by oncogenic insults, such as Ras overexpression or by inactivation of PTEN tumor suppressor, triggers an ARF/p53-dependent tumor-suppressive effect which can significantly restrict cancer progression. Given the important role of the ARF/p53 pathway in cellular senescence and tumor suppression, drugs that stabilize p53 expression have been developed and tested in clinical trials. However, a major hurdle for p53 targeting in cancer treatment arises from the frequent deficiency or mutation of ARF or p53 in human cancers, which, in turn, profoundly compromises their tumor-suppressive ability. Recent discoveries of novel regulators involved in ARF/p53-independent cellular senescence not only reveal novel paradigms for cellular senescence but also provide alternative approaches for cancer therapy.     

Cellular senescence and its effector programs

Cellular senescence is a stress response that accompanies stable exit from the cell cycle. Classically, senescence, particularly in human cells, involves the p53 and p16/Rb pathways, and often both of these tumor suppressor pathways need to be abrogated to bypass senescence. In parallel, a number of effector mechanisms of senescence have been identified and characterized. These studies suggest that senescence is a collective phenotype of these multiple effectors, and their intensity and combination can be different depending on triggers and cell types, conferring a complex and diverse nature to senescence. Series of studies on senescence-associated secretory phenotype (SASP) in particular have revealed various layers of functionality of senescent cells in vivo. Here we discuss some key features of senescence effectors and attempt to functionally link them when it is possible.     

Alteration of cell cycle regulators correlates with survival in epithelial ovarian cancer patients

The p16-cyclinD1/CDK4-pRb pathway (RB pathway) and p14ARF-MDM2-p53 pathway (p53 pathway) work at the G1-S checkpoint, and the ATM-chk2-CDC25-cyclinB1/cdk1 pathway works at the G2-M checkpoint. The disruption of these pathways is thought to be related to the prognosis of human cancer. In this study, we analyzed the status of these pathways in 107 epithelial ovarian cancer (EOC) patients by immunohistochemistry and evaluated the relationship of these results with chemotherapy response and the prognosis. Altered RB, p53, and G2 pathways were detected in 50.5% (54/107), 51.4% (55/107), and 33.6% (36/107) of cases, respectively. The overall survival (OS) of 77.3% for patients with a normal RB pathway was significantly higher than the OS of 50.0% for patients with an altered RB pathway (by Kaplan-Meier analysis, P = 0.0021). The OS of 66.2% for patients with a normal G2 pathway was significantly higher than the OS of 58.3% for patients with an altered G2 pathway (P = 0.0416). However, the status of the p53 pathway was not related to OS. By univariate and multivariate analyses, advanced stage, high histological grade, altered RB pathway, and altered G2 pathway were significant predictors of poor OS. However, there was no significant relationship between pathway status and chemotherapy response. The status of the RB pathway and of the G2 pathway were independent prognostic factors of EOC.     

Interferon-γ induces cellular senescence through p53-dependent DNA damage signaling in human endothelial cells

Cellular senescence is a stress-response phenomenon in which cells lose the ability to proliferate; it is induced by telomere shortening, activation of oncogenes or tumor suppressor genes, or exposure to a sub-lethal dose of DNA damaging agents or oxidative stresses. cDNA microarray analysis reveals that the levels of interferons (IFNs) and IFN-inducible genes were altered during replicative senescence in human umbilical vascular endothelial cells (HUVECs). However, the role of IFNs in cellular senescence of HUVECs remains unidentified. This study demonstrated that prolonged treatment with IFN-γ induced cellular senescence in HUVECs, as confirmed by G0/G1 cell cycle arrest, up-regulation of p53 and p21 protein levels, increased SA-β-gal staining, and the accumulation of phospho-H₂AX foci. IFN-γ-induced cellular senescence was observed only in p16-knockdown cells or p16-null mouse embryonic fibroblasts (MEFs), but not in p53-knockdown cells or p53-null MEFs. IFN-γ treatment increased ROS production, and an antioxidant, N-acetylcysteine, inhibited IFN-γ-induced cellular senescence. Knockdown of ATM kinase or IFI16 rescued IFN-γ-induced cellular senescence. Therefore, these results suggest that IFN-γ might play an important role in cellular senescence through a p53-dependent DNA damage pathway and contribute to the pathogenesis of atherosclerosis via its pro-senescent activity.     

Cell fusion induced by ERVWE1 or measles virus causes cellular senescence

Cellular senescence limits proliferation of potentially detrimental cells, preventing tumorigenesis and restricting tissue damage. However, the function of senescence in nonpathological conditions is unknown. We found that the human placental syncytiotrophoblast exhibited the phenotype and expressed molecular markers of cellular senescence. During embryonic development, ERVWE1-mediated cell fusion results in formation of the syncytiotrophoblast, which serves as the maternal/fetal interface at the placenta. Expression of ERVWE1 caused cell fusion in normal and cancer cells, leading to formation of hyperploid syncytia exhibiting features of cellular senescence. Infection by the measles virus, which leads to cell fusion, also induced cellular senescence in normal and cancer cells. The fused cells activated the main molecular pathways of senescence, the p53- and p16–pRb-dependent pathways; the senescence-associated secretory phenotype; and immune surveillance-related proteins. Thus, fusion-induced senescence might be needed for proper syncytiotrophoblast function during embryonic development, and reuse of this senescence program later in life protects against pathological expression of endogenous fusogens and fusogenic viral infections.     

Dinstinct ROS and biochemical profiles in cells undergoing DNA damage-induced senescence and apoptosis

Cellular senescence and apoptosis are both caused by DNA damage stresses, and their severity appears to decide between the two cellular outcomes. In recent studies, it is suggested that these two states may be closely linked and be switched by certain molecular determinants such as p21WAF1 and caspase (Abdelhadi, 2003). However, it is unknown how the pathways to senescence and apoptosis are determined. In addition, although DNA damage stresses frequently accompany cellular accumulation of reactive oxygen species (ROS), how ROS are involved in the decision between the two pathways is unknown. In the present study, MCF-7 cells were induced to senescence or apoptosis by the treatment of varying doses of adriamycin. And, through a series of time course studies, ROS generation profiles and changes in the status of the proteins involved in growth regulation and apoptosis were determined. Significant levels of ROS were produced in senescing cells but not in apoptotic cells. Therefore, senescence is associated with ROS accumulation, but apoptosis is caused independently of ROS. In addition, cells in these two states exhibited quite distinct time course profiles of the proteins, p53, p21WAF1, and E2F1.     

Cyclin dependent kinase inhibitors and replicative senescence]

Replicative senescence is defined for human diploid fibroblasts in culture as a cell growth arrest appearing beyond 50 +/- 10 population doublings and associated with telomeres' shortening. This phenomenon shows an increased expression of growth cell inhibitors: p21Waf1 described as an universal CDK inhibitor and p16INK4a as a specific inhibitor for both G1 phase kinases CDK4 and CDK6. The cell proliferation inhibitor p14ARF, product of INK4a/ARF locus is involved in replicative senescence too. Overexpression or homozygotic deletion of these inhibitors demonstrated their role in senescence induction. These proteins are involved in two different metabolic pathways, the first including p53, represented by E2F, ARF, MDM2, p53, p21Waf1, and the second concerning pRb and p16INK4a. These two pathways present numerous interactions and the polymerase (PARP) in relation with p53 and activated by telomere shortening might represent via p21Waf1 a link between this shortening and cell cycle control. An another metabolic pathway involving PTEN and p27KIP1 is discussed in senescent-like phenotype induction, but its activity in replicative senescent is uncertain.