p16INK4A Positively Regulates p21WAF1 Expression by suppressing AUF1-dependent mRNA decay

Background

p16^INK4a and p21^WAF1 are two independent cyclin-dependent kinase inhibitors encoded by the CDKN2A and CDKN1A genes, respectively. p16^INK4a and p21^WAF1 are similarly involved in various anti-cancer processes, including the regulation of the critical G1 to S phase transition of the cell cycle, senescence and apoptosis. Therefore, we sought to elucidate the molecular mechanisms underlying the link between these two important tumor suppressor proteins.

Methodology/Principal Findings

We have shown here that the p16^INK4a protein positively controls the expression of p21^WAF1 in both human and mouse cells. p16^INK4a stabilizes the CDKN1A mRNA through negative regulation of the mRNA decay-promoting AUF1 protein. Immunoprecipitation of AUF1-associated RNAs followed by quantitative RT-PCR indicated that endogenous AUF1 binds to the CDKN1A mRNA in a p16^INK4A-dependent manner. Furthermore, while AUF1 down-regulation increased the expression level of the CDKN1A mRNA, the concurrent knockdown of AUF1 and CDKN2A, using specific silencing RNAs, restored the normal expression of the gene. Moreover, we used EGFP reporter fused to the CDKN2A AU-rich element (ARE) to demonstrate that p16^INK4A regulation of the CDKN1A mRNA is AUF1- and ARE-dependent. Furthermore, ectopic expression of p16^INK4A in p16^INK4A-deficient breast epithelial MCF-10A cells significantly increased the level of p21^WAF1, with no effect on cell proliferation. In addition, we have shown direct correlation between p16^INK4a and p21^WAF1 levels in various cancer cell lines.

Conclusion/Significance

These findings show that p16^INK4a stabilizes the CDKN1A mRNA in an AUF1-dependent manner, and further confirm the presence of a direct link between the 2 important cancer-related pathways, pRB/p16^INK4A and p14^ARF/p53/p21^WAF1.     

A New lncRNA,APTR, Associates with and Represses the CDKN1A/p21 Promoter by Recruiting Polycomb Proteins

Long noncoding RNAs (lncRNAs) have emerged as a major regulator of cell physiology, but many of which have no known function. CDKN1A/p21 is an important inhibitor of the cell-cycle, regulator of the DNA damage response and effector of the tumor suppressor p53, playing a crucial role in tumor development and prevention. In order to identify a regulator for tumor progression, we performed an siRNA screen of human lncRNAs required for cell proliferation, and identified a novel lncRNA, APTR, that acts in trans to repress the CDKN1A/p21 promoter independent of p53 to promote cell proliferation. APTR associates with the promoter of CDKN1A/p21 and this association requires a complementary-Alu sequence encoded in APTR. A different module of APTR associates with and recruits the Polycomb repressive complex 2 (PRC2) to epigenetically repress the p21 promoter. A decrease in APTR is necessary for the induction of p21 after heat stress and DNA damage by doxorubicin, and the levels of APTR and p21 are anti-correlated in human glioblastomas. Our data identify a new regulator of the cell-cycle inhibitor CDKN1A/p21 that acts as a proliferative factor in cancer cell lines and in glioblastomas and demonstrate that Alu elements present in lncRNAs can contribute to targeting regulatory lncRNAs to promoters.     

The study of hypermethylation in blood leukocytes of irradiated parents and their children

Background: Accumulation of evidence about the epigenetic regulation of genome function suggests the necessity to explore new aspects of the genotoxic action of radiation on the human body. Methodology: A methylation-sensitive PCR assay was used to analyze promoter methylation of p16/CDKN2A, p14/ARF, RASSF1A and GSTP1 genes in blood leukocytes from 103 unirradiated volunteers and 104 irradiated subjects (83 Chernobyl Nuclear Power Plant liquidators and 21 nuclear specialists). Additionally, 21 families whose fathers were nuclear specialists were examined. Results: A significantly elevated frequency of individuals with abnormal methylation of p16/CDKN2A and GSTP1 genes was revealed in the exposed group compared to the control group (p = 0.0097 and p = 0.005, respectively). The occurrence of promoter methylation of RASSF1A gene significantly correlated with aging both in the control group (r = 0, 213; p = 0.006) and in the exposed individuals (r = 0, 212; p = 0,031). No methylated genes were found in the offspring of control families. Conclusion: Our study showed for the fist time that prolonged radiation exposure at low and medium doses is associated with hypermethylation of genes involved in the basic protective functions of cells; an effect that is persistent in blood leukocytes for significant periods after irradiation.     

ZNF403，一个新的细胞周期调节因子的功能研究

ZNF403和LCRG1是人类基因ZNF403的2个不同转录剪切本.以往的研究表明LCRG1在喉癌细胞株Hep-2中具有抑瘤特性.本研究旨在探明ZNF403和LCRG1不同剪切本之间的关系以及在肿瘤细胞中对ZNF403的功能进行研究.首先,采用实时荧光定量PCR对这2个转录本的相对表达水平进行分析,结果表明,ZNF403表达水平在不同细胞株中明显高于LCRG1(>10倍),为该基因的主要转录表达产物.随后分别采用MTT细胞生长分析法和裸鼠体内成瘤实验在体外和体内对ZNF403的功能进行分析,结果显示ZNF403的基因沉默可以同时在体内和体外抑制喉癌细胞Hep-2细胞的生长.为了探明其作用机制,本研究还采用细胞信息学、流式细胞周期分析术和高通量PCR点阵分析方法进一步分析,结果表明,ZNF403的基因沉默可显著抑制细胞DNA的复制并延缓细胞周期进入到有丝分裂期.同时发现ZNF403可调节一系列的细胞周期调节蛋白如MCM2、p21、ATM、MRE11A等.综上研究提示ZNF403为一新的细胞周期调节因子,其功能的缺失与肿瘤发生发展密切相关.     

Inhibition of G1 to S Cell Cycle Progression by BCCIPb

The BCCIPa protein was identified as a BRCA2 and CDKN1A (p21, or p21Waf1/Cip1) Interacting Protein. It binds to a highly conserved domain proximate to the C-terminus of BRCA2 protein and the C-terminal domain of the CDK-inhibitor p21. Previous reports showed that BCCIPa enhances the inhibitory activity of p21 toward CDK2 and that BCCIPa inhibits the growth of certain tumor cells. Here we show that a second isoform, BCCIPb, also binds to p21 and inhibits cell growth. The growth inhibition by BCCIPb can be partially abrogated in p21 deficient cells. Overexpression of BCCIPb delays the G1 to S progression and results in an elevated p21 expression. These data suggest BCCIPb as a new regulator for the G1-S cell cycle progression and cell growth control.     

Inhibition of G1 to S cell cycle progression by BCCIP beta

The BCCIP alpha protein was identified as a BRCA2 and CDKN1A (p21, or p21(Waf1/Cip1)) Interacting Protein. It binds to a highly conserved domain proximate to the C-terminus of BRCA2 protein and the C-terminal domain of the CDK-inhibitor p21. Previous reports showed that BCCIP alpha enhances the inhibitory activity of p21 toward CDK2 and that BCCIP alpha inhibits the growth of certain tumor cells. Here we show that a second isoform, BCCIP beta, also binds to p21 and inhibits cell growth. The growth inhibition by BCCIP beta can be partially abrogated in p21 deficient cells. Overexpression of BCCIP beta delays the G1 to S progression and results in an elevated p21 expression. These data suggest BCCIP beta as a new regulator for the G1-S cell cycle progression and cell growth control.