p16(INK4A) Positively Regulates p21(WAF1) 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).