Abstract
BackgroundThe cyclin-D/CDK4,6/p16INK4a/pRB/E2F pathway, a key regulator of the critical G1 to S phase transition of the cell cycle, is universally disrupted in human cancer. However, the precise function of the different members of this pathway and their functional interplay are still not well defined.Methodology/Principal FindingsWe have shown here that the tumor suppressor p16INK4a protein positively controls the expression of cyclin D1 and E2F1 in both human and mouse cells. p16INK4a stabilizes the mRNAs of the corresponding genes through negative regulation of the mRNA decay-promoting AUF1 protein. Immunoprecipitation of AUF1-associated RNAs followed by RT-PCR indicated that endogenous AUF1 binds to the cyclin D1 and E2F1 mRNAs. Furthermore, AUF1 down-regulation increased the expression levels of these genes, while concurrent silencing of AUF1 and p16INK4a, using specific siRNAs, restored normal expression of both cyclinD1 and E2F1. Besides, we have shown the presence of functional AU-rich elements in the E2F1 3′UTR, which contributed to p16/AUF1-mediated regulation of E2F1 post-transcriptional events in vivo. Importantly, genome-wide gene expression microarray analysis revealed the presence of a large number of genes differentially expressed in a p16INK4a -dependent manner, and several of these genes are also members of the AUF1 and E2F1 regulons. We also present evidence that E2F1 mediates p16-dependent regulation of several pro- and anti-apoptotic proteins, and the consequent induction of spontaneous as well as doxorubicin-induced apoptosis.Conclusion/SignificanceThese findings show that the cyclin-dependent kinase inhibitor p16 INK4a is also a modulator of transcription and apoptosis through controlling the expression of two major transcription regulators, AUF1 and E2F1.
Highlights
The cyclin D-CDK4,6/p16/pRB/E2F cascade has been found to be altered in more than 80% of human tumors [1,2]
In the present report we have shown that p16 is a cell cycle checkpoint protein, but it is a master regulator of gene expression in absence of cellular stresses
P16 positively regulates the expression of cyclin D1 and the transcription factor E2F1 at the post-transcriptional level
Summary
The cyclin D-CDK4,6/p16/pRB/E2F cascade has been found to be altered in more than 80% of human tumors [1,2]. During G1 phase, pRB is inactivated by sequential phosphorylation events mediated by various cyclindependant kinases (CDKs) leading to the release of the E2F transcription factors, the activation of many genes and progression of the cell cycle [4]. P16 promoter methylation and transcriptional silencing have been detected even in histologically normal mammary tissue of cancerfree women This suggests that p16 inactivation may represent a cancerous precondition and an early event in promoting genomic instability that leads to tumorigenesis [15]. Over-expression of cyclin D1 contributed to malignancy by up-regulation of FGFR1 via the pRB/E2F pathway [19]. We have shown that the transcription factor E2F1 is a target of the RNA binding AUF1 protein, and that the tumor suppressor p16 positively controls cyclin D1 and E2F1 through negative regulation of AUF1. Provide clear evidence that p16 is a master regulator of gene expression and apoptosis in human cells
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call