Cellular senescence and tumor suppressor gene p16

Abstract

Cellular senescence is an irreversible arrest of cell growth. Biochemical and morphological changes occur during cellular senescence, including the formation of a unique cellular morphology such as flattened cytoplasm. Function of mitochondria, endoplasmic reticulum and lysosomes are affected resulting in the inhibition of lysosomal and proteosomal pathways. Cellular senescence can be triggered by a number of factors including, aging, DNA damage, oncogene activation and oxidative stress. While the molecular mechanism of senescence involves p16 and p53 tumor suppressor genes and telomere shortening, this review is focused on the mechanism of p16 control. The p16-mediated senescence acts through the retinoblastoma (Rb) pathway inhibiting the action of the cyclin dependant kinases leading to G1 cell cycle arrest. Rb is maintained in a hypophosphorylated state resulting in the inhibition of transcription factor E2F1. Regulation of p16 expression is complex and involves epigenetic control and multiple transcription factors. PRC1 (Pombe repressor complex (1) and PRC2 (Pombe repressor complex (2) proteins and histone deacetylases play an important role in the promoter hypermethylation for suppressing p16 expression. While transcription factors YY1 and Id1 suppress p16 expression, transcription factors CTCF, Sp1 and Ets family members activate p16 transcription. Senescence occurs with the inactivation of suppressor elements leading to the enhanced expression of p16.

Figure 1

Molecular mechanism of senescence. Induction of reactive oxygen species (ROS) induces mitochondrial breakdown as well as double stranded breaks. Activation of p16 and p53 induce growth arrest at G1/S and activation of NFkB leads to increased expression of cytokines and chemokines. Telomere shortening involved in aging or under stress conditions could result in double stranded breaks. ROS production as well as aging induced stress responses could be blocked by sirtuins, NAD dependent protein deacetylases [125]. Aggregates and crystals of denatured proteins (as in β amyloid crystals in Alzeimer’s disease) are visualized in the endoplasmic reticulum. Lipofuscin (Lf), a brown pigment consisting of insoluble membrane free protein aggregates, formed due to ROS and stress inhibits normal autophagy as well as proteosomal pathway of protein recycling machinery. Calorie restriction (CR) and sirtuins (SIRT) inhibit the effects of ROS and senescence. Activation Inhibition

Figure 2

Regulation of p16 expression and senescence. Bmi-1 binds to PRC2 (Pombe Repressor Complex) proteins, which then bind to p16 promoter resulting in methylation and repression of transcription. Bmi-1 expression is up-regulated by C-MYC which in turn is positively regulated by the E2F-1 transcription factor. P16 levels are up-regulated by the Ets-1 and HBP-1 transcription factors. Binding of p16 to Cyclin D1 and cdk4/6 inhibits the cyclin dependent kinase activity leading to reduced levels of E2F1 released from the Rb. However, HDAC is involved in the release of E2F1 which in turn could up-regulate p14^ARF. Mdm2 activity is then inhibited leading to the activation of p53 and induction of senescence through p21. RAS mediated senescence is inhibited by phosphorylation at Ser 62 of C-MYC by cyclin E dependent cdk2. The cyclin E/Cdk2 function however can be abrogated by p53 induced p21 protein. Cisplatin induced senescence is mediated by the activation of nuclear p16 and p53 proteins. Activation Inhibition

Figure 3

Transcriptional regulation of p16/p14ARF by PRC1 and PRC2 complexes. A) Binding of PRC1 and PRC2 complex proteins to the p16/p14ARF promoter results in formation of heterochromatin leading to suppression of transcription. Release of these factors results in the formation of euchromatin and transcription of p16/p14ARF genes. B) Locus of p16/p14ARF genes showing exons involved in alternate splicing. Promoter regions are shown in red. The map is not drawn to scale.