Oncogene- and tumor suppressor gene-mediated suppression of cellular senescence

Knockdown of AGR2 induces cellular senescence in prostate cancer cells

Anterior-gradient 2 (AGR2), overexpressed in many tumors including prostate cancer (PCa), is implicated in stimulation of cell proliferation, adhesion, anti-apoptosis and cell cycle regulation. Here, a potential role of AGR2 in cellular senescence was investigated. We first observed that AGR2 was overexpressed in Chinese Han PCa tissues and had a positive correlation with cyclin D1 and p-Rb but not with p16(INK4a). AGR2 expression profiles varied among cell lines, with PC3 cells being the highest level, LNCaP and DU145 relatively less. The expression of cyclin D1 showed similar pattern to the AGR2 in cell lines. Knockdown of AGR2 caused a decrease in cell viability in PC3 cells, whereas forced expression of AGR2 led to an increased cell proliferation of LNCaP and DU145 cells. Importantly, AGR2 depletion resulted in accumulation of cells at the G(0)/G(1) phase and induction of cellular senescence in all three PCa cell lines as indicated by an increase of flat, enlarged and senescence-associated β-galactosidase (SA-β-Gal) positive cells. Senescent response to AGR2 silencing was also evidenced by elevated γH2AX and fluorescent punctuate formation of tri-methyl-histone H3 in AGR2-depleted cells. Further studies indicated that LNCaP underwent a p21(CIP1)-dependent cellular senescence in response to AGR2 depletion that requires inactivation of ERK signaling, whereas PC-3 was also p21(CIP1) dependent but involved in suppression of PI3K/Akt. Unlike LNCaP and PC-3, senescent response of DU145 was found to be mainly p27(KIP1) dependent that may require upregulation of PTEN and inhibition of PI3K/Akt signaling. Thus, these findings suggest a novel role of AGR2 in regulation of cellular senescence.

Inhibition of tumor suppressor protein p53-dependent transcription by a tetramerization domain peptide via hetero-oligomerization

Tumor suppressor protein p53 induces cell cycle arrest, apoptosis, and senescence in response to cellular stresses. The p53 tetramer formation is essential for its functions. Despite of these crucial functions of p53 for integrity of genome, activation of the p53 signal pathway causes low induced pluripotent stem (iPS) cell generation efficiency. In this study, we report transient inhibition of p53-dependent transcription using a p53 tetramerization domain peptide that contains cell penetrating and nuclear localization signals. The peptide was efficiently introduced into cells and inhibited p21 expression via hetero-tetramerization with endogenous p53 protein. This method can be applied towards safe and efficient iPS cell generation.