SIRT1 knockdown up-regulates p53 and p21/Cip1 expression in renal adenocarcinoma cells but not in normal renal-derived cells in a deacetylase-independent manner.

SIRT1, an NAD+-dependent deacetylase, causes deacetylation and down-regulation of its target p53. Given that p53 is an upstream regulator of the transcription of the cyclin-dependent kinase inhibitor p21/Cip1, SIRT1 is hypothesized to play a stimulatory role in carcinoma cell proliferation. We previously reported that down-regulation of SIRT1 caused the increase in p21/Cip1 in a post-transcriptional manner, suggesting that p53 is not involved in the p21/Cip1 increase and raising the question whether SIRT1 exhibits the activity other than deacetylase. In the present study, we examined whether SIRT1 down-regulation and the inhibitor for SIRT1 deacetylase activity affects p21/Cip1 and p53 expression in renal adenocarcinoma cells and normal renal cells. SIRT1 knockdown caused an increase in p53 and p21/Cip1 protein levels in renal adenocarcinoma ACHN cells but not normal renal-derived HK-2 cells. The increase in p53 in ACHN cells is unlikely to contribute to the upregulation of p21/Cip1 expression, given that SIRT1 knockdown did not increase p21/Cip1 mRNA levels in these cells. In contrast to the SIRT1-knock down assay, SIRT1 deacetylase inhibitor did not affect p53 or p21/Cip1 protein levels in ACHN cells. Therefore, SIRT1-knockdown likely stimulates p53 and p21/Cip1 protein expression in a deacetylase-independent manner.

AU-1 from Agavaceae plants causes transient increase in p21/Cip1 expression in renal adenocarcinoma ACHN cells in an miR-34-dependent manner.

Here, we show that AU-1, spirostanol saponin isolated from Agavaceae plants, causes a transient increase in cyclin-dependent kinase inhibitor (CDKI) p21/Cip1 through the upregulation of miRNAs, miR-34 and miR-21. AU-1 stimulated p21/Cip1 expression without exerting cytotoxicity against different types of carcinoma cell lines. In renal adenocarcinoma ACHN cells, AU-1 transiently elevated the expression level of p21/Cip1 protein without marked increases in p21/Cip1 mRNA levels. Rapid and transient increases in miR-34 and miR-21, both of which are known to upregulate p21/Cip1, were observed in AU-1-treated cells. Inhibitor for miR-34 and for miR-21 significantly blocked the AU-1-caused increase in p21/Cip1, indicating that elevation of p21/Cip1 protein by AU-1 is dependent on these microRNAs. We further clarified that NAD-dependent deacetylase SIRT1, a direct target of miR-34, is decreased by the treatment with AU-1. Furthermore, we found that SIRT1-knockdown increases p21/Cip1 protein levels in an miR-21-dependent manner. On the other hand, ectopic expression of p21/Cip1 resulted in the lowered expression of miR-34 and miR-21, suggesting that reciprocal regulation exists between p21/Cip1 and these miRNAs. We propose that the following feedback network composed of miR-34/SIRT1/miR-21/p21 is triggered by the treatment with AU-1: in cells treated with AU-1, transient elevation of miR-34 leads to the downregulation of SIRT1, thereby miR-21 is freed from SIRT1-dependent suppression. Then, elevated miR-21 upregulates p21/Cip1 protein, followed by the suppression of miR-34 expression.