The p53 tumor-suppressor protein functions as a transcriptional activator, and several p53-inducible genes that play a role in the induction of apoptosis in response to p53 have been described. We have identified a novel gene named PUMA (p53 upregulated modulator of apoptosis) as a target for activation by p53. This gene encodes two BH3 domain-containing proteins (PUMA-alpha and PUMA-beta) that are induced in cells following p53 activation. PUMA-alpha and PUMA-beta show similar activities; they bind to Bcl-2, localize to the mitochondria to induce cytochrome c release, and activate the rapid induction of programmed cell death. Antisense inhibition of PUMA expression reduced the apoptotic response to p53, and PUMA is likely to play a role in mediating p53-induced cell death through the cytochrome c/Apaf-1-dependent pathway.

The p53 tumor-suppressor protein prevents cancer development through various mechanisms, including the induction of cell-cycle arrest, apoptosis, and the maintenance of genome stability. We have identified a p53-inducible gene named TIGAR (TP53-induced glycolysis and apoptosis regulator). TIGAR expression lowered fructose-2,6-bisphosphate levels in cells, resulting in an inhibition of glycolysis and an overall decrease in intracellular reactive oxygen species (ROS) levels. These functions of TIGAR correlated with an ability to protect cells from ROS-associated apoptosis, and consequently, knockdown of endogenous TIGAR expression sensitized cells to p53-induced death. Expression of TIGAR may therefore modulate the apoptotic response to p53, allowing survival in the face of mild or transient stress signals that may be reversed or repaired. The decrease of intracellular ROS levels in response to TIGAR may also play a role in the ability of p53 to protect from the accumulation of genomic damage.

Butyrate is a well known colonic luminal short chain fatty acid, which arrests cell growth and induces differentiation in various cell types. We examined the effect of butyrate on the expression of WAF1/Cip1, a potent inhibitor of cyclin-dependent kinases, and its relation to growth arrest in a p53-mutated human colon cancer cell line WiDr. Five millimolar butyrate completely inhibited the growth of WiDr and caused G 1 -phase arrest. WAF1/Cip1 mRNA was rapidly induced within 3 h by treatment with 5.0 mM butyrate, and drastic WAF1/Cip1 protein induction was detected. Using several mutant WAF1/Cip1 promoter fragments, we found that the butyrate-responsive elements are two Sp1 sites at ؊82 and ؊69 relative to the transcription start site. We also found that a TATA element at ؊46 and two overlapping consensus Sp1 sites at ؊60 and ؊55 are essential for the basal promoter activity of WAF1/Cip1. These findings suggest that butyrate arrests the growth of WiDr by activating the WAF1/Cip1 promoter through specific Sp1 sites in a p53-independent fashion.Butyrate is one of the most abundant short chain fatty acids in the large intestine, generated by bacterial fermentation of dietary fibers (1). Butyrate shows potent effects on growth arrest and differentiation in vitro in various malignant tumor cell lines, such as breast cancer cells, hepatoma cells, and others (2-5). In colorectal cancer cells, butyrate inhibits cell growth and induces differentiation marker proteins such as alkaline phosphatase and carcinoembryonic antigen (6 -9). Furthermore, butyrate arrests the cell cycle progression at the G 1 phase (9) and decreases c-myc oncogene expression in human colon cancer cell lines (9, 10). However, the precise mechanism of growth suppression by butyrate in colon cancer cells has not been clarified. WAF1/Cip1 protein potently inhibits the various G 1 cyclindependent kinases activities (11-13) by suppressing the phosphorylation of retinoblastoma (RB) protein, thereby supposedly inhibiting the G 1 -S phase transition (11,14). Besides its role as a kinase inhibitor, it has been reported recently that WAF1/ Cip1 at low doses assembles kinase complexes and promotes a kinase activity (15). Furthermore, the transcription of the WAF1/Cip1 gene is directly activated by wild-type p53 protein (16). Thus, WAF1/Cip1 could play a key role as a downstream mediator of the p53-induced cell growth arrest.Several studies have already shown the p53-independent induction of WAF1/Cip1 by serum, transforming growth factor ␤, and other differentiation-inducers (17)(18)(19)(20). In addition, butyrate has been reported to induce WAF1/Cip1 mRNA independently of p53 during differentiation of hematopoietic cells, hepatoma cells, and colon cancer cells in vitro (18,21). Butyrate can also dephosphorylate the retinoblastoma protein in mouse fibroblasts (22). To investigate the mechanism of butyrateinduced growth arrest, we used a human colon cancer cell line WiDr harboring a point mutation in p53 at codon 273 (23) and examined the effect of butyrate on t...

No abstract