Dissociation of DNA binding and in vitro transcriptional activities dependent on the C terminus of P53 proteins

Preferential binding of hot spot mutant p53 proteins to supercoiled DNA in vitro and in cells

Hot spot mutant p53 (mutp53) proteins exert oncogenic gain-of-function activities. Binding of mutp53 to DNA is assumed to be involved in mutp53-mediated repression or activation of several mutp53 target genes. To investigate the importance of DNA topology on mutp53-DNA recognition in vitro and in cells, we analyzed the interaction of seven hot spot mutp53 proteins with topologically different DNA substrates (supercoiled, linear and relaxed) containing and/or lacking mutp53 binding sites (mutp53BS) using a variety of electrophoresis and immunoprecipitation based techniques. All seven hot spot mutp53 proteins (R175H, G245S, R248W, R249S, R273C, R273H and R282W) were found to have retained the ability of wild-type p53 to preferentially bind circular DNA at native negative superhelix density, while linear or relaxed circular DNA was a poor substrate. The preference of mutp53 proteins for supercoiled DNA (supercoil-selective binding) was further substantiated by competition experiments with linear DNA or relaxed DNA in vitro and ex vivo. Using chromatin immunoprecipitation, the preferential binding of mutp53 to a sc mutp53BS was detected also in cells. Furthermore, we have shown by luciferase reporter assay that the DNA topology influences p53 regulation of BAX and MSP/MST1 promoters. Possible modes of mutp53 binding to topologically constrained DNA substrates and their biological consequences are discussed.

Posttranslational phosphorylation of mutant p53 protein in tumor development

p53 has been called the "cellular gatekeeper" and the "genome guard," because in response to exposure to DNA-damaging agents, it induces cell-cycle arrest in G1 or apoptosis and also directly affects DNA replication. Multiple mechanisms regulate p53 activity and posttranslational modification, including multisite phosphorylation of wild-type p53, in particular. Normal functions of wild-type p53 are abrogated by mutation of this gene, and oncogenic studies have revealed that p53 mutation is among the most common genetic alteration in human cancers. It is generally accepted that mutant p53 protein may not only lose the tumor suppressor functions of wild-type p53 but also acquire additional tumorigenetic roles, including dominant-negative effects and gain of function. Although many studies have revealed such aberrant functions of mutant p53, less is known about the posttranslational phosphorylation status of mutant p53 and novel biological functions of phosphorylation in carcinogenesis.

The tumor suppressor p53 abrogates Smad-dependent collagen gene induction in mesenchymal cells

The pleiotropic cytokine transforming growth factor-beta (TGF-beta) is a potent inducer of collagen synthesis and is implicated in the pathogenesis of fibrosis. Acting in concert with transcriptional coactivators p300/CBP, the Smads mediate TGF-beta stimulation of collagen synthesis in human dermal fibroblasts. Little information exists regarding positive and negative modulation of physiological TGF-beta responses. Because the tumor suppressor p53 is implicated in connective tissue homeostasis, here we examined the regulation of collagen gene expression by p53. Forced expression of ectopic p53 in dermal fibroblasts repressed basal and TGF-beta-stimulated collagen gene expression, whereas the absence of cellular p53 was associated with significantly enhanced transcriptional activity of the Type I collagen gene (COL1A2) and collagen synthesis. Ectopic expression of p53 also repressed TGF-beta stimulation of promoter activity driven by minimal Smad-binding elements, suggesting that p53 modulated Smad-dependent intracellular signaling. Inhibition was not due to altered levels, phosphorylation, or nuclear translocation of cellular Smads. Treatment of fibroblasts with etoposide, a potent inducer of cellular p53, abrogated TGF-beta stimulation of COL1A2 promoter activity and collagen synthesis in a p53-dependent manner. Overexpression of the transcriptional coactivator p300 rescued TGF-beta stimulation of COL1A2 promoter activity in fibroblasts overexpressing p53. Furthermore, the ligand-induced interaction of cellular Smad3 with p300 or with its cognate Smad-binding DNA element and recruitment of p300 to the DNA-protein complex assembled on the Smad-binding element were markedly reduced in p53-overexpressing fibroblasts. Collectively, these results indicate, for the first time, that p53 is a potent and selective endogenous repressor of TGF-beta-regulated collagen gene expression in dermal fibroblasts. The ligand-dependent interaction of Smad3 with p300 may be one of the targets of p53-mediated inhibition of TGF-beta responses. These findings suggest that a novel and important physiologic function for the tumor suppressor p53 is the regulation of fibrotic cellular responses.

CP-31398, a novel p53-stabilizing agent, induces p53-dependent and p53-independent glioma cell death

CP-31398 is a prototype small molecule that stabilizes the active conformation of p53 and promotes p53 activity in cancer cell lines with mutant or wild-type p53. Here, we report that CP-31398 induces p53 reporter gene activity and p21 expression in all of 11 glioma cell lines harboring wild-type or mutant p53, but not in p53-null LN-308 cells. Upon prolonged exposure to CP-31398, all glioma cell lines undergo caspase-independent and bcl-x(L)-insensitive cell death with EC(50) concentrations of 10-36 microM. By comparing p53 wild-type U87MG and p53-null LN-308 cells expressing the temperature-sensitive p53(V135A) mutant, we delineate two pathways of CP-31398-induced cell death: an early, p53-dependent pathway that requires (new p53) protein synthesis and a late, p53-independent pathway characterized by aurintricarboxylic acid -sensitive calcium release and epiphenomenal free radical formation. Post-transcriptional repression of p53 synthesis by an intracellularly transcribed short interfering RNA confirmed the presence of these two pathways of cell death. These observations point out some of the liabilities of CP-31398 as a prototype p53-based therapeutic and define a rationale for further refinement of small molecules that specifically target the p53 pathway, but lack the p53-independent effects.