[Show abstract][Hide abstract]ABSTRACT: Carcinogenesis is a multistage process, involving oncogene activation and tumor suppressor gene inactivation as well as complex interactions between tumor and host tissues, leading ultimately to an aggressive metastatic phenotype. Among many genetic lesions, mutational inactivation of p53 tumor suppressor, the "guardian of the genome," is the most frequent event found in 50% of human cancers. p53 plays a critical role in tumor suppression mainly by inducing growth arrest, apoptosis, and senescence, as well as by blocking angiogenesis. In addition, p53 generally confers the cancer cell sensitivity to chemoradiation. Thus, p53 becomes the most appealing target for mechanism-driven anti- cancer drug discovery. This review will focus on the approaches currently undertaken to target p53 and its regulators with an overall goal either to activate p53 in cancer cells for killing or to inactivate p53 temporarily in normal cells for chemoradiation protection. The compounds that activate wild type (wt) p53 would have an application for the treat- ment of wt p53-containing human cancer. Likewise, the compounds that change p53 conformation from mutant to wt p53 (p53 reactivation) or that kill the cancer cells with mutant p53 using a synthetic lethal mechanism can be used to selectively treat human cancer harboring a mutant p53. The inhibitors of wt p53 can be used on a temporary basis to reduce the normal cell toxicity derived from p53 activation. Thus, successful development of these three classes of p53 modulators, to be used alone or in combination with chemoradiation, will revolutionize current anticancer thera- pies and benefit cancer patients.
Preview · Article · Feb 2010 · Translational oncology
[Show abstract][Hide abstract]ABSTRACT: p53 is regulated at multiple levels. We report here that p53, in multiple lines of human cancer cells, is down-regulated by cardiac glycoside drugs digoxin and ouabain, potent inhibitors of Na(+)/K(+)-ATPase. These drugs reduced the basal levels of p53 protein at nanomolar concentrations in a dose-, time-, and cancer cell line-dependent manner, but independent of p53 status of wild-type or mutant. The drugs also reduced the levels of p53 induced by its activators as well as p53 transfected into human cancer cells, regardless of its status. Interestingly, the drugs had no effect on endogenous p53 in two immortalized human cell lines. Mechanistically, p53 reduction occurred not at the mRNA levels but at the protein levels, as a result of reduced protein synthesis rather than enhanced degradation. The cellular sensitivity to drug-induced p53 reduction was not associated with the levels of alphasubunits of Na(+)/K(+)-ATPase in different cell lines. Although lowering extracellular K(+) did not reduce p53 as did ouabain and digoxin, it did potentiate both digoxin- and ouabain-induced p53 reduction in sensitive lines. Finally, p53 reduction seems to be triggered by activation of Src/mitogen-activated protein kinase (MAPK) signaling pathways upon drug binding to the Na(+)/K(+)-ATPase and can be completely blocked by the inhibitors of Src or MAP/ERK kinase. This is the first report that cardiac glycoside drugs, by initiating the Src/MAPK signaling pathways, reduce the p53 levels via inhibition of p53 protein synthesis. The drugs may be useful in the treatment of human cancers with a gain-of-function p53 mutation.
[Show abstract][Hide abstract]ABSTRACT: Noxa is a pro-apoptotic Bcl-2 homology 3 (BH3)-only containing protein. Here we report the identification of two splicing variants of the human Noxa gene, which consists of three exons and two introns. Alternative splicing of exon 2 yields three transcripts. Transcript-1 joins exons 1 and 3 to encode Noxa of 54 amino acids. Transcript-2, consisting of exon-1, partially spliced exon-2, and exon-3, encodes NSV-1 (Noxa splicing variant-1) of 136 amino acids, whereas transcript-3, containing all three exons, encodes NSV-2 of 70 amino acids, which is 100% identical to the first 70 amino acids of NSV-1. All three transcripts, controlled by the same promoter with two p53 consensus-binding sites, were inducible by p53. Although NSV-1 and NSV-2 mRNA were readily detectable by RT-PCR in multiple human cancer cell lines, no endogenous variant proteins were detected under physiological or stressed conditions. Even under forced expression, both variants were barely detectable without proteasome inhibitor, MG132. The protein half-life of the two variants is approximately 40 to 60 mins with MG132, suggesting they are rapidly degraded via proteasome-dependent and independent pathways. Unlike Noxa, BH3-less variants failed to potentiate apoptosis induced by etoposide. Thus, Noxa variants are unlikely to play a role in apoptosis regulation.
Preview · Article · May 2008 · Anticancer research