Differential protection and transactivation of P53, P21, Bcl2, PCNA, cyclin G, and MDM2 genes in rat liver and the HepG2 cell line upon exposure to pifithrin.

In response to genotoxic agents, normal tissue cells are instructed by p53 either to perform DNA repair or to undergo apoptosis. Studies showed that chemo and/or radiotherapy damage both normal and cancerous cells indiscriminately. To this end, severe side effects inflicted by p53 activation in normal tissues, would possibly be abrogated by p53 inhibition. Pifithrin-alpha (PFT-alpha) is a reversible inhibitor of p53-mediated apoptosis, p53-dependent gene transcription, as well as down stream responsive gene function. The objective of this study was (1) to evaluate PFT-alpha for differential cellular protection in response to arsenic trioxide and cadmium chloride exposure of normal and neoplastic cells, and (2) to evaluate the transcriptional activation of p53 and p53-responsive genes in rat liver cells and HepG2 carcinoma cell line. Cell survival was detected by fluorescein diacetate (FDA) and fluorospectroscopy. Mean LC50 and (SD) for HepG2 cells following exposure to arsenic were 13.7 (+/-1.0) microg/ml with PFT- alpha and 13.4 (+/- 0.5) microg/ml without PFT-alpha (p>0.05). For rat liver cells it was 670 (+/- 8.15) microg/ml with and 573.15 (+/-1.0) microg/ml without PFT-alphha (p<0.05). On exposure to cadmium Chloride, LC50's were 6.95 (+/-2.5) microg/ml for HepG2 cell line in presence of PFT-alpha and 7.35 (+/-1.9) microg/ml in its absence (p>0.5). The results revealed significant differences from controls only upon exposure of rat liver cells to arsenic trioxide in presence of PFT-alpha. PFT-alpha inhibited the transactivation of p53 in rat liver cells and resulted in repression of Bcl2, PCNA, MDM2, Cyclin G and P21 genes by arsenic trioxide. HepG2 cells exposed to arsenic trioxide and PFT-alpha showed expression of only the P53 and PCNA genes. We conclude that PFT-alpha exhibits cytoprotective effect, modifies the detrimental influences of known genotoxic agents in normal cells and has the potential for use as an adjuvant to cancer therapy.