Pifithrin-α decreases the radioprotective efficacy of a Podophyllum hexandrum (Himalayan mayapple) fraction (REC-2006) in HepG2 cells

Inhibition of the tumour suppressor p53 by PFT (pifithrin-α) promotes p53-mediated apoptosis and protects against doxorubicin-induced apoptosis. The present study was carried out to evaluate the effect of PFT on the radioprotective potential of Podophyllum hexandrum fraction (REC-2006) in HepG2 (p53⁺⁺) cell line. REC-2006 (10^-5 μg/ml) treatment at 2 h before irradiation (10 Gy) rendered 80 ± 3% protection in HepG2 cells, whereas PFT debilitated the radioprotective potential of REC-2006. REC-2006 increased the expression of Hsp70 (heat-shock protein 70), HSF1 (heat-shock factor 1) and Bcl-2 in irradiated HepG2 cells, whereas PFT when treated with REC-2006 decreased the expression of Hsp70, HSF1 and Bcl-2 in HepG2 cells. REC-2006 facilitated post-irradiation DNA repair by pausing cell-cycle progression at G₁- and G₂ -phase, whereas no such cell-cycle arrest was observed in irradiated HepG2 cells pretreated with PFT in irradiated HepG2 cells. No change was observed inMdm2 (murine double minute 2) and Ras-GAP (Ras-GTPase-activating protein) expression with or without PFT treatment. Decrease in the expression of caspase 3 and Bax was observed in HepG2 cells when REC-2006 treatment was given 2 h before irradiation; however, PFT treatment increased the expression of Bax leading to apoptosis. It can be concluded that p53 expression plays a major role in the REC-2006-mediated protection against acute irradiation in HepG2 cells. PFT treatment reduced the radioprotective efficacy of REC-2006 by inhibiting the expression of HSF1 and Hsp70 and thereby the expression of Bcl-2, by up-regulating the cell-cycle-regulatory proteins and therefore reducing the span of time for DNA repair and also by inducing Bax-mediated apoptosis. PFT did not, however, show any effect on p53 regulating protein (Mdm2) and pro-survival protein (Ras-GAP).