Sensitization of human cervical carcinoma cells to cis-diamminedichloroplatinum(II) by bryostatin 1.

Bryostatins are an important class of protein kinase C (PKC) activators. We have investigated the effect of bryostatin 1 on the antiproliferative activity of cis-diamminedichloroplatinum(II) (CP). A 24-h pretreatment of HeLa cells with 1 nM bryostatin 1 increased cellular sensitivity to CP by 4-fold. The effect of bryostatin 1 on the IC50 of CP (concentration of drug required to inhibit cell proliferation by 50%) was concentration-dependent and biphasic; the maximum effect of bryostatin 1 was seen with 1 nM, but higher concentrations of bryostatin 1 (greater than or equal to 10 nM) produced less CP sensitization. Although bryostatin 1 and phorbol esters caused an equivalent stimulation of HeLa cell PKC in cell-free systems, bryostatin 1 was less effective than phorbol esters in sensitizing cells to CP. Additionally, higher concentrations of bryostatin 1 (greater than or equal to 10 nM) antagonized CP sensitization by phorbol esters. Bryostatin 1 was even more potent than 12-O-tetradecanoylphorbol-13-acetate in inducing PKC down-regulation, and the maximum down-regulation was achieved with 10 nM bryostatin 1. Bryostatin 1 also increased cellular sensitivity to a CP analogue, cis-dichloro(ethylenediamine)platinum(II). A 24-h pretreatment with 1 nM bryostatin 1 increased cellular cis-[3H]DEP by 60%. The concentration- and time-dependent enhancement in CP sensitivity by bryostatin 1 was related to the increase in cis-[3H]DEP level. Thus, cellular accumulation of CP may be regulated by a PKC-dependent phosphorylation event.