Inhibition of protein kinase Cα enhances anticancer agent-induced loss of anchorage-independent growth regardless of protection against apoptosis by Bcl-2

In the present study, we investigated the effects of several selective protein kinase C (PKC) inhibitors (Go6976, Go6983, bisindolylmaleimide I, and rottlerin) in combination with conventional anticancer drugs on apoptosis and long-term anchor-age-independent growth of both parental and Bcl-2-overexpressing mammary adenocarcinoma MTLn3 cells. In normal MTLn3 cells, doxorubicin- and etoposide-induced apoptosis was not affected by any of the PKC inhibitors. However, Bcl-2-mediated cytoprotection against apoptosis was slightly counteracted by Go6976, a selective inhibitor of PKCalpha, as well as by transient overexpression of dominant-negative PKCalpha. Doxorubicin and etoposide both inhibited anchorage-independent growth; for doxorubicin, this occurred at concentrations that did not yet cause apoptosis. Overexpression of Bcl- 2 did not overcome these growth-inhibitory effects. The effects of doxorubicin on colony formation were potentiated by Go6976, Go6983, and bisindolylmaleimide I but not rottlerin. In contrast, etoposide-induced loss of clonogenicity was primarily enhanced by Go6976. Go6976 alone, but not Go6983, bisindolylmaleimide I, or rottlerin, inhibited colony formation in soft agar. This effect of Go6976 correlated with inhibition of cell cycle progression. Overall, the data indicate that pharmacological inhibitors of PKCalpha in combination with anticancer drugs, act additively to inhibit long-term anchorage-independent tumor cell growth, independent of apoptosis induction. Importantly, similar additive effects are observed in Bcl-2 overexpressing cells.