Overexpression of the Csk homologous kinase facilitates phosphorylation of Akt/PKB in MCF-7 cells.

The serine/threonine kinase Akt has recently been the focus of intense research. Akt activation requires the phosphorylation of both Thr-308 and Ser-473. Src kinase was shown to induce activation of Akt, while Lyn kinase seems to inhibit this activation. In the present study, we investigated the effect of overexpressing the Csk homologous kinase (CHK), an inhibitor of Src-family kinases, on the phosphorylation of Akt induced by two different factors: heregulin or cisplatin. We used MCF-7 cells stably overexpressing the wild-type CHK [CHK(wt)] or dead-kinase CHK [CHK(dk)]. We observed that in MCF-7 CHK(wt) cells Lyn kinase activity was more profoundly inhibited than Src kinase activity. When the cells were stimulated with heregulin or cisplatin, Akt phosphorylation occurred more rapidly in MCF-7 CHK(wt) cells in comparison to the other clones used. Interestingly, MCF-7 CHK(wt) cells in vitro were markedly more resistant to cisplatin than the other clones used in the experiments, and surprisingly chemical inhibition of Akt phosphorylation did not influence this resistance. In summary, our results show facilitation of Akt phosphorylation by the overexpression of CHK, and provide new insight into the putative role of CHK in human cancer.