Induction of protein kinase C down‐regulation by the phorbol ester TPA in a calpain/protein kinase C complex

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Abstract
Using a calpain/protein kinase C (PKC) complex, we were able to reproduce, in vitro, the induction of PKC down‐regulation by the phorbol ester 12‐O‐tetradecanoyl‐phorbol‐13‐acetate (TPA) which had been previously observed in cells. We show that TPA initiates this phenomenon by promoting a calpain‐dependent conversion of PKC to the Ca2+ phospholipid‐independent protein kinase M (PKM), at physiological calcium concentrations. This effect of TPA was dependent upon the presence of phosphatidylserine and was observed only when PKC was the substrate for the protease, inactivation of calpain by autolysis not being modified by the presence of TPA. Moreover, PKM generated from the calpain‐PKC complex was resistant to calpain, even after addition of TPA. These results suggest that TPA induces a conformational change in PKC, increasing the affinity of the kinase for calpain and consequently permitting its proteolysis for the basal level of calcium in cells.