The unimportance of being (protein kinase C) epsilon

The purpose of our study was to determine the mechanism through which phorbol esters and smooth muscle myosin phosphatase inhibitors can induce contraction of smooth muscle in the absence of Ca2+. Protein kinase C-epsilon (PKC-epsilon) was previously implicated in this process based largely on its supposed absence in the ferret portal vein, and a correlation was drawn between the presence of this isoform and the ability of smooth muscle to contract independently of Ca2+ and phosphorylation of the 20 kDa regulatory light chains of myosin (MLC20). We demonstrate here, with two antibodies, one to the NH2 terminus and the other to the COOH terminus of PKC-epsilon, that epsilon is present in both ferret portal vein and rabbit portal vein smooth muscle, neither of which exhibits phorbol ester-induced contraction in the absence of Ca2+. However, in the presence of clamped submaximal Ca2+, phorbol es ter increased MLC20 phosphorylation from 17.7+/-1.7% to 46.4+/-3.6% in ferret portal vein smooth muscle and evoked an increase in force. Prolonged (48 h) incubation of ferret portal vein with phorbol esters completely down-regulated PKC-epsilon, as shown by Western blots, and abolished the phorbol ester-evoked contraction at submaximal Ca2+, but not Ca2+-independent, contractions induced by the phosphatase inhibitor microcystin. Contractions induced by microcystin in Ca2+-free solution were associated with increased phosphorylation of myosin light chain kinase (MLCK). Activation of MLCK by autophosphorylation in the absence of Ca2+ occurs in vitro (1). We conclude that PKC-epsilon is neither necessary nor sufficient for Ca2+-independent regulation of myosin II in smooth muscle, but contractions induced by agents that inhibit smooth muscle myosin phosphatase in the absence of Ca2+ may be mediated by MLCK autophosphorylated or activated by another Ca2+-independent kinase.