Insulin stimulation of PKCδ triggers its rapid degradation via the ubiquitin-proteasome pathway

Insulin rapidly upregulates protein levels of PKC delta in classical insulin target tissues skeletal muscle and liver. Insulin induces both a rapid increase in de novo synthesis of PKC delta protein. In this study we examined the possibility that insulin may also inhibit degradation of PKC delta. Experiments were performed on L6 skeletal muscle myoblasts or myotubes in culture. Phorbol ester (PMA)- and insulin-induced degradation of PKC delta were abrogated by proteasome inhibition. Both PMA and insulin induced ubiquitination of PKC delta, but not of that PKC alpha or PKC epsilon and increased proteasome activity within 5 min. We examined the role of tyrosine phosphorylation of PKC delta in targeting PKC delta for degradation by the ubiquitin-proteasome pathway. Transfection of cells with PKC delta(YF)-F-311, which is not phosphorylated, resulted in abolition of insulin-induced ubiquitination of PKC delta and increase in proteasome activity. We conclude that insulin induces degradation of PKC delta via the ubiquitin-proteasome system, and that this effect requires phosphorylation on specific tyrosine residues for targeting PKC delta for degradation by the ubiquitin-proteasome pathway. These studies provide additional evidence for unique effects of insulin on regulation of PKC delta protein levels. (C) 2010 Elsevier B.V. All rights reserved.