Insulin Increases Nuclear Protein Kinase Cδ in L6 Skeletal Muscle Cells

Protein kinase C (PKC) isoforms are involved in the transduction of a number of signals important for the regulation of cell growth, differentiation, apoptosis, and other cellular functions. PKC proteins reside in the cytoplasm in an inactive state translocate to various membranes to become fully activated in the presence of specific cofactors. Recent evidence indicates that PKC isoforms have an important role in the nucleus. We recently showed that insulin rapidly increases PKCδ RNA and protein. In this study we initially found that insulin induces an increase in PKCδ protein in the nuclear fraction. We therefore attempted to elucidate the mechanism of the insulin-induced increase in nuclear PKCδ. Studies were performed on L6 skeletal myoblasts and myotubes. The increase in nuclear PKCδ appeared to be unique to insulin because it was not induced by other growth factors or rosiglitazone. Inhibition of transcription or translation blocked the insulin-induced increase in nuclear PKCδ, whereas inhibition of protein import did not. Inhibition of protein export from the nucleus reduced the insulin-induced increase in PKCδ in the cytoplasm and increased it in the nucleus. The increase in nuclear PKCδ induced by insulin was reduced but not abrogated by treatment of isolated nuclei by trypsin digestion. Finally, we showed that insulin induced incorporation of 35S-methionine into nuclear PKCδ protein; this effect was not blocked by inhibition of nuclear import. Thus, these results suggest that insulin may induce nuclear-associated, or possibly nuclear, translation of PKCδ protein.