Activation of the nuclear transcription factor SP-1 by insulin rapidly increases the expression of protein kinase C delta in skeletal muscle

Abstract

SP-1, a ubiquitous transcription factor involved in regulation of target genes participating in specific signaling pathways, is utilized by insulin for induction of gene transcription. Transcriptional activation generally occurs only after several (14–24) hours. A major element rapidly activated by insulin in skeletal muscle is PKCδ, which plays a positive regulatory role in insulin signaling. We recently reported that insulin stimulation of skeletal muscle increases PKCδ RNA expression and PKCδ protein levels within 5 min. These effects were blocked by inhibitors of either translation or transcription. In this study, we investigated the possibility that SP-1 may participate in this unusually rapid effect. Studies were performed on myoblasts and myotubes of the L6 skeletal muscle cell line. Insulin rapidly increased SP-1 levels and stimulated SP-1 phosphorylation in the nuclear fraction of L6 myotubes. The increase in nuclear SP-1 was blocked by inhibition of nuclear import. Inhibition of SP-1, either pharmacologically or by suppression of SP-1 by RNAi, nearly completely abrogated insulin-induced increase in PKCδ promoter activity. Insulin induced a rapid association of SP-1 with the PKCα promoter. In addition, SP-1 inhibition blocked insulin-induced increases in both PKCδ RNA expression and PKCδ protein levels. We conclude that insulin rapidly stimulates SP-1, which mediates the ability of this hormone to induce the rapid transcription of a major target gene utilized in the insulin signaling cascade.