Involvement of PKCα in insulin-induced PKCδ expression: Importance of SP-1 and NFκb transcription factors

Protein kinase C delta (PKC{delta}) is a key molecule in insulin signaling essential for insulin-induced glucose transport in skeletal muscle. Recent studies in our laboratory have shown that insulin rapidly stimulates PKC{delta} activity and increases PKC{delta} protein and RNA levels, and that the SP-1 transcription factor is involved in insulin-induced transcription of the PKC{delta} gene. Activation of SP-1 involves serine phosphorylation and translocation to the nucleus. In this study we examined the possibility that PKC{alpha} might be involved in serine phosphorylation and activation of SP-1. We found that insulin rapidly phosphorylates and translocates SP-1. In the cytoplasm, SP-1 was constitutively associated with PKC{alpha}, and insulin stimulation caused these proteins to dissociate. In contrast, in the nucleus insulin induced an increase in association between PKC{alpha} and SP-1. PKC{alpha} inhibition blocked insulin-induced serine phosphorylation of SP-1 and its association with PKC{alpha} in the nucleus. Inhibition of PKC{alpha} also reduced the insulin-induced increase in PKC{delta} RNA and protein in the cytoplasmic and nuclear fractions. We also attempted to determine if another transcription factor might be involved in regulation of PKC{delta} expression. We earlier showed that insulin did not affect nuclear NF{kappa}B levels. Inhibition of NF{kappa}B, however, increased insulin-induced increase in PKC{delta} RNA andmore » protein in the cytoplasmic and nuclear fractions. Surprisingly, this inhibition reduced the insulin-induced increase in cytoplasmic and nuclear PKC{alpha} RNA and protein. Inhibition of PKC{delta} reduced I{kappa}B{alpha} phosphorylation as well as NF{kappa}B activation. Thus, PKC{alpha} regulates insulin-induced PKC{delta} expression levels and this regulation involves activation of SP-1 and NF{kappa}B.« less