Insulin regulates protein kinase CβII expression through enhanced exon inclusion in L6 skeletal muscle cells -: A novel mechanism of insulin- and insulin-like growth factor-I-induced 5′ splice site selection

The protein kinase C beta (PKC beta) gene encodes two isoforms, PKC beta I and PKC beta II, as a result of alternative splicing. The unique mechanism that underlies insulin-induced alternative splicing of PKC beta pre-mRNA was examined in L6 myotubes. Mature PKC beta II mRNA and protein rapidly increased >3-fold following acute insulin treatment, while PKC beta I mRNA and protein levels remained unchanged. Mature PKC beta II mRNA resulted from inclusion of the PKC beta II-specific exon rather than from selection of an alternative polyadenylation site. Increased PKC beta II expression was also not likely accounted for by transcriptional activation of the gene or increased stabilization of the PKC beta II mRNA, and suggest that PKC beta II expression is regulated primarily at the level of alternative splicing, Insulin effects on exon inclusion were observed as early as 15 min after insulin treatment; by 20 min, a new 5'-splice site variant of PKC beta II was also observed. After 30 min, the longer 5'-splice site variant became the predominate species through activation of a downstream 5' splice site, Similar lar results were obtained using IGF-I. Although the role of this new PKC beta II mRNA species is presently unknown, inclusion of either PKC beta II-specific exon results in the same PKC beta II protein.