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Protein Kinase C Expression Is Increased upon Differentiation of Human Skeletal Muscle Cells: Dysregulation in Type 2 Diabetic Patients and a Possible Role for Protein Kinase C in Insulin-Stimulated Glycogen Synthase Activity¹

Departments of Biochemistry and Molecular Biology (C.E.C., D.R.C.) and Internal Medicine (D.R.C.), University of South Florida College of Medicine, and The James A. Haley Veterans Hospital (J.E.W., D.R.C.), Tampa, Florida 33612; and San Diego Veterans Hospital (S.N., R.R.H.), and Department of Medicine, University of California (T.P.C., S.N., R.R.H.), San Diego, La Jolla, California 92093

Address all correspondence and requests for reprints to: Denise R. Cooper, Ph.D., J. A. Haley Veteran’s Hospital (VAR 151), 13000 Bruce B. Downs Boulevard, Tampa, Florida 33612. E-mail: dcooper{at}com1.med.usf.edu

Protein kinase C (PKC) is a key enzyme in regulating a variety of cellular functions, including growth and differentiation. PKC is the most abundant PKC isoform expressed in skeletal muscle; however, its role in differentiation and metabolism is not clear. We examined the effect of muscle cell differentiation on PKC expression in human skeletal muscle cells from normal and type 2 diabetic subjects. Low levels of PKC messenger RNA (mRNA) and protein were detected in human myoblasts from both types of subjects. Upon differentiation into myotubes, PKC mRNA and protein were increased 12-fold in myotubes from normal subjects. In human skeletal muscle cells obtained from type 2 diabetic subjects, increases in PKC mRNA and protein were not observed upon differentiation into myotubes although expression of other markers of differentiation and fusion increased. Cells from type 2 diabetic subjects also exhibited decreased insulin-stimulated glycogen synthase activity. To determine whether the up-regulation of PKC was important for the metabolic actions of insulin, PKC was overexpressed in L6 rat skeletal muscle cells. Increased expression of PKC occurred with differentiation of skeletal muscle myoblasts to myotubes. Glycogen synthase activity was further increased in L6 myotubes stably transfected with the complementary DNA for PKC. The decreased expression of PKC found in cells from type 2 diabetic subjects may be linked to insulin resistance and decreased glycogen synthase activity.

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