Role of PKC isoforms in glucose transport in 3T3-L1 adipocytes: insignificance of atypical PKC

Abstract

To elucidate the involvement of protein kinase C (PKC) isoforms in insulin-induced and phorbol ester-induced glucose transport, we expressed several PKC isoforms, conventional PKC-alpha, novel PKC-delta, and atypical PKC isoforms of PKC-lambda and PKC-zeta, and their mutants in 3T3-L1 adipocytes using an adenovirus-mediated gene transduction system. Endogenous expression and the activities of PKC-alpha and PKC-lambda/zeta, but not of PKC-delta, were detected in 3T3-L1 adipocytes. Overexpression of each wild-type PKC isoform induced a large amount of PKC activity in 3T3-L1 adipocytes. Phorbol 12-myristrate 13-acetate (PMA) activated PKC-alpha and exogenous PKC-delta but not atypical PKC-lambda/zeta. Insulin also activated the overexpressed PKC-delta but not PKC-alpha. Expression of the wild-type PKC-alpha or PKC-delta resulted in significant increases in glucose transport activity in the basal and PMA-stimulated states. Dominant-negative PKC-alpha expression, which inhibited the PMA activation of PKC-alpha, decreased in PMA-stimulated glucose transport. Glucose transport activity in the insulin-stimulated state was increased by the expression of PKC-delta but not of PKC-alpha. These findings demonstrate that both conventional and novel PKC isoforms are involved in PMA-stimulated glucose transport and that other novel PKC isoforms could participate in PMA-stimulated and insulin-stimulated glucose transport. Atypical PKC-lambda/zeta was not significantly activated by insulin, and expression of the wild-type, constitutively active, and dominant-negative mutants of atypical PKC did not affect either basal or insulin-stimulated glucose transport. Thus atypical PKC enzymes do not play a major role in insulin-stimulated glucose transport in 3T3-L1 adipocytes.

Dehydroepiandrosterone Stimulates Glucose Uptake in Human and Murine Adipocytes by Inducing GLUT1 and GLUT4 Translocation to the Plasma Membrane

“…Insulin was shown to activate PKC-␣ and, to a lesser extent, PKC-␤ and PKC-/ in 3T3-L1 adipocytes (29,30) and PKC-/ in rat adipocytes (37). However, PKC-␣ and PKC-activation by insulin has not been observed in all studies (51,52). In this study, insulin was found to affect the subcellular distribution of PKC-␣ and PKC-/, whereas it did not influence translocation or tyrosine phosphorylation of PKC-␤.…”

Insulin Receptor Signals Regulating GLUT4 Translocation and Actin Dynamics

“…The phosphorylation and activation of PKC λ/ζ are stimulated by insulin [75,76] and the expression of constitutively active PKC λ/ζ mutants reportedly increases whereas dominantinterfering mutants and microinjection of blocking antibodies inhibit insulin-induced GLUT4 translocation [70][71][72]. However, other studies found no evidence of direct involvement of PKC λ/ζ in this process [77]. Despite PKC λ/ζ not possessing a PH domain, these enzymes are recruited to the plasma membrane, and are activated by PDK-1-dependent phosphorylation in response to insulin stimulation [78,79].…”

Move over protein kinase C, you've got company: alternative cellular effectors of diacylglycerol and phorbol esters

“…These include the following: (1) overexpression of wild-type and dominant interfering PKC mutants [e.g. PKCζ as a regulator of RelA transcriptional activity (Anrather et al, 1999); PKCα and PKCδ as regulators of glucose transport (Tsuru et al, 2002); PKCα and PKCθ as regulators of calcineurin-induced transactivation (Ishaq et al, 2002); (for a review, see Dempsey et al, 2000)]; (2) interference with PKC expression using ribozymes [e.g. PKCα as a regulator of glioma cell growth (Sioud and Sorensen, 1998)]; (3) interference with expression using antisense oligonucleotides [for reviews of the literature with an emphasis on therapeutically relevant approaches see (Tamm et al, 2001;Goekjian and Jirousek, 2001;Swannie and Kaye, 2002)]; and (4) interference with PKC function using peptides that induce or block PKC interactions with targeting proteins (for reviews, see Csukai and Mochly-Rosen, 1999;Jaken and Parker, 2000).…”