Protein kinase C isoforms ɛ, η, δ and ζ in murine adipocytes: expression, subcellular localization and tissue-specific regulation in insulin-resistant states

Abstract

The Ca2+-insensitive protein kinase C (PKC) isoforms ε, η, δ and ζ are possible direct downstream targets of phosphatidylinositol 3-kinase (PI3-K), and might therefore be involved in insulin signalling. Although isoform-specific changes in PKC expression have been reported for skeletal muscle and liver in insulin-resistant states, little is known about these isoforms in adipocytes. Therefore we studied (1) expression and subcellular localization of these isoforms in murine adipocytes, (2) translocation of specific isoforms to membranes in response to treatment with insulin and phorbol 12-myristate 13-acetate (PMA) and (3) regulation of expression in insulin-resistant states. The PKC isoforms ε, η, δ and ζ are expressed in adipocytes. Immunoreactivity for all isoforms is higher in the membranes than in the cytosol, but subcellular fractionation by differential centrifugation shows an isoform-specific distribution within the membrane fractions. PMA treatment of adipocytes induces translocation of PKC-ε and -δ from the cytosol to the membrane fractions. Insulin treatment does not alter the subcellular distribution of any of the isoforms. 3T3-L1 adipocytes express PKC-ε and -ζ, and PKC-ε expression increases with differentiation from preadipocytes to adipocytes. PKC-ε expression decreases in an adipose-specific and age/obesity-dependent manner in two insulin-resistant models, the brown-adipose-tissue-deficient mouse and db/db mouse compared with control mice. We conclude that, although none of the isoforms investigated seems to be activated by insulin, the decrease in PKC-ε expression might contribute to metabolic alterations in adipocytes associated with insulin resistance and obesity.