GLUCOSE-INDUCED TRANSLOCATION OF PROTEIN-KINASE-C ISOFORMS IN RAT-1 FIBROBLASTS IS PARALLELED BY INHIBITION OF THE INSULIN-RECEPTOR TYROSINE KINASE

Rat-1 fibroblasts stably overexpressing high levels,of human insulin r eceptor were used as a model system to study the effects of hyperglyce mia on insulin receptor tyrosine kinase (IRK) activity and protein kin ase C (PKC) translocation in parallel in the intact cell. Glucose (10- 25 mm) induced a significant reduction of IRK activity (tyrosine phosp horylation of IR-beta-subunit and IR-substrate-1) within 10 min. This effect was paralleled by a rapid translocation of several PKC isoforms (cPKC alpha, nPKC delta, nPKC epsilon, nPKC zeta) to the plasma membr ane within 1 min. Kinetics of IRK inhibition and PKC translocation are consistent with the idea that the glucose effect on IRK is mediated b y PKC activation. This hypothesis is supported by further observations . Addition of the protein kinase C inhibitor H-7 can prevent the effec t of glucose on IRK. Inhibition of IRK is also observed after stimulat ion of the cells with the phorbol ester 12-0-tetradecanoylphorbol-13-a cetate, which can substitute for a physiological activator of PKC. Glu cose (25 mm) increases the P-32 incorporation in serine residues of th e beta-subunit of IRK. We conclude that high levels of glucose induce inhibition of IRK in vivo. There is indirect evidence that this effect is mediated by a glucose-induced PKC translocation/activation and ser ine phosphorylation of the insulin receptor.