A re-assessment of the role of protein kinase C in glucose-stimulated insulin secretion

Abstract

Isolated rat islets of Langerhans which had been pretreated with 200 nM-phorbol 12-myristate 13-acetate (PMA) for 20-24 h, a treatment reported in other cell types to deplete cells of protein kinase C activity, were found not to contain detectable Ca2+/phospholipid-dependent protein kinase activity. These islets did not secrete insulin in response to a subsequent exposure to PMA (0.1 or 1 microM) during a 30 min incubation, although insulin secretion could be stimulated by 20 mM-glucose, a response which was enhanced by 20 microM-forskolin. PMA-pretreated islets that had been permeabilized by high-voltage discharge showed unimpaired secretory responses to an increase in Ca2+ concentration, cyclic AMP and forskolin. These results suggest that (i) pretreatment of islets with tumour-promoting phorbol esters may be a useful means of investigating the role of protein kinase C in stimulus-secretion coupling in the pancreatic beta-cell and (ii) protein kinase C may not play an essential role in glucose-induced insulin secretion.

Regulation of the diabetes-associated autoantigen IA-2 in INS-1 pancreatic beta-cells.

“…Although IA-2 is expressed in only low amounts in islets of Langerhans and insulinoma cells (17), the development of a competitive RT-PCR technique allowed us to quantify IA-2 mRNA levels with high sensitivity. In the first set of experiments, we analyzed 2 different transduction systems, the rise in cAMP and the PKC signaling system, which both have been shown to be involved in the modulation of insulin secretion in normal islets and INS-1 cells (18)(19)(20)(21). We here demonstrate that an increase of intracellular cAMP levels, induced either by the activation of adenylate cyclase (forskolin) or by the inhibition of phosphodiesterase (IBMX), results in a strong upregulation of IA-2 gene expression in INS-1 cells.…”

“…To investigate the effect of the PKC signaling pathway, we studied IA-2 expression under conditions thought to activate (short-term treatment with phorbol ester) or deplete/inactivate PKC (phorbol ester treatment for >18 h) (20,27,28). In pancreatic ␤-cells, PKC is activated by glucose and Ca 2+ -mobilizing agents, resulting in a series of cellular events involved in the modulation of the insulin response (29).…”

A major role for protein kinase C in calcium‐activated exocytosis in permeabilised adrenal chromaffin cells

“…In intact cells, phorbol esters stimulated no secretion [7,17] or a low level of secretion [8,18]. In permeabilised chromaffin cells, phorbol esters potentiated calcium-activated secretion [7-91.…”

Potentiation of insulin secretion by phorbol esters is mediated by PKC-α and nPKC isoforms

“…However, very little is known about the role of PKC isoforms that are expressed in ␤-cells and specifically how and which PKC isoforms couple intracellular lipid signals to the stimulation of exocytosis (26,29). Downregulation of PKC activity, because of chronic stimulation (12-24 h) by phorbol esters, has been used widely as a tool to examine the involvement of PKC in insulin secretion (1,2,13,17,18,25,26,29,31). Some studies have treated PKC as a single enzyme activity and have correlated the reduction of total PKC activity to a given change in the secretory response of the ␤-cell (1,2,13,17,18,25).…”

“…Downregulation of PKC activity, because of chronic stimulation (12-24 h) by phorbol esters, has been used widely as a tool to examine the involvement of PKC in insulin secretion (1,2,13,17,18,25,26,29,31). Some studies have treated PKC as a single enzyme activity and have correlated the reduction of total PKC activity to a given change in the secretory response of the ␤-cell (1,2,13,17,18,25). More recent studies have demon-strated that chronic stimulation by phorbol ester resulted in the differential downregulation of PKC isoform mass (26,29).…”