ICER-1γ overexpression drives palmitate-mediated connexin36 down-regulation in insulin-secreting cells

Channels formed by the gap junction protein connexin36 (Cx36) contribute to the proper control of insulin secretion. We investigated the impact of chronic hyperlipidemia on Cx36 expression in pancreatic beta-cells. Prolonged exposure to the saturated free fatty acid palmitate reduced the expression of Cx36 in several insulin-secreting cell lines and isolated mouse islets. The effect of palmitate was fully blocked upon protein kinase A (PKA) inhibition by H89 and (Rp)-cAMP, indicating that the cAMP/PKA pathway is involved in the control of Cx36 expression. Palmitate treatment led to overexpression of the inducible cAMP early repressor (ICER-1 gamma), which bound to a functional cAMP-response element located in the promoter of the CX36 gene. Inhibition of ICER-1 gamma overexpression prevented the Cx36 decrease, as well as the palmitate-induced beta-cell secretory dysfunction. Finally, freshly isolated islets from mice undergoing a long term high fat diet expressed reduced Cx36 levels and increased ICER-1 gamma levels. Taken together, these data demonstrate that chronic exposure to palmitate inhibits the Cx36 expression through PKA-mediated ICER-1 gamma overexpression. This Cx36 down-regulation may contribute to the reduced glucose sensitivity and altered insulin secretion observed during the pre-diabetic stage and in the metabolic syndrome.