Functional And Mechanistic Studies Of RILP In Regulating Insulin Secretion

Insulin secretion is tightly regulated at multiple levels.Excess insulin will be degraded to maintain insulin homeostasis,but the mechanisms remain elusive.RILP(Rab7-interacting lysosomal protein)regulates the endocytic trafficking and the morphogenesis of the late endosomes and lysosomes.RILP interacting with Rab protein regulates the secretion of melanin,hepatitis c virus,but its role in the insulin secretion pathway has not been investigated.In this study,we found over-expression of RILP inhibited insulin secretion in both the ? cell lines and isolated islets.Consequently,the adenovirus-mediated expression of RILP in islets suppressed the ability to recover the glucose homeostasis in the type 1 diabetes mice.Of physiological relevance is that RILP expression was up-regulated in the mouse pancreas suffering from diabetes.Over-expression of RILP induced insulin granules clustering at the peri-nuclear region,and a decreased number of proinsulin-containing granules.Further examination demonstrated that RILP significantly promoted proinsulin degradation and decreased insulin secretion.Conversely,RILP depletion sustained proinsulin and increased insulin secretion.Furthermore,the proinsulin degradation induced by the expression of RILP was inhibited by lysosomal inhibitors,indicating that RILP regulated the degradation of proinsulin through the lysosomal pathway.Moreover,knockdown of Rab7 or blocking the interaction of RILP with Rab7 also inhibited the proinsulin degradation caused by overexpression of RILP,indicating that the degradation process is dependent on Rab7.Also,we found that Rab26,associating with insulin granules,interacts with RILP,and knockdown of Rab26 will reverse the decrease of insulin secretion caused by RILP.This study reveals that RILP may cluster insulin granule perinuclear through the interaction with insulin granule-associated Rab26 to restrict insulin secretion,on the other hand,RILP-Rab7 complex drive the accumulated insulin granules to the lysosomal degradation,to inhibit insulin secretion.Therefore,RILP may be as a new key regulatory factor mediating immature insulin granules to lysosomal degradation.This study provides a new regulatory mechanism and pathway for insulin biogenesis and functional balance.