Inflammatory cytokines rewire the proinsulin interaction network in human islets

Abstract

Aberrant biosynthesis and secretion of the insulin precursor proinsulin occurs in both Type I and Type II diabetes (T1D, T2D). Inflammatory cytokines are implicated in pancreatic islet stress in both forms of diabetes but the mechanisms remain unclear. Here we examined how the diabetes associated cytokines interleukin-1{beta} and interferon-{gamma} alter proinsulin interactions with proteins that regulate its folding, trafficking, and secretion. Human islets treated with cytokines exhibited secretion of proinsulin, IL6 and nitrite, as well as evidence of endoplasmic reticulum (ER) stress. Unbiased proinsulin Affinity Purification-Mass Spectrometry revealed a proinsulin interactome reshaped by cytokines relative to controls. Cytokine treatment increased proinsulin binding to multiple ER chaperones and oxidoreductases, including the major ER chaperone BiP. Moreover, increased BiP binding was an adaptive response required to maintain proinsulin folding in the inflammatory environment. Cytokines also regulated novel interactions between proinsulin and T1D and T2D GWAS candidate proteins not previously known to interact with proinsulin (e.g., Ataxin-2) and these GWAS proteins formed a tight network with each other. Finally, cytokines induced proinsulin interactions with a cluster of microtubule motor proteins. Consistent with a role for these proteins in proinsulin trafficking and release, chemical destabilization of microtubules with Nocodazole exacerbated cytokine induced proinsulin secretion. Together, the data quantitatively map the proinsulin interactome rewired by cytokines, shedding new light on how human proinsulin biosynthesis is dysregulated by an inflammatory environment.