Context
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 and dysfunction in both forms of diabetes but the mechanisms remain unclear.
Objective
We sought to determine the effect of the diabetes associated cytokines on proinsulin folding, trafficking, secretion, and b-cell function.
Design
Human islets were treated with interleukin-1β and interferon-γ for forty-eight hours, followed by analysis of IL6, nitrite, proinsulin and insulin release, RNAseq, and unbiased profiling of the proinsulin interactome by Affinity Purification-Mass Spectrometry (AP-MS).
Results
Cytokine treatment induced secretion of IL6, nitrites, and insulin, as well as aberrant release of proinsulin. RNAseq showed that cytokines upregulated genes involved in ER stress and consistent with this, AP-MS revealed cytokine induced proinsulin binding to multiple ER chaperones and oxidoreductases. Moreover, increased binding to the chaperone BiP was required to maintain proper 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). Finally, cytokines induced proinsulin interactions with a cluster of microtubule motor proteins and chemical destabilization of microtubules with Nocodazole exacerbated cytokine induced proinsulin secretion.
Conclusion
Together, the data shed new light on mechanisms by which diabetes associated cytokines dysregulate β-cell function. For the first time we show that even short term exposure to an inflammatory environment reshapes proinsulin interactions with critical chaperones and regulators of the secretory pathway.