Translation of diabetogenic viruses and secretory granule proteins in beta cells

Introduction: Glucose entry in pancreatic β-cells triggers insulin secretion and the biosynthesis of insulin secretory granules (SGs). Previously we showed that glucose and cAMP independently promote the binding of PTBP1 to mRNAs for proteins, thus enhancing their stability and translation. PTBP1 is also key for IRES-mediated translation of picornaviruses, including coxsackieviruses B (CVB). It has been suggested that CVB infection may trigger or precipitate type 1 diabetes. Since PTBP1 is required for translation of both picornaviruses and most T1D autoantigens we asked if and how CVB infection of insulin-expressing cells affects the expression of SG proteins.

Methods: Insulinoma cells and isolated mouse islets were infected with CVB5 or treated with various inhibitors of cap-dependent translation. Translation of SGs was investigated in pulse chase experiments. eIF4e was depleted with siRNAs.

Results: CVB5 infection blocked cap-dependent mRNA translation by cleaving eIFG4 and PABP. Surprisingly, this did not prevent glucose stimulation from enhancing translation of proinsulin, proICA512, proChromogranin A, proPC1/3 and proPC2. Using a furin inhibitor we showed that elevation of these proforms results from de novo synthesis and not from inhibition of their conversion into the corresponding mature SG proteins. Similar results were observed upon knockdown of eIF4e.

Conclusions: Our findings demonstrate the β-cells, like CVBs, exploit cap-independent translation to rapidly up-regulate the biosynthesis of SG proproteins in response to glucose. Remarkably, however, in CVB5-infected cells the total amount of the corresponding mature granule proteins as well as of SGs were dramatically reduced. As this depletion could not be attributed to increased granule exocytosis, which was inhibited instead, we propose that CVB infection of β-cells does not inhibit glucose-stimulated biosynthesis of SG proteins, but targets SG protein to degradation.