We studied the role of protein degradation pathways in the regulation of insulin production and secretion and hypothesised that autophagy regulates proinsulin degradation, thereby modulating beta cell function.

Proinsulin localisation in autophagosomes was demonstrated by confocal and electron microscopy. Autophagy was inhibited by knockdown of autophagy-related (ATG) proteins and using the H⁺-ATPase inhibitor bafilomycin-A1. Proinsulin and insulin content and secretion were assessed in static incubations by ELISA and RIA.

Confocal and electron microscopy showed proinsulin localised in autophagosomes and lysosomes. Beta-Atg7 ^−/− mice had proinsulin-containing sequestosome 1 (p62 [also known as SQSTM1])⁺ aggregates in beta cells, indicating proinsulin is regulated by autophagy in vivo. Short-term bafilomycin-A1 treatment and ATG5/7 knockdown increased steady-state proinsulin and hormone precursor chromogranin A content. ATG5/7 knockdown also increased glucose- and non-fuel-stimulated insulin secretion. Finally, mutated forms of proinsulin that are irreparably misfolded and trapped in the endoplasmic reticulum are more resistant to degradation by autophagy.

In the beta cell, transport-competent secretory peptide precursors, including proinsulin, are regulated by autophagy, whereas efficient clearance of transport-incompetent mutated forms of proinsulin by alternative degradative pathways may be necessary to avoid beta cell proteotoxicity. Reduction of autophagic degradation of proinsulin increases its residency in the secretory pathway, followed by enhanced secretion in response to stimuli.