Chaperone-Driven Degradation of a Misfolded Proinsulin Mutant in Parallel With Restoration of Wild-Type Insulin Secretion

Abstract

In heterozygous patients with a diabetic syndrome called mutant INS gene–induced diabetes of youth (MIDY), there is decreased insulin secretion when mutant proinsulin expression prevents wild-type (WT) proinsulin from exiting the endoplasmic reticulum (ER), which is essential for insulin production. Our previous results revealed that mutant Akita proinsulin is triaged by ER-associated degradation (ERAD). We now find that the ER chaperone Grp170 participates in the degradation process by shifting Akita proinsulin from high–molecular weight (MW) complexes toward smaller oligomeric species that are competent to undergo ERAD. Strikingly, overexpressing Grp170 also liberates WT proinsulin, which is no longer trapped in these high-MW complexes, enhancing ERAD of Akita proinsulin and restoring WT insulin secretion. Our data reveal that Grp170 participates in preparing mutant proinsulin for degradation while enabling WT proinsulin escape from the ER. In principle, selective destruction of mutant proinsulin offers a rational approach to rectify the insulin secretion problem in MIDY.

Misfolded proinsulin in the endoplasmic reticulum during development of beta cell failure in diabetes

“…46 However, additional work is still needed to establish a direct link between proinsulin misfolding and pancreatic beta cell failure in human type 2 diabetes and to develop therapies to limit accumulation of misfolded proinsulin. 66,75 …”

“…46 However, additional work is still needed to establish a direct link between proinsulin misfolding and pancreatic beta cell failure in human type 2 diabetes and to develop therapies to limit accumulation of misfolded proinsulin. 66,75 In this review, we highlighted features that promote proinsulin misfolding, including its increased synthesis, altered ER environment, the presence of proinsulin structural defects, and deficiency of clearance of misfolded proinsulin. We propose that these processes are fertile areas for studies of molecular and cellular mechanisms that may predispose to beta cell failure during the onset and progression of diabetes mellitus.…”

“…72,73 Moreover, rather than playing a direct role in proinsulin folding, PDI may participate in ERAD of proinsulin 74 in conjunction with other ER luminal components. 75 As discussed further below, this could have significant indirect consequences for the folding of newly synthesized proinsulin in the ER of pancreatic beta cells.…”

“…74 Additional work has revealed that the ER chaperone GRP170 also acts in the upstream ERAD pathway of misfolded proinsulin in such a way as to decrease the abundance of high-molecular-weight proinsulin aggregates, promoting smaller proinsulin complexes that have better competence for retrotranslocation and proteasomal disposal. 75 Interestingly, GRP170 is particularly highly expressed in islet beta cells, and its expression seems to parallel that of proinsulin. 137 Most remarkable, in cells expressing both wild-type proinsulin and the proinsulin-C(A7)Y mutant, GRP170 overexpression assisted in the selective degradation of the mutant, liberating the wild-type proinsulin for export from the ER and production of native insulin.…”

“…137 Most remarkable, in cells expressing both wild-type proinsulin and the proinsulin-C(A7)Y mutant, GRP170 overexpression assisted in the selective degradation of the mutant, liberating the wild-type proinsulin for export from the ER and production of native insulin. 75 This is of obvious potential interest in limiting proinsulin aggregation in order to improve beta cell function for the prevention of diabetes.…”