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

Abstract

The endoplasmic reticulum (ER) is broadly distributed throughout the cytoplasm of pancreatic beta cells, and this is where all proinsulin is initially made. Healthy beta cells can synthesize 6000 proinsulin molecules per second. Ordinarily, nascent proinsulin entering the ER rapidly folds via the formation of three evolutionarily conserved disulfide bonds (B7-A7, B19-A20, and A6-A11). A modest amount of proinsulin misfolding, including both intramolecular disulfide mispairing and intermolecular disulfide-linked protein complexes, is a natural by-product of proinsulin biosynthesis, as is the case for many proteins. The steady-state level of misfolded proinsulin-a potential ER stressor-is linked to (1) production rate, (2) ER environment, (3) presence or absence of naturally occurring (mutational) defects in proinsulin, and (4) clearance of misfolded proinsulin molecules. Accumulation of misfolded proinsulin beyond a certain threshold begins to interfere with the normal intracellular transport of bystander proinsulin, leading to diminished insulin production and hyperglycemia, as well as exacerbating ER stress. This is most obvious in mutant INS gene-induced Diabetes of Youth (MIDY; an autosomal dominant disease) but also likely to occur in type 2 diabetes owing to dysregulation in proinsulin synthesis, ER folding environment, or clearance.

Irp2 regulates insulin production through iron-mediated Cdkal1-catalyzed tRNA modification

“…In mammals, proinsulin synthesis constitutes~30-50% of the total protein synthesis in the β cell 36 , and the ms 2 t 6 A modification in tRNA Lys UUU may be critical for efficient and accurate proinsulin translation. In agreement with studies in Cdkal1 β-cell KO mice 20 , proinsulin accumulation in Irp2-deficient INS-1 cells is associated with UPR activation of PERK-dependent eIF2α phosphorylation, which is known to reduce global translation [26][27][28] . Because β-cell mass was not reduced in 2.5-and 7.5-month-old Irp2 −/− mice, this suggests that phosphorylated eIF2α-mediated translational attenuation may allow Irp2-deficient β cells to adapt to cellular iron deficiency, thereby providing protection against UPR activation of apoptotic signaling pathways.…”

“…The UPR is activated in Irp2-depleted insulinoma cells. Accumulation of proinsulin in the endoplasmic reticulum (ER) lumen can lead to its misfolding or unfolding triggering ER stress and activating the unfolded protein response (UPR) 26 . Increased proinsulin levels in Irp2-deficient INS-1 cells suggested that the UPR might be activated in these cells.…”

Role of Proinsulin Self-Association in MutantINSgene-induced Diabetes of Youth

“…More than 60% of the identified mutant alleles have been predicted or experimentally confirmed to impair oxidative folding of proinsulin in the ER (1,22). Importantly, these misfolded MIDY mutants not only fail to exit from the ER, but also physically attack co-expressed proinsulin-WT, impairing the folding and ER export of co-expressed proinsulin-WT, decreasing insulin production, and initiating insulin-deficient diabetes (12,20,23).…”

Biosynthesis, structure, and folding of the insulin precursor protein

“…The above considerations culminate in a key question: how long is an untranslocated preproinsulin molecule allowed to spend in the cytosol before it is deemed unable to complete translocation and is targeted for degradation, and what mechanism initiates the switch from translocation to degradation? Once this is known, it should be possible to test the question of whether ER stress causes accumulation of undegraded preproinsulin molecules and/or contributes to β‐cell failure in type 1 or type 2 diabetes?…”