Human-IAPP disrupts the autophagy/lysosomal pathway in pancreatic β-cells: protective role of p62-positive cytoplasmic inclusions

Abstract

In type II diabetes (T2DM), there is a deficit in b-cells, increased b-cell apoptosis and formation of intracellular membranepermeant oligomers of islet amyloid polypeptide (IAPP). Human-IAPP (h-IAPP) is an amyloidogenic protein co-expressed with insulin by b-cells. IAPP expression is increased with obesity, the major risk factor for T2DM. In this study we report that increased expression of human-IAPP led to impaired autophagy, due at least in part to the disruption of lysosome-dependant degradation. This action of IAPP to alter lysosomal clearance in vivo depends on its propensity to form toxic oligomers and is independent of the confounding effect of hyperglycemia. We report that the scaffold protein p62 that delivers polyubiquitinated proteins to autophagy may have a protective role against human-IAPP-induced apoptosis, apparently by sequestrating protein targets for degradation. Finally, we found that inhibition of lysosomal degradation increases vulnerability of b-cells to h-IAPPinduced toxicity and, conversely, stimulation of autophagy protects b-cells from h-IAPP-induced apoptosis. Collectively, these data imply an important role for the p62/autophagy/lysosomal degradation system in protection against toxic oligomer-induced apoptosis. Cell Death and Differentiation (2011) 18, 415-426; doi:10.1038/cdd.2010.111; published online 3 September 2010In type II diabetes (T2DM), hyperglycemia is because of inadequate insulin secretion in response to relative insulin resistance. The islet in T2DM is characterized by a deficit in b-cells, 1,2 increased b-cell apoptosis attributable to endoplasmic reticulum (ER) stress 3,4 and intracellular b-cell toxic aggregates of the amyloidogenic protein islet amyloid polypeptide (IAPP), 5 the expression of which increases with insulin resistance. 6 IAPP is co-expressed and co-secreted with insulin by b-cells 7 with its best-characterized physiological role being to inhibit insulin secretion through a direct paracrine effect on b-cells. 8 IAPP has the propensity to form amyloid fibrils in species at risk of T2DM (humans, non-human primates and cats). 9 In contrast to human-IAPP (h-IAPP), the rodent form of IAPP (r-IAPP) is non-amyloidogenic. Transgenic expression of h-IAPP in b-cells of rodents at rates present in insulin resistance leads to the development of diabetes because of ER stress-induced b-cell apoptosis with formation of membrane-damaging intracellular IAPP oligomers comparable to those present in humans with T2DM. 3,5,[10][11][12] Conserved mechanisms protect long-lived cells with a high protein synthetic burden (such as b-cells) from accumulation of intracellular protein aggregates and the adverse consequences termed proteotoxicity. 9 A quality control system in the ER recognizes misfolded proteins and targets them for degradation by the ubiquitin/proteasome system. 13 A second pathway of protein degradation, autophagy, also implies a role for ubiquitin in removal of misfolded proteins. 14 Macroautophagy (hereafter referred to as autophagy) permits selective autodi...

High-fat diet increases autophagic flux in pancreatic beta cells in vivo and ex vivo in mice

“…LC3II has been previously analysed using islets from various models ex vivo, but in the absence of the additional incubation with chloroquine, definitive conclusions about flux are problematic. Our results also confirm prior findings that, although p62/SQSTM1 might be a useful marker of autophagic flux in some situations, in the context of beta cell lipotoxicity its interpretation is confounded by a countermanding stimulation of p62/Sqstm1 gene expression [15].…”

Amyloidogenic peptide oligomer accumulation in autophagy-deficient β cells induces diabetes

“…Lysosomal proteolysis and autophagy have been reported to modulate hIAPP-induced cellular injury in vitro (34,35). However, the role of autophagy in the development of hIAPP-induced diabetes in vivo has not been directly demonstrated.…”

Autophagy in adipose tissue and the beta cell: implications for obesity and diabetes

“…Several studies have shown that NEFA inhibit the expression of lysosome enzymes [78], lysosomal acidification and autophagic flux [81]. Similarly, the amyloidogenic peptide hIAPP has been found to inhibit autophagy [82]. This might lead to accumulation of toxic hIAPP oligomers that exacerbate beta cell dysfunction and apoptosis.…”