CHOP Contributes to, But Is Not the Only Mediator of, IAPP Induced β-Cell Apoptosis

Abstract

The islet in type 2 diabetes is characterized by β-cell loss, increased β-cell apoptosis, and islet amyloid derived from islet amyloid polypeptide (IAPP). When protein misfolding protective mechanisms are overcome, human IAPP (h-IAPP) forms membrane permeant toxic oligomers that induce β-cell dysfunction and apoptosis. In humans with type 2 diabetes (T2D) and mice transgenic for h-IAPP, endoplasmic reticulum (ER) stress has been inferred from nuclear translocation of CCAAT/enhancer-binding protein homologous protein (CHOP), an established mediator of ER stress. To establish whether h-IAPP toxicity is mediated by ER stress, we evaluated diabetes onset and β-cell mass in h-IAPP transgenic (h-TG) mice with and without deletion of CHOP in comparison with wild-type controls. Diabetes was delayed in h-TG CHOP(-/-) mice, with relatively preserved β-cell mass and decreased β-cell apoptosis. Deletion of CHOP attenuates dysfunction of the autophagy/lysosomal pathway in β-cells of h-TG mice, uncovering a role for CHOP in mediating h-IAPP-induced dysfunction of autophagy. As deletion of CHOP delayed but did not prevent h-IAPP-induced β-cell loss and diabetes, we examined CHOP-independent stress pathways. JNK, a target of the IRE-1pTRAF2 complex, and the Bcl-2 family proapoptotic mediator BIM, a target of ATF4, were comparably activated by h-IAPP expression in the presence and absence of CHOP. Therefore, although these studies affirm that CHOP is a mediator of h-IAPP-induced ER stress, it is not the only one. Therefore, suppression of CHOP alone is unlikely to be a durable therapeutic strategy to protect against h-IAPP toxicity because multiple stress pathways are activated.

β Cell–specific increased expression of calpastatin prevents diabetes induced by islet amyloid polypeptide toxicity

“…β Cell endoplasmic reticulum stress, revealed by an increase in CHOP nuclear translocation, was confirmed in hTG mice (34) but markedly suppressed by calpastatin overexpression (1.13% ± 0.24% vs. 0.27% ± 0.13% in hTG and hTG:hCAST mice, respectively, P < 0.01; Figure 4, B and C). Moreover, BCL-2 homology domain 3-only (BH3-only) protein Bim, shown to be central mediator of the endoplasmic reticulum stress-induced apoptosis, was, as expected, increased in islets from hTG mice relative to control mice (35), but this increase was prevented in hTG:hCAST mice (2.54 ± 0.37 vs. 1.45 ± 0.19 in hTG and hTG:hCAST mice, respectively, P < 0.01; Figure 4D). …”

“…The proapoptotic BH3-only Bcl-2 family member Bim that is increased by hIAPP toxicity not only contributes to mitochondrial membrane disruption (48), but may also inhibit autophagy (49). Of interest, while Ddit3 (CHOP) deletion did not attenuate the hIAPP-induced Bim increase (35), suppression of calpain hyperactivation did, implying that hIAPP-induced Bim augmentation is downstream of hyperactive calpain. The fidelity and function of the mitochondrial network is particularly critical in pancreatic β cells, not only for cell viability, but also their function.…”

“…35. The entire pancreatic section was imaged at ×40 magnification (×4 objective) using an Olympus IX70 inverted microscope.…”

“…have been employed that partially ameliorate toxicity of hIAPP, including deletion of CHOP (35), these interventions have generally been, at best, partially effective. Based on genetic studies, the most compelling target that has emerged to date is the autophagy/lysosomal pathway (21-23).…”

The β-cell assassin: IAPP cytotoxicity

“…CHOP activated proteins promote apoptosis via inhibition of protective Bcl-2 but the balance of pro-and anti-apoptotic proteins is delicate, can be reversed quickly and can be modulated by external stimuli. The involvement of CHOP in hIAPP-induced cell death and onset of diabetes has been demonstrated in transgenic mice overexpressing hIAPP (Huang et al 2007b, Gurlo et al 2016). …”