Chronic Palmitate But Not Oleate Exposure Induces Endoplasmic Reticulum Stress, Which May Contribute to INS-1 Pancreatic β-Cell Apoptosis

Abstract

Chronic free fatty acid (FFA) exposure induces pancreatic beta-cell death, which may contribute to the development of type 2 diabetes. The mechanisms involved in FFA-induced cell death are not completely understood. Here we have investigated the effect of FFA on endoplasmic reticulum (ER) stress pathways in INS-1 pancreatic beta-cells. INS-1 cells exposed to palmitate for 16-24 h under serum-free conditions showed marked apoptosis and increased protein levels of phosphorylated eukaryotic translation initiation factor 2alpha (eIF2alpha), activating transcription factor 4 (ATF4), X box-binding protein 1 (XBP-1), and C/EBP homologous transcription factor (CHOP) compared with control cells. The CHOP transcription factor has been implicated in mediating ER stress-induced apoptosis. Unexpectedly, the levels of the ER chaperone proteins Grp78/BiP and PDI were not affected by palmitate treatment, suggesting that the cell protective aspects of the unfolded protein response (UPR) are not up-regulated by palmitate. Palmitate-treated cells had markedly altered distribution of ER chaperones and altered ER morphology, suggesting that accumulation of misfolded proteins might trigger the ER stress response. In contrast, oleate treatment did not significantly induce the UPR pathways, nor was it as detrimental to INS-1 beta-cells. The results suggest that activation of the UPR may significantly contribute to palmitate- but not oleate-induced pancreatic beta-cell death.

A chemical chaperone 4-PBA ameliorates palmitate-induced inhibition of glucose-stimulated insulin secretion (GSIS)

“…Lethal doses of palmitate enhanced phospho-eIF2a and subsequently induced ATF4 and CHOP in beta cells [11]. Another report demonstrated that palmitate could activate IRE and ATF6 as well as upregulate the ER chaperone machinery including BiP [30].…”

“…On the other hand, down-regulation of the IRS/PI3 kinase/Akt signaling pathway was reported to be critical for induction of FFA-induced lipotoxicity [10]. Recently, endoplasmic reticulum (ER) stress was also suggested to be a mediator of FFA-induced beta cell death [11,12].…”

The endoplasmic reticulum in pancreatic beta cells of type 2 diabetes patients

“…When these mechanisms do not remedy the stress situation, apoptosis is initiated. Since a number of articles have recently reported that ER stress may be implicated in beta cell dysfunction and death in diabetes [10][11][12][13][14][15][16][19][20][21][22][23][24][25][26], in the present study we evaluated features of ER function/dysfunction in human beta cells from type 2 diabetic organ donors. First, we confirmed that diabetic beta cells have functional and survival defects, as previously shown by us and others [27,28,[35][36][37][38].…”

“…The mechanisms causing the loss of beta cell function and mass in type 2 diabetes are still debated [8][9][10]. It has been recently proposed that alterations of endoplasmic reticulum (ER) function in the beta cell contribute to cell dysfunction and death [10][11][12][13][14][15][16]. The ER is central to several important cellular functions, including post-translational modifications, folding and assembly of newly synthesised secretory proteins, and calcium storage [14][15][16].…”

“…In MIN6 cells, a mouse beta cell-derived cell line, nitric oxide depletes ER calcium stores and leads to apoptosis through induction of the ER stress associated apoptosis factor C/EBP homologous protein (CHOP) [18]. In addition, NEFAs cause induction of specific ER stress markers such as X-box binding protein 1 (XBP-1, also known as XBP1), immunoglobulin heavy chain binding protein (BiP, also known as heat shock 70 kDa protein 5 [glucose-regulated protein, 78 kDa] [HSPA5]), activating transcription factor 4 and 6 (ATF4 and ATF6), and CHOP (also known as damage-inducible transcript 3 [DDIT3]) in INS-1E cells, while cytokines cause a decrease in ER calcium and severe ER stress in INS-1E and primary beta cells [10,11,13,[19][20][21]. In vivo, a mutation in eukaryotic translation initiation factor 2 alpha kinase 3 (EIF2AK3, also known as PERK) impairs the development of a normal UPR and causes neonatal diabetes in Wolcott-Rallison syndrome [22].…”