Hepatic Nuclear Factor 1α (HNF1α) Dysfunction Down-regulates X-box-binding Protein 1 (XBP1) and Sensitizes β-Cells to Endoplasmic Reticulum Stress

Abstract

Correct endoplasmic reticulum (ER) function is critical for the health of secretory cells, such as the pancreatic ␤-cell, and ER stress is often a contributory factor to ␤-cell death in type 2 diabetes. We have used an insulin-secreting cell line with inducible expression of dominant negative (DN) HNF1␣, a transcription factor vital for correct ␤-cell development and function, to show that HNF1␣ is required for Xbp1 transcription and maintenance of the normal ER stress response. DN HNF1␣ expression sensitizes the ␤-cell to ER stress by directly downregulating Xbp1 transcription, whereas Atf6 is unaffected. Furthermore, DN HNF1␣ alters calcium homeostasis, resulting in elevated cytoplasmic calcium and increased store-operated calcium entry, whereas mitochondrial calcium uptake is normal. Loss of function of XBP1 is toxic to the ␤-cell and decreases production of the ER chaperone BiP, even in the absence of ER stress. DN HNF1␣-induced sensitivity to cyclopiazonic acid can be partially rescued with the chemical chaperone tauroursodeoxycholate. Rat insulin 2 promoter-DN HNF1␣ mouse islets express lower levels of BiP mRNA, synthesize less insulin, and are sensitized to ER stress relative to matched control mouse islets, suggesting that this mechanism is also operating in vivo. Maturity onset diabetes of the young 3 (MODY3)2 is caused by heterozygous mutations in the transcription factor Hnf1␣ (1), which lead to a primary defect in ␤-cell function without insulin resistance. Almost 200 MODY3 mutations in Hnf1␣ have been characterized, with missense mutations occurring most frequently, whereas the appearance of frameshift or nonsense mutations is also observed. MODY3 mutations often act through haploinsufficiency (2), but they can also act through a dominant negative mechanism, as is the case for the (frequently occurring) frameshift mutation P291 fsinsC (3, 4). HNF1␣ was originally discovered as a regulator of hepatic gene expression but has since been found to be expressed in the pancreas (5) and involved in transcription of ␤-cell-specific genes (6, 7). However, genome-wide studies have also demonstrated that its effects on gene expression in the liver and pancreatic islet are strikingly different (8). To characterize the role of HNF1␣ in ␤-cells, a dominant negative (DN) mutant has been used, which carries a substitution of 83 amino acids in its DNA-binding domain, whereas its dimerization domain remains intact (Fig. 1A). This artificial DN mutant forms non-functional dimers with the endogenous protein, preventing it from activating its target genes (9). This has led to the discovery that in insulinsecreting INS-1 cells, suppression of HNF1␣ activity following induction of the DN gene impairs metabolism-secretion coupling (7, 10) and reduces insulin secretion in response to glucose, as well as reducing transcription of the insulin I gene (11). Expression of the transgene also leads to apoptosis in these cells after 48 h of induction (12).During insulin biosynthesis, the nascent chain is translocated into the endop...

Alpha‐synuclein aggregates activate calcium pump SERCA leading to calcium dysregulation

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Tauroursodeoxycholate, a Chemical Chaperone, Prevents Palmitate-induced Apoptosis in Pancreatic β-cells by Reducing ER Stress

“…TUDCA could inhibit ER stress induced by thapsigargin, and restore the decreased glucose stimulation index of islets ( Lee et al, 2010 ). TUDCA could also partially rescue dominant negative HNF1α-induced ER stress in β-cells ( Kirkpatrick et al, 2011 ). Most lately, it was reported that TUDCA prevented high glucose-induced β-cells dysfunction and reduced ER stress markers ( Tang et al, 2012 ).…”

Growth hormone activates X-box binding protein 1 in a sexually dimorphic manner through the extracellular signal-regulated protein kinase and CCAAT/enhancer-binding protein β pathway in rat liver

“…It has been reported that transcription factors other than ATF6 induce Xbp1 expression. For example, HNF1α and HNF4α were found to induce Xbp1 expression in pancreatic β-cells [38,39].…”