Sustained production of spliced X-box binding protein 1 (XBP1) induces pancreatic beta cell dysfunction and apoptosis.
OBJECTIVE
Pro-inflammatory cytokines involved in the pathogenesis of type 1 diabetes deplete endoplasmic reticulum (ER) Ca2+ stores, leading to ER-stress and beta cell apoptosis. However, the cytokine-induced ER-stress response in beta cells is atypical and characterised by induction of the pro-apoptotic PKR-like ER kinase (PERK)-C/EBP homologous protein (CHOP) branch of the unfolded protein response, but defective X-box binding protein 1 (XBP1) splicing and activating transcription factor 6 activation. The purpose of this study was to overexpress spliced/active Xbp1 (XBP1s) to increase beta cell resistance to cytokine-induced ER-stress and apoptosis.


METHODS
Xbp1s was overexpressed using adenoviruses and knocked down using small interference RNA in rat islet cells. In selected experiments, Xbp1 was also knocked down in FACS-purified rat beta cells and rat fibroblasts. Expression and production of XBP1s and key downstream genes and proteins was measured and beta cell function and viability were evaluated.


RESULTS
Adenoviral-mediated overproduction of Xbp1s resulted in increased XBP1 activity and induction of several XBP1s target genes. Surprisingly, XBP1s overexpression impaired glucose-stimulated insulin secretion and increased beta cell apoptosis, whereas it protected fibroblasts against cell death induced by ER-stress. mRNA expression of Pdx1 and Mafa was inhibited in cells overproducing XBP1s, leading to decreased insulin expression. XBP1s knockdown partially restored cytokine/ER-stress-driven insulin and Pdx1 inhibition but had no effect on cytokine-induced ER-stress and apoptosis.


CONCLUSIONS
XBP1 has a distinct inhibitory role in beta cell as compared with other cell types. Prolonged XBP1s production hampers beta cell function via inhibition of insulin, Pdx1 and Mafa expression, eventually leading to beta cell apoptosis.
Allagnat
F
F
Laboratory of Experimental Medicine, Université Libre de Bruxelles, Route de Lennik, 808, CP 618, 1070 Brussels, Belgium.
Christulia
F
F
Ortis
F
F
Pirot
P
P
Lortz
S
S
Lenzen
S
S
Eizirik
D L
DL
Cardozo
A K
AK
D000704:Analysis of Variance; D000818:Animals; D017209:Apoptosis / Q000187:drug effects / Q000502:physiology*; D015153:Blotting, Western; D002452:Cell Count; D002470:Cell Survival / Q000187:drug effects; D002478:Cells, Cultured; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D005455:Fluorescent Antibody Technique; D018398:Homeodomain Proteins / Q000235:genetics / Q000378:metabolism; D007211:Indoles / Q000494:pharmacology; D007328:Insulin / Q000235:genetics / Q000378:metabolism; D000078790:Insulin Secretion; D050417:Insulin-Secreting Cells / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D007371:Interferon-gamma / Q000494:pharmacology; D016209:Interleukin-8 / Q000494:pharmacology; D050977:Maf Transcription Factors / Q000235:genetics / Q000378:metabolism; D008297:Male; D034622:RNA Interference; D034741:RNA, Small Interfering; D051381:Rats; D017208:Rats, Wistar; D000071701:Regulatory Factor X Transcription Factors; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015534:Trans-Activators / Q000235:genetics / Q000378:metabolism; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D014162:Transfection; D000071717:X-Box Binding Protein 1
D004268:DNA-Binding Proteins; D018398:Homeodomain Proteins; D007211:Indoles; D007328:Insulin; D016209:Interleukin-8; D050977:Maf Transcription Factors; D034741:RNA, Small Interfering; D000071701:Regulatory Factor X Transcription Factors; D015534:Trans-Activators; D014157:Transcription Factors; D000071717:X-Box Binding Protein 1; C000605545:Xbp1 protein, rat; C084010:pancreatic and duodenal homeobox 1 protein; D007371:Interferon-gamma; C000543:cyclopiazonic acid