RESUMEN
The Jagn1 protein was indentified in a SILAC proteomic screen of proteins that are increased in insulinoma cells expressing a folding-deficient proinsulin. Jagn1 mRNA was detected in primary rodent islets and in insulinoma cell lines and the levels were increased in response to ER stress. The function of Jagn1 was assessed in insulinoma cells by both knock-down and overexpression approaches. Knock-down of Jagn1 caused an increase in glucose-stimulated insulin secretion resulting from an increase in proinsulin biosynthesis. In contrast, overexpression of Jagn1 in insulinoma cells resulted in reduced cellular proinsulin and insulin levels. Our results identify a novel role for Jagn1 in regulating proinsulin biosynthesis in pancreatic ß-cells. Under ER stress conditions Jagn1 is induced which might contribute to reducing proinsulin biosynthesis, in part by helping to relieve the protein folding load in the ER in an effort to restore ER homeostasis.
Asunto(s)
Estrés del Retículo Endoplásmico , Proteínas de la Membrana/metabolismo , Proinsulina/biosíntesis , Animales , Técnicas de Silenciamiento del Gen , Células HeLa , Humanos , Insulina/metabolismo , Secreción de Insulina , Insulinoma/metabolismo , Islotes Pancreáticos/metabolismo , Ratones , Proteínas Mutantes/metabolismo , Proteómica , RatasRESUMEN
BACKGROUND: The Akita mutation (C96Y) in the insulin gene results in early onset diabetes in both humans and mice. Expression of mutant proinsulin (C96Y) causes endoplasmic reticulum (ER) stress in pancreatic ß-cells and consequently the cell activates the unfolded protein response (UPR). Since the proinsulin is terminally misfolded ER stress is irremediable and chronic activation of the UPR eventually activates apoptosis in some cells. Here we analyzed the IRE1-dependent activation of genes in response to misfolded proinsulin production in an inducible mutant proinsulin (C96Y) insulinoma cell line. RESULTS: The IRE1 endoribonuclease inhibitors 4µ8c and MKC-3946 prevented the splicing of the XBP1 mRNA in response to ER stress caused by mutant proinsulin production. Microarray expression analysis and qPCR validation of select genes revealed that maximal upregulation of many UPR genes in response to mutant proinsulin production required IRE1, although most were still increased above control. Interestingly, neither degradation of misfolded proinsulin via ER-associated degradation (ERAD), nor apoptosis induced by prolonged misfolded proinsulin expression were affected by inhibiting IRE1. CONCLUSIONS: Although maximal induction of most UPR genes requires IRE1, inhibition of IRE1 does not affect ERAD of misfolded proinsulin or predispose pancreatic ß-cells expressing misfolded proinsulin to chronic ER stress-induced apoptosis.
Asunto(s)
Apoptosis/efectos de los fármacos , Células Secretoras de Insulina/citología , Proteínas de la Membrana/antagonistas & inhibidores , Proinsulina/genética , Inhibidores de Proteínas Quinasas/farmacología , Proteínas Serina-Treonina Quinasas/antagonistas & inhibidores , Respuesta de Proteína Desplegada/efectos de los fármacos , Animales , Línea Celular Tumoral , Regulación de la Expresión Génica/efectos de los fármacos , Células Secretoras de Insulina/efectos de los fármacos , Células Secretoras de Insulina/metabolismo , Proteínas de la Membrana/metabolismo , Mutación Puntual , Proinsulina/metabolismo , Proteínas Serina-Treonina Quinasas/metabolismo , Proteolisis/efectos de los fármacos , RatasRESUMEN
RNA-binding proteins (RBPs) have essential roles in post-transcriptional regulation of gene expression. They bind sequence elements in specific mRNAs and control their splicing, transport, localization, translation, and stability. A complete understanding of RBP function requires identification of the target RNAs that an RBP regulates, the mechanisms by which the RBP regulates these targets, and the biological consequences for the cell in which these transactions occur. Antibodies are key tools in such studies: first, mRNA targets of RBPs can be identified by co-immunoprecipitation of RBPs with their associated RNAs followed by microarray analysis or sequencing; second, partner proteins can be identified by immunoprecipitation of the RBP followed by mass spectrometry; third, the mechanisms and functions of RBPs can be inferred from loss-of-function studies employing antibodies that block RBP-RNA interactions. One potentially powerful approach to making antibodies for such studies is the generation of synthetic antibodies using phage display, which involves in vitro selection using a human-designed antibody library to generate antibodies that recognize a target protein. Using two well-characterized Drosophila RNA-binding proteins, Staufen and Smaug, for proof-of-principle, we demonstrate that synthetic antibodies can be generated and used either to perform RNA-coimmunoprecipitations (RIPs) to identify RBP-bound mRNAs, or to block RBP-RNA interactions. Given that synthetic antibody selection protocols are amenable to high-throughput antibody production, these results demonstrate that synthetic antibodies can be powerful tools for genome-wide studies of RBP function.