Glucose induces early growth response gene (Egr-1) expression in pancreatic beta cells

A copy deoxyribonucleic acid (cDNA) clone of the immediate early growth response gene, egr-1 (Krox-24, Zif268, NGFI-1), was isolated through subtractive hybridization screening to identify glucose-induced genes in pancreatic beta cells. Glucose rapidly and transiently induced egr-1 mRNA in the SV40-transformed murine beta-cell line, MIN6. Glucose also increased egr-1 mRNA expression in INS-1, betaTC3 and RINm5F beta-cell lines, although with different kinetics. Expression of the 82 kDa Egr-1 protein was induced both in MIN6 cells stimulated with glucose in vitro and in primary rat islet cells stimulated in vivo or in vitro. This response is unique to beta cells since glucose did not affect egr-1 expression in NIH-3T3 fibroblasts or glucose-sensitive hepatocytes. In beta cells egr-1 induction is specifically associated with insulin secretion, as it was not observed after stimulation with serum or insulin but was elicited by insulin secretagogues, including membrane depolarizing agents and cAMP agonists. Moreover, induction of egr-1 by glucose was inhibited by EDTA, indicating dependence on influx of extracellular Ca2+. Other immediate early response genes, c-fos and junB, were also induced following glucose stimulation with kinetics similar to egr-1, whereas c-jun and junD expression were not affected. Since the zinc-finger protein encoded by egr-1 is highly homologous to transcription factors that control expression of glucose-regulated genes in yeast, Egr-1 could mediate delayed adaptive responses of beta cells to sustained glucose stimulation through transcriptional regulation.