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Glucose Regulation of Insulin Gene Transcription and Pre-mRNA Processing in Human Islets

¹ Department of Medicine, University of Virginia, Charlottesville, Virginia
² Department of Surgery, University of Virginia, Charlottesville, Virginia
³ Human Islet Laboratory, Department of Surgery, University of Pennsylvania, Philadelphia, Pennsylvania

Address correspondence and reprint requests to Raghavendra G. Mirmira, University of Virginia Health System, 450 Ray C. Hunt Dr., Box 801407, Charlottesville, VA 22908. E-mail: mirmira{at}virginia.edu

Glucose is the primary regulator of insulin granule release from pancreatic islets. In rodent islets, the role of glucose in the acute regulation of insulin gene transcription has remained unclear, primarily because the abundance and long half-life of insulin mRNA confounds analysis of transcription by traditional methods that measure steady-state mRNA levels. To investigate the nature of glucose-regulated insulin gene transcription in human islets, we first quantitated the abundance and half-lives of insulin mRNA and pre-mRNAs after addition of actinomycin D (to stop transcription). Our results indicated that intron 1–and intron 2–containing pre-mRNAs were 150- and 2,000-fold less abundant, respectively, than mature mRNA. 5' intron 2–containing pre-mRNAs displayed half-lives of only 60 min, whereas all other transcripts displayed more extended lifetimes. In response to elevated glucose, pre-mRNA species increased within 60 min, whereas increases in mature mRNA did not occur until 48 h, suggesting that measurement of mature mRNA species does not accurately reflect the acute transcriptional response of the insulin gene to glucose. The acute increase in pre-mRNA species was preceded by a sixfold increase in histone H4 acetylation and a twofold increase in RNA polymerase II recruitment at the insulin promoter. Taken together, our data suggest that pre-mRNA species may be a more reliable reflection of acute changes to human insulin gene transcriptional rates and that glucose acutely enhances insulin transcription by a mechanism that enhances chromatin accessibility and leads to recruitment of basal transcriptional machinery.

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