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Insulin-dependent regulation of Glut4 gene expression in ventricular cardiomyocytes: evidence for a direct effect on Glut4 transcription.

The present study examined the effect of insulin on Glut4 transcription in isolated ventricular cardiomyocytes. Upon exposure to the hormone (2 x 10(-7) M) the cellular Glut4 mRNA content increased to 224 +/- 46% of control within 3.5 hours. Nuclear run-on analysis indicated a parallel increase of transcription of the Glut4 gene to 208 +/- 49% of control. Direct incubation of cardiac nuclei with insulin resulted in a comparably significant increase of Glut4 transcription. Cross-linkage experiments with 125I-labelled insulin demonstrated the absence of soluble nuclear insulin receptors, which were readily detected in plasma and microsomal membranes. These findings suggest that expression of the cardiac Glut4 gene is subject to regulation by insulin at the transcriptional level, a process possibly involving nuclear association of the hormone.