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Regulation of Insulin Gene Transcription by a Ca²⁺-Responsive Pathway Involving Calcineurin and Nuclear Factor of Activated T Cells

Department of Molecular Biology and Immunology, University of North Texas Health Science Center at Fort Worth, Fort Worth, Texas 76107-2699

Address all correspondence and requests for reprints to: Dr. Richard A. Easom, Department of Molecular Biology and Immunology, University of North Texas Health Science Center at Fort Worth, 3500 Camp Bowie Boulevard, Fort Worth, Texas 76107-2699. E-mail: reasom{at}hsc.unt.edu

Immunosuppressants such as FK506 (tacrolimus), the primary cellular target of which is calcineurin, decrease ß-cell insulin content and preproinsulin mRNA expression. This study offers an explanation for this effect by establishing that calcineurin is an important regulator of insulin gene expression through the activation of a transcription factor, nuclear factor of activated T cells. Three putative nuclear factor of activated T cells binding sites were located within the proximal region of the rat insulin I gene promoter (-410 to +1 bp). Expression of nuclear factor of activated T cells in both clonal (INS-1) and primary (islet) ß-cells was confirmed by immunoblot and immunocytochemical analyses. Moreover, nuclear factor of activated T cells DNA-binding activity was detected in INS-1 and islet nuclear extracts by EMSAs. Activation of the insulin gene promoter by glucose or elevated extracellular K⁺ (to depolarize the ß-cell) was totally prevented by FK506 (5–10 µM). K⁺-induced promoter activation was suppressed (>65%) by a 2-bp mutation of a single nuclear factor of activated T cells binding site in -410 rInsI. Both stimulants also activated a minimal promoter-reporter construct containing tandem nuclear factor of activated T cells consensus sequences. The effects of FK506 on K⁺-induced nuclear factor of activated T cells reporter or insulin gene promoter activity were not mimicked by rapamycin, indicating specificity toward calcineurin. These findings suggest that the activation of calcineurin by ß-cell secretagogues that elevate cytosolic Ca²⁺ plays a fundamental role in maintenance of insulin gene expression via the activation of nuclear factor of activated T cells.

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