Regulation of insulin gene transcription by a Ca2+- responsive pathway involving calcineurin and nuclear factor of activated T cells

Immunosuppressants such as FK506 (tacrolimus), the primary cellular target of which is calcineurin, decrease beta -cell insulin content and preproinsu lin mRNA expression. This study offers an explanation for this effect by es tablishing that calcineurin is an important regulator of insulin gene expre ssion through the activation of a transcription factor, nuclear factor of a ctivated T cells. Three putative nuclear factor of activated T cells bindin g sites were located within the proximal region of the rat insulin I gene p romoter (-410 to +1 bp). Expression of nuclear factor of activated T cells in both clonal (INS-1) and primary (islet) beta -cells was confirmed by imm unoblot and immunocytochemical analyses. Moreover, nuclear factor of activa ted T cells DNA-binding activity was detected in INS-1 and islet nuclear ex tracts by EMSAs. Activation of the insulin gene promoter by glucose or elev ated extracellular K+ (to depolarize the beta -cell) was totally prevented by FK506 (5-10 muM). 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-repor ter construct containing tandem nuclear factor of activated T cells consens us sequences. The effects of FK506 on K+-induced nuclear factor of activate d T cells reporter or insulin gene promoter activity were not mimicked by r apamycin, indicating specificity toward calcineurin. These findings suggest that the activation of calcineurin by beta -cell secretagogues that elevat e cytosolic Ca2+ plays a fundamental role in maintenance of insulin gene ex pression via the activation of nuclear factor of activated T cells.