Received for publication August 15, 2007.
Revised November 26, 2007.
Accepted for publication November 26, 2007.

Activation of the dual-leucine-zipper-bearing kinase (DLK) and induction of beta-cell apoptosis by the immunosuppressive drug cyclosporin A

Abstract

Posttransplant diabetes is an untoward effect often observed under immunosuppressive therapy with cyclosporin A. Besides the development of peripheral insulin resistance and a decrease in insulin gene transcription a beta-cell toxic effect has been described. However, its molecular mechanism remains unknown. In the present study the effect of cyclosporin A and the dual leucine zipper bearing kinase (DLK) on beta-cell survival was investigated. Cyclosporin A decreased the viability of the insulin producing pancreatic islet cell line HIT in a time- and concentration-dependent manner. Upon exposure to the immunosuppressant fragmentation of DNA, the activation of the effector caspase-3 as well as a decrease of full length caspase-3 and Bcl_XL were observed in HIT cells and in primary mature murine islets, respectively. Cyclosporin A and tacrolimus, both potent inhibitors of the calcium/calmodulin-dependent phosphatase calcineurin, stimulated the enzymatic activity of cellular DLK in an in vitro kinase assay. Immunocytochemistry revealed that the overexpression of DLK, but not its kinase-dead mutant, induced apoptosis and enhanced cyclosporin A-induced apoptosis to a higher extent than the drug alone. Moreover, in the presence of DLK the effective concentration for cyclosporin A-caused apoptosis was similar to its known IC50 value for the inhibition of calcineurin activity in beta-cells. These data suggest, that cyclosporin A through inhibition of calcineurin activates DLK, thereby leading to beta-cell apoptosis. This action may thus be a novel mechanism through which cyclosporin A precipitates posttransplant diabetes.

Key words: Protein Kinases (other), Protein ser/thr Phosphatases, MAP Kinase, Apoptosis, Endocrine cells