Downregulation of Ca2+ and Mg2+ transport proteins in the kidney explains tacrolimus (FK506)-induced hypercalciuria and hypomagnesemia

FK506 (tacrolimus) and dexamethasone are potent immimosuppressants known to induce significant side effects on mineral homeostasis, including hypercalciuria and hypo-magnesemia. However, the underlying molecular mechanisms remain unknown. The present study investigated the effects of FK506 and dexamethasone on the expression of proteins involved in active Ca2+ reabsorption: the epithelial Ca2+ channel TRPV5 and the cytosolic Ca2+-binding protein calbindin-D-28K. In addition, the renal expression of the putative Mg2+ channel TRPM6, suggested to be involved in transcellular Mg2- reabsorption, was determined. Administration of FK506 to ra s by daily oral gavage during 7 d significantly enhanced the urinary excretion of Ca2+ and Mg2+ and induced a significant hypomagnesemia. FK506 significantly decreased the renal mRNA expression of TRPV5 (62 +/- 7% relative to controls, calbindin-D-28K (9 +/- 1%), and TRPM6 (52 +/- 8%), as determined by real-time quantitative PCR analysis. Further- more, semiquantitative immunohistochemistry showed reduced renal protein abundance of TRPV5 (24 +/- 5%) and calbindin-D-28K (29 +/- 4%), altogether suggesting that down-regulation of these transport proteins is responsible for the FK506-induced Ca2+ and Mg2+ wasting. In contrast, dexamethasone significantly enhanced renal TRPV5 (150 +/- 15%), calbindin-D-28K (177 +/- 23%), and TRPM6 (156 +/- 20%) mRNA levels along with TRPV5 (211 +/- 8%) and calbindin-D-28K (176 +/- 5%) protein abundance in the presence of significantly increased Ca2+ and Mg2+ excretion. This indicated that these proteins are directly or indirectly regulated by dexamethasone. In conclusion, FK506 and dexamethasone induce renal Ca2+ and Mg2+ wasting, albeit by different mechanisms. Downregulation of specific Ca2+ and Mg2+ transport proteins provides a molecular mechanism for FK506-induced hypercalciuria and hypomagnesemia, whereas dexamethasone positively regulates these proteins.