Downregulation of Ca(2+) and Mg(2+) transport proteins in the kidney explains tacrolimus (FK506)-induced hypercalciuria and hypomagnesemia.

FK506 (tacrolimus) and dexamethasone are potent immunosuppressants known to induce significant side effects on mineral homeostasis, including hypercalciuria and hypomagnesemia. 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 Ca(2+) reabsorption: the epithelial Ca(2+) channel TRPV5 and the cytosolic Ca(2+)-binding protein calbindin-D(28K). In addition, the renal expression of the putative Mg(2+) channel TRPM6, suggested to be involved in transcellular Mg(2+) reabsorption, was determined. Administration of FK506 to rats by daily oral gavage during 7 d significantly enhanced the urinary excretion of Ca(2+) and Mg(2+) 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. Furthermore, semiquantitative immunohistochemistry showed reduced renal protein abundance of TRPV5 (24 +/- 5%) and calbindin-D(28K) (29 +/- 4%), altogether suggesting that downregulation of these transport proteins is responsible for the FK506-induced Ca(2+) and Mg(2+) 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 Ca(2+) and Mg(2+) excretion. This indicated that these proteins are directly or indirectly regulated by dexamethasone. In conclusion, FK506 and dexamethasone induce renal Ca(2+) and Mg(2+) wasting, albeit by different mechanisms. Downregulation of specific Ca(2+) and Mg(2+) transport proteins provides a molecular mechanism for FK506-induced hypercalciuria and hypomagnesemia, whereas dexamethasone positively regulates these proteins.