Modulation of Na -Ca Exchanger Expression by Immunosuppressive Drugs Is Isoform-Specific

The Na -Ca exchanger (NCX) is a major Ca -regulating protein encoded by three genes: NCX1, NCX2, and NCX3. They share a sequence homology of approximately 65%. NCX1 protein is expressed ubiquitously, and NCX2 and NCX3 are expressed almost exclusively in the brain. We have shown previously (Kimchi-Sarfaty et al., 2002) that treatment of NCX1transfected human embryonic kidney (HEK) 293 cells with the immunosuppressive cyclosporin A (CsA) and its nonimmunosuppressive analog PSC833 (valspodar) results in down-regulation of surface expression and transport activity of the protein without a decrease in expression of cell NCX1 protein. In this study, we show that cyclosporin A and PSC833 treatment of NCX2and NCX3-transfected HEK 293 cells also resulted in dose-dependent down-regulation of surface expression and transport activity of the two brain NCX proteins; however, whereas CsA had no effect on total cell NCX protein expression, PSC833 reduced mRNA and cell protein expression of NCX2 and NCX3. Moreover, tacrolimus (FK506), which had no effect on NCX1 protein expression, down-regulated NCX2 and NCX3 surface expression and transport activity without any significant effect on cell protein expression. Sirolimus (rapamycin) had no effect on NCX2 and NCX3 protein expression, yet it reduced NCX2 and NCX3 transport activity. Because all of the experimental conditions in our studies were identical, presumably the different drug response is related to structural differences between NCX isoforms. Clinical studies suggested that immunosuppressive regimes of patients who have received transplants resulted in complications related to Ca . Expression of NCX genes is tissue-specific. Hence, our results can potentially provide a tool for choosing the immunosuppressive protocol to be used. Cyclosporin A (CsA), FK506 (tacrolimus), and rapamycin (sirolimus) are widely used to prevent organ rejection by patients who have received transplants (Hariharan et al., 2000; Levy, 2000; First, 2004; Tsang et al., 2007). They bind to their respective immunophilin receptors, the cyclophilins and FK506 binding proteins (FKBPs) that are highly conserved families of proteins present in all cells and compartments (Barik, 2006). The immunosuppressive action of CsA and FK506 is based on the interaction of the immunophilinCsA/FK506 complex with the Ca and calmodulin-dependent phosphatase calcineurin (Liu et al., 1992) followed by inhibition of the dephosphorylation of nuclear factor of activated T cells and its translocation to the nucleus, which leads to subsequent suppression of the immune reaction. Rapamycin also interacts with FKBPs; however, the FKBP-rapamycin complex does not inhibit calcineurin but inhibits the target of rapamycin protein, which is a cell cycle-specific serine/threonine kinase involved in cell growth, proliferation, protein transcription, initiation, and translation (Proud,