Tacrolimus: All roads lead to hypertension

   Tacrolimus, also known as FK506, is an immune-suppressive drug commonly used to prevent organ rejection in transplant patients ¹. Tacrolimus acts by binding to the 12 KDa FK506-binding protein (FKBP12), forming a complex that inhibits the phosphatase calcineurin ². Among the targets of calcineurin, the dephosphorylation of Nuclear Factor of Activated T-cell (NFAT) allows its nuclear translocation and transcription, leading to a pro-inflammatory response ³. In addition to the characterized impact of FK506 on immune suppression, less is known about the mechanism by which it can induce hypertension in patients and disease models ^4,5.

  Hypertension caused following FK506 treatment is very similar to that observed in Pseudohypoaldosteronism type II (PHA II) patients. As PHA II arises from mutations disrupting the protein amount and activity of the WNK pathway ^6,7, this suggested a possible intersection between FK506-induced hypertension and deregulation of WNK signaling. This was shown to be the case when studies using FK506 in mice showed that long-term (days) treatment resulted in increased WNK3, WNK4, SPAK and Na⁺, Cl^- cotransporter (NCC) mRNA and protein levels and activity ⁸, an effect that is inhibited by loss of FKBP12 ⁹, pointing to a direct mechanism involving calcineurin. Similar effects were observed with another calcineurin inhibitor, Cyclosporine A, which also caused increased WNK4 mRNA and protein in mouse kidneys ¹⁰. Further data has shown that both Cyclosporine A and FK506 can activate Na⁺, K⁺, 2 Cl^- cotransporter 2 (NKCC2) in the kidney ^11-13, due to a direct dephosphorylation and inhibition of NKCC2 by calcineurin ¹¹. This effect of FK506 on NKCC2 phosphorylation occurs following short-term (1 hour) treatment.

     These studies point to multiple mechanisms by which calcineurin can regulate the WNK signaling pathway and through which calcineurin inhibitors can induce hypertension. While the long-term effects have been well characterized, the mechanism by which they induce the increased expression of WNK pathway components is not known. Furthermore, the short term effects have only recently been studied, and so far only show an impact on NKCC2 in the kidney ¹¹. Whether calcineurin also has a similar effect on NKCC1 has not been demonstrated, but that may have a wider impact as NKCC1 is ubiquitously expressed. This is particularly important considering the vascular contribution to blood pressure in general and to FK506-induced hypertension in particular ⁵, as well as recent studies implicating WNKs in vasoconstriction ^14-16. It is clear that the "road" to calcineurin-induced hypertension goes in large part through the WNK signaling pathway, and further studies will uncover new mechanisms of FK506 action and reveal novel WNK regulatory pathways. 

References:

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