Mycophenolic acid is a new Nox2 inhibitor.

Organ transplantation owes its spectacular development in recent decades to the introduction of immunosuppressive drugs capable of preventing allograft rejection. However, vasculopathy remains a major cause of long-term graft failure; thus, a better understanding of its mechanisms is required to improve tolerance. Because endothelial dysfunction is an early manifestation of vascular disorders, Krötz et al,1 in this issue of Hypertension, hypothesized that endothelial cells (ECs) may be directly affected by immunosuppressive treatment. Therefore, they tested 3 major classes of these drugs with distinct mechanisms of action and effects on the vasculature. The first group includes cyclosporine A (CsA) and FK506 (tacrolimus). These are inhibitors of calcineurin, a ubiquitous calcium-dependent serine and threonine phosphatase (or PP2B). Calcineurin activates transcription factors of the nuclear factor of activated T cells family responsible for expression of surface receptors, cytokines, and chemokines in lymphocytes.2 Calcineurin inhibitors thus induce immunosuppression by blocking lymphocyte proliferation. However, adverse effects include nephrotoxicity, hyperlipidemia, endothelial dysfunction, and hypertension. In the endothelium, calcineurin inhibitors can increase endothelin-1 release,2 prevent dephosphorylation and activation of endothelial nitric oxide synthase (eNOS),3 and increase superoxide production.4 The second type of drug used by Krötz et al1 is mycophenolic acid (MPA), an inhibitor of inosine monophosphate dehydrogenase, the rate-limiting enzyme in de novo guanosine synthesis. Lymphocytes depend on this enzyme, in contrast to most other cells, including ECs, which can also use an alternative salvage pathway for guanosine synthesis. Furthermore, whereas resting lymphocytes and other cell types express one type of inosine monophosphate dehydrogenase, activated lymphocytes express another isoform with 5 times greater sensitivity to MPA. Therefore, MPA depletes GTP preferentially in activated lymphocytes, leading to inhibition of DNA synthesis and proliferation, increased apoptosis of lymphocytes and monocytes, and immunosuppression.5 MPA is not nephrotoxic and does not affect lipidemia and blood pressure. It inhibits proliferation of vascular smooth muscle, experimental atherosclerosis, and graft vasculopathy. In the endothelium, favorable effects of MPA include inhibition of endothelin-1 formation, reduced expression of adhesion molecules, and enhanced prostaglandin I2 release. However, because GTP is also required for de novo synthesis of tetrahydrobiopterin, an essential NOS cofactor, MPA also reduces NO formation. Although this effect may seem detrimental to endothelial function, it seems to preferentially affect inducible, rather than endothelial, NOS.5 The last immunosuppressor used by Krötz et al1 is rapamycin (sirolimus), an inhibitor of mammalian target of rapamycin, a serine and threonine protein kinase effector of phosphatidylinositol 3-kinase involved in protein synthesis and cell cycle progression. Rapamycin blocks interleukinmediated proliferation signals in lymphocytes, leading to immunosuppression, and also has antineoplastic activity. Rapamycin induces hyperlipidemia, but is not nephrotoxic per se, although it can increase CsA toxicity by reducing its degradation. In the vasculature, rapamycin may inhibit restenosis after angioplasty and allograft vasculopathy. However, it can lead to endothelial dysfunction by blocking vascular reendothelialization.6 It is apparent from this brief summary of their pharmacology that some immunosuppressors may protect, whereas others impair endothelial function. To understand the mechanisms by which these drugs exert their differential effects, Krötz et al1 focused on the well-known ability of reactive oxygen species to inhibit endothelium-dependent relaxation by decreasing NO availability. Although eNOS itself may become uncoupled and produce superoxide, another major source of superoxide frequently upregulated in vascular disease is the reduced nicotinamide-adenine dinucleotide phosphate oxidase (Nox) family. These enzymes are composed in ECs of catalytic subunits (Nox1, Nox2, or Nox4) and regulatory subunits p22phox, p47phox, p67phox, and Rac1.7 In their study, Krötz et al1 hypothesized that immunosuppressors directly affect expression and activity of some of these subunits and subsequent superoxide production in ECs. These authors demonstrated that although rapamycin has no effect on superoxide, treatment of ECs for 6 to 24 hours with the calcineurin inhibitors CsA or FK506 is sufficient to double superoxide generation. This response was abolished by preincubation with the specific Nox inhibitor gp91ds-tat, indicating that it is entirely mediated by the oxidase. As suggested by the authors, one can speculate that calcineurin normally dephosphorylates and therefore inhibits p47phox, the subunit responsible for activation of Nox2 (and Nox1, which was not expressed here), but this remains to be determined. This result is important, because it suggests that targeting endothelial Nox2 or its activators may attenuate the major drawback of long-term treatment with calcineurin inhibitors. The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Division of Cardiology, Emory University, Atlanta, Ga. Correspondence to Kathy Griendling, Emory University, Division of Cardiology, 1639 Pierce Dr, WMB 319, Atlanta, GA 30322. E-mail [email protected] (Hypertension. 2007;49:25-26.) © 2006 American Heart Association, Inc.