Endothelial Cell Mineralocorticoid Receptors: turning cardiovascular risk factors into cardiovascular dysfunction

Clinically, mineralocorticoid receptor (MR) antagonists are widely prescribed for the treatment of hypertension and heart failure due to their diuretic action in the aldosterone-sensitive distal nephron. Clinical trials of MR antagonism in patients with various degrees of heart failure severity have also demonstrated a pronounced reduction of cardiovascular mortality in MR antagonist-treated patients¹. The underlying mechanisms of these cardiovascular benefits are still debated and are probably diverse. Potential beneficial effects of MR antagonism on extracellular matrix remodeling, arrhythmia susceptibility, coronary flow reserve, cardiovascular inflammation and vascular function have all been suggested. Indeed the vasculature has been recently highlighted as a primary target of aldosterone and MR antagonists. MR is expressed in the human vascular endothelial cells (EC) and smooth muscle cells as is the 11betaHSD2 enzyme that allows for selective aldosterone versus cortisol activation of MR. The effects of MR activation on vascular reactivity in healthy humans remains controversial due to conflicting results from clinical studies with many demonstrating a constrictive response and some showing vascular relaxation (reviewed in²). The discrepancies may be due to differences in the vascular health of the study participants as well as differences in study design. However, when patients with underlying cardiovascular diseases are studied, the data are quite consistent with MR-activation promoting increased systemic vascular resistance and reduced forearm blood flow and MR antagonism improving endothelium-dependent vasodilatation, independent of changes in blood pressure. The aggregate of the data supports that in healthy vessels, acute MR activation may evoke endothelium-dependent, NO-mediated vasodilatation while, in the presence of endothelial dysfunction, vascular injury, or high vascular oxidative stress (as in patients with cardiovascular risk factors or heart failure), MR activation promotes vasoconstriction (reviewed in²)

Recently two experimental studies, using a mouse model with targeted inactivation of MR in the endothelium, have attempted to address this controversy more directly^{3, 4}. Both studies demonstrate that in the healthy animal, EC-specific deletion of the MR has no effect on systemic blood pressure or mesenteric resistance vessel contractile or relaxation function indicating that endothelial MR does not contribute substantially to these parameters in the absence of disease stimuli. However, both studies reveal that in the setting of cardiovascular risk factors, vascular function is negatively affected by the presence of MR in endothelial cells. Diet-induced obesity or aldosterone infusion was used in the Schaffer study while mineralocorticoid/salt-induced hypertension was used to induce cardiovascular dysfunction in the Rickard et al study⁴. The decline in endothelium-dependent relaxation to acetylcholine caused by induction of obesity or aldosterone infusion was blunted in the aortae of obese mice lacking EC MR, mimicking the beneficial effect of chronic treatment with eplerenone, a specific MR antagonist³. This was independent from pro-inflammatory changes in aortic endothelial cells. EC MR was also mandatory for mineralocorticoid/salt-induced hypertension to induce endothelial dysfunction⁴. Conversely, chronic increase of MR expression specifically in the endothelium has been previously showed to increase the vasoactive response to Angiotensin II and Endothelin 1 as well as basal blood pressure and AngII/ET1-induced hypertension⁵. The mechanisms remain to be determined but EC MR likely contributes to vascular oxidative stress and nitric oxide production as one mechanism that regulates vascular contraction and relaxation in the setting of cardiovascular risk factors. Indeed, EC deletion blunted the increased expression induced by aldosterone of the NADPH oxidase subunit p22phox and COX1 in endothelial cells³. This suggests that EC MR is necessary for aldosterone to induce oxidative stress and associated vascular dysfunction, at least in the setting of obesity-induced vascular disease.

In the present issue, Rickard and al. provide compelling evidences that MR expression in the endothelium is required for mineralocorticoid/salt-hypertension to induce cardiac fibrosis⁴. This study therefore provides new insights into the crucial role of MR activation in the endothelium to sustain the inflammatory process induced by mineralocorticoid-salt challenge leading to the stimulation of extra-cellular matrix remodeling in the heart. The underlying mechanisms remain to be completely elucidated but it is suggested that protection could be due to the absence of macrophage invasion into the cardiac tissue in the EC MR deficient mice. EC MR has been found to regulate endothelial ICAM1 expression to promote leukocyte-EC adhesion, a necessary step for cardiovascular inflammation⁶. Indeed, EC MR deletion attenuated the increase in cardiac ICAM1 expression induced by DOC-salt and prevented cardiac inflammation and cardiac fibrosis. Of note, one limitation of these models is the use of the Tie2/Tek promoter to drive expression of the Cre recombinase. Indeed, the Tie2/Tek promoter sequence has been previously found to target not only endothelial cells but also myeloid cells resulting in decreased macrophage MR expression when used with the MR recombinant mice³. Thus it cannot be excluded that prevention of cardiac remodeling also relies on deletion of macrophage MR in the DOC-salt model since macrophage MR has already been found to contribute to the pro-inflammatory macrophage phenotype that is necessary for cardiac and vascular fibrosis^{7, 8}. It is possible that both blunted macrophage infiltration and decreased macrophage MR activation may participate to the decreased cardiac remodeling induced by mineralocorticoid-salt challenge in this EC MR KO model.

In addition to the contribution to cardiovascular inflammation by regulation of adhesion molecules or impaired relaxation related to oxidative stress, EC MR might contribute to vascular function by other mechanisms that remain to be explored. Local aldosterone infusion into the eye induced a major vasodilation of the retinal choroid vascular bed, mimicking a vision threatening disease called Central Serous Chorioretinitis, providing the rationale for a spectacular benefit of eplerenone in these patients⁹. The underlying mechanism relied on the increased activity of the endothelial KCa2.3 potassium ion channel involved in endothelium-dependent vasodilation⁹. Endothelial MR also participates to endothelium stiffening via the modulation of ENaC subunits expression/activity in the endothelium¹⁰. Recent work revealed that aldosterone unexpectedly induced down-regulation of thrombin generation by the endothelium, an effect caused by an enhancement of thrombomodulin-mediated protein C activation. This was blunted by drospirenone a contraceptive with potent MR antagonist property associated with increased thrombosis in healthy women highlighting a novel role of endothelial MR with potential therapeutic issues¹¹.

In conclusion, the data presented by Rickard et al in the current issue support a model in which EC MR may be beneficial or neutral (see Figure) in the healthy vasculature (no effect on blood pressure and neutral or beneficial in terms of endothelial function or endothelial-dependent vasodilation) while playing a crucial role in the setting of cardiovascular risk factors including obesity, hypertension, and likely other risk factors that remain to be tested. EC MR activation by aldosterone (or other activators such as corticosteroids in the eye) appears to mediate at least some of the detrimental effects of these risk factors on vascular function and cardiovascular fibrosis. These findings might help explain the substantial benefits of MR inhibition in patients with heart failure. Whether endothelial MR also participates to the pathophysiology of other diseases like atherosclerosis, sepsis, tissue ischemia including myocardial infarction, stroke, or renal ischemia, remains to be explored. Since the use of MR antagonists could be limited by their side effects (hyperkalemia, gynecomastia), further exploration of the molecular mechanisms by which EC MR mediates the detrimental effects of known cardiovascular risk factors could identify novel treatment strategies that retain the substantial cardiovascular benefits of MR antagonists without the limiting side effects.

Model highlighting targets modulated by mineralocorticoid receptor activation in the endothelium and the physiological and pathophysiological consequences in healthy individuals and in the presence of risk factors.