Differential regulation of the mineralocorticoid receptor by corticosteroids and high salt in cardiovascular disease.
Creators: name: Lam, Yan Mei Emily
Publisher: name: Monash University
Year: 2017
Subjects: Uncategorized

description: Cardiovascular disease is one of the major causes of death among elderly people in the Australian community that translates as an enormous economic burden on the healthcare system. Therefore understanding the mechanisms which contribute to this disease is vital to its treatment. Given that the majority of patients with heart failure do not have elevated plasma aldosterone, novel mechanisms involved in activating the mineralocorticoid receptor in the failing heart with the development of vascular inflammation and cardiac fibrosis are now being investigated. Two clinical trials present a new cardioprotective therapeutic opportunity using MR antagonists given in conjunction with current best practice therapy for moderate-to-severe heart failure and heart failure post-myocardial infarction. Although a role for the MR in hypertension has long been acknowledged, it has recently been shown to play key roles in the morbidity and mortality associated with progressive cardiac failure. A knowledge of the molecular and cellular events associated with MR activation is needed if the potential of this therapy is to be fully realized. To further investigate this possible regulation of MR activation, I utilized the mineralocorticoid/salt model. In the past decades the model of MR activation with a high salt diet (the DOC/salt model) has been used as a model of progressive cardiac failure in hypertension. Accumulating evidence from this and other models has demonstrated MR activation, by either exogenous mineralocorticoid (DOC) or endogenous glucocorticoid in the absence of 11 BHSD2 protection, produces oxidative stress, vascular damage and cardiac fibrosis. However while it is known that salt is a critical component in the fibrotic response, the mechanisms behind this pathogenesis are unclear. Therefore the aims of this thesis were to investigate MR-mediated cardiac gene expression in response to MR activation by each ligand, with or without a high salt diet. Furthermore, regulation of ligand-mediated MR transactivation responses by elevated sodium concentration was investigated in cardiac and renal cells to the mechanisms of action of salt on the MR. This thesis also explores the involvement of 11 BHSD 1 dehydrogenase activity in determining the activation of the MR and the development of vascular inflammation and cardiac fibrosis. The work in this thesis shows that ligand-mediated activation of the MR in the presence of a high salt diet induces a distinct set of genes via MR activation. Novel genes, determined by micro array analysis, selectively regulated by DOC plus salt, include angiotensin I converting enzyme 2 (ACE2) and xanthine dehydrogenase (XDH), calcium binding protein (S lOOA5) and steroidogenic acute regulating protein (StAR). Elevation of known MR-regulated genes were shown after the administration of the l1~HSD enzyme (both isoforms) inhibitor carbenoxolone (CBX) plus salt to intact rats, allowing endogenous glucocorticoids to activate the MR which produces similar outcomes as exogenous mineralocorticoids. These findings suggest that salt loading ill mineralocorticoid-treated animals produces a distinct response at the earliest time point in the pathology. These early genes were differentially regulated in response to selective cardiac MR activation by DOC or endogenous corticosterone (CBX treatment) in the presence of an elevated salt intake. These data suggest that it is possible to identify ligand-selective modulators of the MR. The identification of ligand plus salt-dependent genes will further our understanding of the pathology of cardiovascular disease, and novel mechanisms of MR activation. The current in vitro study investigates the regulation of MR transactivation by elevated sodium concentration in the presence of aldosterone or cortisol. Studies demonstrated that elevated sodium content in the culture media modestly enhances aldosterone-mediated MR transactivation in H9C2 cardiac cells but not HEK293 renal cells. Mannitol (a control for changes in osmotic pressure) caused a modest increase in the sensitivity of the response to aldosterone in HEK293, but not H9C2 cells. In contrast, neither sodium nor mannitol changed responses to cortisol in either cell lines. These findings suggest that short-term elevated sodium may promote MR-mediated inflammatory responses and oxidative damage in cardiovascular pathology. Further studies have also investigated the combined effect of salt and oxidative stress on ligand-specific regulation of MR signaling. These studies will further our understanding of the key mediators responsible for establishing cardiovascular disease. The association of 11 ~HSD1 activity with the development of cardiac fibrosis was further investigated by determining whether specific inhibition of 11 ~HSD1 by Cpd544 compound could promote inflammation and fibrosis. It is hypothesized that (i) 11 ~HSD1 is acting as a dehydrogenase in the vessel wall and (ii) reduction of pre-receptor inactivation of glucocorticoids can activate the MR in the blood vessel wall. No cardiac and renal hypertrophy was observed at 8 days. The 11 ~HSD1 inhibitor produced a significant elevation in collagen deposition compared to control DOC and CBX modestly raised cardiac collagen levels compared to control but did not reach significance. MR blockade showed a slight reduction in collagen levels compared to Cpd544, but this did not reach significance. Values for the macrophage marker ED-l and the inflammatory marker, OPN were significantly elevated by Cpd544, DOC and CBX treatments. Whereas DOC and CBX significantly elevated COX-2 expression in small and medium coronary vessels, Cpd544 did not. Co-administration of MR blockade (Kcan) with Cpd544 significantly reduced ED-I, OPN and COX-2 expression in these vessels. Serum corticosterone levels were not significantly changed by 11 PHSD 1 inhibition suggesting local tissue changes rather than changes in the HPA axis activity. While inhibition of I1PHSDI may be beneficial for the treatment of the metabolic syndrome, it is possible that it may also cause or accelerate cardiovascular damage which may involve activation of the MR; the exact mechanisms remain to be elucidated.