Cardiovascular 11β-HSD1: its role in myocardial physiology and pathophysiology
White, Christopher Iain
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Glucocorticoid production by the adrenal gland is regulated by hypothalamicpituitary- adrenal (HPA) axis activity. Within cells, glucocorticoid levels are modulated by 11β-hydroxysteroid dehydrogenase (11β-HSD), which interconverts active and intrinsically inert glucocorticoids. Glucocorticoids have widespread physiological effects and, in the cardiovascular system, they play a crucial role in heart development and maturation, blood pressure control, and myocardial calcium cycling. Mice which are unable to regenerate the physiological glucocorticoid, corticosterone, from 11-dehydrocorticosterone due to deletion of the type 1 11β-HSD isozyme (11β-HSD1) have previously been shown to have smaller, lighter hearts but unaltered systolic function. Moreover, a single nucleotide polymorphism (SNP) in the Hsd11b1 gene has been associated with reduced left ventricular mass in humans, suggesting a role for 11β-HSD1 in regulating cardiac size. After myocardial infarction (MI), 11β-HSD1 deficient mice have an augmented inflammatory response, increased numbers of pro-reparative alternatively-activated macrophages in the heart, enhanced peri-infarct angiogenesis and improved cardiac function compared to C57BL/6 controls. However, the role of ‘cardiovascular’ 11β-HSD1 in the development of these phenotypes, both basally and after MI, is unknown. It was hypothesised that ‘cardiovascular’ 11β-HSD1 deficiency would result in smaller hearts, and that this selective deletion would lead to altered calcium handling protein expression and diastolic abnormalities. Furthermore, it was hypothesised that ‘cardiovascular’ 11β-HSD1 deletion would reproduce the beneficial post-MI phenotype previously seen in global 11β-HSD1 deficient mice. The first aim was to characterise the cardiac phenotype of mice with global deletion of 11β-HSD1 (DelI mice), and mice in which deletion is restricted to cardiomyocytes and vascular smooth muscle cells (SMAC mice). SMAC mice have ‘floxed’ 11β- HSD1 alleles and a Cre recombinase transgene inserted into the Sm22α gene. Sm22α is expressed in vascular smooth muscle cells, and transiently in cardiomyocytes during development. Thus, Cre expression in these cells results in deletion of exon three of the Hsd11b1 gene and gives rise to a non-functional protein. Controls for DelI mice were C57BL/6 mice, and controls for SMAC mice were their Cre- littermates. DelI, but not SMAC, mice have smaller, lighter hearts, which may be explained by their shorter cardiomyocytes measured following isolation using a Langendorff preparation. Cardiomyocyte cross-sectional area is unchanged. In vivo measurement of cardiac function using ultrasound imaging showed systolic function is comparable between DelI mice and SMAC mice and their respective controls. However, there is mild diastolic dysfunction in both DelI and SMAC mice, characterised by reduced E wave deceleration and an increased mitral valve deceleration time. This phenotype occurred following pharmacological inhibition of 11β-HSD1, by administration of UE2316, a selective 11β-HSD1 inhibitor, to adult C57BL6/SJL mice. While ventricular collagen content is unaltered in DelI, SMAC and UE2316-treated mice compared to their respective controls, expression of sarcoplasmic reticulum Ca2+ ATPase (SERCA) is reduced, suggesting that altered calcium handling, rather than changes in stiffness, may underlie this phenotype. The second aim was to determine whether the beneficial acute outcomes seen previously in 11β-HSD1 deficient mice after MI could be reproduced by selective cardiovascular deletion of the enzyme. Seven days after MI, compared to Cre- littermate controls, SMAC mice have similar peri-infarct angiogenesis, total macrophage and alternatively-activated macrophage infiltration into the heart, infarct size, ventricular dilatation and systolic function. This suggests 11β-HSD1 deletion in another cell type, or types, is responsible for the phenotype seen in global 11β-HSD1 deficient mice. The final aim was to assess the impact of global 11β-HSD1 deficiency and ‘cardiovascular’ 11β-HSD1 deletion on the development of heart failure, using magnetic resonance imaging to determine structure and function. Eight weeks after MI, mice globally deficient in 11β-HSD1 have attenuated expression of ANP and β- MHC, RNA markers of heart failure, and show attenuated pulmonary oedema, reduced chamber dilatation, preserved systolic function and smaller infarcts compared to control. None of these parameters are altered in SMAC mice relative to control. In conclusion, the data presented in this thesis shows that cardiovascular 11β-HSD1 influences physiological cardiac function, potentially through regulation of calcium handling. 11β-HSD1 in other cells influences cardiomyocyte length, resulting in smaller hearts in its absence. Despite this, global 11β-HSD1 deficiency prevents heart failure development after MI, suggesting that pharmacological inhibition of 11β-HSD1 may be of benefit in treating this condition. Cardiovascular 11β-HSD1 does not, however, account for the changes in infarct healing or remodelling associated with this beneficial outcome, therefore these effects must be related to 11β-HSD1 deficiency elsewhere, such as fibroblasts or myeloid cells.