Role of the glucocorticoid receptor in cardiac growth and remodeling
Richardson, Rachel Victoria
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The glucocorticoid receptor (GR) is expressed throughout the cardiovascular system and glucocorticoids (GC) are known to influence cardiovascular processes ranging from angiogenesis and vascular tone to cardiomyocyte hypertrophy and inflammation. Genetic variation in the human GR gene that associates with relative glucocorticoid resistance is also linked to hypertension and increased risk of cardiovascular disease. Mice with global GR haploinsufficiency (GR+/-) are similarly glucocorticoid resistant, with increased hypothalamic-pituitary-adrenal (HPA) axis activity and elevated blood pressure in adulthood. Previous work from the laboratory has demonstrated that the GR is essential for normal growth and maturation of the foetal heart in late gestation and in vitro studies show that GC can alter cardiomyocyte function and induce cardiomyocyte hypertrophy. I hypothesised that reduced GR density during development would have consequences for cardiovascular function and disease risk in adulthood and that cardiovascular GR signalling is important for postnatal growth of the heart, as well as physiological and pathological cardiac remodeling in adulthood. I tested this hypothesis in GR+/- mice with global alteration in GR density as well as in SMGRKO mice, with deletion of GR in cardiomyocytes and vascular smooth muscle. To investigate the association between GC resistance and cardiovascular disease risk, I have characterised the cardiac phenotype of GR+/- mice, basally and following physiological and pathological cardiac remodeling induced by a swim training programme and Angiotensin II treatment, respectively. Survival to weaning was reduced by 35% in GR+/- mice compared with wild-type (WT) littermates. Ultrasound analysis revealed impairment of systolic cardiac function in utero (E17.5) and at postnatal day (P) 2. However, by P7 cardiac function had normalised in surviving GR+/- mice and remained equivalent to WT littermates in adulthood. Heart weight and morphology were normal in GR+/- mice in adulthood but cardiomyocyte cross sectional area was reduced, in combination with an increase in nuclei per unit area implying an increased number of cardiomyocytes. This could arise from a delay in the developmental transition from hyperplasic to hypertrophic growth of cardiomyocytes and suggests that GR+/- mice may have a reduced ability to respond to the increased cardiac workload at birth and during the early postnatal period. Further cardiac challenge may be posed by the elevated blood pressure, compensatory increase in HPA axis activity and aldosterone levels previously reported in GR+/- mice. Adaptation to pathological cardiac challenge was assessed in adult GR+/- mice and WT littermates in response to AngII treatment, which has a direct hypertrophic effect on cardiomyocytes and, at higher doses, elevates blood pressure. GR+/- and WT mice showed an equivalent, dose-dependent increase in cardiomyocyte hypertrophy and cardiac fibrosis in response to AngII, as well as similar alterations in expression of Ca2+ handling genes. Functionally, these changes to the myocardium resulted in matched reductions in ejection fraction in GR+/- and WT mice. In contrast, when cardiac hypertrophy was induced by the physiological challenge of swim training, normal cardiac function was maintained in both GR+/- mice and WT controls. The physiological cardiac hypertrophy induced by swim training was not associated with cardiac fibrosis or pathological changes to left ventricle (LV) gene expression profiles. GR+/- mice have elevated HPA axis activity at baseline and swim training increased adrenal gland weight to a greater extent in GR+/- mice suggesting that raised GC levels due to compensatory HPA activation in GR+/- mice, may mask the role of GR in cardiac remodeling. To remove the effects of compensatory HPA axis activation and to achieve a greater degree of GR deficiency in the cardiovascular system, homozygous SMGRKO mice were investigated. Similar to GR+/- mice, survival to weaning compared with control littermates was impaired, by 46% and 65% in males and females respectively. Doppler measurements of transmitral inflow and transaortic outflow of blood showed a detrimental increase in the myocardial performance index (MPI), a load-independent measure of combined systolic and diastolic function. This was due to prolongation of the isovolumetric contraction time, indicating impairment of the initial LV contractile phase. Heart/body weight ratio was increased in both and male and female SMGRKO mice. Interestingly, cross sectional area was reduced in adult female SMGRKO mice cardiomyocyte, as was found in the GR+/- mice. In contrast, in male SMGRKO mice, cardiomyocyte cross sectional area and nuclei per unit area were equivalent to control littermates at 6 weeks of age, when heart/body weight ratio was already elevated. By 12 weeks of age, cardiomyocyte cross sectional area was greater in male SMGRKO mice than control littermates. In addition, levels of mRNA encoding myosin heavy chain-β, a marker of pathological cardiac hypertrophy, were greater in the LV of male but not female SMGRKO mice at 12 weeks. These findings suggest that cardiomyocyte hyperplasia in early neonatal life, possibly in combination with physiological elongation of cardiomyocytes, may underlie the elevated heart weight in female SMGRKO mice, whereas in male SMGRKO mice there is a transition to potentially pathological hypertrophy of cardiomyocytes. This may occur at puberty, in response to increased androgens, when marked LV growth occurs in males. Histopathology showed LV fibrosis in SMGRKO mice in both sexes, accompanied by elevated levels of mRNA encoding pro-fibrotic and matrix-remodeling genes in the LV. Intriguingly, levels of mRNA encoding the mineralocorticoid receptor (MR) were elevated in both sexes, which may be causal in the development of fibrosis. Indeed, in the LV, levels of mRNA encoding MR were already elevated in 6 week old SMGRKO males, at a time when cardiac collagen levels were only modestly increased. Levels of mRNA encoding the ryanodine receptor, which is essential for cardiac excitation contraction coupling, were reduced in the LV of female, but not male SMGRKO mice. The mechanisms underlying gender differences could be further investigated by comparing neonatal cardiac development in male and female SMGRKO mice. In conclusion, deletion of GR in cardiomyocytes and vascular smooth muscle causes gender specific pathological cardiac remodeling, demonstrating the essential role of cardiovascular GR signalling in cardiac maturation and function. Global GR deficiency alters the trajectory of cardiac development and increases risk of mortality. In surviving GR+/- mice, compensatory adaptations occur in response to the functional impairment seen in utero but subtle cardiac abnormalities remain in adulthood which, together with the elevated blood pressure and GC levels, may contribute to cardiovascular disease risk.