Macrophage mineralocorticoid receptor regulate cardiac remodelling and blood pressure

Chronic mineralocorticoid receptor (MR) activation is a pathological situation seen in clinical conditions such as primary aldosteronism and chronic heart failure. In both diseases, mineralocorticoid antagonists have demonstrated proven efficacy in reducing cardiovascular events and improving survival respectively. However the mechanisms underlying their protective actions are not fully understood. In many MR-dependent experimental models, tissue perivascular inflammation and interstitial remodelling are accompanied by mononuclear cell infiltration which is composed mainly of macrophages. These macrophages have been shown to promote oxidative stress and inflammation that may play a major role in the pathophysiology of MR-mediated cardiovascular disease. Since MR and the absence of 11βHSD2 have been characterised in the macrophage, the direct role for recruited macrophages and the macrophage MR in response to mineralocorticoid excess warrants further investigation. In 2009, our group published the findings that directly implicated the macrophage MR, using myeloid MR-null mice, in MR-dependent cardiovascular injury. This was followed by another study that suggested MR signalling in the macrophage controls macrophage polarization in vitro. We therefore sought to demonstrate in successive transgenic mouse studies that firstly, recruited macrophages are crucial players in the MR-mediated cardiovascular injuries. In Chapter 3, mice null for the major macrophage chemoattractant, MCP-1 (gene CCL2), failed to recruit macrophages into cardiovascular tissues. Cardiac and aortic tissues from these mice exhibited significantly reduced expression of inflammatory and remodelling markers, which translated into attenuation of cardiac fibrosis and lower blood pressure. The study confirmed the key role for recruited macrophages in this model of disease. Secondly, MR signalling appears to control the polarization status and hence the functional phenotype of cardiovascular tissue macrophages in vivo. In Chapter 4, myeloid and T cells were isolated from the heart, aorta and the spleen of myeloid MR-null and wild-type (WT) mice to examine markers of macrophage and T cell phenotype and activation status. Isolated myeloid and T cells from the heart and the aorta of myeloid MR-null mice displayed markers consistent with an anti-inflammatory phenotype, in contrast to the proinflammatory WT, which could account for the overall reduction in tissue inflammation and remodelling. There were notable differences between these tissue-derived cells and the splenic-derived cells, highlighting contribution from the local tissue environment. This study not only verified the important role for macrophage MR signalling observed in earlier studies, but also highlighted the molecular mechanisms of the MR-dependent alteration in macrophage phenotype in cardiovascular tissues vs. the spleen. The data also suggests an active role for T cells, which is dependent upon the interaction with macrophages. Thirdly, the underlying cellular mechanism of the MR-dependent proinflammatory or anti-inflammatory phenotype in WT or MR-null macrophages respectively relies on the differential regulation of the MAPK signalling pathway. In Chapter 5, while WT bone marrow macrophages (BMM) demonstrated intact LPS-induced MAPK (JNK, ERK1/2, p38) and NF-κB activation, these responses to LPS were retained in the MR-null BMM except for JNK signalling. Moreover MR-null BMM also showed an attenuated proinflammatory gene expression response to LPS. In summary, the findings of this dissertation validated and extended the role for recruited macrophages in the orchestration of the inflammatory process mediated by MR activation. They also provided novel insights into the pathogenic inflammatory and profibrotic phenotype of the WT tissue macrophages as well as the protective, anti-inflammatory nature of the myeloid MR-null. Since intact MR signalling may be an essential component for normal macrophage response in a proinflammatory environment, the combination of an MR antagonist and JNK inhibitor may be synergistic in the treatment of MR-dependent cardiovascular disease while achieving minimal drug side-effects.