The orphan nuclear receptor Nr4a1 contributes to interstitial cardiac fibrosis via modulation of cardiac fibroblast and macrophage phenotype.

The orphan nuclear receptor Nr4a1 has complex biological functions and has been implicated in numerous diseases, including cardiovascular disease. While protective in atherosclerosis and myocardial ischemia, Nr4a1 has been shown to cause cardiac fibrosis in non-ischemic adverse remodeling of the heart. However, mechanisms underlying these actions are still poorly understood. Accordingly, we sought to: (1) understand the contribution of Nr4a1 to the inflammatory environment including macrophage phenotype; and (2) determine the contribution of Nr4a1 to cardiac fibroblast phenotype in the fibrotic heart. Wild type and Nr4a1^-/- mice were infused with angiotensin II (1500 ng/kg/min) to induce cardiac fibrosis and diastolic dysfunction. Nr4a1 deletion prevented cardiac fibrosis and maintained normal diastolic function. We determined that macrophages lacking Nr4a1 had distinctly different phenotypes to wild type macrophages, with Nr4a1 deletion preventing the induction of a pro-inflammatory macrophage phenotype, instead promoting an anti-inflammatory phenotype. This had functional consequences in that macrophages lacking Nr4a1 showed a reduced ability to induce cardiac fibroblast migration. Interestingly, deletion of Nr4a1 in isolated cardiac fibroblasts also had profound effects on their phenotype and function, with these cells not able to produce excess extracellular matrix proteins, convert to a myofibroblast phenotype, or respond to macrophage stimuli. Nr4a1 causes cardiac fibrosis and subsequent diastolic dysfunction by inducing a pro-inflammatory phenotype in macrophages and by pushing cardiac fibroblasts towards a pro-fibrotic phenotype in response to pro-fibrotic stimuli. Nr4a1 is also critical for macrophage/fibroblast interactions.