Differential Roles of Cardiomyocyte and Macrophage Peroxisome Proliferator-activated Receptor Gamma in Cardiac Fibrosis

Objective: Cardiac fibrosis is an important component of diabetic cardiomyopathy. Peroxisome proliferator-activated receptor gamma (PPARgamma) ligands repress proinflammatory gene expression, including that of osteopontin, a known contributor to the development of myocardial fibrosis. We thus investigated the hypothesis that PPARgamma ligands could attenuate cardiac fibrosis.

Research Design And Methods: Wild-type cardiomyocyte- and macrophage-specific PPARgamma(-/-) mice were infused with angiotensin II (AngII) to promote cardiac fibrosis and treated with the PPARgamma ligand pioglitazone to determine the roles of cardiomyocyte and macrophage PPARgamma in cardiac fibrosis.

Results: Cardiomyocyte-specific PPARgamma(-/-) mice (cPPARgamma(-/-)) developed spontaneous cardiac hypertrophy with increased ventricular osteopontin expression and macrophage content, which were exacerbated by AngII infusion. Pioglitazone attenuated AngII-induced fibrosis, macrophage accumulation, and osteopontin expression in both wild-type and cPPARgamma(-/-) mice but induced hypertrophy in a PPARgamma-dependent manner. We pursued two mechanisms to explain the antifibrotic cardiomyocyte-PPARgamma-independent effects of pioglitazone: increased adiponectin expression and attenuation of proinflammatory macrophage activity. Adenovirus-expressed adiponectin had no effect on cardiac fibrosis and the PPARgamma ligand pioglitazone did not attenuate AngII-induced cardiac fibrosis, osteopontin expression, or macrophage accumulation in monocyte-specific PPARgamma(-/-) mice.

Conclusions: We arrived at the following conclusions: 1) both cardiomyocyte-specific PPARgamma deficiency and activation promote cardiac hypertrophy, 2) both cardiomyocyte and monocyte PPARgamma regulate cardiac macrophage infiltration, 3) inflammation is a key mediator of AngII-induced cardiac fibrosis, 4) macrophage PPARgamma activation prevents myocardial macrophage accumulation, and 5) PPARgamma ligands attenuate AngII-induced cardiac fibrosis by inhibiting myocardial macrophage infiltration. These observations have important implications for potential interventions to prevent cardiac fibrosis.