The role of cardiokines in metabolic heart disease
Tu, Vivian Huikang
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Metabolic heart disease (MHD) caused by obesity or diabetes is characterized by cardiac hypertrophy, diastolic dysfunction, and fibrosis - a maladaptive remodeling of the extracellular matrix. Though the influence of cardiac fibrosis on the left ventricular diastolic dysfunction has been reported, little is known about the cardiac-specific secreted autocrine, paracrine, or endocrine factors termed "cardiokines" in MHD. Transforming growth factor beta (TGF-b1) is a well-known inducer of cardiac fibrosis. However, the role TGF-b2 in mediating cardiac fibrosis has yet to be described. In addition, follistatin-like 3 (FSTL3), an extracellular inhibitor of activin A and myostatin, is found to be elevated in end-stage heart failure patients and obese individuals. FSTL3 has been suggested as a cardiokine, yet its role in MHD has not been established. To identify cardiokines induced by MHD, two relevant mouse models were employed in this study: the high-fat high sucrose (HFHS) diet feeding model and the cardiomyocyte-specific Fatp1 overexpressing transgenic mouse model. Interstitial fibrosis was observed in both models, accompanied by fibrotic gene expression and anti-fibrotic miR-29 suppression. It was found that Tgf-b1 and Tgf-b2 mRNA were upregulated by 85% and 76%, respectively, in the non-myocytes of 1-month HFHS-fed mice, while Fstl3 was increased by 30% in the myocytes. In contrast, in the FATP1 transgenic animals, Tgf-b2 and Fstl3 were elevated by 3.8-fold and 1.9-fold in the myocytes while Tgf-b1 remained unchanged compared to control animals. The in vitro results tested in NIH3T3 and primary fibroblast cultures indicate that both TGF-b1 and TGF-b2 exerted profibrotic effects via activation of SMAD proteins and collagen synthesis, but FSTL3 did not. Plasma samples collected from patients with metabolic syndrome showed increased FSTL3 levels with strong correlations with cardiac hypertrophy and impaired diastolic function. Overall, this study has demonstrated that TGF-b1 and TGF-b2 are the key profibrotic cardiokines induced in MHD. The study has also revealed the role of FSTL3 as a biomarker for LV hypertrophy induced in MHD. The results presented here should facilitate the development of better diagnosis and treatment for this disease in the future.