Cardiac fibrosis is strongly associated with obesity and metabolic dysfunction and may contribute to the increased incidence of heart failure, atrial arrhythmias and sudden cardiac death in obese subjects. Our review discusses the evidence linking obesity and myocardial fibrosis in animal models and human patients, focusing on the fundamental pathophysiologic alterations that may trigger fibrogenic signaling, the cellular effectors of fibrosis and the molecular signals that may regulate the fibrotic response. Obesity is associated with a wide range of pathophysiologic alterations (such as pressure and volume overload, metabolic dysregulation, neurohumoral activation and systemic inflammation); their relative role in mediating cardiac fibrosis is poorly defined. Activation of fibroblasts likely plays a major role in obesity-associated fibrosis; however, inflammatory cells, cardiomyocytes and vascular cells may also contribute to fibrogenic signaling. Several molecular processes have been implicated in regulation of the fibrotic response in obesity. Activation of the Renin-Angiotensin-Aldosterone System, induction of Transforming Growth Factor-β, oxidative stress, advanced glycation end-products (AGEs), endothelin-1, Rho-kinase signaling, leptin-mediated actions and upregulation of matricellular proteins (such as thrombospondin-1) may play a role in the development of fibrosis in models of obesity and metabolic dysfunction. Moreover, experimental evidence suggests that obesity and insulin resistance profoundly affect the fibrotic and remodeling response following cardiac injury. Understanding the pathways implicated in obesity-associated fibrosis may lead to development of novel therapies to prevent heart failure and to attenuate post-infarction cardiac remodeling in obese patients.

Rationale Diabetes is associated with cardiac fibrosis. Matricellular proteins are induced in fibrotic conditions and modulate fibrogenic and angiogenic responses by regulating growth factor signaling. Objective To test the hypothesis that the prototypical matricellular protein thrombospondin (TSP)-1, a potent angiostatic molecule and crucial activator of TGF-β, may play a key role in remodeling of the diabetic heart. Methods and results Obese diabetic db/db mice exhibited marked myocardial TSP-1 upregulation in the interstitial and perivascular space. In order to study the role of TSP-1 in remodeling of the diabetic heart we generated and characterized db/db TSP-1 null (dbTSP) mice. TSP-1 disruption did not significantly affect weight gain and metabolic function in db/db animals. When compared with db/db animals, dbTSP mice had increased left ventricular dilation associated with mild non-progressive systolic dysfunction. Chamber dilation in dbTSP mice was associated with decreased myocardial collagen content and accentuated Matrix Metalloproteinase (MMP)-2 and -9 activity. TSP-1 disruption did not affect inflammatory gene expression and activation of TGF-β/Smad signaling in the db/db myocardium. In cardiac fibroblasts populating collagen pads, TSP-1 incorporation into the matrix did not activate TGF-β responses, but inhibited leptin-induced MMP-2 activation. TSP-1 disruption abrogated age-associated capillary rarefaction in db/db mice, attenuating myocardial upregulation of angiopoietin-2, a mediator that induces vascular regression. In vitro, TSP-1 stimulation increased macrophage, but not endothelial cell, angiopoietin-2 synthesis. Conclusions TSP-1 upregulation in the diabetic heart prevents chamber dilation by exerting matrix-preserving actions on cardiac fibroblasts and mediates capillary rarefaction through effects that may involve angiopoietin-2 upregulation.

Background Heart failure in diabetics is associated with cardiac hypertrophy, fibrosis and diastolic dysfunction. Activation of transforming growth factor (TGF)-β/Smad3 signaling in the diabetic myocardium may mediate fibrosis and diastolic heart failure, while preserving matrix homeostasis. We hypothesized that Smad3 may play a key role in the pathogenesis of cardiovascular remodeling associated with diabetes and obesity. Methods and Results We generated leptin-resistant db/db Smad3 null mice (dbSKO) and db/db Smad3 +/- animals (dbShet). Smad3 haploinsufficiency did not affect metabolic function in db/db mice, but protected from myocardial diastolic dysfunction, while causing left ventricular chamber dilation. Improved cardiac compliance and chamber dilation in dbShet animals was associated with decreased cardiomyocyte hypertrophy, reduced collagen deposition and accentuated matrix metalloproteinase (MMP) activity. Attenuation of hypertrophy and fibrosis in dbShet hearts was associated with reduced myocardial oxidative and nitrosative stress. dbSKO mice had reduced weight gain and decreased adiposity associated with attenuated insulin resistance, but also exhibited high early mortality, in part due to spontaneous rupture of the ascending aorta. Ultrasound studies showed that both lean and obese Smad3 null animals had significant aortic dilation. Aortic dilation in dbSKO mice occurred despite reduced hypertension, and was associated with perturbed matrix balance in the vascular wall. Conclusions Smad3 mediates diabetic cardiac hypertrophy, fibrosis and diastolic dysfunction, while preserving normal cardiac geometry and maintaining the integrity of the vascular wall.

Tumor necrosis factor ␣-converting enzyme (TACE, also known as ADAM17) was recently involved in the pathogenesis of insulin resistance. We observed that TACE activity was significantly higher in livers of mice fed a high-fat diet (HFD) for 1 month, and this activity was increased in liver > white adipose tissue > muscle after 5 months compared with chow control. In mouse hepatocytes, C 2 C 12 myocytes, and 3T3F442A adipocytes, TACE activity was triggered by palmitic acid, lipolysaccharide, high glucose, and high insulin. TACE overexpression significantly impaired insulin-dependent phosphorylation of AKT, GSK3, and FoxO1 in mouse hepatocytes. To test the role of TACE activation in vivo, we used tissue inhibitor of metalloproteinase 3 (Timp3) null mice, because Timp3 is the specific inhibitor of TACE and Timp3 ؊/؊ mice have higher TACE activity compared with wild-type (WT) mice. Timp3 ؊/؊ mice fed a HFD for 5 months are glucose-intolerant and insulin-resistant; they showed macrovesicular steatosis and ballooning degeneration compared with WT mice, which presented only microvesicular steatosis. Shotgun proteomics analysis revealed that Timp3 ؊/؊ liver showed a significant differential expression of 38 proteins, including lower levels of adenosine kinase, methionine adenosysltransferase I/III, and glycine N-methyltransferase and higher levels of liver fatty acid-binding protein 1. These changes in protein levels were also observed in hepatocytes infected with adenovirus encoding TACE. All these proteins play a role in fatty acid uptake, triglyceride synthesis, and methionine metabolism, providing a molecular explanation for the increased hepatosteatosis observed in Timp3 ؊/؊ compared with WT mice. Conclusion: We have identified novel mechanisms, governed by the TACE-Timp3 interaction, involved in the determination of insulin resistance and liver steatosis during overfeeding in mice. (HEPATOLOGY 2010;51:103-110.) P andemic obesity is now considered the underlying basis for the increasing prevalence of chronic metabolic-inflammatory diseases including type 2 diabetes, nonalcoholic fatty liver disease (NAFLD) and atherosclerosis. 1 Although NAFLD is an emerging metabolic complication of obesity, its pathogenic mechanisms are still unclear. 1 The contribution of insulin resistance to the development of fatty liver occurs in part by deficient control of lipid storage in white adipose tissue and in part by altered