Targeted disruption of the iNOS gene improves adipose tissue inflammation and fibrosis in leptin-deficient ob/ob mice: role of tenascin C

S Becerril,J Gómez-Ambrosi,M Llorente,B Ramírez,V Catalán,L Méndez-Giménez,X Unamuno,N Sáinz,A Rodríguez,G Frühbeck

doi:10.1038/s41366-018-0005-5

Abstract

Background/Objectives:Obesity is related to a dynamic extracellular matrix (ECM) remodeling, which involves the synthesis and degradation of different proteins, such as tenascin C (TNC) in the adipose tissue (AT). Given the functional relationship between leptin and inducible nitric oxide synthase (iNOS), our aim was to analyze the impact of the absence of the iNOS gene in AT inflammation and ECM remodeling in ob/ob mice.Subjects/Methods:The expression of genes involved in inflammation and ECM remodeling was evaluated in 10-week-old male double knockout (DBKO) mice simultaneously lacking the ob and iNOS genes as well as in ob/ob mice classified into three groups [control, leptin-treated (1 mg kg−1 day−1) and pair-fed].Results:Leptin deficiency increased inflammation and fibrosis in AT. As expected, leptin treatment improved the obesity phenotype. iNOS deficiency in ob/ob mice improved insulin sensitivity, AT inflammation, and ECM remodeling, as evidenced by lower AT macrophage infiltration and collagen deposition, a downregulation of proinflammatory and profibrogenic genes Tnf, Emr1, Hif1a, Col6a1, Col6a3, and Tnc, as well as lower circulating TNC levels. Interestingly, leptin upregulated TNC expression and release in 3T3-L1 adipocytes, and iNOS knockdown in 3T3-L1 fat cells produced a significant decrease in basal and leptin-induced Tnc expression.Conclusions:Ablation of iNOS in leptin-deficient mice improved AT inflammation and ECM remodeling-related genes, attenuating fibrosis, and metabolic dysfunction. The activation of iNOS by leptin is necessary for the synthesis and secretion of TNC in adipocytes, suggesting an important role of this alarmin in the development of AT inflammation and fibrosis.

Highlights

Obesity constitutes a chronic low-grade inflammatory condition, leading to robust morphological and functional changes within adipose tissue (AT), including the Electronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.Ablation of inducible nitric oxide synthase (iNOS) in ob/ob mice improves adipose tissue fibrosisThese inflammatory markers have direct effects on cellular metabolism, leading to decreased AT expandability, development of fibrosis, and dysfunction[8,9,10,11]
We confirmed the ability of leptin to stimulate tumor necrosis factor α (Tnf) mRNA expression in murine 3T3-L1 adipocytes, but not in undifferentiated preadipocytes
Our data revealed that leptin promoted the synthesis and release of Tenascin C (TNC), an endogenous activator of Toll-like receptor 4 (TLR4) that is key for the production of proinflammatory cytokines and extracellular matrix (ECM) remodeling[19,22]

Summary

Introduction

Ablation of iNOS in ob/ob mice improves adipose tissue fibrosis. These inflammatory markers have direct effects on cellular metabolism, leading to decreased AT expandability, development of fibrosis, and dysfunction[8,9,10,11]. The excessive accumulation of ECM components (tenascins, collagens, or fibrin, among others) damages AT homeostasis and reduces tissue plasticity leading to adipocyte dysfunction, ectopic lipid deposition in peripheral tissues, tissue inflammation, and metabolic disorders[15,16]. TNC is associated with tissue injury as well as inflammation and modulates fibrotic and inflammatory responses in diverse diseases, such as liver fibrosis, arthritis, or obesity by inducing the production of proinflammatory cytokines[19,20,21]. Our group has recently described that TNC participates in the etiopathology of obesity via AT inflammation[22]

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