Osteopontin Deletion Prevents the Development of Obesity and Hepatic Steatosis via Impaired Adipose Tissue Matrix Remodeling and Reduced Inflammation and Fibrosis in Adipose Tissue and Liver in Mice

Osteopontin (OPN) is a multifunctional extracellular matrix (ECM) protein involved in multiple physiological processes. OPN expression is dramatically increased in visceral adipose tissue in obesity and the lack of OPN protects against the development of insulin resistance and inflammation in mice. We sought to unravel the potential mechanisms involved in the beneficial effects of the absence of OPN. We analyzed the effect of the lack of OPN in the development of obesity and hepatic steatosis induced by a high-fat diet (HFD) using OPN-KO mice. OPN expression was upregulated in epididymal white adipose tissue (EWAT) and liver in wild type (WT) mice with HFD. OPN-KO mice had higher insulin sensitivity, lower body weight and fat mass with reduced adipose tissue ECM remodeling and reduced adipocyte size than WT mice under a HFD. Reduced MMP2 and MMP9 activity was involved in the decreased ECM remodeling. Crown-like structure number in EWAT as well as F4/80-positive cells and Emr1 expression in EWAT and liver increased with HFD, while OPN-deficiency blunted the increase. Moreover, our data show for the first time that OPN-KO under a HFD mice display reduced fibrosis in adipose tissue and liver, as well as reduced oxidative stress in adipose tissue. Gene expression of collagens Col1a1, Col6a1 and Col6a3 in EWAT and liver, as well as the profibrotic cytokine Tgfb1 in EWAT were increased with HFD, while OPN-deficiency prevented this increase. OPN deficiency prevented hepatic steatosis via reduction in the expression of molecules involved in the onset of fat accumulation such as Pparg, Srebf1, Fasn, Mogat1, Dgat2 and Cidec. Furthermore, OPN-KO mice exhibited higher body temperature and improved BAT function. The present data reveal novel mechanisms of OPN in the development of obesity, pointing out the inhibition of OPN as a promising target for the treatment of obesity and fatty liver.骨桥蛋白（Osteopontin, OPN）是一种多功能细胞外基质（extracellular matrix, ECM）蛋白，参与多种生理进程。肥胖状态下，内脏脂肪组织中的OPN表达会显著升高；而OPN缺失可使小鼠免于发生胰岛素抵抗（insulin resistance）与炎症（inflammation）。本研究旨在阐明OPN缺失发挥有益作用的潜在机制。我们借助骨桥蛋白基因敲除（OPN-KO）小鼠，分析了OPN缺失对高脂饮食（high-fat diet, HFD）诱导的肥胖及肝脂肪变性（hepatic steatosis）进程的影响。 在高脂饮食喂养的野生型（wild type, WT）小鼠中，其附睾白色脂肪组织（epididymal white adipose tissue, EWAT）与肝脏内的OPN表达均会上调。相较于WT小鼠，高脂饮食喂养的OPN-KO小鼠胰岛素敏感性更高、体重与脂肪量更低，同时脂肪组织细胞外基质重塑（adipose tissue ECM remodeling）程度更弱，脂肪细胞大小（adipocyte size）更小。细胞外基质重塑减弱与基质金属蛋白酶2（MMP2）、基质金属蛋白酶9（MMP9）活性降低密切相关。 高脂饮食会使EWAT内的冠状样结构（Crown-like structure）数量增多，同时EWAT与肝脏内的F4/80阳性细胞（F4/80-positive cells）比例及Emr1基因（Emr1）表达水平升高，而OPN缺失可抑制这一变化。此外，本研究首次发现，高脂饮食喂养的OPN-KO小鼠脂肪组织与肝脏的纤维化（fibrosis）程度更低，脂肪组织的氧化应激（oxidative stress）水平也有所降低。 高脂饮食会使EWAT与肝脏内的胶原蛋白Col1a1、Col6a1及Col6a3（collagens Col1a1, Col6a1 and Col6a3）的基因表达上调，同时EWAT内的促纤维化细胞因子Tgfb1（profibrotic cytokine Tgfb1）表达升高，而OPN缺失可阻断上述表达上调现象。OPN缺失还可通过降低脂肪积累相关分子（如Pparg、Srebf1、Fasn、Mogat1、Dgat2及Cidec）的表达，从而改善肝脂肪变性。此外，OPN-KO小鼠体温更高，棕色脂肪组织（brown adipose tissue, BAT）功能也得到改善。本研究揭示了OPN参与肥胖发生发展的全新机制，提示抑制OPN可作为治疗肥胖与脂肪肝（fatty liver）的潜在有效靶点。