The Association Between Osteocalcin And NAFLD In Patients With Coronary Artery Disease And The Regulatory Pathways Through Which Osteocalcin Improved NAFLD In Mice

Objective: Osteocalcin plays roles in energy,glucose,and lipid metabolism.Consequently,the relationship between osteocalcin and nonalcoholic fatty liver disease(NAFLD),as well as the specific underlying mechanisms is of interest.The present study not only aimed to explore the possible correlation between serum osteocalcin levels and NAFLD in patients with coronary artery disease(CAD),but also aimed to investigate whether osteocalcin protects against high-fat diet induced NAFLD in mice by regulating Nuclear factor erythroid 2-related factor 2(Nrf2)and c-Jun N-terminal kinase(JNK)pathways.Methods: During July 2008 and January 2010,174 inpatients of the Cardiology Department of the Sixth People's Hospital affiliated with Shanghai Jiao Tong University who underwent coronary angiography(CAG)and were diagnosed with CAD were recruited for this study.The presence of fatty liver disease was confirmed by abdominal ultrasonography.NAFLD was diagnosed using the working definition of the revised guidelines for the management of NAFLD published by the Chinese Liver Disease Association(2010).Serum osteocalcin levels were determined using electrochemiluminescent immunoassays.Besides,6-week-old male C57/BL6 J mice were fed a high-fat diet for 12 weeks to induce NAFLD and were treated with recombinant uncarboxylated osteocalcin(30 ng/g)or vehicle by daily intraperitoneal injection during this period.Liver enzymes and hepatic triglyceride content were carried out.Hepatic redox state was examined.The role of osteocalcin on Nrf2 and JNK were investigated by Western blot and Real-time PCR.Results: Patients with NAFLD had lower serum osteocalcin levels than those without NAFLD [16.2(14.2–23.8)vs.20.7(15.6–26.2)ng/mL,P<0.05].After adjustment for gender and age,serum osteocalcin levels correlated with the presence of NAFLD(r=-0.260,P=0.010),FPG level(r=-0.230,P=0.023)and HbA1 c level(r=-0.229,P=0.023).Osteocalcin(?=-0.097,P=0.025),BMI(?=0.345,P<0.001),HbA1c(?=0.641,P=0.004)and TG(?=1.002,P<0.001)were the independent factor for the presence of NAFLD.Osteocalcin treatment protected mice from diet-induced hepatic triglyceride accumulation and liver injury(P<0.01~0.001).Increased levels of malondialdehyde,8-iso-prostaglandin F2? and a higher ratio of oxidized/ reduced glutathione in the liver of mice fed a high-fat diet were decreased by osteocalcin(P<0.01~0.001).Osteocalcin treatment not only activated Nrf2 nuclear translocation and up-regulated the expression of antioxidant enzyme genes(catalase,superoxide dismutase 1,and glutathione peroxidase 1)(P<0.05),but also inhibited the activation of JNK in the liver(P<0.05).G protein coupled receptor family C,group 6,subtype A(GPRC6A),the putative receptor of osteocalcin,was found in the liver.Conclusions: Serum osteocalcin levels were negatively associated with the presence of NAFLD in patients with CAD.Osteocalcin improves NAFLD in mice by activating the Nrf2 pathway to alleviate oxidative stress,and inhibiting JNK pathway.