The Effects Of Adipose-specific FH Deficiency On Adipocytes And Whole-body Energy Metabolism In Mice

Obesity is a worldwide epidemic and a major risk factor for insulin resistance, type 2diabetes and cirrhosis. Fat mass is ultimately determined by the balance of triglyceride(TG)synthesis and degradation(lipolysis). Obesity and type 2 diabetes are associated with low mitochondrial function and low mitochondrial DNA(mtDNA) content in white adipose tissue(WAT). Mouse models of obesity, including ob/ob, db/db and high fat feeding, are characterized by low mitochondrial function in WAT. It remains unclear whether compromised mitochondrial function is a cause or consequence of the metabolic alterations associated with obesity, insulin resistance and diabetes. In order to explore the effects of primary mitochondrial dysfunction on adipocytes and whole-body energy homeostasis, we generated a mouse model which have adipose-specific deficiency of fumarate hydratase(FH),an essential Krebs cycle enzyme(AFHKO mice), and acquired some specific results by systemic studies:1. By using Adipoq-Cre/Loxp system, the AFHKO mice were successfully created.These mice were born in the expected Mendelian ratio and were viable. In AFHKO mice, FH m RNA and protein expression and enzymatic activity in perigonadal white adipose tissue(WAT) and brown adipose tissue(BAT) were significantly reduced, while FH protein expression was unaffected in other organs of AFHKO mice, confirming tissue-specific gene excision.2. FH deficiency caused mitochondrial dysfunction. AFHKO mitochondria showed abnormally ultrastructural morphology with swelling mitochondria and fewer disordered cristae, ATP content was significantly decreased; in AFHKO perigonadal WAT, mtDNA, CS activity and transcripts related to Krebs cycle and OXPHOS were significantly increased,while which were decreased in AFHKO BAT.3. AFHKO mice were resistant to aging- and high fat diet-induced obesity and insulin resistance. On normal chow diet, both male and female AFHKO mice were resistant to weight gain since 13-week-old, and remained body weight since 5-month-old. AFHKO mice were also resistant to high fat diet-induced obesity. DEXA results revealed that the difference of body mass was exclusively due to lower fat mass. Histology data showed that AFHKO whiteadipocytes were smaller than controls, while AFHKO brown adipocytes were mainly monolocular. 10-month normal diet fed and 5-month high fat diet fed AFHKO mice had increased insulin sensitivity and glucose tolerance. 10-month normal diet fed AFHKO mice had elevated phosphorylation of Akt at ser473 in skeletal muscle than that of controls. Also,AFHKO mice had lower plasma levels of insulin, leptin, TG and NEFA at 5 and 10 months.Despite the lean phenotype of AFHKO mice, their level of circulating adiponectin was lower than in controls at 5 and 10 months, western blot result showed that adiponectin in AFHKO adipose tissue showed a high level of succination. In AFHKO perigonadal WAT, high expression was observed for genes related to glucose uptake, glycolysis and lipid metabolism.In contrast, in AFHKO BAT, most of these parameters were lower. Some of the inflammatory genes were higher in AFHKO perigonadal WAT.4. FH deficiency leaded to impaired adaptive thermogenesis. At 4o C with food, body temperature was similar in AFHKO and control mice, however, when removed food at 4o C,temperature dropped significantly in AFHKO mice. During fasting at 21o C, AFHKO mice had similar body temperature to controls 5 hour after food removal, but after 48 h, AFHKO mice were significantly lower. Compatible with defective adaptive thermogenesis in AFHKO BAT, UCP1 mRNA was only 5.3% of control levels, the ARβ3 transcript was 67.2% of controls and UCP1 protein was undetectable. With stimulation of fasting and CL-316,243,AFHKO had normal lipolysis.5. AFHKO were resistant to hepatic steatosis. Liver histology, mass and TG content were similar in 5-month-old chow-fed AFHKO and control mice. After 10 months of chow feeding and HFD feeding, control mice developed marked steatosis. In contrast, AFHKO mice retained the same level of hepatic fat content and similar histological features as5-month-old mice. O2 consumption, CO2 release and energy expenditure of AFHKO mice were slight higher than controls, but did not reach statistics significant.6. Depletion of adipocyte ATP reduced TG synthesis. AFHKO adipocytes had lower capacity for TG synthesis in both basal and insulin-stimulated conditions. Three respiratory chain inhibitors: rotenone(complex I), antimycin A(complex III) and oligomycin(complex V)were applied to treat isolated adipocytes. In each case, a reduction in cellular ATP content and in TG synthesis were observed. A strong negative relationship was observed between ATP content and TG synthesis.In this study, we created AFHKO mice by using Adipoq-Cre/Loxp systhem, and explored the effects of primary mitochondrial dysfunction in adipocyte derived by FH deficiency on adipose tissue and whole-body homeostasis. Adipocyte-specific FH deficiency leads to energy depletion, reduced triglyceride synthesis and small white adipocytes and to protection fromobesity, insulin resistance and hepatic steatosis. This study provided new evidence for understanding the causal relationship between adipose mitochondrial dysfunction and obesity development, and provided a potential therapeutic target for treatment and prevention of obesity and diabetes.