Fasting-mediated alteration studies in insulin action on lipolysis and lipogenesis in obese women

Abstract

The effects of fasting on insulin-induced antilipolysis and lipogenesis were investigated in vitro in isolated human fat cells of 11 obese females. Glycerol release and lipogenesis were determined simultaneously in the same test tube and related to methylglucose transport and specific insulin binding. Insulin binding and sensitivity and the responsiveness (maximum effect) of insulin-induced antilipolysis were enhanced by fasting. The latter was strongly correlated with an enhancement in the lipolysis rate. The effects of fasting on antilipolysis were not dependent on the glucose concentration, unlike insulin-stimulated lipogenesis. At 1 mumol/l of glucose, where hexose transport is rate limiting, sensitivity and responsiveness of insulin-induced lipogenesis were inhibited by fasting. Similar results were obtained with methylglucose transport. At 1-10 mmol/l of glucose, where hexose metabolism is rate limiting, insulin stimulated lipogenesis before fasting but was totally ineffective after fasting. In conclusion, fasting induces multiple alterations in insulin action on lipolysis and lipogenesis in adipocytes. Antilipolysis is enhanced because of stimulation at the receptor and postreceptor levels, which may be associated with an enhanced rate of lipolysis. Fasting inhibits the lipogenic effect of insulin due to postreceptor changes involving both transport and metabolism of glucose, making lipogenesis unresponsive to insulin at physiological glucose concentrations.

Insulin signal transduction and glucose transport in human adipocytes: effects of obesity and low calorie diet

“…We used an indirect method [16] to assess glucose transport that measures the incorporation of (3-3 H)glucose into adipocyte lipids [19]. This method requires less tissue than the direct method and results are comparable with studies using 3-Omethylglucose in human adipocytes [16,20].…”

“…We used an indirect method [16] to assess glucose transport that measures the incorporation of (3-3 H)glucose into adipocyte lipids [19]. This method requires less tissue than the direct method and results are comparable with studies using 3-Omethylglucose in human adipocytes [16,20]. Isolated adipocytes were incubated at a concentration of 2% (vol/vol) in Krebs Ringer phosphate buffer (pH 7.4) containing albumin (40 mg/ml), (3-3 H)glucose (5×10 6 dpm/ml), unlabelled glucose (1 µmol/l), and human insulin in different concentrations (0-70 nmol/l).…”

“…Incubations were stopped by rapidly chilling the incubation vials to 4°C. Incorporation of radiolabelled glucose into adipocyte lipids was assessed [16]. Incorporation of radiolabelled glucose into lipids (i.e., lipogenesis) reflects glucose transport because at micromolar glucose con-Thus, a greater understanding of molecular defects contributing to impaired insulin signal transduction and glucose transport in insulin responsive cells, such as adipocytes, is of importance to identify relevant targets for therapeutic strategies to enhance insulin action.…”

“…A very low calorie diet is associated with further deterioration of insulin action on glucose transport in adipose tissue from obese subjects [8,16]. Effects of obesity and VLCD on insulin signal transduction at the level of IRS-1, IRS-2, PI 3-kinase and glucose incorporation into lipid in isolated subcutaneous adipocytes from obese women was studied.…”

“…The obese women were investigated twice and thus adi-centrations, glucose transport is the rate-limiting step for lipogenesis in human adiocytes [16]. Lipogenesis was expressed as the amount of glucose incorporated per adipocyte number [16].…”