Insulin and Rosiglitazone Regulation of Lipolysis and Lipogenesis in Human Adipose Tissue In Vitro

Abstract

Lipolysis is an important process determining fuel metabolism, and insulin regulates this process in adipose tissue. The aim of this study was to investigate the long-term effects of insulin, an insulin enhancer (rosiglitazone [RSG]), and insulin in combination with RSG on the regulation of lipolysis and lipogenesis in human abdominal subcutaneous fat. Lipolysis and lipogenesis were assessed by protein expression studies of hormone-sensitive lipase (HSL) (84 kDa) and lipoprotein lipase (LPL) (56 kDa), respectively. In addition, lipolytic rate was assessed by glycerol release assay and tumor necrosis factor (TNF)-␣ release measured by enzyme-linked immunosorbent assay (n ‫؍‬ 12). In subcutaneous adipocytes, increasing insulin doses stimulated LPL expression, with maximal stimulation at 100 nmol/l insulin (control, 1.0 ؎ 0.0 [mean ؎ SE, protein expression relative to control]; 1 nmol/l insulin, 0.87 ؎ 0.13; 100 nmol/l insulin, 1.68 ؎ 0.19; P < 0.001). In contrast, insulin at the 100 nmol/l dose reduced the expression of HSL (100 nmol/l insulin, 0.49 ؎ 0.05; P < 0.05), while no significant reduction was observed at other doses. Higher doses of insulin stimulated both HSL (1,000 nmol/l insulin, 1.4 ؎ 0.07; P < 0.01) and LPL (control 1.00 ؎ 0.0; 1,000 nmol/l insulin, 2.66 ؎ 0.27; P < 0.01) protein expression. Cotreatment with RSG induced an increased dose response to insulin for LPL and HSL (P < 0.05); RSG alone also increased LPL and HSL expression (P < 0.05). Insulin stimulated TNF-␣ secretion in a dose-dependent manner (P < 0.01); the addition of RSG (10 ؊8 mol/l) reduced TNF-␣ secretion (P < 0.05). In summary, chronic treatment of human adipocytes with insulin stimulates lipolysis and LPL protein expression. The addition of RSG reduced the lipolytic rate and TNF-␣ secretion. The increase in lipolysis is not explained by changes in HSL expression. These data, therefore, may explain in part why hyperinsulinemia coexists with increased circulating nonesterified free fatty acids and increased adiposity in obese and/or type 2 diabetic patients.

Differential regulation of CIDEA and CIDEC expression by insulin via Akt1/2- and JNK2-dependent pathways in human adipocytes

“…Therefore, we examined lipolysis in human adipocytes treated with JNK1 or JNK2 siRNA. Consistent with previous studies ( 45,52 ), insulin did not affect the basal levels of glycerol release (supplementary Fig. I).…”

PPARγ agonism increases rat adipose tissue lipolysis, expression of glyceride lipases, and the response of lipolysis to hormonal control

“…With regard to HSL, other studies have reported a strong PPARγ-induced increment in expression in human subcutaneous mature adipocytes [29] and in rodent brown adipocytes [30] incubated short-term with rosiglitazone. Also, HSL was recently shown to be a transcriptional target of PPARγ in differentiating pre-adipocytes and other tissues, but not in visceral WAT in vivo [31].…”

The effects of rosiglitazone on fatty acid and triglyceride metabolism in type 2 diabetes

“…Lipoprotein lipase (LPL) mRNA expression and mass increased [32,33] or were unchanged [33,34], depending on the dose of the thiazolidinedione and model used. The activity of LPL generally decreased with treatment [33,35], although this reduction was not always statistically significant [35].…”