Obesity is associated with increased adipose tissue macrophage (ATM) infiltration, and rodent studies suggest that inflammatory factors produced by ATMs contribute to insulin resistance and type 2 diabetes. However, a relationship between ATM content and insulin resistance has not been clearly established in humans. Since thiazolidinediones attenuate adipose tissue inflammation and improve insulin sensitivity, we examined the temporal relationship of the effects of pioglitazone on these two parameters. The effect of 10 and 21 days of pioglitazone treatment on insulin sensitivity in 26 diabetic subjects was assessed by hyperinsulinemic-euglycemic clamp studies. Because chemoattractant factors, cytokines, and immune cells have been implicated in regulating the recruitment of ATMs, we studied their temporal relationship to changes in ATM content. Improved hepatic and peripheral insulin sensitivity was seen after 21 days of pioglitazone. We found early reductions in macrophage chemoattractant factors after only 10 days of pioglitazone, followed by a 69% reduction in ATM content at 21 days and reduced ATM activation at both time points. Although markers for dendritic cells and neutrophils were reduced at both time points, there were no significant changes in regulatory T cells. These results are consistent with an association between adipose macrophage content and systemic insulin resistance in humans.

Macrophages are more abundant in adipose tissue from obese individuals than from those of normal weight and may contribute to the metabolic consequences of obesity by producing various circulating factors. One of these factors is plasminogen activator inhibitor-1 (PAI-1), which contributes to both atherosclerosis and insulin resistance. Because nutritional factors appear to regulate PAI-1 expression, we hypothesized that exposure to fatty acids and adipocyte secretory products could stimulate production of PAI-1 by adipose macrophages. Increased free fatty acid (FFA) concentrations in blood for 5 hours in nondiabetic, overweight subjects markedly suppressed insulin-stimulated glucose uptake and raised circulating PAI-1 concentrations, with a concomitant increase in the expression of the PAI-1 gene in adipose tissue. FFAs also rapidly increased PAI-1 gene expression in adipose macrophages and PAI-1 protein immunofluorescence surrounding these cells. By contrast, PAI-1 expression in circulating monocytes was very low and was not affected by raising the concentration of FFAs. Medium from cultured adipocytes stimulated PAI-1 expression in cultured macrophages and potentiated the increase in PAI-1 messenger RNA expression in response to FFAs. Together, our data suggest that adipocyte-derived factors prime adipose macrophages so that they respond to nutritional signals (FFAs) by releasing a key inflammatory adipokine, PAI-1.

The regulation of glucose effectiveness involves a complex interplay of hormonal and metabolic factors, with free fatty acid and glucoregulatory hormones playing key roles. The loss of this regulation in type 2 diabetes mellitus contributes importantly to hyperglycemia, and may largely be caused by increased free fatty acid levels.

Impaired effectiveness of glucose to suppress endogenous glucose production (EGP) is an important cause of worsening hyperglycemia in type 2 diabetes. Elevated free fatty acids (FFAs) may impair glucose effectiveness via several mechanisms, including rapid changes in metabolic fluxes and/or more gradual changes in gene expression of key enzymes or other proteins. Thus, we examined the magnitude and time course of effects of FFAs on glucose effectiveness in type 2 diabetes and whether glucose effectiveness can be restored by lowering FFAs. Glucose fluxes ([3-3 H]-glucose) were measured during 6-h pancreatic clamp studies, at euglycemia (5 mmol/l glucose, t ‫؍‬ 0 -240 min), and hyperglycemia (10 mmol/l, t ‫؍‬ 240 -360 min). We studied 19 poorly controlled subjects with type 2 diabetes (HbA 1c 10.9 ؎ 0.4%, age 50 ؎ 3 years, BMI 30 ؎ 2 kg/m 2 ) on at least two occasions with saline (NA؊ group) or nicotinic acid (NA group) infusions for 3, 6, or 16 h (NA3h, NA6h, and NA16h groups, respectively) to lower FFAs to nondiabetic levels. As a reference group, glucose effectiveness was also assessed in 15 nondiabetic subjects. There was rapid improvement in hepatic glucose effectiveness following only 3 h of NA infusion (NA3h ‫؍‬ 31 ؎ 6% suppression of EGP with hyperglycemia vs. NA؊ ‫؍‬ 8 ؎ 7%; P < 0.01) and complete restoration of glucose effectiveness after 6 h of NA (NA6h ‫؍‬ 41 ؎ 8% suppression of EGP; P ‫؍‬ NS vs. nondiabetic subjects). Importantly, the loss of hepatic glucose effectiveness in type 2 diabetes is completely reversible upon correcting the increased FFA concentrations. A longer duration of FFA lowering may be required to overcome the chronic effects of increased FFAs on hepatic glucose effectiveness.