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Glucocorticoids induce insulin resistance in humans, whereas thiazolidinediones enhance insulin sensitivity. Although the effects of glucocorticoids and thiazolidinediones have been assessed in isolation, interaction between these drugs, which both act as ligands for nuclear receptors, has been less well studied. Therefore, we examined the metabolic effects of dexamethasone and troglitazone, alone and in combination, for the first time in humans. A total of 10 healthy individuals with normal glucose tolerance (age 40 +/- 11 years, BMI 31 +/- 6.1 kg/m(2)) were sequentially studied at baseline, after 4 days of dexamethasone (4 mg/day), after 4-6 weeks on troglitazone alone (400 mg/day), and again after 4 days of dexamethasone added to troglitazone. Key metabolic variables included glucose tolerance assessed by blood glucose and insulin responses to an oral glucose tolerance test (OGTT), insulin sensitivity evaluated via hyperinsulinemic-euglycemic clamp, free fatty acids (FFAs) and FFA suppressibility by insulin during the clamp study, and fasting serum leptin. Dexamethasone drastically impaired glucose tolerance, with fasting and 2-h OGTT insulin values increasing by 2.3-fold (P < 0.001) and 4.4-fold (P < 0.001) over baseline values, respectively. The glucocorticoid also induced a profound state of insulin resistance, with a 34% reduction in maximal glucose disposal rates (GDRs; P < 0.001). Troglitazone alone increased GDRs by 20% over baseline (P = 0.007) and completely prevented the deleterious effects of dexamethasone on glucose tolerance and insulin sensitivity, as illustrated by a return of OGTT glucose and insulin values and maximal GDR to near-baseline levels. Insulin-mediated FFA suppressibility (FFA decline at 30 min during clamp/FFA at time 0) was also markedly reduced by dexamethasone (P = 0.002). Troglitazone had no effect per se, but it was able to normalize FFA suppressibility in subjects coadministered dexamethasone. Futhermore, the magnitudes of response of FFA suppressibility and GDR to dexamethasone were proportionate. The same was true for the reversal of dexamethasone-induced insulin resistance by troglitazone, but not in response to troglitazone alone. Leptin levels were increased 2.2-fold above baseline by dexamethasone. Again, troglitazone had no effect per se but blocked the dexamethasone-induced increase in leptin. Subjects experienced a 1.7-kg weight gain while taking troglitazone but no other untoward effects. We conclude that in healthy humans, thiazolidinediones antagonize the action of dexamethasone with respect to multiple metabolic effects. Specifically, troglitazone reverses both glucocorticoid-induced insulin resistance and impairment of glucose tolerance, prevents dexamethasone from impairing the antilipolytic action of insulin, and blocks the increase in leptin levels induced by dexamethasone. Even though changes in FFA suppressibility were correlated with dexamethasone-induced insulin resistance and its reversal by troglitazone, a cause-and-effect relationship cannot be established. However, the data suggest that glucocorticoids and thiazolidinediones exert fundamentally antagonistic effects on human metabolism in both adipose and muscle tissues. By preventing or reversing insulin resistance, troglitazone may prove to be a valuable therapeutic agent in the difficult clinical task of controlling diabetes in patients receiving glucocorticoids.

To explore the potential role of the uncoupling protein (UCP) family in human obesity and diabetes, we have used the reverse transcription-polymerase chain reaction to quantify UCP mRNA expression in human skeletal muscle. Levels of mRNA for UCP2, and for both short (UCP3S) and long (UCP3L) forms of UCP3, were highly correlated in individuals, indicating that gene transcription of these UCPs may be coordinately regulated by common mechanisms. In normal glucose-tolerant individuals, muscle UCP2 mRNA levels were positively correlated with percentage of body fat and with BMI (r = 0.6 and P < 0.05 for both). UCP3S mRNA levels were also positively correlated with percentage of body fat (r = 0.52, P < 0.05), and UCP3L mRNA tended to increase as a function of obesity (0.05 < P < 0.1). UCP mRNA levels, however, were not correlated with resting metabolic rate. UCP3S and UCP3L mRNA levels (P < 0.05) and the UCP2 mRNA level (P = 0.09) were increased by 1.8- to 2.7-fold in type 2 diabetes, an effect that could not be explained by obesity. No significant difference was found for UCP2, UCP3S, or UCP3L mRNA levels between insulin-sensitive and insulin-resistant nondiabetic subgroups. We conclude that 1) skeletal muscle mRNA levels encoding UCP2 and UCP3 are correlated among individuals and may be coordinately regulated; 2) UCP3 expression is not regulated by differential effects on UCP3L and UCP3S forms of the mRNA; and 3) UCP mRNA expression tends to increase in muscle as a function of obesity but not of resting metabolic rate or insulin resistance, and is increased in patients with type 2 diabetes.

Leptin is an adipocyte-derived hormone that interacts with a putative receptor(s) in the hypothalamus to regulate body weight. The relationship of leptin to metabolic abnormalities associated with obesity together with hormonal and substrate regulation of leptin have not been extensively studied. Therefore, 116 subjects (62 men and 54 women) with a wide range of body weight [body mass index (BMI), 17-54 kg/m2] were characterized on a metabolic ward with regard to body composition, glucose intolerance, insulin sensitivity, energy expenditure, substrate utilization, and blood pressure. Eighty-five of the subjects had normal glucose tolerance (50 men and 35 women), and 31 had noninsulin-dependent diabetes mellitus (12 men and 19 women). In both men and women, fasting leptin levels were highly correlated with BMI (r = 0.87 and r = 0.88, respectively) and percent body fat (r = 0.82 and r = 0.88, respectively; all P < 0.0001). However, men exhibited lower leptin levels at any given measure of obesity. Compared with those in men, leptin levels rose 3.4-fold more rapidly as a function of BMI in women [leptin = 1.815 (BMI)-31.103 in women; leptin = 0.534 (BMI)-8.437 in men] and 3.2 times more rapidly as a function of body fat [leptin = 1.293 (% body fat)-24.817 in women; leptin = 0.402 (% body fat)-3.087 in men]. Hyperleptinemia was associated with insulin resistance (r = -0.57; P < 0.0001) and high waist to hip ratio (r = 0.75; P < 0.0001) only in men. On the other hand, during the hyperinsulinemic euglycemic clamp studies, hyperinsulinemia acutely increased leptin concentrations (20%) only in women. There was no correlation noted between fasting leptin levels and either resting energy expenditure or insulin-induced thermogenesis in men or women (P = NS). In stepwise and multiple regression models with leptin as the dependent variable, noninsulin-dependent diabetes mellitus did not enter the equations at a statistically significant level. The data indicate that there are important gender-based differences in the regulation and action of leptin in humans. Serum leptin levels increase with progressive obesity in both men and women. However, for any given measure of obesity, leptin levels are higher in women than in men, consistent with a state of relative leptin resistance. These findings have important implications regarding differences in body composition in men and women. The observation that serum leptin is not related to energy expenditure rates suggests that leptin regulates body fat predominantly by altering eating behavior rather than calorigenesis.

Ras associated with diabetes (Rad), a new ras-related GTPase, was recently identified by subtractive cloning as an mRNA in skeletal muscle that is overexpressed in NIDDM. To better understand its metabolic significance, we measured skeletal muscle Rad expression in well-characterized insulin sensitive (IS) and insulin resistant (IR) subjects with normal glucose tolerance and in untreated NIDDM patients. We found no differences in expression of Rad mRNA levels among IS, IR, and NIDDM groups using a ribonuclease protection assay (0.22 +/- 0.06, 0.13 +/- 0.01, and 0.16 +/- 0.02 relative units, respectively; NS) and no differences in Rad protein expression using a specific anti-peptide Rad antibody (1.05 +/- 0.18, 1.14 +/- 0.08, and 1.08 +/- 0.21 units/mg protein, respectively; NS). However, Rad protein levels were positively correlated with BMI (r = 0.43, P = 0.03) and percentage body fat (r = 0.55, P < 0.005), two independent measures of obesity, and negatively correlated with resting metabolic rate (r = 0.49, P = 0.01). In multiple regression analyses, percentage body fat and resting metabolic rate independently accounted for 30 and 10% of individual variability in muscle Rad protein expression. In conclusion, Rad expression in skeletal muscle is not altered as a function of insulin resistance or NIDDM in humans. However, these data, for the first time, implicate a role for Rad in regulating body composition and energy expenditure and provide a framework for studies designed to elucidate Rad's cellular functions.