Direct Thiazolidinedione Action on Isolated Rat Skeletal Muscle Fuel Handling Is Independent of Peroxisome Proliferator–Activated Receptor-γ−Mediated Changes in Gene Expression

Abstract

Thiazolidinediones (TZDs) are believed to induce insulin sensitization by modulating gene expression via agonistic stimulation of the nuclear peroxisome proliferator–activated receptor-γ (PPAR-γ). We have shown earlier that the TZD troglitazone inhibits mitochondrial fuel oxidation in isolated rat skeletal muscle. In the present study, rat soleus muscle strips were exposed to TZDs to examine whether the inhibition of fuel oxidation is mediated by PPAR-γ activation. Our findings consistently indicated direct, acute, and PPAR-γ−independent TZD action on skeletal muscle fuel metabolism. Rapid stimulation of lactate release by 20 μmol/l troglitazone within 30 min suggested that direct TZD action on skeletal muscle in vitro does not rely on changes in gene expression rates (12.6 ± 0.6 [control] vs. 16.0 ± 0.8 μmol · g⁻¹ · h⁻¹ [troglitazone]; P < 0.01). This conclusion was supported by the failure of actinomycin D and cycloheximide to block the effects of troglitazone. Mitochondrial fuel oxidation was consistently inhibited by six different TZDs (percent inhibition of CO₂ production from palmitate after 25 h: troglitazone, −61 ± 2%; pioglitazone, −43 ± 7% ; rosiglitazone, −22 ± 6%; BM13.1258, −47 ± 9%; BM15.2054, −51 ± 4%; and T-174, −59 ± 4% [P < 0.005 each]), but not by PPAR-γ agonistic compounds not belonging to the TZD class (JTT-501, −5 ± 7% [NS]; prostaglandin J₂, 17 ± 7% [P < 0.05]), which further argues against dependence on PPAR-γ activation. In summary, our findings provided good evidence that direct inhibition of mitochondrial fuel oxidation in isolated skeletal muscle is a group-specific effect of TZDs and is independent of PPAR-γ−mediated gene expression.