Troglitazone stimulates the insulin-induced tetrahydrobiopterin synthesis in vascular endothelial cells

We investigated the effects of troglitazone, which is a ligand of nuclear transcription factor peroxisome proliferator-activated receptor-gamma (PPAR-gamma), on insulin-induced tetrahydrobiopterin (BH4) synthesis in mouse brain microvascular endothelial cells (MBMECs). BH, content was determined as biopterin, by reversed-phase high performance liquid chromatography using fluorometric detection. Measurement of mRNA level of GTP cydohydrolase I(GTPCH), which is a rate-limiting enzyme for de novo BH, synthesis, was performed by reverse transcription-polymerase chain reaction (RT-PCR). Addition of insulin (10 mug/ml) to endothelial cells increased intra cellular BH4 content and GTPCH mRNA level, and the insulin-induced increase in BH, content and GTPCH mRNA level was further stimulated by co-treatment with troglitazone (0.3-3 muN). However, the expression of PPAR-gamma mRNA in untreated and insulin-treated endothelial cells was not detected. Moreover, bezafibrate (1-10 muM), a ligand of PPAR-delta, did not affect BH4 synthesis in insulin-treated endothelial cells. These findings suggest that troglitazone stimulates insulin-induced BH4 synthesis in endothelial cells with the induction of GTPCH through PPAR-gamma- and PPAR-delta -independent mechanism.