Effect of thiazolidinediones on adenosine transport in vascular smooth muscle cells

Abstract

Objective: Thiazolidinediones are a new class of anti-diabetic agents which increase insulin sensitivity by binding to the peroxisome proliferators-activated receptor γ (PPARγ) and stimulate the expression of insulinresponsive genes involved in glucose and lipid metabolism. These drugs also have vasodilatory and antiproliferative effects on vascular smooth muscle cells (VSMCs). However the mechanisms for these actions are not understood. Adenosine is a vasodilator. Equilibrative nucleoside transporter (ENT)-1, which is an Na+- independent and nitrobenzylmercaptopurine riboside (NBMPR)-sensitive nucleoside transporter, plays an important role in adenosine functions because it fine-tunes the local concentrations of adenosine in the vicinity of adenosine receptors. In the present study, the effects of three thiazolidinediones, troglitazone, pioglitazone and ciglitazone, on ENT1 in VSMCs were studied. Methods: The ENT1 activity in VSCMs was determined by the initial rate of [3H]adenosine uptake and [3H]NBMPR binding assay. The mRNA and protein expressions of ENT1 were detected by the reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting, respectively. Results: Incubating VSMCs for 48 hr with thiazolidinediones had no effect on ENT1 mRNA and protein levels. However, troglitazone inhibited [3H]adenosine uptake and [3H]NBMPR binding of VSMCs with IC50 values of 2.35 ± 0.35 µM and 3.99 ± 0.57 µM, respectively. This effect of troglitazone on ENT1 was PPARγ- independent and kinetic studies revealed that troglitazone was a competitive inhibitor of ENT1. In contrast, pioglitazone and ciglitazone had minimal effects on [3H]adenosine uptake by VSMCs. Troglitazone differs from pioglitazone and ciglitazone as its side-chain contains a vitamin E moiety. This difference in structure of troglitazone did not account for its inhibitory effect on ENT1 because vitamin E did not inhibit [3H]adenosine uptake by VSMCs. Conclusions: Troglitazone inhibits ENT1. It is suggested that troglitazone may alter the vascular effect of adenosine.