Effect of thiazolidinediones on equilibrative nucleoside transporter-1 in human aortic smooth muscle cells

Thiazolidinediones are a new class of anti-diabetic agents which increase insulin sensitivity by binding to the peroxisome proliferator-activated receptor γ (PPARγ) and stimulating the expression of insulin-responsive genes involved in glucose and lipid metabolism. These drugs also have vasodilatory and anti-proliferative effects on vascular smooth muscle cells. However the mechanisms for these actions are not fully understood. Adenosine is a vasodilator and a substrate of equilibrative nucleoside transporters (ENT). The present study studied the effects of three thiazolidinediones, troglitazone, pioglitazone and ciglitazone, on ENT1 in the human aortic smooth muscle cells (HASMCs). Although incubating HASMCs for 48 h with thiazolidinediones had no effect on ENT1 mRNA and protein levels, troglitazone acutely inhibited [ ³H]adenosine uptake and [³H]NBMPR binding of HASMCs with IC₅₀ values of 2.35 ± 0.35 and 3.99 ± 0.57 μM, respectively. The 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 HASMCs. Troglitazone differs from pioglitazone and ciglitazone in that its side-chain contains a Vitamin E moiety. The difference in structure of troglitazone did not account for its inhibitory effect on ENT1 because Vitamin E did not inhibit [³H]adenosine uptake by HASMCs. Using the nucleoside transporter deficient PK15NTD cells stably expressing ENT1 and ENT2, it was found that troglitazone inhibited ENT1 but had no effect on ENT2. From these results, it is suggested that troglitazone may enhance the vasodilatory effect of adenosine by inhibiting ENT1. Pharmacologically, troglitazone is a novel inhibitor of ENT1.