Thiazolidinedione addition reduces the serum retinol-binding protein 4 in type 2 diabetic patients treated with metformin and sulfonylurea

Retinol-binding protein 4 (RBP4) has been found to induce insulin resistance and to be increased in type 2 diabetes. Thiazolidinediones (TZDs) can improve insulin sensitivity through the activation of peroxisome proliferators-activated receptor-gamma (PPAR-γ) and have been suggested as an adjunct to metformin (MF) and sulfonylurea (SU) in type 2 diabetes in a consensus statement from the ADA and EASD. Therefore, we investigated whether TZD could affect serum RBP4 level in type 2 diabetes already treated with MF and/or SU. Eighty-one type 2 diabetic patients were divided into 2 groups: (1) TZD group (n = 55): Pioglitazone 30 mg/day was given as an add-on medication; (2) SU group (n = 26): Gliclazide MR 30–120 mg or glimepiride 2–8 mg/day was prescribed. The average period of study was 97.1 days. Serum RBP4 and adiponectin were measured by enzyme-linked immunosorbent assay and radioimmunoassay, respectively. The addition of pioglitazone (TZD group) markedly decreased homeostasis model assessment of insulin resistance (HOMA-IR) (P = 0.021) compared with the SU group (P = 0.688). The change of RBP4 in the TZD group (−3.87 ± 11.27 μg/mL) significantly differed from that in the SU group (2.52 ± 8.24 μg/mL, P < 0.012). The increase of adiponectin in the TZD group (11.49 ± 7.85 μg/mL) was apparently higher than that in the SU group (1.54 ± 5.62 μg/mL, P < 0.001). Despite the change of glycosylated hemoglobin (HbA1c) did not differ (−0.77 ± 1.3 vs −0.50 ± 1.7, P = 0.446), the addition of pioglitazone could significantly lower serum RBP4 and HOMA-IR values, whereas an increased dosage of sulfonylurea agents did not alter HOMA-IR, RBP4, or adiponectin in type 2 diabetic patients who had been treated with metformin and/or sulfonylurea.