Differential Expression of Suppressor of Cytokine Signaling and Interferon Gamma in Lean and Obese Patients with Type 2 Diabetes Mellitus

BACKGROUND: The model of obesity-induced insulin resistance has long been used to explain the development of type 2 diabetes mellitus (T2DM) in obese individuals (body mass index (BMI) > 25 kg/m(2)), but this model failed to explain the development of the disease in lean individuals (BMI < 18.5 kg/m(2)). Defects in the insulin signaling pathway have been postulated to play a role in these patients, particularly in suppressors of cytokine signaling (SOCS) proteins, which are involved in the downregulation of insulin transduction. The expression of SOCS is also known to be induced by cytokines such as interferon gamma (IFN-γ). It is still not clear whether these pathways operate differently in lean versus obese patients with T2DM. Therefore, this pilot study was designed to study the expression of SOCS1, SOCS3, and IFN-γ in lean and obese patients with T2DM. OBJECTIVES: The levels of IFN-γ in serum and the messenger RNA (mRNA) expression of SOCS (SOCS1 and SOCS3) and IFN-γ genes in whole blood in lean and obese patients with T2DM. METHODS: Sixty newly diagnosed T2DM patients (not on any pharmacotherapy) were enrolled and divided into 2 groups of lean (BMI < 18.5 kg/m(2)) and obese (BMI > 25 kg/m(2)) patients (n = 30 per group). Serum IFN-γ was measured by enzyme-linked immunosorbent assay (ELISA), and mRNA expression of IFN-γ, SOCS1, and SOCS3 was measured by real-time polymerase chain reaction (PCR) using the ∆∆ Ct method. RESULTS: Serum IFN-γ levels were 10.83 ± 5.81 pg/mL in the lean group and 9.35 ± 5.14 pg/mL in the obese group (P = 0.02). Fasting serum insulin levels were 16.07 ± 8.39 µIU/mL in the lean group and 27.11 ± 4 .91 µIU/mL in the obese group (P = 0.001). There was a 3.16-fold increase in mRNA expression of IFN-γ and a 1.3-fold increase in mRNA expression of SOCS1 in the lean group compared to the obese group. mRNA expression of SOCS3 was similar in both groups. CONCLUSIONS: The level of IFN-γ increased at both transcriptional and translational levels, and mRNA expression of SOCS1 was higher in the lean group than in the obese group. The SOCS protein is a known negative regulator in insulin signaling pathways. Thus, our findings and available scientific literature suggest that IFN-γ might impair the insulin signaling pathway to a greater extent in lean patients than in obese patients via induction of SOCS1. This signaling pathway could be a major contributing factor to hyperglycemia in lean patients with T2DM compared with obese counterparts. This suggests that different therapeutic approaches to these groups might be of greater benefit in the treatment of T2DM.