Quantification of PPAR-γ protein in monocyte/macrophages from healthy smokers and non-smokers: A possible direct effect of nicotine

Previous observations demonstrated that Peroxisome Proliferator-Activated Receptor-gamma (PPAR-γ), a key regulator of adipocyte differentiation, is expressed in a large variety of cells, including cells of the monocyte/macrophage lineage. This study was aimed to quantify both the constitutive and ligand-induced PPAR-γ expression in monocytes and monocyte-derived macrophages (MDM) isolated from healthy smokers and non-smokers, and to evaluate the possible direct effect of nicotine. PPAR-γ protein was detected by Western blot and quantification was performed by calculating the ratio between PPAR-γ and β-actin protein expression. Cytokine release was measured with enzyme-linked immunoassay kits. Constitutive PPAR-γ protein was detected in human monocytes and its expression was up-regulated along with differentiation to MDM. The endogenous ligand 15-deoxy-delta12,14-prostaglandin J2 and the synthetic agonist ciglitazone enhanced PPAR-γ expression, the former being effective also at low micromolar concentrations. Both agonists significantly inhibited the basal secretion of pro-inflammatory cytokines (e.g., TNF-α, IL-6), ciglitazone being more potent. Monocytes and MDM from healthy smokers presented a significantly enhanced (4-fold and 2.5-fold, respectively) constitutive PPAR-γ expression, as compared to those from healthy non-smokers. However, ligand-induced PPAR-γ expression and inhibition of cytokine secretion were similar in healthy smokers and non-smokers. Nicotine dose-dependently enhanced PPAR-γ expression with a maximum at 10 μM, and inhibited release of pro-inflammatory cytokines; these effects were reversed by α-bungarotoxin. Nicotine and PPAR-γ agonists did not exert synergistic effects. In conclusion, monocytes and MDM from healthy smokers present a constitutively enhanced PPAR-γ expression; this effect is reproduced, to some extent, by nicotine in vitro.