Leptin, the product of the ob gene, regulates food intake, energy balance and lymphocyte functions. Receptor mutated animals have reduced production of proinflammatory cytokines. To analyze the direct effects of leptin (0.1 nM /1 mM) on cytokine production from colonic cells, we examined pro-inflammatory (IL-8, MCP-1, IL-6 and TNFalpha) and anti-inflammatory (IL-10) cytokine production in normal human colonic epithelial fresh cells and in colon cancer cell lines (HT-29 and Caco-2) using an ELISA and the Rnase Protection Assay (RPA). Leptin increased the release of IL-8 and MCP-1 and slightly reduced IL-10 release, but did not affect both basal-stimulated and LPS-stimulated IL-6 and TNF-alpha in the supernatant of confluent cells. Up to 24 h after cell exposure to leptin, IL-8 significantly increased and IL-10 decreased, and these effects were dose and time dependently documented in cancer cell lines (HT29 and Caco2). To investigate the mechanisms of this pro-inflammatory effect, by luciferase reporter gene assay, we showed an increase in the activity of NF-kappaB in response to leptin in a dose-dependent manner. This activity was inhibited in IkappaB super-repressor transfected cells as assessed by luciferase expression and IL-8 and MCP-1 production (ELISA and RPA). Furthermore, PPAR-gamma was reduced after leptin treatment as assessed by western blot while the addition of PPAR agonists (Rosiglitazone and MCC-555) induced a decrease of IL-8 and MCP-1 expression, suggesting PPAR-gamma suppression is required for cytokine productions. The failure of the PPAR-gamma antagonist (GW-9662) to inhibit cytokine production and to alter NF-kappaB activity suggest that PPAR-gamma suppression is required for cytokine productions. We demonstrate that leptin regulates pro-inflammatory cytokine production by colonic epithelial cells via PPAR-gamma modulation.