In human monocytes 100 ng/mL leptin increased both statin-inhibitable free radical and cholesterol production in vitro. In our recent study, we aimed to elucidate the concentration dependence of observed leptin-effect. Following leptin stimulation cholesterol synthesis was measured in the presence of inhibitors to determine affected signal pathways. Leptin at low (10-100 ng/mL) concentrations increased [(14)C]acetate incorporation, whereas at 250 ng/mL and higher concentrations it suppressed cholesterol synthesis. HMG CoA reductase, phosphatidyl-3-kinase (PI3K) and mitogen activated protein kinase (MAPK) were involved in mediating leptin effects at low concentrations, whereas the cholesterol synthesis suppression was abolished by inhibitors of protein kinase C (PKC) and PI3K.