The effects of leptin, in concentrations seen in obesity, on collagen production and turnover in non-immortalized human hepatic stellate cell (HSC), were unknown. The profibrogenic effects of leptin in these cells were studied. Hepatic stellate cells were obtained from resected livers. Collagen I/III gene expression and protein production were measured by quantitative real-time polymerase chain reaction and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, respectively. The signal transduction pathways involved were evaluated by specific blockers of the phosphatidylinositol 3-kinase (PI3K), mitogen-activated protein kinase kinase (MEK), and Janus kinase 2 (JAK2). The effects on matrix metalloproteinase 1 (MMP-1) and tissue inhibitor of metalloproteinase 1 (TIMP-1) were assessed by their gene transcript levels, collagenolytic activity of cell culture supernatants, and MMP-1 protein levels. At concentrations seen in nonobese individuals ([leptin] < 10 ng/mL), leptin did not affect collagen production. At concentrations seen in obesity (30-50 ng/mL), leptin increased collagen I and III messenger RNA (mRNA) transcript levels by 286% +/- 55% (P < .001) and 167% +/- 62% (P < .007) and protein production by 45.8% +/- .02% and 84.39% +/- .01%, respectively. These effects were blocked by JAK2 inhibition as well as PI3K inhibition. Although MEK inhibition blocked leptin-induced procollagen I and III mRNA levels, there were no significant effects on collagen I and III protein levels. Leptin (10-50 ng/mL) had no significant effects on MMP-1 or TIMP-1 mRNA levels, collagenolytic activity, or MMP-1 protein levels. In conclusion, leptin, at levels seen in obese individuals, produces an increase in collagen production by HSC acting through the JAK and PI3K pathways. At these concentrations, leptin does not affect MMP-1 or TIMP-1 expression or collagenolytic activity of HSC.