Friedman-Einat, M., Institute of Animal Science, Agricultural Research Organization, Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel
Camoin, L., Institut Cochin, 22 rue Méchain, 75014 Paris, France
Faltin, Z., Institute of Horticulture, Agricultural Research Organization, Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel
Rosenblum, C.I., Department of Obesity Research, Merck Research Laboratories, P.O. Box 2000, Rahway, NJ 07065, United States
Kaliouta, V., Institute of Animal Science, Agricultural Research Organization, Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel
Eshdat, Y., Institute of Horticulture, Agricultural Research Organization, Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel
Strosberg, A.D., Institut Cochin, 22 rue Méchain, 75014 Paris, France

Blood levels of the satiety hormone leptin are directly correlated to fat stores in obese and lean people. Therefore, leptin resistance is the logical explanation for the phenomenon of common obesity. However, the important question of whether or not the intrinsic leptin activity could differ between obese and lean people has not been examined before. In the present study, serum leptin activity was measured by an in vitro assay of leptin signaling in a modified culture of HEK-293 cells. The system is based on activation of a luciferase reporter gene through a leptin receptor-dependent activation of the signal transducer and activator of transcription (STAT3). Serum samples from 20 obese and 20 non-obese individuals with leptin levels ranging from 3 to 75 ng/ml, as determined by radioimmunoassay (RIA), were used. A high correlation was observed for each serum sample between leptin RIA values and leptin activity in the bioassay. The results indicate that obesity in the 20 obese patients among the 40 individuals examined cannot be accounted for by alterations in leptin activity in our assay. The assay system provides a tool to screen for possible rare cases exhibiting alteration in leptin activity either due to a change in leptin itself or through interaction with other serum factors. © 2002 Elsevier Science B.V. All rights reserved.