Abstract
Evidence both from mice and cultured cells suggests an important role of soluble leptin receptors in obesity and leptin signaling. However, the direct effects of soluble receptors on leptin uptake by cells are not clear. This study shows that soluble leptin receptors antagonize the permeation of leptin across the mouse blood‐brain barrier by reducing the binding and endocytosis of leptin. This is illustrated by analysis of radioactively labeled and fluorescent‐tagged leptin in normal mice and in cultured cells overexpressing various forms of leptin receptors. Three constructs of soluble leptin receptors were generated in this study: ObRe (805 aa), ObR839, and ObR852. ^125^I‐leptin was injected intravenously and its influx rate from blood to brain determined by multiple‐time regression analysis. Pre‐incubation with ObR839 caused a significant reduction of leptin influx across the blood‐brain barrier. Endocytosis assays and fluorescent image analysis further showed that ObRe, ObR839, and ObR852 failed to mediate leptin internalization and trafficking within the cells. Instead, these soluble receptors inhibited surface binding and endocytosis of leptin. Thus, we provide novel direct evidence both in vivo and in vitro that soluble receptors of leptin serve as antagonists of the transport of leptin. J. Cell. Physiol. 214: 301–305, 2008. © 2007 Wiley‐Liss, Inc.