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Transcellular Transport of Leptin by the Short Leptin Receptor Isoform ObRa in Madin-Darby Canine Kidney Cells¹

Department of Medicine, Division of Endocrinology and Metabolism, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts 02215

Address all correspondence and requests for reprints to: Christian Bjørbæk, 325 Research North, Beth Israel Deaconess Medical Center, 99 Brookline Avenue, Boston, Massachusetts 02215. E-mail: cbjorbae{at}caregroup.harvard.edu

Leptin is an adipocyte-derived hormone that acts in specific regions of the brain to regulate body weight and neuroendocrine function. The mechanism by which leptin enters the brain is unknown. We previously reported that rat brain microvessels, which constitute the blood-brain barrier, contain large amounts of messenger RNA encoding a short form of the leptin receptor (ObRa), suggesting that this site may be important for receptor-mediated transport of leptin into the brain. The purpose of this study was to determine whether ObRa is capable of transcellular transport of intact leptin. A transwell system in which Madin-Darby Canine Kidney (MDCK) cells stably expressing ObRa are grown in a monolayer was used to determine receptor distribution on apical or basolateral cell surfaces and the capacity for directional transport of ¹²⁵I-leptin. Binding of ¹²⁵I-leptin was greater on the apical vs. the basolateral cell surface and transport of ¹²⁵I-leptin occurred only in the apical to basolateral direction. 11% of transported radioactivity appearing in the basolateral chamber represented intact leptin as assessed by TCA precipitation analysis and by SDS-PAGE. Parental MDCK cells did not express leptin receptors and did not bind or transport ¹²⁵I-leptin. Epidermal growth factor (EGF) binding and transport via endogenous EGF receptors in MDCK cells also was assessed. In contrast to leptin, specific binding of ¹²⁵I-EGF occurred primarily on the basolateral cell surface and transport of ¹²⁵I-EGF occurred predominantly in the basolateral to apical direction. These data show that ObRa is preferentially targeted to the apical cell membrane in MDCK cells and that leptin transport occurs, albeit at a low rate, in a unidirectional manner in the apical to basolateral direction. These findings may be relevant to the putative role of ObRa in receptor-mediated transport of leptin from the circulation into the brain.

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