Generation of soluble leptin receptor by ectodomain shedding of membrane-spanning receptors in vitro and in vivo

Leptin is an adipocyte-derived hormone with potent effects on food intake and body weight. Genetically obese rodents with mutations of leptin or leptin receptor develop morbid obesity and diabetes. The receptor for leptin, OB-R, is alternatively spliced to at least five transcripts, encoding receptors designated OB-Ra, -b, -c, -d, and -e. OB-Re does not encode a transmembrane domain and is secreted. In humans, transcripts corresponding to OB-Re have not been discovered. However, soluble leptin receptor does circulate in human plasma and represents the major leptin-binding activity. In this report, we attempted to determine whether the soluble leptin receptor may also be derived from membrane-spanning receptor isoforms by ectodomain shedding. Using stable cell lines expressing both OB-Ra, the most abundant leptin receptor isoform, and OB-Rb, the signaling form of the leptin receptor, we demonstrate that soluble leptin receptor protein can indeed be generated by proteolytic cleavage of these two receptor isoforms in vitro. Experiments using adenoviruses expressing dually tagged OB-Ra or Ob-Rb also demonstrate that soluble leptin receptor may be derived from ectodomain shedding of both receptor isoforms in vivo. Because our earlier and other studies have shown that the soluble receptors modulate the levels as well as activity of leptin, our findings suggest that regulated shedding of the ectodomain of membrane-spanning leptin receptors may represent a novel mechanism of modulating leptin's biological activity.