The soluble leptin receptor is crucial for leptin action: evidence from clinical and experimental data.

OBJECTIVE: The soluble leptin receptor (sOB-R) was recently identified as the main leptin-binding protein in human blood. The aim of our study was to elucidate the effects of physiologically relevant amounts of sOB-R on leptin-induced proliferation in a cell model. SUBJECTS AND MEASUREMENTS: To determine molar ratios between sOB-R and leptin in vivo, we measured both parameters in the serum of 529 healthy children and adolescents. For our in vitro cell model, mouse pre-B cells, transfected with the long form of the murine leptin receptor (BAF3/L46), were incubated with recombinant human leptin at two different basal levels (0.5 and 0.1 nM) and with the sOB-R at varying levels. The proliferative response of the cells was quantified by a (3)H-thymidine uptake assay.
RESULTS: Significantly higher molar sOB-R/leptin ratios were observed during the first years of life, up to a 7.67-fold excess of sOB-R (quartiles: 4.43/10.27) in boys, compared to the states of prepuberty and puberty. An up to 10-fold molar excess of the sOB-R, reflecting the in vivo situation, resulted in a significant suppression of leptin action in the cell model. In contrast, gradually decreasing ratios of lower than two, as calculated during the progression of childhood and in early puberty, corresponded to proliferative rates in vitro as determined at basal leptin concentrations.
CONCLUSION: At a distinct molar excess, sOB-R may suppress leptin action through inhibition of specific leptin binding to membrane-bound receptors in vitro. In vivo, sOB-R may further function to delay leptin clearance and increase the available leptin pool in the circulation. In the case of a massive excess of sOB-R, is it likely to be inhibitory to leptin interaction with the tissue membrane-bound OB-R.