Objective: The soluble leptin receptor (sOb-R) is generated by ectodomain shedding of the membrane-associated Ob-R and may modulate leptin action in metabolic dysregulations. The objective of our study was to investigate whether or not sOb-R generation may be induced by indicators of energy availability or metabolic regulators in a cell model.
Material and methods: The four human Ob-R isoforms were cloned, characterized by binding experiments with 125-iodinated leptin and transfected into HEK 293 cells. Cells were stimulated with levels of leptin, glucose, phorbol 12-myristate 13-acetate (PMA), insulin, human growth hormone, dexamethasone, troglitazone and AICAR. Ob-R gene expression was controlled by an isoform specific TaqMan assay. sOb-R levels in cell supernatant were measured by an in-house immunofunctional assay.
Results: Leptin incubation of our Ob-R transfected (P<0.01) and non-transfected cells (P<0.05) lead to a concentration-dependent significant decrease of sOb-R. Total glucose deprivation and PMA stimulation were associated with increased sOb-R results, whereas low levels of glucose restored the normal shedding function of the cells. No significant effect on sOb-R was observed after incubation with the remaining substances.
Conclusions: Presumed that sOb-R levels in supernatant of cells reflect the density of the Ob-R membrane receptor, the leptin-dependent decrease of sOb-R suggests receptor down-regulation or leptin resistance. This suggestion is in-line with in vitro data of obese subjects. In contrast, the increase of sOb-R by glucose deprivation may support the hypothesis that Ob-R shedding is associated with reduced energy availability of the cell. The consequence of both effects for the net action of leptin has to be determined in further experiments. Additionally, the increase of sOb-R concentration after PMA stimulation underlines that the shedding process is PKC dependent.