Secretion of soluble leptin receptors by exocrine and endocrine cells of the gastric mucosa

Leptin is a hormone secreted by the gastric mucosa into the lumen of the stomach. It is present in its intact form in the intestine where it regulates nutrient absorption and intestinal mucosa integrity. We have identified the binding protein that protects leptin from the harsh conditions of the gastric juice. Immunoprecipitations and Western blot analyses demonstrated that leptin is present in the gastric mucosa and the gastric juice, bound to a protein corresponding to the extracellular domain of the leptin receptor. In the absence of this soluble receptor, leptin is rapidly degraded. Immunocytochemistry on rat gastric mucosa identified the cells and intracellular compartments involved in secretion of this complex. Leptin receptor extracellular domain and leptin are present along the rough endoplasmic reticulum-Golgi-granules secretory pathways and form a complex in the secretory granules of Chief and specific endocrine cells. The long-form membrane leptin receptor OB-Rb, the protease activator furin, and proprotein convertase 7 were found in Chief cell granules but not in those of endocrine cells. The shedding of the receptor occurs in the immature granules. It is concluded that in the immature secretory granules of Chief cells, furin activates proprotein convertase 7 that, in turn, cleaves the extracellular portion of membrane-bound leptin receptors. Leptin bound to its soluble receptor forms a complex that is resistant to the gastric juice. Endocrine cells, on the other hand, generate a soluble leptin receptor by mechanisms different from those of the exocrine cells.