Abstract
Leptin is a hormone that plays an important role in overall body energy homeostasis, and the obesity receptor, OB-R, is widely distributed in the organism. In the intestine, a multitude of leptin actions have been reported, but it is currently unclear to what extent the hormone affects the intestinal epithelial cells by an endocrine or exocrine signaling pathway. To elucidate this, the localization of endogenous porcine leptin and OB-R in enterocytes and colonocytes was studied. By immunofluorescence microscopy, both leptin and OB-R were mainly observed in the basolateral membrane of enterocytes and colonocytes but also in the apical microvillar membrane of the cells. By electron microscopy, coclustering of hormone and receptor in the plasma membrane and localization in endosomes was frequently detected at the basolateral surface of the epithelial cells, indicative of leptin signaling activity. In contrast, coclustering occurred less frequently at the apical cell surface, and subapical endosomal localization was hardly detectable. We conclude that leptin action in intestinal epithelial cells takes place at the basolateral plasma membrane, indicating that the hormone uses an endocrine pathway both in the jejunum and colon. In contrast, the data obtained did not provide evidence for an exocrine, lumenal action of the hormone in the intestine.