Urocortin trafficking in cerebral microvessel endothelial cells.

Urocortin, a potent peptide inhibitor of feeding behavior, can enter the brain from blood by leptin-facilitated permeation across the blood-brain barrier. Here, we show in cultured RBE4 cerebral microvessel endothelial cells that urocortin endocytosis is increased by leptin in a time- and dose-dependent manner. Fluorescently labeled urocortin (Alexa488-urocortin) shows vesicular trafficking localized in early endosomes at 1 min and the Golgi complex at 20 min. The endocytosis at 20 min was increased by 10 microg/mL, but not 2 microg/mL, of leptin. The facilitating effect of leptin at the dose of 10 microg/mL was seen at 20 and 30 min but not at 10 min. This increase could be abolished by excess unlabeled urocortin in radio-tracer uptake studies, indicating selective rather than nonsaturable entry. The specificity of the effect was further supported by the lack of changes in gamma-glutamyl transpeptidase activity and endothelial nitric oxide synthase upon stimulation by high doses of leptin and urocortin. Leptin did not affect the level of expression of the urocortin corticotropin-releasing hormone receptor (CRHR) after 30 min of treatment but appeared to slow the turnover of CRHRs induced by urocortin. In MDCK cells overexpressing CRHR2, leptin facilitated urocortin uptake, whereas ObRa coexpression did not exert an additional effect. Thus, urocortin endocytosis is a saturable process leading to vesicular intracellular transport that can be enhanced by cell-surface leptin.