Clathrin‐independent internalization of the human histamine H 1 ‐receptor in CHO‐K1 cells

The aim of the present study was to investigate the cellular pathway involved in histamine‐stimulated internalization of the human H 1 ‐receptor in CHO‐K1 cells expressing N‐terminal myc‐tagged H 1 ‐receptor (Myc‐H 1 ) or N‐terminal myc‐C‐terminal green fluorescent protein (Myc‐GFP H 1 ) versions of the receptor. Studies of 3 H‐mepyramine binding and histamine‐stimulated 3 H‐inositol phosphate accumulation in these cells showed that the Myc‐H 1 and Myc‐GFP H 1 ‐receptors had identical pharmacology to the wild‐type H 1 ‐receptor. The Myc‐H 1 ‐receptor was rapidly internalized in CHO‐K1 cells following stimulation with histamine (0.1 m M ). This response occurred within 15 min, and could be prevented by the quaternary H 1 ‐receptor antagonist α ‐pirdonium. Similar data were obtained with the Myc‐GFP H 1 ‐receptors. Internalization of the Myc‐GFP H 1 ‐receptor was maintained in the absence of extracellular calcium and was not inhibited by the CAM kinase II inhibitor KN‐62 (10  μ M ). Phorbol dibutyrate, an activator of protein kinase C, was also able to stimulate internalization of the H 1 ‐receptor. However, inhibition or downregulation of protein kinase C (which significantly modified histamine‐stimulated inositol phosphate responses) was without effect on the internalization of the H 1 ‐receptor stimulated by histamine. Hypertonic sucrose did not prevent histamine‐induced internalization of the Myc‐GFP H 1 ‐receptor, but was able to attenuate internalization of transferrin via clathrin‐mediated endocytosis in the same cells. In contrast, preincubation of cells with filipin or nystatin, which disrupts caveolae and lipid rafts, completely inhibited the histamine‐induced internalization of the Myc‐GFP H 1 ‐receptor, but was without effect on the sequestration of transferrin. The H 1 ‐receptor and cholera toxin subunit B were colocalized under resting conditions at the cell surface. Immunohistochemical studies with an antibody to caveolin‐1 confirmed that this protein was also localized predominantly to the plasma membrane. However, following stimulation of CHO‐Myc‐GFP H 1 cells with histamine, there was no evidence for internalization of caveolin‐1 in parallel with the H 1 ‐receptor. These data provide strong evidence that the H 1 ‐receptor is internalized via a clathrin‐independent mechanism and most likely involves lipid rafts.British Journal of Pharmacology (2005) 146 , 612–624. doi: 10.1038/sj.bjp.0706337