Cytokeratin 18 is expressed on the hepatocyte plasma membrane surface and interacts with thrombin-antithrombin complexes.
During experiments to identify putative hepatic receptors for thrombin-antithrombin (TAT) complexes, a 45-kDa protein was identified by ligand blotting. Following gel purification, amino acid sequencing revealed the 45-kDa TAT-binding polypeptide to be cytokeratin 18 (CK18). The presence of CK18 on the surface of intact rat hepatoma cells was demonstrated by binding of 125I-anti-CK18 antibodies. Anti-CK18 antibodies reduced the binding and internalization of 125I-TAT by rat hepatoma cells. Immunocytochemical analysis, to determine the location of CK18 in vivo, revealed a periportal gradient of CK18 staining; with hepatocytes around the portal triads demonstrating striking pericellular staining. In addition, anti-CK18 IgG associated with perfused livers to a significantly greater extent than preimmune IgG. Taken together, these data provide evidence that CK18 is found on the extracellular surface of hepatocytes and could play a role in TAT removal. Finally, these data, in conjunction with recent reports of CK8 (Hembrough, T. A., Li, L., and Gonias, S. L. (1996) J. Biol. Chem. 271, 25684-25691) and CK1 cell membrane surface expression (Schmaier, A. H. (1997) Thromb. Hemostasis 78, 101-107), indicate a novel role for these proteins as putative cellular receptors or cofactors to cellular receptors.
Wells
M J
MJ
Department of Pathology, McMaster University, Hamilton, Ontario L8N 3Z5, Canada.
Hatton
M W
MW
Hewlett
B
B
Podor
T J
TJ
Sheffield
W P
WP
Blajchman
M A
MA
D000818:Animals; D000906:Antibodies / Q000378:metabolism; D000990:Antithrombin III / Q000378:metabolism*; D001667:Binding, Competitive; D006528:Carcinoma, Hepatocellular / Q000378:metabolism; D002417:Cattle; D002462:Cell Membrane / Q000378:metabolism; D015696:Gene Products, tat / Q000378:metabolism; D006801:Humans; D007633:Keratins / Q000096:biosynthesis*; D008099:Liver / Q000166:cytology / Q000378:metabolism*; D008113:Liver Neoplasms / Q000378:metabolism; D010447:Peptide Hydrolases / Q000378:metabolism*; D010477:Perfusion; D011817:Rabbits; D051381:Rats; D013499:Surface Properties; D014407:Tumor Cells, Cultured
D000906:Antibodies; D015696:Gene Products, tat; C046193:antithrombin III-protease complex; D007633:Keratins; D000990:Antithrombin III; D010447:Peptide Hydrolases