ADHESION OF CARCINOMA-CELLS TO RAT HEPATOCYTES AND RAT FIBRONECTIN ISINHIBITED BY THE OPAR MONOCLONAL-ANTIBODY, WHICH IS DIRECTED AGAINST A RAT LIVER-SPECIFIC CARBOHYDRATE EPITOPE

The OPAR mouse monoclonal antibody (mAb) directed against rat hepatocy tes was previously shown to inhibit adhesion of TA3/Ha mammary carcino ma cells to hepatocytes. The antigen is abundantly present at the surf ace of hepatocytes beneath the endothelium of liver capillaries where we have observed invasion of carcinoma cells to occur, The OPAR mAb re acted with three major bands on a Western blot of liver plasma membran e proteins, The same proteins were also seen upon immunoprecipitation from iodinated liver plasma membrane proteins, We have isolated OPAR a ntigens by lectin wheat germ agglutinin (WGA) and OPAR affinity chroma tography. Amino acid sequence analysis revealed that two of the bands were alpha 1-macroglobulin and C4-binding protein, which are serum com ponents produced by hepatocytes. The presence of the epitope on distin ct proteins and our previous observation that it can be detected in th e Golgi apparatus but not in the endoplasmic reticulum, suggested that OPAR reacts with a liver-specific glycoconjugate, Loss of OPAR reacti vity after neuraminidase and N-glycosidase F treatment showed that the epitope contains sialic acid residues on N-linked sugar moieties, OPA R also reacted with rat fibronectin, and inhibited adhesion of TA3/St cells to fibronectin. This explains the inhibition by the OPAR mAb of TA3/St cell adhesion to hepatocytes, which we have shown to be due mai nly to interaction with hepatocyte surface-associated fibronectin, How ever, adhesion of the related TA3/Ha cells to hepatocytes, which is me diated by the alpha(6) beta(4) integrin, and does not involve binding to fibronectin, is also inhibited, This suggests that alpha(6) beta(4) on liver-metastasizing carcinoma cells binds to an OPAR epitope-carry ing glycoprotein produced by hepatocytes.