METABOLISM OF CARCINOEMBRYONAL ANTIGEN

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Carcinoembryonic antigen (CEA) can function as both a homotypic and heterotypic cell adhesion molecule. Our studies have shown that CEA may be involved in the development of hepatic metastases from colorectal cancers possibly as a consequence of its cell adhesion properties. This study focuses on the structure of CEA binding proteins in the liver. The major hepatic CEA binding protein is a 80kD subunit Kupffer cell surface molecule that probably functions as a homodimer. A similar protein is found on lung alveolar macrophages, but not on peritoneal macrophages or circulating monocytes. Peptide sequences will be obtained from purified 80kD binding protein and from derived peptides by microsequencing from PVDF membranes. This data will be used to construct oligonucleotide probes and as a check on sequences obtained from cloning the gene for the 8OkD binding protein. Both polyclonal and monoclonal antibodies will be made to the 80kD protein or to derived peptides. The gene responsible for this protein will be cloned and sequenced from both rat and human Kupffer cell cDNA libraries by probing expression vectors with synthetic oligonucleotides, antibodies or using a functional CEA binding assay. This information will be used to pinpoint functional domains and will be important in understanding the role of this binding protein in directing site specific (lung and liver) metastases. The binding site for the 8OkD protein has been located to the junction of the N-terminal and first loop domains of CEA. This 10 amino acid sequence contains a pentapeptide with sequence homology to a pentapeptide (PELPK) found in complement subcomponent Cls, stromelysin and collagenase 1. Stromelysin and collagenase are extracellular matrix degrading enzymes that have been implicated in tumor cell invasion and metastases. The peptide sequence at the junction of the N-terminal and first loop domain of CEA that binds to the 8OkD Kupffer cell protein will therefore, be narrowed down to the minimum structure required for activity. The potential role of the 8OkD protein as a binding site for stromelysin collagenase and complement Cls will be investigated. This study expands our knowledge of mechanisms by which Kupffer cells process glycoproteins and the potential role of the 8OkD binding protein in the mechanism by which colorectal cancer cells attach and invade in the liver.