CEA is the major PHA-L-reactive glycoprotein in colon carcinoma cell lines and tumors: Relationship between K-ras activation and β1-6 branching of N-linked carbohydrate on CEA
Oncogene Protein p21(ras)
Previously we have shown that a positive correlation existed between the presence of beta1-6 branching of N-linked carbohydrate (detected as PHA-L reactivity) and the level of Ras activation in colon carcinoma cell lines. In these cell lines the major PHA-L-reactive species was found to be 180 kDa. Here we identified this species to be carcinoembryonic antigen (CEA) by demonstrating that: (a) CEA immunoreactivity and PHA-L reactivity colocalized on blots of crude cellular membranes from these cell lines, and that (b) immunoprecipitation of CEA resulted in quantitative coprecipitation of PHA-L reactivity at 180 kDa. Metabolic labeling of cell line HTB39 with [(3)H]mannose revealed that CEA was the predominantly labeled glycoprotein. This indicated that CEA was the major PHA-L-reactive species due its high level of expression. The amount of PHA-L reactivity present on CEA, expressed as the PHA-L/CEA ratio, was found to vary between cell lines. This ratio was found to correlate closely with the level of Ras activation in these cells. In cellular membrane isolated from primary colon carcinoma, the major PHA-L-reactive species was also 180 kDa. This reactivity colocalized with CEA immunoreactivity, indicating that the major beta1-6-branching glycoprotein in membranes from primary colon carcinoma was CEA. Similar to that seen in cell lines, the amount of PHA-L reactivity on CEA in human tumor samples varied, suggesting that a similar paradigm of Ras-induced expression of beta1-6 branching may occur in human colon carcinoma.