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.