ABSTRACT
Signet-ring cell carcinoma is one of the most malignant tumors, classified histologically as a poorly differentiated adenocarcinoma. The ErbB2/ErbB3 complex is often constitutively activated, which suggests that the ErbB2/ErbB3 signaling pathway may be important for malignancy of this tumor. However, the mechanism underlying this activation has not been understood. Here, we show that ErbB2 and Muc4 bind in signet ring carcinoma cells, which was not seen in highly differentiated adenocarcinoma cell lines. ErbB3 was suggested to be a substrate of ErbB2 because knockdown of ErbB2 resulted in less phosphorylation of ErbB3. Inhibition of expression of Muc4 at the cell surface by the treatment of the cells with benzyl-GalNac, an inhibitor of mucin secretion, blocked phosphorylation of ErbB3, suggesting that activity of ErbB2 depends on the expression of Muc4. These results supply the biochemical backgrounds in recent studies suggesting the contribution of Muc4 in the tumorigenesis.
Subject(s)
Mucins/metabolism , Receptor, ErbB-2/metabolism , Adenocarcinoma/genetics , Carcinoma, Signet Ring Cell/pathology , Cell Differentiation , Cell Line , Cell Line, Tumor , Humans , Kidney , Mucin-4 , Mucins/genetics , Phosphorylation , RNA, Neoplasm/genetics , Receptor, ErbB-2/genetics , Receptor, ErbB-3/metabolism , Recombinant Proteins/metabolism , TransfectionABSTRACT
HCC2998 is a highly differentiated human colon carcinoma cell line, which has been shown to be converted to a poorly differentiated one after expression of a constitutively active phosphatidylinositol 3-kinase (PI3' kinase). These cells express aberrant sizes of a regulatory subunit of PI3' kinase, p85alpha, with molecular weights of 50 and 76 kDa at a very low level. To elucidate how these cells express these proteins, we analyzed mutations within the p85alpha gene. DNA sequencing analysis revealed that these mutant proteins were generated by independent point mutations in the two alleles of the p85alpha gene: one in the coding sequence, and the other in the acceptor sequence for splicing. Introduction of wild-type p85alpha into HCC2998 cells induced slight rounding of the cells and enhancement of mucin secretion. At the same time, a membrane receptor, ErbB3, was phosphorylated on tyrosine, which in turn, binds to PI3' kinase. Since ErbB3 is upstream of PI3' kinase, it is likely that there is an autocrine loop in which PI3' kinase is activated by ErbB3, which may contribute to dedifferentiation of the cells.
