A GPIB-ALPHA-RELATED PROTEIN IS EXPRESSED BY FRESH HUMAN BREAST-CARCINOMA TISSUE AND IS REGULATED BY A PKC-SENSITIVE MECHANISM

Fresh frozen breast carcinoma tissues were examined for the presence o f GPIb alpha by immunohistochemistry. GPIb alpha was detected in six o f seven primary invasive intraductal breast carcinoma tissues whereas staining was negative in seven of seven nonmalignant breast specimens. When biotin-labeled, triton-lysed, phorbol-12-myristate 13-acetate (P MA)-incubated breast carcinoma MCF-7 cells were immunoprecipitated wit h a MoAb directed against the platelet GPIb/IX complex, expression of GPIb was significantly enhanced in comparison to control preparations. Furthermore, incubation of MCF-7 cells for 84 h with 16 nmol/L PMA, b ut not with its biologically inactive derivative MePMA, induced a thre e-to fourfold increase in the surface expression of both GPIb alpha an d GPIb/IX by flow cytometry. This PMA-enhanced GPIb alpha expression w as almost completely abrogated when MCF-7 cells were first preincubate d with the specific protein kinase C (PKC) inhibitor, H-7, prior to PM A treatment. Finally, PMA-incubated MCF-7 cells demonstrated a 63% (N = 6; P < 0.001) increase in tumor-induced platelet agglutination when added to platelets in comparison to control tumor cells. This enhancem ent could be abrogated by H-7. These findings confirm the expression o f a protein with homology to platelet GPIb alpha expressed by fresh hu man breast carcinoma tissues, demonstrate that PMA enhances GPIb membr ane expression by MCF-7 cells, and suggest that PKC plays a role in th is process. (C) 1997 Academic Press.