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 of GPIb alpha by immunohistochemistry. GPIb alpha was detected in six of 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 (PMA)-incubated breast carcinoma MCF-7 cells were immunoprecipitated with 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, but not with its biologically inactive derivative MePMA, induced a three- to fourfold increase in the surface expression of both GPIb alpha and GPIb/IX by flow cytometry. This PMA-enhanced GPIb alpha expression was almost completely abrogated when MCF-7 cells were first preincubated with the specific protein kinase C (PKC) inhibitor, H-7, prior to PMA 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 enhancement could be abrogated by H-7. These findings confirm the expression of a protein with homology to platelet GPIb alpha expressed by fresh human breast carcinoma tissues, demonstrate that PMA enhances GPIb membrane expression by MCF-7 cells, and suggest that PKC plays a role in this process.