Fig. 1. Antibody ligation of CD9 inhibits the cell proliferation of human cancer cell lines. (A) The indicated cell lines were serum-starved for 24 hours and subsequently cultured for 48 hours in the presence of 50 µg ml^–1 ALB6, isotype-matched mouse IgG1 or medium alone, and their proliferation was measured by a [³H]-thymidine incorporation assay. Data are represented as the means±s.e.m. from six replications and are expressed as a percentage of the values found for cells cultured in medium alone. *P<0.01 vs control. (B) MKN-28 cells were serum-starved for 24 hours and subsequently incubated for 48 hours with the indicated concentrations of ALB6, and their proliferation was measured by a [³H]-thymidine incorporation assay. Data are represented as the means±s.e.m. from six replications and are expressed as a percentage of the values found for cells treated with isotype-matched mouse IgG1 (50 µg ml^–1). *P<0.01 vs control. (C) After MKN-28 cells (2x10⁴ cells per well) were cultured on six-well plates for 24 hours, the medium was replaced with fresh serum-deprived medium containing 50 µg ml^–1 ALB6 or isotype-matched mouse IgG1. Their cell viability after ALB6 or isotype-matched mouse IgG1 treatment for the indicated periods was estimated by the trypan-blue dye-exclusion method. Data are represented as the mean cell viability (%) ±s.e.m. (n=4). *P<0.01 vs control. (D) After MKN-28 cells (1x10⁴ cells per well) were cultured on six-well plates for 24 hours, the medium was replaced with fresh serum-deprived medium containing different concentrations of ALB6. Their viable cell numbers were estimated by the trypan-blue dye-exclusion method after 96 hours of culture. Data are represented as mean cell viability (%) ±s.e.m. (n=4). *P<0.01 vs control. Similar results were observed in five independent experiments. (E) Effect of HB-EGF on ALB6-induced growth suppression in MKN-28 cells. The indicated cell lines were serum-starved for 24 hours and subsequently cultured in the presence of 50 µg ml^–1 ALB6 or isotype-matched mouse IgG1 together with or without 20 ng ml^–1 HB-EGF for 48 hours and their proliferation was measured by a [³H]-thymidine incorporation assay. Data are presented as means±s.e.m. from six repeats of the experiment and are expressed as a percentage of the values found for isotype-matched mouse IgG1 (50 µg ml^–1) treatment cells. (F) Effect of EGFR-Fc on ALB6-induced growth suppression in MKN-28 cells. The indicated cell lines were serum-starved for 24 hours and subsequently cultured in the presence of 50 µg ml^–1 ALB6 or isotype-matched mouse IgG1 together with or without 20 µg ml^–1 EGFR-Fc for 48 hours, and their proliferation was measured by a [³H]-thymidine incorporation assay. Data are presented as means±s.e.m. from six repeats of the experiment and are expressed as a percentage of the values found for isotype-matched mouse IgG1 (50 µg ml^–1) treatment cells.