Specific cancer markers are powerful tools for early cancer diagnosis, prognosis prediction and monitoring of disease progression. To date, no general cancer marker is available for different cancer types. Monoclonal antibody (mAb) 376.96 has been used for detection of micrometastatic tumor cells for its ability to recognize a wide range of tumor cells, but not to most normal cells. Yet, the cellular antigen for 376.96 mAb is still unknown. In this study, using immunoprecipitation, immunoblotting, and siRNA technologies, we discovered that the antigen is human immunoregulatory molecule 4Ig-B7H3 (4Ig-hB7H3). ELISA data demonstrated that 376.96 mAb has a high specificity and sensitivity to 4Ig-hB7H3. Furthermore, immunoblot results from subcellular fractionation and tunicamycin treatment indicated that 4Ig-hB7H3 is a ~100-kDa N-linked glycoprotein that is expressed only on the cell membrane of many different cancer types. These data indicates that 4Ig-hB7H3 presents as a valuable cancer marker to discriminate between tumor cells and normal cells.

Interestingly, tumor promoter phorbol 12-myristate 13-acetate (PMA) significantly enhanced the expression of 4Ig-hB7H3 in FEMX-I (melanoma), MA11 (breast cancer), and OHS (osteosarcoma) cells, suggesting a relationship between 4Ig-hB7H3 and tumor biological functions. Functional studies showed that in both melanoma and breast cancer cells down-regulation of 4Ig-hB7H3 by siRNA-hB7H3 reduced cell adhesion ability 2-fold. Down-regulation also significantly inhibited migration and matrigel-invasion by more than 3-fold, while no apparent impact on cell proliferation and on sensitivity of the tumor cells to NK cell-mediated cytoxicity was observed. In summary, these data indicate that 4Ig-hB7H3 is an important cancer marker. The results shed new light on the novel role of the immunoregulatory molecule 4Ig-hB7H3 in tumor metastasis and progression and it is critical for improving the cancer diagnosis and cancer treatment in the future.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA