Carbonic Anhydrase IX Promotes the Tumorigenicity of Adult T-Cell Leukemia/Lymphoma-Derived Cells in NOG Mice

Carbonic anhydrase IX (CA9) is a membrane-associated carbonic anhydrase that regulates pH through its extra-cellular catalytic domain and thereby fosters tumor cell survival, growth, and invasion. The expression of CA9 is induced by hypoxia via HIF-1α. CA9 overexpression is known to be closely associated with aggressive behaviors and poor prognoses in various types of human solid tumors. In particular, CA9 is a critical mediator of the expansion of breast cancer stem cells in hypoxic niches by sustaining the mesenchymal and 'stemness' phenotypes of these cells. However, as for hematological malignancies, there are few reports suggesting a relation between its tumorigenecity and CA9. Here we demonstrated that CA9 plays a crucial role in promoting the tumorigenicity of the ATL-derived cells.

Recently we established a highly tumorigenic subline, ST1-N6, from an ATL-derived cell line, ST1, by serial transplantation in immunodeficient NOG mice. ST1-N6 cells developed subcutaneous tumors under a shorter duration and smaller cell number than parental ST1 cells. DNA microarray gene expression analysis revealed an upregulated expression of CA9 in ST1-N6 cells as compared with ST1 cells. After serial sorting of ST1 cells by CA9 expression, we obtained ST1-CA9^high and ST1-CA9^low sublines. Like ST1-N6 cells, ST1-CA9^high cells showed stronger tumorigenicity than ST1-CA9^low or parental ST1 cells. Knockdown of CA9 expression with shRNAs suppressed the tumorigenicity of ST1-CA9^high cells, whereas ST1 cells introduced with CA9 significantly showed enhanced tumorigenicity. Another ATL-derived cell line, TL-Om1 also showed increased tumorigenicity in NOG mice when introduced with CA9. These observations indicate that CA9 contributes to promoting the tumorigenic potential of ATL-derived cells in xenotransplantation.

To investigate CA9 expression on fresh ATL cells, we prepared peripheral blood from six acute ATL patients, and tested for quantitative expression of CA9 in sera of six ATL patients and three healthy donors by ELISA. The amounts of serum CA9 were 31.1~95.4 pg/ml in the healthy donors, while they were 175.6~1, 971.6 pg/ml in the ATL patients. We next prepared ATL tissues from nine ATL patients, and two of them contained a few CA9^high cells, which were cells expressing CD25, suggesting that a part of ATL lymphoma cells expressed CA9. These results suggested that the amount of serum CA9 tends to be higher in ATL patients than healthy donors.

Based on these findings, we suppose that CA9 plays an important role in the malignant development of ATL.