Xenograft Models of Multiple Myeloma Reveal That PU.1 Serves As a Tumor Suppressor for Multiple Myeloma

PU.1 is an essential transcription factor for hematopoiesis and important for differentiation of both myeloid and lymphoid lineages. In mice conditionally knocked-out of 3.4 kb length of the enhancer region located in14 kb 5’ upstream of the PU.1 gene (URE), PU.1 is down-regulated in myeloid cells and B cells by 20% of that of wild type, and such mice develop acute myeloid leukemia and CLL-like diseases. These data strongly suggest that PU.1 has tumor suppressor activity in hematopoietic cells. We previously reported that human PU.1 is down-regulated in the majority of myeloma cell lines through the methylation of the promoter and the 17 kb upstream enhancer region (URE) of the PU.1 gene that is homologous to that in 14 kb 5’ upstream of the murine PU.1 gene. Conditionally expressed PU.1 with tet-off system induced cell growth arrest and apoptosis in two myeloma cell lines, KMS12PE and U266, suggesting that the down-regulation of PU.1 is necessary for myeloma cell growth. We have also reported that PU.1 is expressed in normal plasma cells and in contrast, PU.1 is down-regulated in primary myeloma cells from a subset of myeloma patients, who appear to have poor prognosis.

In the present study, to test whether PU.1 has tumor suppressor activity in vivo, we generated xenograft mouse models. 0.6 - 1 x 10⁷ KMS12PE cells were subcutaneously injected in 16 immunodeficient mice (Rag2-/- Jak3-/- bulb/c). The mice were then administered doxycycline through drinking water. Half of the mice (N=8) stopped taking doxycycline when the tumor sizes reached 1 cm in diameter, whereas the other half (N=8) kept taking doxycycline. Although the tumors in the mice taking doxycycline continued to grow, the tumor growth in the mice not taking doxycycline significantly slowed down. Flow cytometry analysis of the tumors in the mice that stopped taking doxycycline revealed that the cells from the tumor had completely lost PU.1 expression. Moreover, when U266 cells conditionally expressing PU.1 were subcutaneously injected to another 10 mice and the same experiment was conducted, although the tumors in the mice taking doxycycline (N=5) kept growing, the tumors in the mice not taking doxycycline (N=5), did not grow any further. The present data suggest that PU.1 serves as a tumor a suppressor in the multiple myeloma cell lines as examined in vivo.