A xenograft model reveals that PU.1 functions as a tumor suppressor for multiple myeloma in vivo.

We previously demonstrated that PU.1 expression is down-regulated in the majority of myeloma cell lines and primary myeloma cells from patients. We introduced the tet-off system into the human myeloma cell lines U266 and KMS12PE that conditionally express PU.1 and demonstrated that PU.1 induces cell cycle arrest and apoptosis in myeloma cells in vitro. Here, we established a mouse xenograft model of myeloma using these cell lines to analyze the effects of PU.1 on the phenotype of myeloma cells in vivo. When doxycycline was added to the drinking water of mice engrafted with these myeloma cells, all mice had continuous growth of subcutaneous tumors and could not survived more than 65 days. In contrast, mice that were not exposed to doxycycline did not develop subcutaneous tumors and survived for at least 100 days. We next generated mice engrafted with subcutaneous tumors 5-10 mm in diameter that were induced by exposure to doxycycline. Half of the mice stopped taking doxycycline-containing water, whereas the other half kept taking the water. Although the tumors in the mice taking doxycycline continued to grow, tumor growth in the mice not taking doxycycline was significantly suppressed. The myeloma cells in the tumors of the mice not taking doxycycline expressed PU.1 and TRAIL and many of such cells were apoptotic. Moreover, the expression of a cell proliferation marker Ki67 was significantly decreased in tumors from the mice not taking doxycycline, compared with that of tumors from the mice continuously taking doxycycline. The present data strongly suggest that PU.1 functions as a tumor suppressor of myeloma cells in vivo.

Lactate, a putative survival factor for myeloma cells, is incorporated by myeloma cells through monocarboxylate transporters 1.

BACKGROUND: Lactate levels within tumors are correlated with metastases, tumor recurrence, and radioresistance, thus apparently contributing to poor outcomes in patients with various cancers. We previously reported that high-level production of lactate by multiple myeloma (MM) cell lines is associated with high-level LDH activity within such MM cells. However, the kinetics of lactate remains to be studied. In the present study, we attempted to elucidate the mechanism of lactate incorporation into MM cells. METHODS: Six MM cell lines and stromal cells obtained through long-term culture of bone marrow samples from MM patients were employed. Incorporation of lactate was quantified using C(14)-labeled lactate. The role of MCT1, a member of the monocarboxylate transporters (MCTs), expressed on MM cells, was examined in the presence of its inhibitor (α-cyano-4-hydroxycinnamic acid: CHC) and by using gene-silencing technique. RESULTS: MM cell lines as well as stromal cells were found to produce lactate. Incorporation of C(14)-labeled lactate into MM cells occurred in all 6 MM cell lines analyzed. Inhibition of MCT1 by using CHC or MCT1-targeting siRNA reduced lactate incorporation and caused apoptosis in MM cells. This apoptosis was enhanced when the activity of pyruvate dehydrogenase kinase was blocked by dichroloacetate. Survival of normal peripheral blood mononuclear cells was not influenced by MCT1 inhibition. CONCLUSIONS: The present data suggest that lactate is produced by MM cell lines and stromal cells, and contributes to the survival of such MM cells in autocrine or paracrine manners. Suppression of lactate incorporation by targeting MCT1 may provide a novel therapeutic strategy for MM which may be applicable for other B-cell neoplasms.