MM.1S

Characterization of the MM.1 Cell Lines

MM is a clonal B-lymphocyte malignancy characterized by accumulating terminally differentiated antibody-producing cells in the bone marrow. The MM.1 cells grow in suspension or lightly attached to the flask singly or in small clusters. Their growth media is supplemented only by 10% fetal calf serum, glutamine, fungizone, and penicillin-streptomycin. The cells have a doubling time of 72 hours and grow best at high density.

The MM.1 cells have typical myeloma cell morphology. They are round with eccentrically located nuclei, many of which are binucleated or multinucleated (Fig. 1). They have well-developed rough endoplasmic reticulum and prominent nucleoli. Cell staining showed that MM.1 was a cytochemical characteristic of myeloma cells. Immunocytochemical staining showed that the MM.1 cells contain cytoplasmic λ- and not a κ-light chain (Table 1).

The MM.1 cells are characteristic plasma cells. They are negative for CD19 and CD20 found on most normal and malignant B lymphocytes. They have low expression of CD10 (common lymphoblastic leukemia antigen), which is found on immature B cells, and have low expression of CD45. The MM.1 cells express the plasma cell surface marker CD38 (Table 1) and the plasma cell–associated antigen (PCA-1). The MM.1 cells also express HLA-DR (D-related human leukocyte antigen); CD59, which is a GPI-linked complement receptor seen on the surface of some lymphocytes; CD52; and CD25, the interleukin-2 (IL-2) receptor.

Fig. 1 Morphological features of the MM.1S cultured cells. (Greenstein S, et al., 2003)

Table 1. Expression of cell surface markers in the MM.1S cell lines (Greenstein S. et al., 2003).

TGF-β Induces Growth Suppression in MM.1S Cells

Transforming growth factor-β (TGF-β) is important in essential cell phenotypes, particularly in cell proliferation, differentiation, and apoptosis. The present study found that cell proliferation was markedly inhibited in TGF-β-treated MM.1S cells.

MM.1S cells were grown in the presence or absence of TGF-β. Cell viability was assessed using an MTS assay. It was found that the growth of MM.1S cells was inhibited by TGF-β in a time-dependent manner (P<0.05; Fig. 2A). This was confirmed by a reduction in cell count (P<0.05; Fig. 2B). MM.1S cells were seeded overnight at 3×10⁵/ml, during which time almost 60% of cells were synchronized in the G1 phase of the cell cycle, as measured by PI staining (Fig. 2C). Cells were then stimulated or not with TGF-β for 48 h. Control cells progressed from G1 to S phase through the cell cycle, whereas TGF-β-treated cells exhibited efficient G1 cell cycle suppression (P<0.05; Fig. 2C). The present study found that cell proliferation was markedly inhibited in TGF-β-treated MM.1S cells.

To study the contribution of E2F1 in mediating this TGF-β response, RNA interference was used to reduce the expression of endogenous E2F1. It was found that the effect of TGF-β on cell viability in the MM.1S cell lines tested was almost completely prevented when E2F1 expression was silenced, indicating that E2F1 is required for mediating the TGF-β-induced response in the MM.1S cell line (P<0.05; Fig. 3).

Fig. 2 TGF-β induces efficient G1 cell cycle suppression in MM.1S cells. (Liu X, et al., 2017)

Fig. 3 TGF-β-mediated cell growth suppression is impaired in E2F1-null MM.1S cells. (Liu X, et al., 2017)