Identification and characterization of mouse bone marrow stromal cell lines immortalized by temperature-sensitive SV40 T antigen: supportive activity for osteoclast differentiation

Osteoblasts are derived from mesenchymal/stromal cells in bone marrow, and gain the ability to support osteoclastogenesis during differentiation though the expression of receptor activator of NF-κB ligand (RANKL) and macrophage-colony stimulating factor (M-CSF). However, the properties (differentiation stage and expression of osteoblast marker genes) of stromal or osteoblastic cells that have the capacity to support osteoclast differentiation are unclear. Therefore, we sought to establish and characterize bone marrow-derived stromal cell lines (TSB) from temperature-sensitive SV40 T-antigen transgenic mice to define them at the clonal level. Of the 24 randomly selected cell lines, only 2 cell lines, TSB13 and TSB20, could support osteoclast differentiation in the presence of 1α,25(OH)2D3. In both cell lines, RANKL mRNA was induced and osteoprotegerin (OPG) mRNA was decreased in response to treatment with 1α,25(OH)2D3 for 2 days. Other RNA expression analyses of osteoblast-specific marker genes demonstrated the following characteristics of TSB13 and TSB20: (1) alkaline phosphatase (ALP) and type I collagen genes are expressed; (2) osteocalcin and osteopontin genes are expressed at low levels, and their expression levels are upregulated after induction of differentiation by a temperature shift from 33°C to 37°C, or 1α,25(OH)2D3 treatment. Consequently, the long-term culture of TSB13 and TSB20 cell lines strongly stimulated osteocalcin expression and effectively induced calcified nodule formation in the presence of phosphate. The results suggest that the supportive cells for osteoclastogenesis are restricted to a specialized population of bone marrow stromal cells, and the high ratio of RANKL vs. OPG expression found in this population after 1α,25(OH)2D3 treatment might be a general property of osteoclast-supporting cells.