Osteoblastic cells regulate mi gene expression in cultured bone marrow cells.

The microphthalmia (mi) gene encodes a bHLH-Zip protein and certain mutations in this gene are known to result in osteopetrosis. We examined mi gene expression in murine bone marrow (BM) cells and calvaria-derived osteoblastic (OB) cells. The mi message level in BM cells was three- to four-fold higher than that in OB cells. The mi message level in BM cells alone was not enhanced by 1,25(OH)2 vitamin D3 (vitamin D) which induces TRAP-positive multinucleated cells, but was enhanced two-fold by the addition of conditioned medium obtained from cultures of OB cells. The mi message levels in BM cells were also enhanced by macrophage colony-stimulating factor (M-CSF). The mi message levels in cocultures of BM cells and osteoblastic MC3T3-E1 cells, which do not support osteoclastogenesis, were similar to those in the cocultures of BM cells and OB cells. Cocultures of BM cells with MC3T3-E1 cells did not yield TRAP-positive multinucleated cells, but they did maintain osteoclast progenitor cells. These findings suggest that mi is expressed in BM cells that may be under the control of OB cells.