Involvement of Ets transcription factors and targets in osteoblast differentiation and matrix mineralization

The osteoblast-like MC3T3-E1 cell line provides an. excellent in vitro mode l of bone development. This system undergoes three orderly time-dependent p hases characterized by proliferating preosteoblasts, matrix accumulation by postmitotic differentiating osteoblasts, and mineralization of the matrix, which results in the formation of multilayered bone nodules. The Ets famil y transcription factors regulate genetic programs that affect the prolifera tion and differentiation of osteoblasts. Of the eight Ets family transcript ion factors examined by our laboratory, only Ets1 and Ets2 were found to be expressed at significant levels in this osteogenic system. Ets1 is express ed in proliferating preosteoblastic cells, whereas Ets2, silent during this phase, is expressed by differentiating and mature osteoblasts. In addition , the expression of Ets1 can be induced in MC3T3-E1 and fetal rat calvaria cells by retinoic acid (RA) which is known to exert profound effects on ske letal growth and development and bone turnover and induce specific cellular responses in bone cells. Thus, the multiple functions of RA in bone cells are likely to be mediated in part by Ets1. We show that the expression of E ts2 precedes and then parallels osteopontin expression and that the OPN pro moter contains Ets binding sites and is a transcriptional target of Ets2. i n order to identify other potential Ets target genes, we analyzed promoter regions of genes revealed by serial analysis of gene expression as present in the differentiation stage. The functional analysis of these genes has th e potential to provide much needed information as to their function in oste ogenesis and mineralization of the extracellular matrix and in bone-related diseases. (C) 2000 Academic Press.