Gene array analysis of osteoblast differentiation

We have used gene array technology to chart changes in gene expression during differentiation of the mouse calvarial-derived MC3T3-E1 cell line to an osteoblast-like phenotype. Expression was analyzed on a mouse gene array panel of 588 cDNAs representing tightly regulated genes with key roles in various biological processes. When compared with NIH3T3 fibroblasts, MC3T3-E1 cells showed generally higher expression of cyclins and Bcl-2 family members, as well as specific expression of products such as the CD44 antigen, which is consistent with their calvarial origin. MC3T3-E1 cells also showed a surprisingly high level of p53. Differentiation in MC3T3-E1 cells involves withdrawal from the cell cycle by day 7, accompanied by matrix accumulation and, ultimately, mineralization. Gene expression patterns in induced MC3T3-E1 cells generally reflected these stages. Cyclins were sharply down-regulated, and expression of certain antiproliferative factors and tissue-restricted genes was induced. Many of the observed changes, such as the induction of follistatin, bone morphogenetic protein receptor 1A, transforming growth factor beta, and matrix remodeling factors, reflect expected patterns and support the physiological relevance of the results. Other observed changes were not anticipated and offer new insight into the osteoblast differentiation process. An example is the sharp induction of the Tob antiproliferative factor, which has previously been associated specifically with terminal differentiation in muscles. Another example is the induction of the DNA damage-associated proteins EI24 and Gadd45, apparently as a normal aspect of osteoblast differentiation. The oxidative stress-induced protein A170 and the transcription factor Nrf2, which regulates metabolic responses to oxidative stress, were also induced. This response may reflect the in vivo requirement for vascularization during bone growth and fracture repair. Other induced factors include tumor necrosis factor receptor-associated factor-1 (1-TRAF), which is a nuclear factor kappaB activator, cellular retinoic acid-binding protein II (CRABP-II), and the transcription factors S-II, SP2, and SEF2 (ITF2/E2:2). SEF2 is the first basic helix-loop-helix protein found to be up-regulated during osteoblast differentiation. Northern blots confirm the induction of SEF2.