Bone matrix mRNA expression in differentiating fetal bovine osteoblasts

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Abstract
In the accompanying study, we report an in vitro culture system from bovine bone cells that can be applied to investigate bone cell growth and differentiation. In this system, bovine bone cells placed in mineralization medium formed multilayers (days 2–3), began deposition of mineral (days 5–6), and eventually acquired a mineralized matrix sheet (days 14–20) through the stages of mineralizing nodules and trabecular-like structure. In the current study we used this system to investigate the relative expression of bone matrix genes that may play an important role in bone development and metabolism. α1(I)-collagen, alkaline phosphatase, osteonectin, biglycan (PgI), decorin (PgII), osteopontin, and bone sialoprotein mRNA gene expression were measured on days 0, 2, 6, 10, and 20 (date when the cells were placed in mineralization medium as day 0). Total RNA was purified and analyzed by northern blot using radiolabeled cDNA encoding these genes. To comprehend the relationship between gene expression and mineralization, total calcium content in the cultures was also measured. During the culture period we observed several very different gene expression profiles. The expression of both α1(I)-collagen and biglycan increased 3- to 4-fold by day 6 and then returned to basal levels by day 20. The osteonectin gene was highly expressed throughout the culture, with no significant increase in induction found during any time of culture. A significant induction of alkaline phosphatase (13.8-fold) gene expression was observed by day 6. Osteopontin showed a similar profile to that of alkaline phosphatase but had a much greater level of relative expression (26-fold) compared to day 0. Interestingly, downregulation during mineral accumulation seemed a common occurrence among many of the genes measured. In contrast, the bone sialoprotein gene showed a significant and distinct expression pattern, increasing rapidly after the onset of mineralization on day 6 and ultimately reaching 140-fold that of day 0. Decorin (Pg II) showed an increasing pattern, with the final relative level of induction 5-fold on day 20. These data suggest that the development of the mature osteoblastic phenotype, complete with the ability to produce a thick mineralized matrix, requires the differential regulation of a series of genes and their gene products over the culture period.