OSTEOGENIC POTENTIAL OF MURINE OSTEOSARCOMA CELLS - COMPARISON OF BONE-SPECIFIC GENE-EXPRESSION IN IN-VITRO AND IN-VIVO CONDITIONS

Bone tissue formation and the expression of osteoblast-specific genes were compared in vitro and in vivo for two well characterized murine c lonal osteosarcoma cell lines (K7 and K8). In vitro studies were carri ed out under conditions that promoted extracellular matrix morphogenes is and mineralization. The K8 cells showed 8-fold greater alkaline pho sphatase activity and mineral accumulation than did K7 cells during 21 days of in vitro growth. The K8 cell line showed high levels of bone sialoprotein (BSP), collagen type I (COL I), and alkaline phosphatase (APase) mRNA expression throughout its growth in vitro. In contrast, K 7 cells showed an almost complete absence of BSP and COL I and very lo w levels of APase throughout the culture period. in vitro, both cell l ines expressed very low levels of osteocalcin (OC). For in vivo studie s, we used a three-dimensional culture device that permitted analysis of tissue formation by the cells after their implantation into syngene ic mice. The K8 cells consistently generated extensive mineralized wov en bone after their subcutaneous implantation. The striking features d istinguishing the bone formed by the implanted cells from normal recip ient bone were the complete absence of osteoclasts or matrix resorptio n, the absence of OC protein, and very low levels of OC mRNA expressio n in the tissues formed by these cell lines. BSP, APase, and COL I exp ressions were maintained at high levels in the KB-produced tissues. In contrast to their near absence in vitro, APase, BSP, and COL I were e xpressed by K7 cells and increased with time in vivo. These findings d emonstrate that the K7 cells in vitro are less differentiated than K8 cells, but that K7 cells in vivo undergo osteogenic maturation. Thus, expression of bone-specific genes in these osteogenic cell lines was d ependent on systemic or local factors in recipient animals and was dis tinct for these cell lines when grown under in vitro conditions. OC pr otein does not appear to be needed for the mineralization of the extra cellular matrix but may be needed to provide the necessary signals for the resorption and remodeling of the tissue.