1,25-Dihydroxyvitamin D3 and Transforming Growth Factor-beta Act Synergistically to Override Extinction of LiversolidusBonesolidusKidney Alkaline Phosphatase in Osteosarcoma Hybrid Cells
In this study, a somatic cell genetic approach was used to study the regulation of liversolidusbonesoliduskidney alkaline phosphatase (ALPL) gene expression in osteoblasts. ALPL plays an important role in skeletal mineralization and serves as a good index of bone formation. A series of intertypic hybrids constructed by fusion of the human osteosarcoma TE-85 with the mouse fibrosarcoma La-t- demonstrated a 10-fold reduction of ALPL steady-state mRNA and enzyme activity, a phenomenon termed extinction. Hybrid subclones which reexpressed ALPL contained reduced numbers of fibroblast chromosomes compared to earlier passages. This suggests that a trans-acting negative regulatory factor expressed from the fibroblast genome regulates ALPL expression. Two factors known to influence ALPL expression are 1,25-dihydroxyvitamin D3 (1,25D3) and transforming growth factor-beta1 (TGFbeta1). 1,25D3 is involved in mobilizing bone calcium stores and TGFbeta1 plays a critical role in bone remodeling. The extinguished hybrids were exposed to 1,25D3, TGFbeta1, and a combination of these factors. For two hybrids, the combination induced reexpression of ALPL activity to levels comparable to the TE-85 parent, indicating a competition between the factors and the extinguisher(s). Neither factor alone could induce ALPL reexpression to the levels observed with the combination. In only one hybrid, the combination of factors synergistically increased ALPL expression. These data help define the cis sequence element(s) in the ALPL promoter which are involved in the negative regulation of this gene.