RETINOIC ACID SUPPRESSES THE OSTEOGENIC DIFFERENTIATION CAPACITY OF MURINE OSTEOBLAST-LIKE 3 A/1D-1M CELL-CULTURES/

Bone is a target tissue for action of retinoids though their precise r ole remains unclear. This study investigated the effects of retinoic a cid (RA) on the marrow stromal 3/A/1D-1M osteoblast-like cells, derive d from the in vivo transplantation of 3/A/1D-1 chondroprogenitor cells . Long-term treatment with 1 mu M RA for 7 weeks induced a marked decr ease in bone-like nodule number and ultrastructural alterations in the striation and the size of the collagen fibres. RA at concentrations v arying from 10 nM to 3.16 mu M had a dose-dependent inhibition effect on alkaline phosphatase (AP) activity with an IC50 of 0.7 mu M. Treatm ent with 1 mu M RA for up to 17 days induced a time-dependent inhibiti on of AP activity, while the beginning of RA treatment (4 or 52 h of c ulture) produced a differential magnitude of inhibition. These: variat ions were unrelated to modifications of the expression of RAR receptor at the protein level, as assessed by Western blot analysis. Exposure to 1 mu M RA for 6 or 24 h administered at day 14 produced an inhibiti on of AP activity, which reached a maximum after 48 h, with a recovery time of 8 days in both cases. Long-term treatment with RA at 1 mu M c ompletely abolished the level of osteocalcin mRNA on both days 12 and 16, as revealed by Northern blot analysis. However, such RA-treated ce lls retained the constitutive expression of type II procollagen transc ripts. These results suggest that RA inhibits several aspects of osteo genic differentiation capacity, a loss of phenotype, which, in associa tion with the maintenance of type II procollagen cartilage-related cha racteristic, could be a prerequisite step for cellular plasticity.