Type β transforming growth factor (TGFβ) regulation of alkaline phosphatase expression and other phenotype‐related mRNAs in osteoblastic rat osteosarcoma cells

Abstract

TGFβ1 from porcine platelets increased alkaline phosphatase (AP) activity in the rat osteoblastic cell line ROS 17/2.8 about three-fold. This effect was dose-dependent with an ED₅₀ of about ∼0.2 ng/ml and was larger during logarithmic growth than at confluence. TGFβ1 inhibited cell growth by about 30% with similar dose dependence. Thirty min exposure to TGFβ1 was sufficient to increase AP activity 3 days later by about two-fold but did not affect cell growth, suggesting dissociation between effects on proliferation and differentiation. The rise in AP activity started 6 h after TGFβ1 addition and was blocked by cycloheximide and actinomycin D. TGFβ1 also increased AP mRNA by two- to three-fold and this effect was not blocked by cycloheximide. The half-life of AP mRNA, estimated following the addition of 5,6-dichloro-1-β-D-ribofuranosylbenzimidazole was about ten h in both control and TGFβ1-treated cells. The mRNAs for type I procollagen and osteonectin were also increased by TGFβ1 but fibronectin mRNA was decreased. TGFβ2 effects on AP and cell growth were similar to those of TGFβ1, except for lack of activity following transient exposure. At saturating concentrations, TGFβ2 (2 ng/ml) or dexamethasone (10⁻⁷M), which has similar effects on these cells, did not further augment the effects of TGFβ1 (at 2 ng/ml). Above findings suggest that TGFβ promotes osteoblatic differentiation in rat osteosarcoma cells at least in part by acting at the pretranslational level.