Thymosin β₄ is expressed in ROS 17/2.8 osteosarcoma cells in a regulated manner

The differential expression of mRNAs between the closely related rat osteosarcoma cell lines ROS 17/ 2.8 and ROS 25/1 was used to identify genes whose expression is associated with the osteoblast phe-notype. Thymosin β₄ cDNA was cloned from an ROS 17/2.8 complimentary DNA library on the basis of its differential hybridization with radiolabeled cDNA prepared from ROS 17/2.8 and ROS 25/1 cells. Northern blot analysis confirmed that thymosin β₄, hitherto a putative immunodulatory hormone, was indeed differentially expressed. Steady state mRNA levels were severalfold higher in ROS 17/2.8 cells exhibiting an osteoblast-like phenotype, compared with the less osteoblast-like ROS 25/1. Thymosin β₄ transcripts were also detected in rat UMR 106 osteo-sarcoma cells and in intact neonatal and fetal rat calvaria. Sequence analysis of the cDNA indicated that thymosin β₄ transcripts may arise by processing at a more distal polyadenylation signal. Treatment of ROS 17/2.8 cells with dexamethasone increased, while addition of 1,25-dihydroxyvitamin D₃ decreased thymosin β₄ mRNA. The phenotype-dependent expression in the ROS cells and the response to steroid hormone suggest that thymosin β₄ expression contributes to the osteoblast phenotype.