Proline-rich transcript of the brain (prtb) is a serum-responsive gene in osteoblasts and upregulated during adhesion

To characterize the temporal expression of genes that play a functional role during the process of osteoblast adhesion, we used differential display (DD-PCR) on mRNA isolated from attached vs. suspended osteoblasts. A 200-bp fragment displaying upregulated expression after 30 and 60 min adhesion was isolated, sequenced, and showed 97% homology to prtb, previously showed to be expressed in mouse brain. Northern analysis confirmed a two-fold increase in prtb message during adhesion to tissue culture polystyrene, both in the presence or absence of surface-adsorbed serum proteins. Serum stimulation alone was also able to induce prtb expression, although to a lesser extent, in suspension cells. Strong prtb expression was also detected in both brain and bone of adult rats. Furthermore, prtb expression analysis during MC3T3-E1 cell differentiation revealed high expression levels independent of proliferation (day 0-7), matrix maturation (day 7-14), and mineralization (day 14-31). Time course analysis of prtb expression during adhesion of sensitized osteoblasts to serum-protein coated surfaces showed robust mRNA expression at 5 min post-plating and a peak at 10 min. The two known serum-inducible immediate early genes c-fos and c-jun showed similar expression kinetics, with c-jun mRNA levels peaking at 15 min and c-fos at 20 min. Based on these data, we hypothesize that prtb may function as an immediate early, serum-responsive, and adhesion-inducible gene with possible involvement in processes such as cell cycle control, adhesion, and proliferation.