Expression and growth factor regulation of platelet-derived growth factor B transcripts in primary osteoblast cell cultures.

Platelet-derived growth factor (PDGF), an important bone cell mitogen, exists as a homo- or heterodimer product of the PDGF-A and -B genes. Normal unstimulated cells of the osteoblast lineage express the PDGF-A gene, but it is not known whether they express the PDGF-B gene. We examined the expression of PDGF-B messenger RNA (mRNA) levels in cultures of osteoblast-enriched cells from 22-day-old fetal rat calvariae (Ob cells) and determined whether they were modified by transforming growth factor-beta 1 (TGF beta 1), basic fibroblast growth factor (bFGF), insulin-like growth factor I (IGF-I), and PDGF-BB. Ob cells expressed PDGF-B transcripts of 3.5 kilo-bases, as determined by Northern blot analysis. Treatment of Ob cells with TGF beta 1 at 0.01-1.2 nM caused a dose-dependent increase in steady state PDGF-B mRNA, an effect that was initially observed after 2 h and was maximal after 6h. Cycloheximide induced PDGF-B transcripts and decreased the effect of TGF beta 1. TGF beta 1 did not modify the half-life of PDGF-B mRNA in transcriptionally arrested Ob cells and increased the rate of PDGF-B gene transcription in nuclear run-on assays. In contrast, treatment with PDGF-BB at 3.3 nM, bFGF at 6 nM, or IGF-I at 100 nM for 2-24 h did not modify PDGF-B mRNA levels in Ob cells. In conclusion, normal Ob cells express the PDGF-B gene, and TGF beta 1 induces its transcription, whereas bFGF, IGF-I, and PDGF-BB do not enhance the levels of PDGF-B mRNA. PDGF-BB may act not only as a systemic but also as a local regulator of bone cell function.