BMP-2 differentially modulates FGF-2 isoform effects in osteoblasts from newborn transgenic mice.

We previously generated separate lines of transgenic mice that specifically overexpress either the Fibroblast Growth Factor (FGF-2) low molecular weight isoform (TgLMW) or the high molecular weight isoforms (TgHMW) in the osteoblast lineage. Vector/control (TgVector) mice were also made. Here we report use of isolated calvarial osteoblasts (COBs) from those mice to investigate whether the FGF-2 protein isoforms differentially modulate bone formation in vitro. Our hypothesis states that FGF-2 isoforms specifically modulate bone morphogenetic protein 2 (BMP-2) function and subsequently, bone differentiation genes and their related signaling pathways. We found a significant increase in alkaline phosphatase (ALP) positive colonies in TgLMW COBs compared with TgVector controls. BMP-2 treatment significantly increased mineralized colonies in TgVector and TgLMW COBs. BMP-2 caused a further significant increase in mineralized colonies in TgLMW COBs compared with TgVector COBs but did not increase ALP positive colonies in TgHMW COBs. Time course studies showed that BMP-2 caused a sustained increase in SMAD1/5/8, Runx-2, and Osterix protein in TgLMW COBs. BMP-2 caused a sustained increase in phospho-p38 MAP kinase in TgVector but only a transient increase in TgLMW and TgHMW COBs. BMP-2 caused a transient increase in phospho-p44/42 MAP kinase in TgVector COBs, no increase in TgLMW COBs but a sustained increase was found in TgHMW COBs. Basal expression of FGF receptor 1 (FGFR1) protein was significantly increased in TgLMW COBs relative to TgVector COBs and, while BMP-2 caused a transient increase in FGFR1 expression in TgVector COBs and TgHMW COBs, there was no further increase TgLMW COBs. Interestingly, while basal expression of FGF receptor 2 (FGFR2) was similar in COBs from all genotypes, BMP-2 treatment caused a sustained increase in TgLMW COBs but decreased FGFR2 in TgVector COBS and TgHMW COBs.