BONE MORPHOGENETIC PROTEIN-2 (BMP-2) RECEPTOR EXPRESSION AND FUNCTION IN GONADOTROPH CELL LINES
SY Ng, D Sitara, KK Sidhu, RC Fowkes & JM Burrin
The cellular and molecular mechanisms underlying the actions of BMP-2 to induce gonadotroph-specific gene expression during pituitary development are poorly understood. We have characterised BMP receptor (BMP-R) expression in two gonadotroph models, the alphaT3-1 and LbetaT2 cells, which represent different stages of gonadotroph development. Signal transduction via BMP-R requires a heterodimeric complex between type I (A or B) and type II receptors. RT-PCR for BMP-R, using sub-type and isoform-specific primers, demonstrated expression of BMP-R type IA and type II in both cell lines. However, expression of type IB was limited to alphaT3-1 cells. Recent evidence has shown that the mode of BMP-R oligomerisation determines different signalling pathways, and in osteoblast-like cells involves activation of the mitogen activated protein kinase (MAPK) or Smad pathways. Western blotting for phosphorylated p42/p44 MAPK and p38 SAPK was performed on alphaT3-1 cell extracts, treated with BMP-2 (100ng/ml) for up to 24 hours. This treatment failed to elicit significant changes on the activation of these two signalling molecules, suggesting an alternative signalling pathway (i.e. Smads) may regulate BMP-2 effects in alphaT3-1 cells. To examine putative BMP-2 target genes, we identified the expression of a BMP-2 responsive gene, BMP-2-induced factor-1 (Bif-1, a novel transcription factor involved in osteoblast development) in both gonadotroph cell lines. Long and short isoforms of Bif-1 were present in both cell lines by RT-PCR, and expression of the long isoform was serum-dependent. Over-expression of Bif-1 in alphaT3-1 cells failed to significantly alter basal or hormone-stimulated glycoprotein hormone alpha-subunit gene expression, as determined by reporter gene assays. These studies demonstrate for the first time the presence of different isoforms of BMP-R's in gonadotroph cells of different developmental origin and suggests that BMP-2 signalling pathways may be both common and distinct from those found in osteoblasts. Supported by Barts Cancer Research Committee.