Abstract
Purpose: :
It has been well established that FGFs play a central role in the early stages of epithelial-to-fiber differentiation. We have recently published evidence that BMP signaling is required for FGF-induced fiber differentiation of normally developed epithelial cells in both serum-free primary cultures of embryonic chick lens cells (referred to as DCDMLs) and in transgenic mice in vivo (Boswell et al., 2008 Dev Biol). The goal of this study was to use DCDMLs to investigate the molecular basis for the cross-talk between the BMP and FGF signaling pathways in secondary fiber differentiation.
Methods: :
DCDMLs were cultured for 6 days with or without noggin, a potent antagonist of BMP signaling that acts by very tightly and specifically binding the BMPs (BMP 4 and 7) endogenously produced by lens cells. Activation of ERK and FRS2 was assessed by Western blotting of whole cell lysates using anti-phospho-ERK (Thr202/Tyr204) and anti-phospho-FRS2 (Tyr196) antibodies.
Results: :
Blocking endogenous BMP signaling by culturing DCDMLs with noggin for 6 days strongly (by ~75%) inhibited the ability of subsequently added FGF to induce activation of ERK. Noggin did not affect cell viability, proliferation, epithelial phenotype, or the ability of FGF to bind to FGF receptors, nor did it reduce activation of ERK in response to 50 nM phorbol ester. The latter indicates that noggin reduces the expression of a component upstream of Raf1 in the canonical FGF-to-ERK signaling pathway. Consistent with this interpretation, noggin pretreatment reduced FGF-induced activation of FRS2, a docking protein that is constitutively bound to FGF receptors that mediates most of the downstream effects of FGF-FGFR interaction.
Conclusions: :
Taken together, these results show that noggin inhibits FGF-to-ERK signaling upstream of FRS2 activation, which strongly indicates that noggin blocks FGF signaling at least in part at the level of FGFR activation. We conclude that an essential role of BMP signaling in FGF-mediated fiber differentiation is to maintain lens cells in an optimally FGF-responsive state by regulating the expression and/or activity of FGF receptors. Supported by NIH R01 EY014622.
Keywords: growth factors/growth factor receptors • signal transduction • differentiation