P. Dvorak et al., EMBRYOGLYCAN ECTODOMAINS REGULATE BIOLOGICAL-ACTIVITY OF FGF-2 TO EMBRYONIC STEM-CELLS, Journal of Cell Science, 111, 1998, pp. 2945-2952
Basic fibroblast growth factor (FGF-2) functions as a natural inducer
of mesoderm, regulator of cell differentiation and autocrine modulator
of cell growth and transformation. The FGF-2 signals are transduced t
hrough receptors with intrinsic protein tyrosine kinase activity. Howe
ver, receptor binding and activation is governed by extracellular matr
ix, cell surface or soluble proteoglycans. This paper focuses on the r
ole of proteoglycans synthesized by embryonic cells, embryoglycans, in
FGF-2 signaling via FGF receptor-1 (FGFR-1). We found that embryoglyc
an ectodomain Lewis X, analog of developmentally regulated embryonic c
ell surface epitope TEC 1, promotes oligomerization of FGF-2 in the ce
ll free chemical crosslinking, In vitro assays show that a large molar
excess of extracellular Lewis X does not inhibit binding of FGF-2 to
embryonic stem (ES) cells, but prevents the mitogenic effect of FGF-2.
Western blot analysis of ES cells revealed the presence of abundant 5
2 kDa and trace amounts of 67 and 125 kDa isoforms of FGFR-1. However,
none of these isoforms undergo any detectable changes in tyrosine pho
sphorylation under the conditions that modulate the mitogenic effect o
f FGF-2. Rather, a primary substrate of all receptor tyrosine kinases,
phospholipase Cy (PLC gamma), is activated by both FGF-2 and Lewis X.
The combination, FGF-2 plus Lewis X, leads to weak inhibition, when c
ompared with the effects of FGF-2 and Lewis X, respectively. In accord
ance, the level of phosphorylation of non-receptor tyrosine kinase c-S
rc is reduced in a reversed pattern to PLC gamma Furthermore, in this
particular cell type we show the presence of activated forms of extrac
ellular signal-related kinase (ERK) in all nontreated and treated cell
s. These findings demonstrate that embryoglycan ectodomains may act as
negative regulators of FGF-2-induced ES cell proliferation, most like
ly through the FGFR-1-independent signaling pathway.