Ligand concentration is a driver of divergent signaling and pleiotropic cellular responses to FGF

Area of Science:

• Cell Biology
• Developmental Biology
• Molecular Signaling

Background:

• Fibroblast growth factors (FGFs) are crucial signaling molecules regulating embryonic development, stem cell behavior, and tissue homeostasis.
• FGFs exert their effects by binding to fibroblast growth factor receptors (FGFRs), initiating downstream signaling cascades.
• The impact of varying FGF concentrations on cellular responses, particularly the dose-dependent signaling, remains incompletely understood.

Purpose of the Study:

• To investigate how different concentrations of FGF2 influence the cellular responses of fibroblasts, including proliferation, survival, and differentiation.
• To elucidate the underlying differential signaling pathways and negative feedback mechanisms that mediate these concentration-dependent effects.
• To understand the role of specific signaling intermediates, such as FRS-2, Erk, and p38 MAP kinase, in mediating FGF2's pleiotropic actions.

Main Methods:

• Fibroblast cell cultures were exposed to a range of FGF2 concentrations.
• Cellular responses including proliferation, apoptosis, and differentiation were quantified.
• Activation of key signaling intermediates (FGFR, FRS-2, Erk, p38 MAP kinase) was analyzed across the FGF2 concentration spectrum.
• Specific kinase inhibitors were used to determine the roles of p38 and Erk in FGF2-induced proliferation and apoptosis.

Main Results:

• FGF2 exhibited concentration-dependent effects: low and high concentrations promoted survival and differentiation, while intermediate concentrations differentially regulated proliferation and apoptosis.
• Proliferation was stimulated by intermediate FGF2 concentrations in the presence of serum but led to apoptosis in serum-free conditions.
• High FGF2 concentrations reversed the effects seen at intermediate levels, mirroring low-concentration responses.
• Distinct signaling patterns were observed: Erk and FRS-2 activation peaked at intermediate concentrations and were downregulated by high FGF2, whereas p38 MAP kinase and FGFR activation increased incrementally.
• FGF2-induced proliferation was dependent on p38 MAP kinase, while FGF2-induced apoptosis under serum-free conditions relied on Erk MAP kinase.

Conclusions:

• FGF2 concentration is a critical determinant of cellular fate, precisely controlling signaling outputs and cellular responses.
• Differential activation of intracellular signaling pathways, modulated by negative feedback, underlies the observed opposing cellular behaviors.
• These findings highlight the importance of FGF concentration gradients in developmental processes like limb formation, influencing cell proliferation, survival, and differentiation.