The effect of growth factor environment on fibroblast morphological response to substrate stiffness

According to conventional understanding regarding dependence of cell behavior on substrate stiffness, tissue cells typically remain round on soft substrates but spread on stiff substrates. The current studies were carried out to learn if the growth factor environment influenced the foregoing relationship. Using standard methods, we prepared planar (2D) polyacrylamide (PA) gels ranging from 0.5 to 40 kPa and covalently cross-linked with fibronectin and collagen at concentrations ranging from 2.5 to 50 μg/ml. We carried out experiments with fibroblasts varying in their ability to form actin stress fibers and focal adhesions. In fetal bovine serum (FBS) containing medium--the growth factor environment in which most studies on cell spreading and substrate stiffness have been carried out--cell spreading increased with increasing substrate stiffness and adhesion ligand density. However, in platelet-derived growth factor (PDGF) containing medium, cell spreading was relatively independent of substrate stiffness and adhesion ligand density except little cell attachment occurred in the complete absence of cross-linked adhesion ligands. If cell contraction was blocked with blebbistatin, then cell spreading in FBS-containing medium became independent of substrate stiffness. The findings suggest that under growth factor conditions that stimulate global cell contraction (FBS), cell spreading cannot occur unless adhesion ligand density and substrate stiffness result in cell-substrate interactions strong enough to resist and overcome the inward tractional force. Under growth factor conditions that stimulate global cell protrusion (PDGF), such resistance is not required.