Heparin is required for fibroblast growth factor (FGF) stimulation of biological responses. Using isothermal titration lorimetry, we show that acidic FGF (aFGF) forms a complex with the soluble extracellular domain of F receptor (FGFR). Heparin exerts its effect by binding to many molecules of aFGF. The resulting aFGF-heparin complex can bind to several receptor molecules, leading to FGFR dimerization. In two cell fines lacking endogenous heparan sulfate, exogenous heparin is required for FGFR dimerization, tyrosine kinase activation, c-fos mRNA transcription, and cell proliferation. Moreover, a synthetic heparin analog that binds monovalently to aFGF blocks FGFR dimerization, activation, and signaling via FGFR. We propose that heparin causes oligomerization of aFGF such that its binding to FGFR results in dimerization and activation. This represents a novel mechanism for transm rane signaling and may account for the action of y heparin-bound growth factors.

Ligand-induced dimerization is a key event in transmembrane signaling by receptors with tyrosine kinase activity. Receptor dimerization leads to an increase in kinase activity, resulting in autophosphorylation and the induction of diverse biological responses (reviewed by Schlessinger and Ullrich, 1992). Certain growth factors, such as plateletderived growth factor (PDGF), colony stimulating factor-l, and stem cell factor, are themselves dimeric proteins and can induce the dimerjzation of their respective receptors if each molecule of the dimer binds independently to a separate receptor molecule (Heldin et al., 1989). Other growth factors are monomeric but contain two receptorbinding sites, enabling them to cross-link two receptor molecules. A crystal structure of the extracellular domain of the human growth hormone receptor in complex with human growth hormone shows that a single molecule of human growth hormone contributes to the stability of the receptor dimer by binding across the interface (de Vos et a!., 1992). Similarly, recent studies of epidermal growth factor (EGF) binding to its receptor suggest that a single ligand molecule can bind simultaneously to two receptor molecules (Lemmon et al., submitted).