Regulation of growth factor activation by proteoglycans: What is the role of the low affinity receptors?

Department of Pharmacology New York University Medical Center New York, New York 10016 Many cell membrane receptors, such as the nicotinic ace- tylcholine receptor and the T cell antigen receptor, are composed of several different subunits, and their correct assembly is necessary for the generation of functional re- ceptors. Most lymphokine receptors are composed of at least two components, and ligand-induced oligomeriza- tion is essential for receptor activation and signal transmis- sion. In recent years, it has become clear that various growth factors and lymphokines can bind to two different classes of cell surface receptors. For example, fibroblast growth factor (FGF) and transforming growth factor p (TGFB) both bind with high affinity to signaling receptors endowed with tyrosine or serinelthreonine kinase activi- ties. However, the same growth factors also bind with lower affinity to cell surface proteoglycans that cannot transmit signals alone, but somehow modulate the ability of the growth factor or the signaling receptor to generate a biological response (Klagsbrun and Baird, 1991; L6pez- Casillas et al., 1993; Yayon et al., 1991; Roghani et al., 1994). Proteoglycans are proteins that are found predomi- nantly on the cell surface and in the extracellular matrix and that contain carbohydrates called glycosaminogly- cans. Glycosaminoglycans are polymers of disaccharide repeats, which are mostly highly sulfated and negatively charged. The main glycosaminoglycans in proteoglycans are chondroitin sulfate, dermatan sulfate, heparan sulfate, heparin, and keratan sulfate (Ruoslahti, 1989). Binding of growth factors to proteoglycans is thought to have an important regulatory role (Ruoslahti and Yamaguchi, 1991). This has been particularly well explored for FGF, in which it has been shown that heparins (or heparan sulfate proteoglycans) are necessary for FGF-induced biologicl neurotrophin binding to ~75 (low affinity) and to the various nerve growth factor (NGF) receptor tyrosine kinases (trkA, trkB, and trkC, which are high affinity), as well as the binding of the insulin- like growth factor IGF2 to the IGF2 receptor (nonsignaling receptor) and to the IGFl receptor (a tyrosine kinase related to the insulin receptor). The existence of nonsignaling as well as signaling receptors for the same ligand is a feature common for many growth factors. How- ever, the physiological role of the nonsignaling receptors is poorly understood. The most discussed model for the role of the low affinity nonsignaling receptors is that they present ligand to high affinity signaling receptors. Binding of the ligand to the high affinity signaling receptors will then activate this receptor and trigger biological responses. However, when a given ligand at low concentration is allowed to bind to cells that display on their surface both low and high affinity receptors, the law of mass action dictates that at equilib- rium the ligand will bind, predominantly, to high affinity receptors rather than to the low affinity receptors. This fact makes it difficult to accept the so-called presentation model. At higher ligand concentration, both the high and low affinity receptors will be occupied. The binding con- stants of the high affinity receptors are usually lo- to 1 OO- fold higher than those of the low affinity receptors. There- fore, even when the density of the low affinity receptors on the cell surface is higher by an order of magnitude, a similar number of high and low affinity receptors will be occupied upon saturation of the high affinity receptors. These arguments demonstrate the need for a different model to understand the role of the low affinity receptors. An alternative and more plausible model is that the primary function of these receptors is to reduce the dimensionality of ligand diffusion from three to two dimensions (Adam and Delbruck, 1968; Richter and Eigen, 1974). When ligands such as lymphokines or growth factors are bound to cell surface receptors, their diffusion is restricted to two dimen- sions ratherthan diffusing in thethree-dimensionalvolume of the extracellular space. Once restricted to just two di- mensions, the ligand molecules are more likely to encoun- ter and bind to the less abundant high affinity signaling receptors. In otherwords, binding of ligands, such as FGF, to abundant low affinity receptors that are restricted two dimensions will increase the local concentration of the bound ligands at the plasma membrane, and the probabil- ity of their interaction with a high affinity receptor will be greatly enhanced. For example, 20,000 receptors per cell