Ligand-induced oligomerization of receptors is a key step in initiating growth factor signaling. Nevertheless, complex biological responses often require additionaltrans-signaling mechanisms involving two or more signaling cascades. For cells of neuronal origin, it was shown that neurotrophic effects evoked by nerve growth factor or other neurotro-phins dependhighlyon the cooperativitywith cytokines that belong to the transforming growth factor b (TGF-b) superfamily. We found that rat pheochromocytoma cells, which represent amodel systemfor neuronal differentiation, are unresponsive to TGF-b1 due to limiting levels of its receptor, TbRII
