| Abstract: | Two chimeric receptors, ER1 and ER2, were constructed. ER1 contains the extracellular and transmembrane (TM) domains derived from epidermal growth factor receptor and the cytoplasmic domain from c-Ros; ER2 is identical to ER1 except that its TM domain is derived from c-Ros. Both chimeras can be activated by epidermal growth factor and are capable of activating or phosphorylating an array of cellular signaling proteins. Both chimeras promote colony formation in soft agar with about equal efficiency. Surprisingly, ER1 inhibits while ER2 stimulates cell growth on monolayer culture. Cell cycle analysis revealed that all phases, in particular the S and G2/M phases, of the cell cycle in ER1 cells were elongated whereas G1 phase of ER2 cells was shortened threefold. Comparison of signaling pathways mediated by the two chimeras revealed several differences. Several early signaling proteins are activated or phosphorylated to a higher extent in ER1 than in ER2 cells in response to epidermal growth factor. ER1 is less efficiently internalized and remains tyrosine phosphorylated for a longer time than ER2. However, phosphorylation of the 66-kDa She protein, activation of mitogen activated protein kinase, and induction of c-fos and c-jun occur either to a lesser extent or for a shorter time in ER1 cells. Cellular protein phosphorylation patterns are also different in ER1 and ER2 cells. In particular, a 190-kDa Shc-associated protein is tyrosine phosphorylated in ER2 but not in ER1 cells. Our results indicate that the TM domains have a profound effect on the signal transduction and biological activity of those chimeric receptors. The results also imply that sustained stimulation of ER1 due to its retarded internalization apparently triggers an inhibitory response that dominantly counteracts the receptor-mediated mitogenic signals. These two chimeras, expressed at similar levels in the same cell type but having opposite effects on cell growth, provide an ideal system to study the mechanism by which a protein tyrosine kinase inhibits cell growth. |
