Internalization of the granulocyte-macrophage colony-stimulating factor receptor is not required for induction of protein tyrosine phosphorylation in human myeloid cells

Granulocyte-macrophage colony-stimulating factor (GM-CSF) exerts its biologic activities through binding to specific high-affinity cell surface receptors. After binding, the ligand/ receptor complex is rapidly internalized in most hematopoietic cells. Using a human factor-dependent cell line, MO7, and normal human neutrophils, we found that the internalization is exquisitely temperature-dependent, such that ligand/ receptor internalization does not detectably occur at 4°C. Activation of the GM-CSF receptor has previously been shown to stimulate a number of postreceptor signal transduction pathways, including activation of a tyrosine kinase and activation of the serine/threonine kinase, Raf-1. The GM-CSF-stimulated increase in tyrosine kinase activity occurs rapidly at both 4°C and 37°C, and therefore is likely to be independent of receptor internalization. At 37°C, the protein tyrosine phosphorylation was transient in MO7 cells, with maximum phosphorylation observed after 5 to 15 minutes, followed by a rapid decline. At 4°C, the protein tyrosine phosphorylation of the same substrates was greater than at 37°C, and no decline in substrate phosphorylation was observed for at least 90 minutes. In contrast to tyrosine phosphorylation, the activation and hyperphosphorylation of Raf-1 observed at 37°C in both MO7 cells and neutrophils was markedly diminished at 4°C. These results indicate that at least one postreceptor signal transduction mechanism, activation of a tyrosine kinase, does not require ligand/receptor internalization, and indicate that receptor internalization may be a consequence, rather than the initiator, of signal transduction.