Regulation of chemotaxis by the platelet-derived growth factor receptor-beta

Epidermal growth factor receptor-mediated cell motility: phospholipase C activity is required, but mitogen-activated protein kinase activity is not sufficient for induced cell movement

We recently have demonstrated that EGF receptor (EGFR)-induced cell motility requires receptor kinase activity and autophosphorylation (P. Chen, K. Gupta, and A. Wells. 1994. J. Cell Biol. 124:547-555). This suggests that the immediate downstream effector molecule contains a src homology-2 domain. Phospholipase C gamma (PLC gamma) is among the candidate transducers of this signal because of its potential roles in modulating cytoskeletal dynamics. We utilized signaling-restricted EGFR mutants expressed in receptor devoid NR6 cells to determine if PLC activation is necessary for EGFR-mediated cell movement. Exposure to EGF (25 nM) augmented PLC activity in all five EGFR mutant cell lines which also responded by increased cell movement. Basal phosphoinositide turnover was not affected by EGF in the lines which do not present the enhanced motility response. The correlation between EGFR-mediated cell motility and PLC activity suggested, but did not prove, a causal link. A specific inhibitor of PLC, U73122 (1 microM) diminished both the EGF-induced motility and PLC responses, while its inactive analogue U73343 had no effect on these responses. Both the PLC and motility responses were decreased by expression of a dominant-negative PLC gamma-1 fragment in EGF-responsive infectant lines. Lastly, anti-sense oligonucleotides (20 microM) to PLC gamma-1 reduced both responses in NR6 cells expressing wild-type EGFR. These findings strongly support PLC gamma as the immediate post receptor effector in this motogenic pathway. We have demonstrated previously that EGFR-mediated cell motility and mitogenic signaling pathways are separable. The point of divergence is undefined. All kinase-active EGFR mutants induced the mitogenic response while only those which are autophosphorylated induced PLC activity. U73122 did not affect EGF-induced thymidine incorporation in these motility-responsive infectant cell lines. In addition, the dominant-negative PLC gamma-1 fragment did not diminish EGF-induced thymidine incorporation. All kinase active EGFR stimulated mitogen-activated protein (MAP) kinase activity, regardless of whether the receptors induced cell movement; this EGF-induced MAP kinase activity was not affected by U73122 at concentrations that depressed the motility response. Thus, the signaling pathways which lead to motility and cell proliferation diverge at the immediate post-receptor stage, and we suggest that this is accomplished by differential activation of effector molecules.

The IRS-1 signaling system

Insulin-receptor substrate 1 (IRS-1) is a principal substrate of the receptor tyrosine kinase for insulin and insulin-like growth factor 1, and a substrate for a tyrosine kinase activated by interleukin 4. IRS-1 undergoes multisite tyrosine phosphorylation and mediates downstream signals by 'docking' various proteins that contain Src homology 2 domains. IRS-1 appears to be a unique molecule; however, 4PS, a protein found mainly in hemopoietic cells, may represent another member of this family.