The
epidermal growth factor (
EGF) receptor is overexpressed in
squamous cell carcinoma and the
EGF receptor has been proposed as a potential target for new therapeutic agents in this tumour type. We have utilized a tyrphostintype inhibitor of the
EGF receptor tyrosine kinase domain (
RG50864) to study
EGF-dependent proliferation and phosphorylation in two human
squamous cell carcinoma cell lines. There were selected on the basis that whereas both cell lines have a large number of
EGF receptors, one is growth inhibited by
EGF (A431) while the proliferation of the other cell line (B2A4) is stimulated by
EGF.
EGF induced receptor autophosphorylation in each of the two cell lines; however, the level of phosphorylation was greater in the A431 cells than in the B2A4 cells. The pattern of
proteins phosphorylated in response to
EGF was different in the two squamous cell lines.
RG50864 antagonized the
EGF-dependent proliferation of B2A4 cells, but was unable to reverse the inhibitory effect of
EGF on A431 cell growth.
RG50864 partially inhibited
EGF receptor autophosphorylation in both cell lines and completely inhibited the
EGF-dependent phosphorylation of other cellular
proteins, one of which co-migrated with MAP2kinase in both cell lines. Moreover, different dose-response relationships for the inhibition of phosphorylation of various
proteins were observed in A431 versus B2A4 cells. As a substrate competitive inhibitor of the
EGF receptor tyrosine kinase, the primary mode of action of
RG50864 may be to prevent the association and/or phosphorylation of multiple specific substrates of the receptor in a fashion which may be cell line dependent. The precise relationship of these phosphorylation events to
tyrphostin sensitivity remains to be established.