Xm. Ouyang et al., HUMAN CANCER-CELLS EXHIBIT PROTEIN-KINASE C-DEPENDENT C-ERBB-2 TRANSMODULATION THAT CORRELATES WITH PHOSPHATASE SENSITIVITY AND KINASE-ACTIVITY, The Journal of biological chemistry, 271(36), 1996, pp. 21786-21792
The c-erbB-2 receptor tyrosine kinase is often overexpressed in human
tumors, but the functional implications of this phenotype remain uncle
ar. We previously used phosphorylation-specific antibodies to define m
ajor differences in c-erbB-2 tyrosine kinase activity between overexpr
essing human tumor cell lines (Epstein, R. J., Druker, B. J., Roberts,
T.M., and Stiles, C. D. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 1043
5-10439). Here we extend this approach to define the relationship betw
een c-erbB-2 tyrosine phosphorylation and protein kinase C (PRC)-depen
dent transmodulation. Phosphorylation-specific antibodies to the juxta
membrane PKC site Thr(686) recognize tyrosine-dephosphorylated wild-ty
pe c-erbB-2 following G8/DHFR 3T3 cell treatment with PKC agonists. B1
04-1-1 cells transformed by activated c-erbB-2 express a subset of tyr
osine-phosphorylated receptors that are homologously phosphorylated on
Thr(686), indicating that Thr(686) phosphorylation alone is insuffici
ent to abrogate receptor tyrosine phosphorylation, Similarly, the c-er
bB-2-overexpressing human cancer cell lines SK-Ov-3 and BT-474 express
constitutively Thr(686)-phosphorylated receptors. SK-Ov-3 cells expre
ss predominantly kinase-inactive c-erbB-2 that is heavily Thr(686)-pho
sphorylated, indicating that Thr(686) phosphorylation in this line is
heterologous in origin. In contrast, BT-474 cells express constitutive
ly autophosphorylated c-erbB-2 despite Thr(686) phosphorylation. These
results indicate that Thr(686) phosphorylation does not directly abol
ish c-erbB-2 activity and suggest that such phosphorylation reflects c
onstitutive PKC activity induced by either receptor-activating mutatio
ns or heterologous growth factors. The latter possibility suggests in
turn that c-erbB-2 interacts in an as yet undefined way with heterolog
ous growth factor receptors in human tumor cells.