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J Biol Chem, Vol. 273, Issue 7, 4065-4072, February 13, 1998
,
From the Divisions of Experimental Medicine and
Hematology/Oncology, Beth Israel Deaconess Medical Center, Harvard
Institutes of Medicine, Boston, Massachusetts 02115, the
Substantial evidence exists supporting direct
roles for ErbB-2/neu and Src kinase activation in breast cancer. The
Csk homologous kinase (CHK) is a recently identified tyrosine kinase
which, like Csk, phosphorylates the C-terminal tyrosine of Src kinases,
resulting in inactivation of these enzymes. Recently, we observed that
CHK is associated with the ErbB-2/neu receptor upon heregulin
stimulation of breast cancer cells. Here, we report that CHK expression
was observed in 70 out of 80 primary breast cancer specimens but not in
normal breast tissues (0/19). Confocal microscopy analysis revealed
co-localization of CHK with ErbB-2 in these primary specimens (6/6). In
addition, we observed that the cytoplasmic domain of the ErbB-2/neu
receptor is sufficient for its interaction with the
CHKSH2 domain. Phosphopeptide inhibition of the
in vitro interaction of CHKSH2 or native CHK
with ErbB-2/neu, as well as site-directed mutagenesis of ErbB-2/neu,
indicated that CHKSH2 binds to Tyr1253 of
ErbB-2/neu. Interestingly, autophosphorylation at this site confers
oncogenicity to this receptor. Moreover, CHK was able to down-regulate
ErbB-2/neu-activated Src kinases. Overexpression of CHK in MCF-7 breast
cancer cells markedly inhibited cell growth and proliferative response
to heregulin as well as decreased colony formation in soft agar. These
studies indicate that CHK binds, via its SH2 domain, to
Tyr1253 of the activated ErbB-2/neu and down-regulates the
ErbB-2/neu-mediated activation of Src kinases, thereby inhibiting
breast cancer cell growth. These data strongly suggest that CHK is a
novel negative growth regulator in human breast cancer.
Department of Chemical Immunology, the Weizmann
Institute of Science, Rehovot 76100, Israel, and the
§ Department of Cell Research and Immunology, Tel Aviv
University, Ramat Aviv 69978, Israel
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