Functional analysis of Csk and CHK kinases in breast cancer cells

In this report, we analyzed the expression and kinase activities of Csk and CHK kinases in normal breast tissues and breast tumors and their involveme nt in HRG-mediated signaling in breast cancer cells. Csk expression and kin ase activity were abundant in normal human breast tissues, breast carcinoma s, and breast cancer cell lines, whereas CHK expression was negative in nor mal breast tissues and low in some breast tumors and in the MCF-7 breast ca ncer cell line. CHK kinase activity was not detected in human breast carcin oma tissues (12 of 12) or in the MCF-7 breast cancer cell line (due to the low level of CHK protein expression), but was significantly induced upon he regulin (HRG) stimulation. We have previously shown that CHK associates wit h the ErbB-2/neu receptor upon HRG stimulation via its SH2 domain and that it down-regulates the ErbB-2/ neu-activated Src kinases. Our new findings d emonstrate that Csk has no effect on ErbB-2/neu-activated Src kinases upon HRG treatment and that its kinase activity is not modulated by HRG. CHK sig nificantly inhibited in vitro cell growth, transformation, and invasion ind uced upon HRG stimulation. In addition, tumor growth of wt CHK-transfected MCF-7 cells was significantly inhibited in nude mice. Furthermore, CHK down regulated c-Src and Lyn protein expression and kinase activity, and the ent ry into mitosis was delayed in the wt CH-K-transfected MCF-7 cells upon HRG treatment. These results indicate that CHK, but not Csk, is involved in HR G-mediated signaling pathways, down-regulates ErbB-2/neu-activated Src kina ses, and inhibits invasion and transformation of breast cancer cells upon H RG stimulation. These findings strongly suggest that CHK is a novel negativ e growth regulator of HRG-mediated ErbB-2/neu and Src family kinase signali ng pathways in breast cancer cells.