CONSTITUTIVELY TYROSINE-PHOSPHORYLATED P52 SHC IN BREAST-CANCER CELLS- CORRELATION WITH ERBB2 AND P66 SHC EXPRESSION

Breast cancer cell lines display a wide variety of growth factor recep tors, and considerable evidence implicates signaling from these recept ors, especially ErbB2, in the important early stages of this disease, contributing to malignant progression. If this is true, then we would hypothesize that a useful prognostic indicator would be the level of a ctivity of a second messenger protein used in common by these receptor s. One such second messenger is the Shc adapter protein, which is acti vated when tyrosine phosphorylated by receptors. Therefore, one predic tion from the hypothesis is that the level of tyrosine-phosphorylated Shc (PY-Shc) in breast cancer cell lines would correlate with total re ceptor tyrosine kinase activity. To begin to test this prediction, we examined Shc tyrosine phosphorylation in a diverse group of breast can cer cell lines that display varied levels of ErbB2. Using Shc immunopr ecipitation and anti-phosphotyrosine immunoblotting analysis, we found a strong correlation between the level of ErbB2 overexpression (r = 0 .91, p < 0.0002) and PY-ErbB2 levels (r = 0.89, p = 0.0005) compared w ith the level of tyrosine phosphorylation of the p52 and p46 Shc isofo rms. Consistent with Shc tyrosine phosphorylation being driven by ErbB 2, ali ErbB2-specific tyrosine kinase inhibitor markedly reduced Shc t yrosine phosphorylation. Unexpectedly, although all cell lines had com parable total amounts of p52 and p46 Shc, the amount of an inhibitory Shc isoform, p66, was inversely related to the level of ErbB2 expressi on (r = - 0.86, p = 0.0013). This suggests that reduced p66 Shc expres sion may play a role in ErbB2-positive breast cancer. In summary, thes e data are consistent with our prediction that the cellular level of P Y-Shc would correlate with the levels of activated ErbB2 displayed by cell lines derived from breast cancers.