METHODS: Pharmacological inhibition of c-Src blocked
estrogen-dependent proliferation in MCF-7 cells and enhanced the inhibitory effects of
tamoxifen or
estrogen-deprivation on cell growth. Maximum inhibition (95%) of cell growth was obtained when
tamoxifen and c-Src blockade were combined. Inhibition of
c-Src kinase decreased levels of the ER targets c-Myc and
cyclin D1 expression but not of Bcl-2. Nevertheless, blocking
c-Src kinase in
tamoxifen-treated MCF-7 cells led to apoptosis. Inhibition of
c-Src kinase altered the ratio of Mcl-1
isoforms in favor of cell death whereas expression of the proapoptotic molecules Bad, Bak, and Bax was not altered. Surprisingly, blocking ER function increased the levels of Bad phosphorylation at
serine 112 (BadpS112), an inactive (nonapoptotic) form of Bad. This inactivation of Bad upon ER blockade seemed to depend on c-Src function as chemical inhibition of
c-Src kinase reduced BadpS112 levels in cells with impaired ER function but not in
estrogen-treated cells.
CONCLUSION: These results indicate a crucial role for
c-Src kinase in the survival of ER-positive
breast cancer cells only when ER function is blocked. Therefore, this study suggests that targeting simultaneously c-Src and ER may effectively inhibit growth of ER-positive
breast cancer.