The genetic changes and mechanisms underlying the progression of
estrogen-dependent breast
cancers to
estrogen-independent,
antiestrogen-resistant, and metastatic breast
cancers are unclear despite being a major problem in endocrine
therapy. To identify genes responsible for this progression, we carried out a genetic screening by an enhanced retroviral
mutagen (ERM)-mediated random mutagenesis in the
estrogen-dependent T47D
breast cancer cells. We found that T47D cells contain only one p27kip1 (p27) allele coding for the p27
cyclin-dependent kinase (CDK) inhibitor. An ERM insertion into the p27 locus of T47D cells disrupted the p27 gene and created
estrogen-independent and
antiestrogen-resistant
breast cancer cells that still maintained functional
estrogen receptors. Disruption of p27 in T47D cells resulted in several changes, and most of these changes could be rescued by p27 restoration. First, CDK2 activity was increased in the absence of
estrogen or in the presence of
estrogen antagonists tamoxifen or
ICI 182780; second, amplified in
breast cancer 1 (AIB1), a
cancer overexpressed transcriptional coactivator, was hyperphosphorylated, which made AIB1 a better coactivator for E2F1; and third,
growth factor receptor binding protein 2-associated binder 2 (Gab2) and Akt activity were increased following E2F1 overactivation, leading to a significant enhancement of cell migration and invasion. Furthermore, the p27-deficient cells, but not T47D control cells, developed lung
metastasis in an ovarian
hormone-independent manner when they were i.v. injected into nude mice. In sum, loss of p27 activated AIB1, E2F1, Gab2, and Akt; increased cell migration and invasion; caused
antiestrogen insensitivity; and promoted
metastasis of
breast cancer cells. These findings suggest that p27 plays an essential role in restriction of
breast cancer progression.