AND-34 is a murine
protein that binds by a cdc25-like
GDP exchange factor domain to the focal adhesion docking
protein p130Cas. Overexpression of either of the human homologues of AND-34 and p130Cas, BCAR3 and BCAR1, respectively, has been reported to induce resistance to
antiestrogens in
breast cancer cell lines. Here we show that overexpression of AND-34 leads to activation of the Rho family
GTPases Cdc42 and Rac. Consistent with these findings, BCAR3 overexpression induced alterations in
F-actin distribution and augmented both autophosphorylation and
kinase activity of the Cdc42/Rac-responsive
serine/threonine kinase PAK1. p130Cas-associated BCAR3
protein was detected in the
estrogen-independent
breast cancer cell line 578-T, but not in
estrogen-dependent MCF7 or ZR-75-1 cells. Stable ZR-75-1 transfectants overexpressing BCAR3, but not vector-only transfectants, grew in the presence of the pure
antiestrogen ICI 182,780. Stable transfection with RacV12, a constitutively active form of Rac1, also induced
antiestrogen resistance in ZR-75-1 cells. Transient transfection of BCAR3 in
estrogen-dependent MCF7 cells induced activation of
luciferase constructs containing the proximal 1745 or 163 bp but not 66 bp of the
cyclin D1 promoter. Such
cyclin D1 promoter activation was inhibited by dominant negative forms of Rac1 and PAK1. Overexpression of the PAK1 autoinhibitory domain (residues 83-149) but not an inactive PAK1 autoinhibitory domain point mutant (L107F) also blocked BCAR3-mediated
cyclin D1 activation. These studies suggest that AND-34/BCAR3 induces
antiestrogen resistance in
breast cancer cell lines by a Rac1- and PAK1-dependent pathway.