To determine whether the
epidermal growth factor receptor 2 (ErbB2) and Akt1 can alter the in vivo growth of MCF-7 cells, parental cells or cells stably transfected with constitutively active Akt1 (myr-Akt1) or dominant-negative Akt1 mutants (K179M-Akt1 and R25C-Akt1) were implanted into athymic nude mice.
Tumor growth was monitored in the presence or absence of the
antiestrogen tamoxifen and the selective ErbB2 inhibitor,
AG825. MCF-7 [parental or empty vector transfected, cytomegalovirus (CMV)] and myr-Akt1 cells formed
tumors upon
estradiol supplementation after 20-30 d (59-, 29-, and 17-fold increase in
tumor volume, respectively).
Tamoxifen and
AG825 blocked the
estradiol effect by 93 and 96% in MCF-7 xenografts, 88 and 81% in CMV xenografts, and 91% in myr-Akt1 xenografts. Furthermore,
AG825 suppressed the growth of established
tumors in CMV and myr-Akt1 inoculated animals by 68 and 75%, respectively, as compared with continued
estrogen supplementation, suggesting a role for ErbB2. When K179M-Akt1 or R25C-Akt1 cells were injected into ovariectomized animals,
tumor growth was reduced upon
estradiol treatment by 95% and 98%, respectively, supporting a role for Akt1. In contrast to ovariectomized animals, in intact animals, myr-Akt1 cells could establish
tumors without
estradiol priming after 40-50 d (20-fold increase in
tumor volume). Loss of Akt1 phosphorylation was associated with
tumor growth inhibition. Immunohistochemical assays showed that in
tumors from parental and CMV xenografts,
estradiol decreased
estrogen receptor-alpha expression and induced
progesterone receptor expression and Akt phosphorylation, effects that were inhibited by
tamoxifen,
AG825, and R25C-Akt1 by 89, 82, and 77% for
progesterone receptor expression and 48, 66, and 73% for pAkt expression, respectively. Cumulatively, our results suggest that Akt1 and ErbB2 are involved in in vivo
tumorigenesis and modulation of
estrogen receptor-alpha expression and activity.