To determine whether 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 - CMV) and myr-Akt1 cells formed tumors upon estradiol supplementation after 20-30 days (59-, 29-, and 17-fold increase in tumor volume, respectively). Tamoxifen and AG825 blocked the estradiol effect by 93% and 96% in MCF-7 xenograts, 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 days (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 ER- expression, induced PR expression and Akt phosphorylation, effects that were inhibited by tamoxifen, AG825, and R25C-Akt1 by 89%, 82%, and 77% for PR 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 ER- expression and activity.