We have shown previously that the AKT2 pathway is essential for cell survival and important in malignant transformation. In this study, we demonstrate elevated
kinase levels of AKT2 and
phosphatidylinositol-3-OH kinase (PI3K) in 32 of 80 primary
breast carcinomas. The majority of the cases with the activation are
estrogen receptor alpha (
ERalpha) positive, which prompted us to examine whether AKT2 regulates
ERalpha activity. We found that constitutively activated AKT2 or AKT2 activated by
epidermal growth factor or
insulin-like growth factor-1 promotes the transcriptional activity of
ERalpha. This effect occurred in the absence or presence of
estrogen. Activated AKT2 phosphorylates
ERalpha in vitro and in vivo, but it does not phosphorylate a mutant
ERalpha in which ser-167 was replaced by Ala. The PI3K inhibitor,
wortmannin, abolishes both the phosphorylation and transcriptional activity of
ERalpha induced by AKT2. However, AKT2-induced
ERalpha activity was not inhibited by
tamoxifen but was completely abolished by ICI 164,384, implicating that AKT2-activated
ERalpha contributes to
tamoxifen resistance. Moreover, we found that
ERalpha binds to the p85alpha regulatory subunit of PI3K in the absence or presence of
estradiol in epithelial cells and subsequently activates PI3K/AKT2, suggesting
ERalpha regulation of PI3K/AKT2 through a nontranscriptional and
ligand-independent mechanism. These data indicate that regulation between the
ERalpha and PI3K/AKT2 pathway (
ERalpha-PI3K/AKT2-
ERalpha) may play an important role in pathogenesis of human
breast cancer and could contribute to
ligand-independent
breast cancer cell growth.