Subcellular Localization of Activated AKT in Estrogen Receptor- and Progesterone Receptor-Expressing Breast Cancers : Potential Clinical Implications

Abstract

Activated v-AKT murine thymoma viral oncogene homolog 1 (AKT)/protein kinase B (PKB) kinase (pAKT) is localized to the plasma membrane, cytoplasm, and/or nucleus in 50% of cancers. The clinical importance of pAKT localization and the mechanism(s) controlling this compartmentalization are unknown. In this study, we examined nuclear and cytoplasmic phospho-AKT (pAKT) expression by immunohistochemistry in a breast cancer tissue microarray (n = 377) with ≈15 years follow-up and integrated these data with the expression of estrogen receptor (ER)α, progesterone receptor (PR), and FOXA1. Nuclear localization of pAKT (nuclear-pAKT) was associated with long-term survival (P = 0.004). Within the ERα+/PR+ subgroup, patients with nuclear-pAKT positivity had better survival than nuclear-pAKT–negative patients (P ≤ 0.05). The association of nuclear-pAKT with the ERα+/PR+ subgroup was validated in an independent cohort (n = 145). TCL1 family proteins regulate nuclear transport and/or activation of AKT. TCL1B is overexpressed in ERα-positive compared with ERα-negative breast cancers and in lung metastasis–free breast cancers. Therefore, we examined the possible control of TCL1 family member(s) expression by the estrogen:ERα pathway. Estradiol increased TCL1B expression and increased nuclear-pAKT levels in breast cancer cells; short- interfering RNA against TCL1B reduced nuclear-pAKT. Overexpression of nuclear-targeted AKT1 in MCF-7 cells increased cell proliferation without compromising sensitivity to the anti-estrogen, tamoxifen. These results suggest that subcellular localization of activated AKT plays a significant role in determining its function in breast cancer, which in part is dependent on TCL1B expression