Effect of estradiol on estrogen receptor-α gene expression and activity can be modulated by the ErbB2/PI 3-K/Akt pathway

Abstract

Epidermal growth factor (EGF), insulin-like growth factor-I (IGF-I), and heregulin-b1 (HRG-b1), can modulate the expression and activity of the estrogen receptor-a (ER-a) via the phosphatidylinositol 3-kinase (PI 3-K)/Akt pathway in the ER-a-positive breast cancer cell line, MCF-7. Estradiol can also rapidly activate PI 3-K/Akt in these cells (nongenomic effect). The recent study examines whether Akt is involved in the ER-a regulation by estradiol (genomic effect). Stable transfection of parental MCF-7 cells with a dominant-negative Akt mutant, as well as the PI 3-K inhibitors wortmannin and LY 294,002, blocked the effect of estradiol on ER-a expression and activity by 70-80 and 55-63%, respectively. Stable transfection of MCF-7 cells with a constitutively active Akt mimicked the effect of estradiol. The changes in ER-a expression and activity were abrogated in response to estradiol by an arginine to cysteine mutation in the pleckstrin homology (PH) domain of Akt (R25C), suggesting the involvement of this amino acid in the interaction between Akt and ER-a. Experiments employing selective ErbB inhibitors demonstrate that the effect of estradiol on ER-a expression and activity is mediated by ErbB2 and not by EGFR. Moreover, anchorage-dependent and -independent growth assays, cell cycle and membrane ruffling analyses showed that Akt exerts estrogen-like activity on cell growth and membrane ruffling and that a selective ErbB2 inhibitor, but not anti-ErbB2 antibodies directed to the extracellular domain, can block these effects. In the presence of constitutively active Akt, tamoxifen only partially inhibits cell growth. In contrast, in cells stably transfected with either a dominant-negative Akt or with R25C-Akt, as well as in parental cells in the presence of a selective ErbB2 inhibitor, the effect of estradiol on anchorage-dependent and -independent cell growth was inhibited by 50-75 and 100%, respectively. Dominant-negative Akt inhibited membrane ruffling by 54%; however, R25C-Akt did not have any effect, suggesting that kinase activity plays an important role in this process. Scatchard analysis demonstrated a 67% reduction in estrogen-binding capacity in cells transfected with constitutively active Akt. No change in binding affinity of estradiol to the receptor was observed upon transfection with either Akt mutant. Taken together, our results suggest that estradiol treatment results in binding to membrane ER-a and interaction with a heterodimer containing ErbB2, leading to tyrosine phosphorylation. This results in the activation of PI 3-K and Akt. Akt, in turn, may interact with nuclear ER-a, altering its expression and activity.

Functional identification of genes causing estrogen independence of human breast cancer cells

“…Within the cytoplasmic compartment of the cell, signaling from the receptor tyrosine kinases along the RAS-ERK and/or the PI3K-AKT pathway involves several proteins (ERBB2, EGFR, PDGFRA and B, NRG1, FGF17, activated HRAS and RAF1, AKT1) contributing to estrogen independency [for review 4,7,20,[25][26][27][28]. Adaptor proteins like GRB7, BCAR1 and BCAR3 may modulate the outcome of these main cytoplasmic signaling routes through selective recruitment of other players [29][30][31].…”

Resistance to endocrine therapy in breast cancer: exploiting estrogen receptor/growth factor signaling crosstalk

“…Many of these interactions lead to the activation of key secondary signaling messengers and downstream kinase pathways, such as the p21Ras/p42/44 MAPK and AKT, leading to the activation of various cellular processes such as proliferation, growth, and survival. In addition to activating their known sets of downstream transcription regulators, these kinase signals can also activate nuclear ER activity as well as other components in ER's transcriptional machinery and thus promote ER-dependent transcription (ER genomic activity) (Sun et al 2001, Shou et al 2002, Stoica et al 2003. This loop of pathway interdependence, or bidirectional crosstalk, augments signaling of both ER and growth factor receptor pathways and enhances propagation and survivability of a breast cancer cell by the shared contribution of multiple pathways.…”

“…1A). Indeed, overexpression of growth factor receptors such as EGFR/HER2 may augment both genomic and nongenomic ER actions in breast cancer experimental systems (Chung et al 2002, Kumar et al 2002, Stoica et al 2003 and may lead to tamoxifen resistance (Benz et al 1992, Miller et al 1994 (Fig. 1B).…”

Estrogen signaling ability in human endometrial cancer through the cancer-stromal interaction

“…Recent studies reveal that estrogen receptors (ERs) are activated not only by estrogen but also by protein phosphorylation by kinases such as mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K)/Akt (Campbell et al 2001, Stoica et al 2003. Activated ER contributes to the proliferation, anti-apoptosis, and metastasis of tumor cells.…”