Glycogen synthase kinase-3 interacts with and phosphorylates estrogen receptor alpha and is involved in the regulation of receptor activity

Pathways to tamoxifen resistance

Therapies that target the synthesis of estrogen or the function of estrogen receptor(s) have been developed to treat breast cancer. While these approaches have proven to be beneficial to a large number of patients, both de novo and acquired resistance to these drugs is a significant problem. Recent advances in our understanding of the molecular mechanisms that contribute to resistance have provided a means to begin to predict patient responses to these drugs and develop rational approaches for combining therapeutic agents to circumvent or desensitize the resistant phenotype. Here, we review common mechanisms of antiestrogen resistance and discuss the implications for prediction of response and design of effective combinatorial treatments.

DUSP22/LMW-DSP2 regulates estrogen receptor-alpha-mediated signaling through dephosphorylation of Ser-118

In the previous study, we demonstrated the involvement of dual specificity phosphatase 22 (DUSP22/LMW-DSP2) in regulating the leukemia inhibitory factor/interleukin-6/signal transducer and activator of transcription 3-mediated signaling pathway. In this study, we show beta-estradiol (E2)-induced DUSP22 mRNA expression in estrogen receptor alpha (ERalpha)-positive breast cancer cells, whereas E2-induced phosphorylation and activation of ERalpha was suppressed by overexpression of DUSP22 but not catalytically inactive mutants. Furthermore, small-interfering RNA-mediated reduction of DUSP22 expression enhanced ERalpha-mediated transcription and endogenous gene expression. In fact, DUSP22 associated with ERalpha in vivo and both endogenous proteins interacted in ERalpha-positive breast cancer T47D cells. These results strongly suggest that DUSP22 acts as a negative regulator of the ERalpha-mediated signaling pathway.

Membrane-initiated actions of estrogens in neuroendocrinology: emerging principles

Hormonal ligands for the nuclear receptor superfamily have at least two interacting mechanisms of action: 1) classical transcriptional regulation of target genes (genomic mechanisms); and 2) nongenomic actions that are initiated at the cell membrane, which could impact transcription. Although transcriptional mechanisms are increasingly well understood, membrane-initiated actions of these ligands are incompletely understood. Historically, this has led to a considerable divergence of thought in the molecular endocrine field. We have attempted to uncover principles of hormone action that are relevant to membrane-initiated actions of estrogens. There is evidence that the membrane-limited actions of hormones, particularly estrogens, involve the rapid activation of kinases and the release of calcium. Membrane actions of estrogens, which activate these rapid signaling cascades, can also potentiate nuclear transcription. These signaling cascades may occur in parallel or in series but subsequently converge at the level of modification of transcriptionally relevant molecules such as nuclear receptors and/or coactivators. In addition, other hormones or neurotransmitters may also activate cascades to crosstalk with estrogen receptor-mediated transcription. The idea of synergistic coupling between membrane-initiated and genomic actions of hormones fundamentally revises the paradigms of cell signaling in neuroendocrinology.

An inherent role of integrin-linked kinase-estrogen receptor alpha interaction in cell migration

Integrin-linked kinase (ILK) and estrogen receptor (ER)-alpha modulate cell migration. However, the crosstalk between ERalpha and ILK and the role of ILK in ERalpha-mediated cell migration remain unexplored. Here, we report that ILK participates in ERalpha signaling in breast cancer cells. We found that ILK binds ERalpha in vitro and in vivo through a LXXLL motif in ILK. Estrogen prevented ERalpha-ILK binding, resulting in phosphatidylinositol 3-kinase (PI3K)-dependent increase in ILK kinase activity. Furthermore, the regulation of ERalpha-ILK interaction was dependent on the PI3K pathway. Unexpectedly, transient knockdown or inhibition of ILK caused hyperphosphorylation of ERalpha Ser(118) in an extracellular signal-regulated kinase/mitogen-activated protein kinase pathway-dependent manner and an enhanced ERalpha recruitment to the target chromatin and gene expression, a process reversed by overexpression of ILK. Compatible with these interactions, estrogen regulated cell migration via the PI3K/ILK/AKT pathway with stable ILK overexpression hyperactivating cell migration. Thus, status of ILK signaling may be an important modifier of ER signaling in breast cancer cells and this pathway could be exploited for therapeutic intervention in breast cancer cells.

Potential role of estrogen receptor alpha (ERalpha) phosphorylated at Serine118 in human breast cancer in vivo

Post-translational modifications of proteins are known to be important in protein activity and ERalpha is known to be phosphorylated at multiple sites within the protein. The exact function of site-specific phosphorylation in ERalpha is unknown, although several hypotheses have been developed using site-directed mutagenesis and cell culture models. Targeting the ERalpha at the level of such post-translational modification pathways would be a new and exciting approach to endocrine therapy in breast cancer, but adequate knowledge is lacking with regard to the relevance of site-specific phosphorylation in ERalpha in human breast cancer in vivo. Recently, antibodies to P-Serine(118)-ERalpha and P-Serine(167)-ERalpha, two major sites of phosphorylation in ERalpha, have become available and some in vivo data are now available to complement studies in cells in culture. However, the in vivo data are somewhat contradictory and limited by the small cohorts used and the lack of standard well-characterized reagents and protocols.

Phosphatidylinositol 3-kinase and glycogen synthase kinase 3 regulate estrogen receptor-mediated transcription in neuronal cells

In addition to 17beta-estradiol binding, estrogen receptor (ER) transcriptional activity could be controlled by intracellular kinase signaling pathways activated by growth factors. In this report we present evidence suggesting that glycogen synthase kinase 3 (GSK3), an effector kinase of the phosphatidylinositol 3-kinase (PI3K) pathway, may affect ERalpha activity in N2a neuroblastoma cells. LiCl, sodium valproate, and SB415286, three inhibitors of GSK3, dose-dependently blocked ERalpha-mediated transcription. In contrast, overexpression of wild-type GSK3, but not of a mutant inactive form, increased ER-dependent gene expression. Pharmacological or genetic inhibition of the PI3K/Akt pathway, whose activity is inversely correlated with that of GSK3, increased ERalpha-mediated transcription, and this effect was blocked by GSK3 inhibitors. As in other cell types, IGF-I increased ERalpha activity in absence of estradiol by a mechanism independent of PI3K. In contrast, IGF-I decreased ERalpha activity in the presence of estradiol, and this effect was mediated by PI3K. We also observed a regulated interaction between beta-catenin, one of the main GSK3 nuclear targets, and ERalpha. Transfection with a nondegradable mutant of beta-catenin blocked the increase in ERalpha transcriptional activity induced by the PI3K inhibitor wortmannin, suggesting a role for beta-catenin in estrogen signaling. In addition, we investigated the regulation of ER protein levels as a potential mechanism for its regulation by the PI3K/GSK3 pathway; GSK3 blockade increased ERalpha protein stability, whereas PI3K inhibition decreased it. In summary, our findings suggest that ER-dependent gene expression in N2a cells is controlled by the PI3K/Akt/GSK3 signaling pathway.

Multiple pathways transmit neuroprotective effects of gonadal steroids

Numerous preclinical studies suggest that gonadal steroids, particularly estrogen, may be neuroprotective against insult or disease progression. This paper reviews the mechanisms contributing to estrogen-mediated neuroprotection. Rapid signaling pathways, such as MAPK, PI3K, Akt, and PKC, are required for estrogen's ability to provide neuroprotection. These rapid signaling pathways converge on genomic pathways to modulate transcription of E2-responsive genes via ERE-dependent and ERE-independent mechanisms. It is clear that both rapid signaling and transcription are important for estrogen's neuroprotective effects. A mechanistic understanding of estrogen-mediated neuroprotection is crucial for the development of therapeutic interventions that enhance quality of life without deleterious side effects.