Estrogen rapid action via protein complex formation involving ERα and Src

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This review provides insight into biomolecular knowledge regarding the non-genomic actions of estrogen in hormone-dependent breast cancer, particularly its role in the rapid stimulation of pathways that transmit signals to increase cell division or decrease programmed cell death. Until recently, attention to estrogenic effects focused primarily on events in the nucleus, where most estrogen receptors (ERα and β) reside. However, a fraction of ERα associated with the cell membrane also participates in rapid estrogen-induced cell membrane-mediated events via formation of a protein complex with many signaling molecules, leading to activation of the mitogen-activated protein kinase and Akt signaling pathways. Understanding the mechanisms underlying these relationships, with the aim of abrogating specific steps, should lead to more targeted strategies to treat hormone-dependent breast cancer.

Introduction

17β-Estradiol (estradiol) stimulates cell growth and prevents apoptosis in estrogen-responsive breast cancer cells. The classic effects of estradiol are mediated through binding to estrogen receptors (ERα and ERβ) and stimulation of transcription at the level of the nucleus. ERα was the first receptor subtype to be identified and is the dominant form in the mammary gland, where ERβ expression is low [1]. In response to estrogen, both receptors form homo- or heterodimers and interact with estrogen-responsive elements contained within the promoter region of estrogen-responsive genes to stimulate gene transcription. Other actions of estradiol involve the tethering of ERα to nuclear transcription factors, such as NF-Y and Sp-1, which in turn bind to DNA and initiate transcription [2]. These events are generally termed ‘estrogen genomic effects’ owing to the involvement of gene transcription and translation. More recent terminology designates these events as ‘nuclear initiated steroid signaling’ [3] or ‘intranuclear events’. However, nuclear transcriptional mechanisms do not explain many estrogen-induced effects that occur rapidly within seconds to minutes, such as activation of the Ca2+ channel and mitogen-activated protein kinase (MAPK) pathway. These rapid effects have subsequently been called ‘non-genomic, extranuclear, membrane-mediated effects’, which take place outside the nucleus or are initiated from the membrane. The newly proposed terminology calls these events ‘membrane initiated steroid signaling’ [3]. A large body of evidence has shown that the non-genomic actions of estrogen are mediated by membrane-associated ERα [4], which resides in or near the cell membrane and interacts with several growth factor signaling pathways. This leads to activation of many cell membrane-initiated signaling molecules (Box 1). Functionally, estrogen-initiated non-genomic events, like those induced by many cytokines, have important physiological consequences leading to DNA synthesis, cell proliferation and protection against cell death 5, 6. Current research on the ER has broadened from focusing on transcriptional activity to defining the mechanisms and relationships of both genomic and non-genomic estrogen effects. Such mechanisms have been shown not only to involve the estradiol–ERα system, but also other steroid hormone family members such as progesterone [7], androgen [8] and 1,25(OH)2D3 [9] and their respective receptors.

At the present time, it is not clear why steroid hormones require membrane-mediated kinase pathway activation in addition to nuclear initiated events. O'Malley suggested that activation of cytoplasmic kinases through membrane initiated events results in the phosphorylation and activation of coactivators, which enhance nuclear transcriptional events [10]. Clinically, ERα has been a molecular target for breast cancer treatment by anti-estrogens, such as tamoxifen and fulvestrant. Further deciphering the ERα-mediated non-genomic pathway in concert with its genomic pathway will greatly enhance our understanding of estrogen effects and aid in designing more effective cancer treatment regimens. This review addresses the role of ERα-interacting molecules, such as MNAR (modulator of non-genomic activity of estrogen receptor), p85α, Shc, caveolins and G protein, on MAPK and Akt pathway activation in rapid estrogen action.

Section snippets

ERα structure and function

ERα contains the following functional domains (Figure 1): a hormone-independent transcription function domain (AF-1), a DNA-binding domain (DBD), a hinge region containing three nuclear localization sequences (NLS) that mediate the translocation of the receptor from cytosol to nucleus, and a conserved hormone-binding domain (HBD), possessing dimerization and hormone-dependent activation functions (AF-2). Binding of estrogen to ERα, leading to ERα dimer formation through the ligand-binding

Estrogen-induced MAPK and Akt activation

Membrane-associated ERα transduces estrogen rapid signals, leading to (i) the activation of many signaling molecules, such as MAPK, Akt, p21ras, Raf-1, and protein kinase C; (ii) alteration of maxi-K channels; (iii) increase in intracellular Ca2? levels; and (iv) release of nitric oxide and stimulation of prolactin secretion (see detailed reviews 25, 26, 27). In addition to activation of the signaling molecules described above, membrane growth factor receptors, such as IGF-1R and epidermal

Src-centered activating particle formation in estrogen rapid action

It is as yet unknown which proteins mediate actions directly downstream of membrane-associated ERα and how the signals are initiated. Because ERα has no intrinsic kinase domain and therefore is not capable of phosphorylating other proteins, the signaling molecules must function directly downstream of, and physically associate with, ERα. At the same time, the signaling molecule must transduce the ERα signal to downstream cascades, leading to the rapid activation of MAPK and Akt. Src tyrosine

Conclusions and future perspectives

Taken together, we have devised a model for the formation of a multiprotein-based ‘activating particle’ involving ERα, Src, MNAR, p85α, Shc and G proteins in the case of ER-responsive breast cancer cells (Figure 4). In this protein complex, MNAR, p85α, Shc and G proteins all act to mediate ERα-induced Src activation. It should be noted that another term ‘signalsome’ has recently been suggested, which describes the formation of protein complexes involving IGF-1R–Shc–ERα or EGFR–G

Acknowledgement

We thank Wei Yue of the Department of Internal Medicine, University of Virginia for her helpful discussions and suggestions in preparing of this manuscript. This work was supported by grants from Department of Defense Breast Cancer Research Program (DAMD 17–02–1-0610 to RXS) and the NIH (CA 65622 to RJS).

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