Genomic Collaboration of Estrogen Receptor α and Extracellular Signal-Regulated Kinase 2 in Regulating Gene and Proliferation Programs

Abstract

The nuclear hormone receptor, estrogen receptor ␣ (ER␣), and mitogen-activated protein kinases (MAPKs) play key roles in hormone-dependent cancers, and yet their interplay and the integration of their signaling inputs remain poorly understood. In these studies, we document that estrogen-occupied ER␣ activates and interacts with extracellular signal-regulated kinase 2 (ERK2), a downstream effector in the MAPK pathway, resulting in ERK2 and ER␣ colocalization at chromatin binding sites across the genome of breast cancer cells. This genomic colocalization, predominantly at conserved distal enhancer sites, requires the activation of both ER␣ and ERK2 and enables ERK2 modulation of estrogen-dependent gene expression and proliferation programs. The ERK2 substrate CREB1 was also activated and recruited to ERK2-bound chromatin following estrogen treatment and found to cooperate with ER␣/ERK2 in regulating gene transcription and cell cycle progression. Our study reveals a novel paradigm with convergence of ERK2 and ER␣ at the chromatin level that positions this kinase to support nuclear receptor activities in crucial and direct ways, a mode of collaboration likely to underlie MAPK regulation of gene expression by other nuclear receptors as well.Estrogen receptor ␣ (ER␣), a member of the large superfamily of nuclear receptors, exerts profound effects on the gene expression, cellular response programs, and phenotypic properties of estrogen target cells, including over 70% of breast cancers. This hormone receptor also plays a central role in breast cancer development and progression. Because of these broad and important actions, ER␣ is usually considered the single most crucial predictor of breast cancer prognosis and is the key target of endocrine therapies. Blocking the activity of this receptor protein by use of selective estrogen receptor modulators (SERMs) or aromatase inhibitors, which reduce estrogen production, has proven highly effective in targeted treatment of hormone-responsive breast cancers (23,24,37) and also in the prevention of breast cancer in women at high risk for the disease (42).Increased activity of the mitogen-activated protein kinase (MAPK) pathway is one of the hallmarks of more aggressive cancers and of endocrine resistance, in which ER␣-positive tumors become refractory to endocrine therapies and relapse. It is believed that the balance of control of cellular physiology switches from ER␣ nuclear-initiated pathways to increased involvement of extranuclear-activated protein kinase pathways in these breast cancers (5,20,23,32,38,39). However, the interplay and integration of the signaling inputs of this nuclear hormone receptor and MAPKs are poorly understood and were therefore aspects we examined here.The MAPK family comprises well-conserved proteins that function as downstream effectors of a multitier signaling cascade, including a MAPK kinase (MAPKK, MEK) and a MAPKK kinase (MAPKKK), with MAPKs phosphorylating serine/threonine residues on target proteins to control a variety of cellular activitie...

Mutation of the palmitoylation site of estrogen receptor α in vivo reveals tissue-specific roles for membrane versus nuclear actions

“…A nonpalmitoylatable Cys447Ala mutant form of ERα, or its C451A mutant mouse counterpart, expressed in cultured cells lacks interaction with caveolin-1 and downstream activation of signaling pathways and cell proliferation (18,19). Numerous cell culture experiments further suggest potentially important kinase-mediated cross-talk between membrane and nuclear ERα that modifies genomic responses to E2 (20,21), including recent studies revealing that MISS dependent on receptor palmitoylation influences receptor nuclear actions (22). Our current understanding of these processes has relied primarily on experimentation in cell culture.…”

“…The function of many transcription factors is regulated through protein kinase-mediated phosphorylation, and these transcription factors may be targets for extranuclear actions of estrogen (20,33). An important role for ERα MISS was reported in cancer cell proliferative responses to E2 (34)(35)(36), and ERK 2, a downstream effector of the MAPK pathway, cooperates in regulating gene transcription (21). Similarly, a cross-talk between membrane-initiated signaling of steroid receptors, such as progesterone or androgen receptor, and gene regulation and cell proliferation was also reported in various culture models of cancer cells (37).…”

Genome-wide activity of unliganded estrogen receptor-α in breast cancer cells

“…In these studies, the experimental setting, together with the fact that breast cancer cell lines show a high grade of genomic rearrangements (14), made it difficult to evaluate ERα binding in cells treated with vehicle alone. As a consequence, most authors have dismissed the question of hormone-independent binding as compromised peak calling or as unspecific background (15,16). However, especially for the clinical problem concerning the response to aromatase inhibitors, identifying possible ERα genomic actions in the absence of ligands would be very relevant.…”

Phosphorylation by p38 Mitogen-Activated Protein Kinase Promotes Estrogen ReceptorαTurnover and Functional Activity via the SCFSkp2Proteasomal Complex

“…he nuclear hormone receptor estrogen receptor ␣ (ER␣) is a master regulator of gene expression and the proliferative program of breast cancer cells (18,29,36,38,50,54) and, hence, is the main target of endocrine therapies. Approximately 70% of human breast tumors express ER␣ and depend on estrogens for growth, rendering these tumors amenable to treatment with drugs such as selective estrogen receptor modulators/antiestrogens (such as tamoxifen) and aromatase inhibitors, which are quite effective and have relatively few side effects.…”