MNAR plays an important role in ERa activation of Src/MAPK and PI3K/Akt signaling pathways.

Estrogens play a critical role in the regulation of cellular proliferation, differentiation, and apoptosis. Evidence indicates that this regulation is mediated by a complex interface of direct control of gene expression (so-called "genomic action") and by regulation of cell-signaling/phosphorylation cascades (referred to as the "non-genomic", or "extranuclear" action). However, the mechanisms of the non-genomic action of estrogens are not well defined.

Signaling by estrogens.

By regulating activities and expression levels of key signaling molecules, estrogens control mechanisms that are responsible for crucial cellular functions. Ligand binding to estrogen receptor (ER) leads to conformational changes that regulate the receptor activity, its interaction with other proteins and DNA. In the cytoplasm, receptor interactions with kinases and scaffolding molecules regulate cell signaling cascades (extranuclear/nongenomic action).

Phosphorylation of MNAR promotes estrogen activation of phosphatidylinositol 3-kinase.

Estrogen actions are mediated by a complex interface of direct control of gene expression (the so-called "genomic action") and by regulation of cell signaling/phosphorylation cascades, referred to as the "nongenomic," or extranuclear, action. We have previously described the identification of MNAR (modulator of nongenomic action of estrogen receptor) as a novel scaffold protein that regulates estrogen receptor alpha (ERalpha) activation of cSrc. In this study, we have investigated the role of MNAR in 17beta-estradiol (E2)-induced activation of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway.

The role of extranuclear signaling actions of progesterone receptor in mediating progesterone regulation of gene expression and the cell cycle.

Human progesterone receptor (PR) contains a motif that interacts with the SH3 domain of Src and mediates rapid activation of Src and downstream MAPK (Erk-1/-2) without relying on the transcriptional activity of the receptor. Here we investigated the role and intracellular location of this nontranscriptional activity of PR. Progestin activation of Src/MAPK occurred outside the nucleus with the B isoform of PR that was distributed between the cytoplasm and nucleus, but not with PR-A that was predominantly nuclear.

Vitamin D receptor ligands for osteoporosis.

1alpha,25-Dihydroxyvitamin D3 (1,25-(OH)2D3), the biologically active metabolite of vitamin D, mediates its actions via the vitamin D receptor (VDR), a member of the superfamily of steroid/thyroid hormone/retinoid receptors. 1,25-(OH)2D3 is required for calcium and phosphorus homeostasis, and for normal skeletal development as well as maintenance of skeletal architecture. Two VDR ligands, calcitriol (1,25-(OH)2D3) and its synthetic analog alfacalcidol (1alpha-hydroxyvitamin D3), have been approved for the treatment of osteoporosis.

Characterization of MNAR expression.

We have previously demonstrated that modulator of nongenomic action of estrogen receptor (MNAR) integrates action of estrogen receptor alpha (ERalpha), and potentially some other nuclear receptors (NRs), in regulation of Src/Ras/Raf/mitogen-activated protein kinase (MAPK) signaling pathway. MNAR is a scaffolding protein that contains 10 LXXLL type motifs that can interact with NRs and 3 PXXP type motifs that can bind to SH3 domains present in kinases and other signaling molecules. Formation of ER-MNAR-cSrc complex leads to activation of Src and downstream Ras/Raf/MAPK pathway.

Regulation of cell signalling cascades by steroid hormones.

Steroid hormones control a wide variety of cellular functions important for cell homeostasis, proliferation, differentiation, and apoptosis. Evidence collected in the last few years indicates that this regulation is mediated by a complex interface of direct control of gene expression (so-called "transcriptional" action, driven by receptors that are localized in cell nucleus) and by regulation of cell signaling/phosphorylation cascades ("extranuclear" action, mediated by receptors that are localized in close proximity to cellular membrane). Regulation of gene expression takes place via ligand-dependent receptor binding to target gene promoters as part of the preinitiation complex, which leads to chromatin remodeling and ultimately regulates the rate of gene expression.

Characterization of the interactions of estrogen receptor and MNAR in the activation of cSrc.

In this study, we have evaluated the molecular mechanism of Src activation after its interaction with estrogen receptor alpha (ERalpha) and a newly identified scaffold protein, called MNAR (modulator of nongenomic activity of ER). Under basal condition, Src enzymatic activity is inhibited by intramolecular interactions. The enzyme can be activated by interaction between the SH2 domain of Src and phosphotyrosine-containing sequences and/or by interaction between the SH3 domain of Src and proteins containing PXXP motifs.

Hierarchical affinities and a bipartite interaction model for estrogen receptor isoforms and full-length steroid receptor coactivator (SRC/p160) family members.

Nuclear receptor (NR)-mediated transcription is driven by dynamic multiprotein coactivator complexes, the composition of which is thought to determine the biological activity of NRs at specific promoters. The extent to which NRs discriminate between a spectrum of potential binding partners is intuitively a function of the inherent affinities of these individual interactions. Using real time interaction analysis with BIAcore, we evaluated the affinities and kinetics of the interactions of full-length members of the SRC/p160 coactivator family with estrogen receptor alpha (ER alpha) and ER beta bound to a variety of ligands.

Estrogen receptor-interacting protein that modulates its nongenomic activity-crosstalk with Src/Erk phosphorylation cascade.

Numerous studies have demonstrated that estrogens induce rapid and transient activation of the Src/Erk phosphorylation cascade. Activation of this cascade triggers vital cellular functions including cell proliferation and differentiation. However, the details of the molecular mechanism of this process remain to be elucidated.