An inherent role of microtubule network in the action of nuclear receptor

Abstract

Estrogen receptor ␣ (ER␣) functions as both a transcription factor and a mediator of rapid estrogen signaling. Recent studies have shown a role for ER␣-interacting membranous and cytosolic proteins in ER␣ action, but our understanding of the role of the microtubule network in the modulation of ER␣ signaling remains unclear. Here we found that endogenous ER␣ associates with microtubules through the microtubule-binding protein hematopoietic PBX-interaction protein (HPIP). Biochemical and RNA-interference studies demonstrated that HPIP influences ER␣-dependent rapid estrogen signaling by acting as a scaffold protein and recruits Src kinase and the p85 subunit of phosphatidylinositol 3-kinase to a complex with ER␣, which in turn stimulates AKT and MAPK. We also found that ER␣ interacts with ␤-tubulin through HPIP. Destabilization of microtubules activated ER␣ signaling, whereas stabilization of microtubules repressed ER␣ transcriptional activity in a HPIP-dependent manner. These findings revealed a role for HPIPmicrotubule complex in regulating 17␤-estradiol-ER␣ responses in mammalian cells and discovered an inherent role of microtubules in the action of nuclear receptor.17␤-estradiol ͉ estrogen receptor ͉ hematopoietic PBX-interaction protein E strogen regulates a plethora of functionally divergent physiological processes including development, homeostasis, and reproduction (1). The diversity of estrogen action results in part from the ability of estrogen receptors (ERs) to act both as transcription factors that regulate gene expression (i.e., genomic effects) and as signaling proteins that rapidly recruit and activate kinase-dependent signaling pathways (rapid effects). There is growing evidence that a subpopulation of the conventional nuclear steroid receptor localized in the vicinity of the cell membrane mediates many of the rapid signaling actions of steroid hormones; however, membrane receptors unrelated to conventional steroid receptors have also been implicated (2, 3). Several studies support the concept that estrogen can activate multiple cytosolic signaling pathways through direct interactions of conventional estrogen receptor (ER␣ or ER␤) with various cytoplasmic and membranous proteins, including kinases and adaptor proteins, by forming different multiprotein complexes (2, 4). In addition, sequestration of ER by MTA1s (metastasisassociated antigen 1 short form) also triggers estrogen rapid signaling. So it appears that relative subcellular distribution of ERs plays a critical role in estrogen signaling. Besides these mechanistic studies, recent reports have suggested that extranuclear estrogen signaling is directly implicated in cell migration through actin cytoskeleton remodeling (5).Microtubules are structural components of the cytoskeleton required for cell motility that regulate a variety of signaling pathways, including the inducible nitric oxide synthase, NF-B, ERK, JNK, Wnt, and Hedgehog signaling pathways (6). The functional role of microtubules in signal transduction has been further elucidated ...

MDM2 restrains estrogen-mediated AKT activation by promoting TBK1-dependent HPIP degradation

“…Moreover, both HPIP and TBK1 were dispensable for EGF-mediated AKT and ERK1/2 activations in MCF7 cells (Supplementary Figure S3). As estrogenmediated AKT-activation relies on HPIP, which tethers ERa to microtubules, 10 we tested whether TBK1 is involved in this signaling cascade. 17b estradiol (E2) activated TBK1, AKT and ERK1/2 in the p53 WT breast cancer cell line MCF7 (Figure 2a).…”

“…8,9 AKT activation by estrogens requires the microtubule-binding protein hematopoietic PBX-interaction protein (HPIP). 10 Initially identified as a corepressor of pre-B-cell leukemia homeobox protein 1 (PBX1), 11 HPIP assembles a signaling complex that connects the p85 subunit of PI3K and ERa to microtubules in order to properly activate AKT. 10 Likewise, HPIP also promotes the growth and differentiation of hematopoietic cells through AKT.…”

“…10 Initially identified as a corepressor of pre-B-cell leukemia homeobox protein 1 (PBX1), 11 HPIP assembles a signaling complex that connects the p85 subunit of PI3K and ERa to microtubules in order to properly activate AKT. 10 Likewise, HPIP also promotes the growth and differentiation of hematopoietic cells through AKT. 12 Because correct regulation of AKT is of paramount importance, multiple mechanisms have evolved to terminate or limit its activation.…”

PBX proteins: much more than Hox cofactors

“…The biological relevance of HPIP's ability to modulate Pbx function remains to be assessed in a more physiological context. A recent report (Manavathi et al, 2006) shows that HPIP is able to physically bind estrogen receptor α (ERα) and that a HPIP-microtubule complex could regulate estradiol-ER responses in mammalian cells. Unfortunately, the involvement of Pbx proteins in such pathway has not been investigated yet by the authors.…”

Functional Regulation of Pre-B-cell Leukemia Homeobox Interacting Protein 1 (PBXIP1/HPIP) in Erythroid Differentiation

“…Recent studies have also revealed the role of HPIP in cell migration and proliferation in breast cancer cells (13,14). Mouse xenograft studies and anchorage-independent growth assays demonstrated the oncogenic nature of HPIP (13).…”