Transcriptional regulation by the repressor of estrogen receptor activity via recruitment of histone deacetylases

Decreased BRCA1 confers tamoxifen resistance in breast cancer cells by altering estrogen receptor-coregulator interactions

The breast cancer susceptibility gene 1 (BRCA1) is mutated in approximately 50% of hereditary breast cancers, and its expression is decreased in 30-40% of sporadic breast cancers, suggesting a general role in breast cancer development. BRCA1 physically and functionally interacts with estrogen receptor-alpha (ERalpha) and several transcriptional regulators. We investigated the relationship between cellular BRCA1 levels and tamoxifen sensitivity. Decreasing BRCA1 expression in breast cancer cells by small interfering RNA alleviated tamoxifen-mediated growth inhibition and abolished tamoxifen suppression of several endogenous ER-targeted genes. ER-stimulated transcription and cytoplasmic signaling was increased without detectable changes in ER or ER coregulator expression. Co-immunoprecipitation studies showed that with BRCA1 knockdown, tamoxifen-bound ERalpha was inappropriately associated with coactivators, and not effectively with corepressors. Chromatin immunoprecipitation studies demonstrated that with tamoxifen, BRCA1 knockdown did not change ERalpha promoter occupancy, but resulted in increased coactivator and decreased corepressor recruitment onto the endogenous cyclin D1 promoter. Our results suggest that decreased BRCA1 levels modify ERalpha-mediated transcription and regulation of cell proliferation in part by altering ERalpha-coregulator association. In the presence of tamoxifen, decreased BRCA1 expression results in increased coactivator and decreased corepressor recruitment on ER-regulated gene promoters.

Enhanced capacity of antigen presentation of HBc-VLP-pulsed RAW264.7 cells revealed by proteomics analysis

Many recent studies have indicated that virus-like particles (VLPs) have many potential applications in the fields of vaccine development and gene therapy. However, we still know little about the subtle mechanisms involved in the presentation of VLPs by antigen presenting cells (APCs). To illustrate the mechanisms, we utilized two-dimensional electrophoresis and tandem MS to compare and identify differentially expressed proteins between hepatitis B virus core antigen VLP (HBc-VLP)-pulsed and control RAW264.7 cells. Of the 25 spots identified as differentially expressed ( p < 0.05) between the two cell lines, 11 (corresponding to 11 unique proteins) were positively identified. Further analysis of two proteins, prohibitin and heat shock protein 70, confirmed that these proteins are expressed at higher levels in HBc-VLP-pulsed RAW264.7 cells compared with control cells. The proteins identified in this study will be useful in revealing the mechanisms that underlie VLP-APC interactions. Overall, this study also provides some useful suggestions for vaccine development and gene therapy.

Prohibitin function within mitochondria: essential roles for cell proliferation and cristae morphogenesis

Prohibitins comprise an evolutionary conserved and ubiquitously expressed family of membrane proteins. Various roles in different cellular compartments have been proposed for prohibitin proteins. Recent experiments, however, identify large assemblies of two homologous prohibitin subunits, PHB1 and PHB2, in the inner membrane of mitochondria as the physiologically active structure. Mitochondrial prohibitin complexes control cell proliferation, cristae morphogenesis and the functional integrity of mitochondria. The processing of the dynamin-like GTPase OPA1, a core component of the mitochondrial fusion machinery, has been defined as a key process affected by prohibitins. The molecular mechanism of prohibitin function, however, remained elusive. The ring-like assembly of prohibitins and their sequence similarity with lipid raft-associated SPFH-family members suggests a scaffolding function of prohibitins, which may lead to functional compartmentalization in the inner membrane of mitochondria.

Prohibitins control cell proliferation and apoptosis by regulating OPA1-dependent cristae morphogenesis in mitochondria

Prohibitins comprise an evolutionarily conserved and ubiquitously expressed family of membrane proteins with poorly described functions. Large assemblies of PHB1 and PHB2 subunits are localized in the inner membrane of mitochondria, but various roles in other cellular compartments have also been proposed for both proteins. Here, we used conditional gene targeting of murine Phb2 to define cellular activities of prohibitins. Our experiments restrict the function of prohibitins to mitochondria and identify the processing of the dynamin-like GTPase OPA1, an essential component of the mitochondrial fusion machinery, as the central cellular process controlled by prohibitins. Deletion of Phb2 leads to the selective loss of long isoforms of OPA1. This results in an aberrant cristae morphogenesis and an impaired cellular proliferation and resistance toward apoptosis. Expression of a long OPA1 isoform in PHB2-deficient cells suppresses these defects, identifying impaired OPA1 processing as the primary cellular defect in the absence of prohibitins. Our results therefore assign an essential function for the formation of mitochondrial cristae to prohibitins and suggest a coupling of cell proliferation to mitochondrial morphogenesis.

A repressive role for prohibitin in estrogen signaling

Nuclear receptor-mediated gene expression is regulated by corepressors and coactivators. In this study we demonstrate that prohibitin (PHB), a potential tumor suppressor, functions as a potent transcriptional corepressor for estrogen receptor alpha (ERalpha). Overexpression of PHB inhibits ERalpha transcriptional activity, whereas depletion of endogenous PHB increases the expression of ERalpha target genes in MCF-7 breast cancer cells. Chromatin immunoprecipitation experiments demonstrate that PHB is associated with the estrogen-regulated pS2 promoter in the absence of hormone and dissociates after estradiol treatment. We demonstrate that PHB interacts with the repressor of estrogen receptor activity (REA), a protein related to PHB, to form heteromers and enhance the protein stability of both corepressors. Interestingly, the corepressor activity of PHB is cross-squelched by the coexpression of REA (and vice versa), suggesting that PHB and REA repress transcription only when they are not paired. We further demonstrate that coiled-coil domains located in the middle of PHB and REA are responsible for their heteromerization, stabilization, and cross-squelching actions. Finally, ablation of PHB function in the mouse results in early embryonic lethality, whereas mice heterozygous for the PHB null allele exhibit a hyperproliferative mammary gland phenotype. Our results indicate that PHB functions as a transcriptional corepressor for ERalpha in vitro and in vivo, and that its heteromerization with REA acts as a novel mechanism to limit its corepressor activity.

Akt2 is implicated in skeletal muscle differentiation and specifically binds Prohibitin2/REA

Akt1 and Akt2 are the major isoforms of Akt expressed in muscle cells and muscle tissue. We have performed siRNA silencing of Akt1 and Akt2 in C2 myoblasts to characterize their specific implication in muscle differentiation. Whereas silencing Akt2, and not Akt1, inhibited cell cycle exit and myoblast differentiation, Akt2 overexpression led to an increased proportion of differentiated myoblasts. In addition, we demonstrate that Akt2 is required for myogenic conversion induced by MyoD overexpression in fibroblasts. We show Akt2, but not Akt1, binds Prohibitin2/Repressor of Estrogen Activator, PHB2/REA, a protein recently implicated in transcriptionnal repression of myogenesis. Co-immunoprecipitation experiments on endogenous proteins showed the Akt2-REA complex does not contain Prohibitin1. We have analyzed expression and localization of PHB2/REA during proliferation and differentiation of mouse and human myoblasts. PHB2/REA shows punctated nuclear staining which partially co-localizes with Akt2 in differentiated myotubes and PHB2 levels decrease at the onset of myogenic differentiation concomitant with an increase in Akt2. There appears to be an inverse correlation between Akt2 and PHB2 protein levels where cells silenced for Akt2 expression show increased level of PHB2/REA and overexpression of Akt2 resulted in decreased Prohibitin2/REA. Taken together, these results, along with our previous observations, clearly show that Akt2 and not Akt1 plays a major and early role in cell cycle exit and myogenic differentiation and this function involves its specific interaction with PHB2/REA.

Deciphering the molecular machinery of stem cells: a look at the neoblast gene expression profile

BACKGROUND

Mammalian stem cells are difficult to access experimentally; model systems that can regenerate offer an alternative way to characterize stem cell related genes. Planarian regeneration depends on adult pluripotent stem cells--the neoblasts. These cells can be selectively destroyed using X-rays, enabling comparison of organisms lacking stem cells with wild-type worms.

RESULTS

Using a genomic approach we produced an oligonucleotide microarray chip (the Dj600 chip), which was designed using selected planarian gene sequences. Using this chip, we compared planarians treated with high doses of X-rays (which eliminates all neoblasts) with wild-type worms, which led to identification of a set of putatively neoblast-restricted genes. Most of these genes are involved in chromatin modeling and RNA metabolism, suggesting that epigenetic modifications and post-transcriptional regulation are pivotal in neoblast regulation. Comparing planarians treated with low doses of X-rays (after which some radiotolerant neoblasts re-populate the planarian body) with specimens irradiated with high doses and unirradiated control worms, we identified a group of genes that were upregulated as a consequence of low-dose X-ray treatment. Most of these genes encode proteins that are known to regulate the balance between death and survival of the cell; our results thus suggest that genetic programs that control neoblast cytoprotection, proliferation, and migration are activated by low-dose X-rays.

CONCLUSION

The broad differentiation potential of planarian neoblasts is unparalleled by any adult stem cells in the animal kingdom. In addition to our validation of the Dj600 chip as a valuable platform, our work contributes to elucidating the molecular mechanisms that regulate the self-renewal and differentiation of neoblasts.

Class IIa histone deacetylases: regulating the regulators

In the last decade, the identification of enzymes that regulate acetylation of histones and nonhistone proteins has revealed the key role of dynamic acetylation and deacetylation in various cellular processes. Mammalian histone deacetylases (HDACs), which catalyse the removal of acetyl groups from lysine residues, are grouped into three classes, on the basis of similarity to yeast counterparts. An abundance of experimental evidence has established class IIa HDACs as crucial transcriptional regulators of various developmental and differentiation processes. In the past 5 years, a tremendous effort has been dedicated to characterizing the regulation of these enzymes. In this review, we summarize the latest discoveries in the field and discuss the molecular and structural determinants of class IIa HDACs regulation. Finally, we emphasize that comprehension of the mechanisms underlying class IIa HDAC functions is essential for potential therapeutic applications.

Alkylating agent methyl methanesulfonate (MMS) induces a wave of global protein hyperacetylation: implications in cancer cell death

Protein acetylation modification has been implicated in many cellular processes but the direct evidence for the involvement of protein acetylation in signal transduction is very limited. In the present study, we found that an alkylating agent methyl methanesulfonate (MMS) induces a robust and reversible hyperacetylation of both cytoplasmic and nuclear proteins during the early phase of the cellular response to MMS. Notably, the acetylation level upon MMS treatment was strongly correlated with the susceptibility of cancer cells, and the enhancement of MMS-induced acetylation by histone deacetylase (HDAC) inhibitors was shown to increase the cellular susceptibility. These results suggest protein acetylation is important for the cell death signal transduction pathway and indicate that the use of HDAC inhibitors for the treatment of cancer is relevant.

Corepressors of agonist-bound nuclear receptors

Nuclear receptors (NRs) rely on coregulator proteins to modulate transcription of target genes. NR coregulators can be broadly subdivided into coactivators which potentiate transcription and corepressors which silence gene expression. The prevailing view of coregulator action holds that in the absence of agonist the receptor interacts with a corepressor via the corepressor nuclear receptor (CoRNR, "corner") box motifs within the corepressor. Upon agonist binding, a conformational change in the receptor causes the shedding of corepressor and the binding of a coactivator which interacts with the receptor via NR boxes within the coregulator. This view was challenged with the discovery of RIP140 which acts as a NR corepressor in the presence of agonist and utilizes NR boxes. Since then a number of other corepressors of agonist-bound NRs have been discovered. Among them are LCoR, PRAME, REA, MTA1, NSD1, and COPR1 Although they exhibit a great diversity of structure, mechanism of repression and pathophysiological function, these corepressors frequently have one or more NR boxes and often recruit histone deacetylases to exert their repressive effects. This review highlights these more recently discovered corepressors and addresses their potential functions in transcription regulation, disease pharmacologic responses and xenobiotic metabolism.

EZH2 regulates the transcription of estrogen-responsive genes through association with REA, an estrogen receptor corepressor

Enhancer of zeste homolog 2 (EZH2) is a histone methyltransferase polycomb group (PcG) protein, which has been implicated in the process of cellular differentiation and cancer progression for both breast and prostate cancer. Although transcriptional repression by histone modification appears to contribute to the process of cellular differentiation, it is unclear what mediates the specificity of PcG proteins. Since EZH2 requires a binding partner for its histone methyltransferase activity, we surmised that evaluating interacting proteins might shed light on how the activity of EZH2 is regulated. Here we describe the identification of a novel binding partner of EZH2, the repressor of estrogen receptor activity (REA). REA functions as a transcriptional corepressor of the estrogen receptor and can potentiate the effect of anti-estrogens. REA expression levels have also previously been associated with the degree of differentiation of human breast cancers. We show here that EZH2 can also mediate the repression of estrogen-dependent transcription, and that moreover, the ability of both REA and EZH2 to repress estrogen-dependent transcription are mutually dependent. These data suggest that EZH2 may be recruited to specific target genes by its interaction with the estrogen receptor corepressor REA. The identification of a novel interaction between EZH2 and REA, two transcription factors that have been linked to breast cancer carcinogenesis, may lead to further insights into the process of deregulated gene expression in breast cancer.

Nicotine causes age-dependent changes in gene expression in the adolescent female rat brain

Humans often start smoking during adolescence. Recent results suggest that rodents may also be particularly vulnerable to nicotine dependence during adolescence. We examined the effect of chronic nicotine exposure on gene expression profiles during adolescence in female rats, who were dosed with nicotine (and control animals were dosed with saline) via subcutaneously implanted osmotic minipumps. Brain samples were collected at four ages: before puberty (postnatal day 25), at about the time of puberty in females (postnatal day 35), and after puberty (postnatal days 45 and 55). The expression of 7931 genes in three brain areas was measured using DNA microarrays. Quantitative RT-PCR was also employed to confirm the expression patterns of selected genes. We used a novel clustering technique (principal cluster analysis) to classify 162 nicotine-regulated genes into five clusters, of which only one (cluster A) showed similar patterns of gene expression across all three brain areas (ventral striatum, prefrontal cortex, and hippocampus). Three clusters of genes (A, B, and C) showed dramatic peaks in their nicotine responses at the same age (p35). The other two clusters (D1 and D2) showed smaller peaks and/or valleys in their nicotine responses at p35 and p45. Thus, the age of maximal gene expression response to nicotine in female rats corresponds approximately to the age of maximal behavioral response and the age of puberty.

Mitochondrial Functions and Estrogen Receptor-dependent Nuclear Translocation of Pleiotropic Human Prohibitin 2

Proteins with multiple cellular functions provide biological diversity to eukaryotic cells. In the current studies, we identified the mitochondrial functions of human prohibitin 2 (PHB2), which was initially identified as a repressor of estrogen-dependent transcriptional activity. The mitochondrial complex of PHB2 consists of PHB1, voltage-dependent anion channel 2, adenine nucleotide translocator 2, and the anti-apoptotic Hax-1, which is a novel binding partner for PHB2. RNA interference-mediated knockdown of PHB2 in HeLa cells resulted in caspase-dependent apoptosis through down-regulation of Hax-1 and fragmentation of mitochondria. We also found that, although PHB2 is predominantly expressed in the mitochondria of HeLa cells, it translocates to nucleus in the presence of estrogen receptor alpha and estradiol. Here, we first demonstrated the roles of mammalian PHB2 in mitochondria and the molecular mechanism of its nuclear targeting and showed that PHB2 is a possible molecule directly coupling nuclear-mitochondrial interaction.

Haploinsufficiency of the corepressor of estrogen receptor activity (REA) enhances estrogen receptor function in the mammary gland

Estrogen receptor (ER)-mediated gene expression plays an essential role in mammary gland morphogenesis, function, and carcinogenesis. The repressor of ER activity (REA) is an ER-interactive protein that counterbalances estrogen-induced ER transcriptional activity. Our previous study showed that genetic deletion of both REA alleles resulted in embryonic lethality. This study demonstrates that REA and ERalpha are coexpressed in mammary epithelial cells. REA heterozygous (REA(+/-)) mutant mice exhibit faster mammary ductal elongation in virgin animals, increased lobuloalveolar development during pregnancy, and delayed mammary gland involution after weaning. These morphological phenotypes of REA(+/-) mice are associated with significantly increased cell proliferation and ER transcriptional activities, as indicated by the estrogen response element (ERE)-luciferase reporter in the WT/ERE-Luc and REA(+/-)/ERE-Luc bigenic mice and by the higher expression levels of estrogen-responsive genes such as progesterone receptor and cyclin D1 in the mammary gland. Our analysis also revealed that REA is an important repressor of ER transcriptional activity in the mammary gland under natural, as well as ovariectomized and estrogen-replaced, hormonal conditions. Our results indicate that REA is a physiological modulator of ER function in the mammary gland and that its correct gene dosage is required for maintenance of normal ER activity and normal mammary gland development. Consequently, a reduction or loss of REA function may cause overactivation of ER and increase breast cancer risk in humans.

Only Akt1 is required for proliferation, while Akt2 promotes cell cycle exit through p21 binding

Protein kinase B (PKB/Akt) is an important modulator of insulin signaling, cell proliferation, and survival. Using small interfering RNA duplexes in nontransformed mammalian cells, we show that only Akt1 is essential for cell proliferation, while Akt2 promotes cell cycle exit. Silencing Akt1 resulted in decreased cyclin A levels and inhibition of S-phase entry, effects not seen with Akt2 knockdown and specifically rescued by microinjection of Akt1, not Akt2. In differentiating myoblasts, Akt2 knockout prevented myoblasts from exiting the cell cycle and showed sustained cyclin A expression. In contrast, overexpression of Akt2 reduced cyclin A and hindered cell cycle progression in M-G1 with increased nuclear p21. p21 is a major target in the differential effects of Akt isoforms, with endogenous Akt2 and not Akt1 binding p21 in the nucleus and increasing its level. Accordingly, Akt2 knockdown cells, and not Akt1 knockdown cells, showed reduced levels of p21. A specific Akt2/p21 interaction can be reproduced in vitro, and the Akt2 binding site on p21 is similar to that in cyclin A spanning T145 to T155, since (i) prior incubation with cyclin A prevents Akt2 binding, (ii) T145 phosphorylation on p21 by Akt1 prevents Akt2 binding, and (iii) binding Akt2 prevents phosphorylation of p21 by Akt1. These data show that specific interaction of the Akt2 isoform with p21 is key to its negative effect on normal cell cycle progression.

Endogenous Inhibition of Histone Deacetylase 1 by Tumor-Suppressive Maspin

Maspin, a noninhibitory serine protease inhibitor, exerts multifaceted tumor-suppressive effects. Maspin expression is associated with better differentiated phenotypes, better cancer prognosis, and better drug sensitivity. Consistently, maspin also correlates with increased expression of Bax and p21WAF1/CIP1. Interestingly, histone deacetylase 1 (HDAC1), a major HDAC responsible for histone deacetylation, was shown to interact with maspin in a yeast two-hybrid screening. In this study, we confirmed the maspin/HDAC1 interaction in human prostate tissues, in prostate cancer cell lines, and with purified maspin. We produced several lines of evidence that support an inhibitory effect of maspin on HDAC1 through direct molecular interaction, which was detected in both the nucleus and the cytoplasm. Both endogenously expressed maspin and purified maspin inhibited HDAC1. In contrast, small interfering RNA (siRNA) silencing of maspin in PC3 cells increased HDAC activity. Accordingly, maspin-transfected DU145 cells exhibited increased expression of HDAC1 target genes Bax, cytokeratin 18 (CK18), and p21(WAF1/CIP1), whereas maspin siRNA decreased CK18 expression in PC3 cells. The maspin effect on HDAC1 correlated with an increased sensitivity to cytotoxic HDAC inhibitor M344. Interestingly, glutathione S-transferase (GST, another maspin partner) was detected in the maspin/HDAC1 complex. Furthermore, a COOH-terminally truncated maspin mutant, which bound to HDAC1 but not GST, did not increase histone acetylation. Although HDACs, especially the highly expressed HDAC1, are promising therapeutic targets in cancer intervention, our data raise a novel hypothesis that the endogenous inhibitory effect of maspin on HDAC1 is coupled with glutathione-based protein modification, and provide new leads toward future developments of specific HDAC1-targeting strategies.

The Prohibitins: emerging roles in diverse functions

The prohibitins, Phb1 and Phb2 are highly conserved proteins in eukaryotic cells that are present in multiple cellular compartments. Initial investigations focused on the role of Phb1 as an inhibitor of cell proliferation hence the original name prohibitin. However both proteins appear to have a diverse range of functions and recent evidence suggests that the prohibitins have very similar but as yet only partially understood functions. In addition to their role as chaperone proteins in the mitochondria, and their ability to target to lipid rafts, their is now compelling evidence that both prohibitins are localized in the nucleus and can modulate transcriptional activity by interacting with various transcription factors, including the steroid hormone receptors, either directly or indirectly. In addition Phb1 and Phb2 are present in the circulation and can be internalized when added to cultured cells suggesting that the circulating prohibitins may have some regulatory role. This review presents some of the recent developments in prohibitin research and focuses on the similarities in the structure and function of these interesting proteins.

Estrogen-related receptors-stimulated monoamine oxidase B promoter activity is down-regulated by estrogen receptors

Although there are studies published about the neuroprotective effect of estrogen, little is known about the mechanisms and cellular targets of the hormone. Recent reports demonstrate that estrogen down-regulates the expression of monoamine oxidase A and B (MAO-A and MAO-B) in the hypothalamus of the Macaques monkey, both of which are key isoenzymes in the neurotransmitter degradation pathway. Additionally, estrogen-related receptor alpha (ERRalpha) up-regulates MAO-B gene expression in breast cancer cells. ERRalpha recognizes a variety of estrogen response elements and shares many target genes and coactivators with estrogen receptor alpha (ERalpha). In this study, we investigate the interplay of ERs and ERRs in the regulation of MAO-B promoter activity. We demonstrate that ERRalpha and ERRgamma up-regulate MAO-B gene activity, whereas ERalpha and ERbeta decrease stimulation in both a ligand-dependent and -independent manner. Ectopically expressed ERRalpha and ERRgamma stimulate the expression of MAO-B mRNA and protein as well as increase the MAO-B enzymatic activity in ER-negative HeLa cells. The ability of ERRs to stimulate MAO-B promoter activity was reduced in ER-positive MCF-7 and T47D cells. Several AGGTCA motifs of the MAO-B promoter are responsible for up-regulation by ERRs. Interestingly, ERalpha or ERbeta alone have no effect on MAO-B promoter activity but can down-regulate the activation function of ERRs, whereas glucocorticoid receptor does not. By using chromatin immunoprecipitation assay, we demonstrate that ERs compete with ERRs for binding to the MAO-B promoter at selective AGGTCA motifs, thereby changing the chromatin status and cofactor recruitment to a repressed state. These studies provide new insight into the relationship between ERalpha, ERbeta, ERRalpha, and ERRgamma in modulation of MAO-B gene activity.

Acetylation of estrogen receptor alpha by p300 at lysines 266 and 268 enhances the deoxyribonucleic acid binding and transactivation activities of the receptor

Using a variety of biochemical and cell-based approaches, we show that estrogen receptor alpha (ERalpha) is acetylated by the p300 acetylase in a ligand- and steroid receptor coactivator-dependent manner. Using mutagenesis and mass spectrometry, we identified two conserved lysine residues in ERalpha (Lys266 and Lys268) that are the primary targets of p300-mediated acetylation. These residues are acetylated in cells, as determined by immunoprecipitation-Western blotting experiments using an antibody that specifically recognizes ERalpha acetylated at Lys266 and Lys268. The acetylation of ERalpha by p300 is reversed by native cellular deacetylases, including trichostatin A-sensitive enzymes (i.e. class I and II deacetylases) and nicotinamide adenine dinucleotide-dependent/nicotinamide-sensitive enzymes (i.e. class III deacetylases, such as sirtuin 1). Acetylation at Lys266 and Lys268, or substitution of the same residues with glutamine (i.e. K266/268Q), a residue that mimics acetylated lysine, enhances the DNA binding activity of ERalpha in EMSAs. Likewise, substitution of Lys266 and Lys268 with glutamine enhances the ligand-dependent activity of ERalpha in a cell-based reporter gene assay. Collectively, our results implicate acetylation as a modulator of the ligand-dependent gene regulatory activity of ERalpha. Such regulation is likely to play a role in estrogen-dependent signaling outcomes in a variety of estrogen target tissues in both normal and pathological states.

Camptothecin induces nuclear export of prohibitin preferentially in transformed cells through a CRM-1-dependent mechanism

Prohibitin is a growth-suppressive protein that has multiple functions in the nucleus and the mitochondria. Our earlier studies had shown that prohibitin represses the activity of E2F transcription factors while enhancing p53-mediated transcription. At the same time, prohibitin has been implicated in mediating the proper folding of mitochondrial proteins. We had found that treatment of cells with camptothecin, a topoisomerase 1 inhibitor, led to the export of prohibitin and p53 from the nucleus to the mitochondria. Here we show that the camptothecin-induced export of prohibitin occurs preferentially in transformed cell lines, but not in untransformed or primary cells. Cells that did not display the translocation of prohibitin were refractive to the apoptotic effects of camptothecin. The translocation was mediated by a putative nuclear export signal at the C-terminal region of prohibitin; fusion of the nuclear export signal (NES) of prohibitin to green fluorescence protein led to its export from the nucleus. Leptomycin B could inhibit the nuclear export of prohibitin showing that it was a CRM-1-dependent event driven by Ran GTPase. Confirming this, prohibitin was found to physically interact with CRM-1, and this interaction was significantly higher in transformed cells. Delivery of a peptide corresponding to the NES of prohibitin prevented the export of prohibitin to cytoplasm and protected cells from apoptosis. These results suggest that the regulated translocation of prohibitin from the nucleus to the mitochondria facilitates its pleiotropic functions and might contribute to its anti-proliferative and tumor suppressive properties.

A cross-species analysis of the rodent uterotrophic program: elucidation of conserved responses and targets of estrogen signaling

Physiological, morphological, and transcriptional alterations elicited by ethynyl estradiol in the uteri of Sprague-Dawley rats and C57BL/6 mice were assessed using comparable study designs, microarray platforms, and analysis methods to identify conserved estrogen signaling networks. Comparative analysis identified 153 orthologous gene pairs that were positively correlated, suggesting conserved transcriptional targets important in uterine proliferation. Functional annotation for these responses were associated with angiogenesis, water and solute transport, cell cycle control, redox control, DNA replication, protein synthesis and transport, xenobiotic metabolism, cell-cell communication, energetics, and cholesterol and fatty acid regulation. The identification of conserved temporal expression patterns of these orthologs provides experimental support for the transfer of functional annotation from mouse orthologs to 44 previously unannotated rat expressed sequence tags based on their homology and co-expression patterns. The identification of comparable temporal phenotypic responses linked to related gene expression profiles demonstrates the ability of systematic comparative genomic assessments to elucidate important conserved mechanisms in rodent estrogen signaling during uterine proliferation.

Histone deacetylase inhibitors and cancer: from cell biology to the clinic

Aberrant gene regulation plays an important role in tumor initiation and progression, and the acetylation of histones is a well understood key component of gene regulation. Histone acetylation involves the opposing activities of the histone acetyltransferases (HATs) and histone deacetylases (HDACs)--histone acetylation is associated with increased transcriptional activity while histone deacetylation is associated with repression of gene expression. In addition, the modification of non-histone proteins by HATs and HDACs is also an important process in regulating gene expression. Several lines of evidence suggest that inappropriate transcriptional activation and repression mediated by HATs and HDACs is a common occurrence in the formation of many different types of cancer. These enzymes thus represent novel molecular targets for which inhibitors are sought that could reprogram transcription and inhibit tumor cell growth and progression. Much of the research has focused on HDAC inhibitors, where several agents have demonstrated in vitro and in vivo activity against different tumor cell models and have entered Phase I clinical trials. HDAC inhibitors are believed to exert their antiproliferative effects by inducing a small set of genes involved in regulating cellular activities such as proliferation and differentiation. Future research is expected to lead to a better understanding of the molecular targets of HDACs and facilitate the development of more potent inhibitors of these enzymes. First results from clinical trials will help to determine the optimal strategy for utilizing these agents for the treatment of cancer patients.

Recruitment of histone deacetylase 4 to the N-terminal region of estrogen receptor alpha

Transcriptional activation of estrogen receptor alpha (ERalpha) is regulated by the ligand-dependent activation function 2 and the constitutively active N-terminal activation function 1. To identify ERalpha N-terminal-specific coregulators, we screened a breast cDNA library by T7 phage display and isolated histone deacetylase 4 (HDAC4). HDAC4 interacts with the ERalpha N terminus both in vitro and in vivo. Presence of the ERalpha DNA binding domain and hinge region reduce HDAC4 recruitment whereas full-length ERalpha enhances recruitment. HDAC4 interaction is selective for the ERalpha and not ERbeta N terminus and occurs in the nucleus. We demonstrate in vivo that HDAC4 is recruited by the N terminus to the promoter of an endogenous estrogen responsive gene. HDAC4 suppresses transcriptional activation of ERalpha by estrogen and selective ER modulators (SERMs) such as tamoxifen in a cell type-dependent manner. Consistently, silencing of HDAC4 promotes the agonist effect of SERMs (tamoxifen and raloxifene) in a cell type-specific manner. These findings indicate a role for HDAC4 in regulating ERalpha activity as a novel N-terminal coregulator and uncover a mechanism by which certain cell types regulate SERM behavior.

Genetic deletion of the repressor of estrogen receptor activity (REA) enhances the response to estrogen in target tissues in vivo

We previously identified a coregulator, repressor of estrogen receptor activity (REA), that directly interacts with estrogen receptor (ER) and represses ER transcriptional activity. Decreasing the intracellular level of REA by using small interfering RNA knockdown or antisense RNA approaches in cells in culture resulted in a significant increase in the level of up-regulation of estrogen-stimulated genes. To elucidate the functional activities of REA in vivo, we have used targeted disruption to delete the REA gene in mice. The targeting vector eliminated, by homologous recombination, the REA exon sequences encoding amino acids 12 to 201, which are required for REA repressive activity and for interaction with ER. The viability of heterozygous animals was similar to that of the wild type, whereas homozygous animals did not develop, suggesting a crucial role for REA in early development. Female, but not male, heterozygous animals had an increased body weight relative to age-matched wild-type animals beginning after puberty. REA mRNA and protein levels in uteri of heterozygous animals were half that of the wild type, and studies with heterozygous animals revealed a greater uterine weight gain and epithelial hyperproliferation in response to estradiol (E2) and a substantially greater stimulation by E2 of a number of estrogen up-regulated genes in the uterus. Even more dramatic in REA heterozygous animals was the loss of down regulation by E2 of genes in the uterus that are normally repressed by estrogen in wild-type animals. Mouse embryo fibroblasts derived from heterozygous embryos also displayed a greater transcriptional response to E2. These studies demonstrate that REA is a significant modulator of estrogen responsiveness in vivo: it normally restrains estrogen actions, moderating ER stimulation and enhancing ER repression of E2-regulated genes.

Formation of membrane-bound ring complexes by prohibitins in mitochondria

Prohibitins comprise a remarkably conserved protein family in eukaryotic cells with proposed functions in cell cycle progression, senescence, apoptosis, and the regulation of mitochondrial activities. Two prohibitin homologues, Phb1 and Phb2, assemble into a high molecular weight complex of approximately 1.2 MDa in the mitochondrial inner membrane, but a nuclear localization of Phb1 and Phb2 also has been reported. Here, we have analyzed the biogenesis and structure of the prohibitin complex in Saccharomyces cerevisiae. Both Phb1 and Phb2 subunits are targeted to mitochondria by unconventional noncleavable targeting sequences at their amino terminal end. Membrane insertion involves binding of newly imported Phb1 to Tim8/13 complexes in the intermembrane space and is mediated by the TIM23-translocase. Assembly occurs via intermediate-sized complexes of approximately 120 kDa containing both Phb1 and Phb2. Conserved carboxy-terminal coiled-coil regions in both subunits mediate the formation of large assemblies in the inner membrane. Single particle electron microscopy of purified prohibitin complexes identifies diverse ring-shaped structures with outer dimensions of approximately 270 x 200 angstroms. Implications of these findings for proposed cellular activities of prohibitins are discussed.

Coregulators and chromatin remodeling in transcriptional control

Despite many years of investigation by numerous investigators, transcriptional regulatory control remains an intensely investigated and continuously evolving field of research. Transcriptional regulation is dependent not only on transcription factor activation and chromatin remodeling, but also on a host of transcription factor coregulators-coactivators and corepressors. In addition to transcription factor activation and chromatin changes, there is an expanding array of additional modifications that titrate transcriptional regulation for the specific conditions of a particular cell type, organ system, and developmental stage, and such events are likely to be greatly influenced by upstream signaling cascades. Here, we will briefly review the highlights and perspectives of chromatin remodeling and transcription controls as affected by cofactor availability, cellular energy state, relative ratios of reducing equivalents, and upstream signaling. We also present the C-terminal binding protein (CtBP) as a novel nuclear receptor (NR) coregulator, which exemplifies the integration of a number of transcriptional regulatory controls.

Histone deacetylase inhibition and estrogen signalling in human breast cancer cells

Estrogens are steroid hormones, which act through specific nuclear estrogen receptors (ERalpha and ERbeta) and are important regulators of breast cancer growth. These receptors control gene expression by recruiting transcriptional cofactors that exhibit various enzymatic activities such as histone acetyltransferase or histone deacetylase (HDAC) which target histone as well as non-histone substrates. The ERalpha itself and some of the transcriptional regulators have been shown to be acetylated proteins. Research performed over the last decade has highlighted the role of HDAC inhibitors (HDACi) as modulators of transcriptional activity and as a new class of therapeutic agents. In human cancer cells, inhibition of HDACs controls the expression of the ERalpha gene and the transcriptional activity in response to partial antiestrogens such as 4-hydroxytamoxifen. Various HDACi strongly inhibit breast cancer cell proliferation and ERalpha-negative (ER-) appear less sensitive than ERalpha-positive (ER+) cell lines. p21WAF1/CIP1 gene expression, in relation with ERalpha levels, could play a role in this differential response of breast cancer cells to hyperacetylating agents.