Protein phosphatases 1 and 2A regulate the transcriptional and DNA binding activities of retinoic acid receptors

Hepatic Cyp2d and Cyp26a1 mRNAs and activities are increased during mouse pregnancy

There is considerable evidence that drug disposition is altered during human pregnancy and based on probe drug studies, CYP2D6 activity increases during human pregnancy. The aim of this study was to determine whether the changes of CYP2D6 activity observed during human pregnancy could be replicated in the mouse, and explore possible mechanisms of increased CYP2D6 activity during pregnancy. Cyp2d11, Cyp2d22, Cyp2d26 and Cyp2d40 mRNA was increased (P < 0.05) on gestational days (GD) 15 and 19 compared with the non-pregnant controls. There was no change (P > 0.05) in Cyp2d9 and Cyp2d10 mRNA. In agreement with the increased Cyp2d mRNA, Cyp2d-mediated dextrorphan formation from dextromethorphan was increased 2.7-fold (P < 0.05) on GD19 (56.8±39.4 pmol/min/mg protein) when compared with the non-pregnant controls (20.8±11.2 pmol/min/mg protein). An increase in Cyp26a1 mRNA (10-fold) and retinoic acid receptor (Rar)β mRNA (2.8-fold) was also observed during pregnancy. The increase in Cyp26a1 and Rarβ mRNA during pregnancy indicates increased retinoic acid signaling in the liver during pregnancy. A putative retinoic acid response element was identified within the Cyp2d40 promoter and the mRNA of Cyp2d40 correlated (P < 0.05) with Cyp26a1 and Rarβ. These results show that Cyp2d mRNA is increased during mouse pregnancy the and mouse may provide a suitable model to investigate the mechanisms underlying the increased clearance of CYP2D6 probes observed during human pregnancy. Our findings also suggest that retinoic acid signaling in the liver is increased during pregnancy, which may have broader implications to energy homeostasis in the liver during pregnancy.

Essential phosphatases and a phospho-degron are critical for regulation of SRC-3/AIB1 coactivator function and turnover

SRC-3/AIB1 is a master growth coactivator and oncogene, and phosphorylation activates it into a powerful coregulator. Dephosphorylation is a potential regulatory mechanism for SRC-3 function, but the identity of such phosphatases remains unexplored. Herein, we report that, using functional genomic screening of human Ser/Thr phosphatases targeting SRC-3's known phosphorylation sites, the phosphatases PDXP, PP1, and PP2A were identified to be key negative regulators of SRC-3 transcriptional coregulatory activity in steroid receptor signalings. PDXP and PP2A dephosphorylate SRC-3 and inhibit its ligand-dependent association with estrogen receptor. PP1 stabilizes SRC-3 protein by blocking its proteasome-dependent turnover through dephosphorylation of two previously unidentified phosphorylation sites (Ser101 and S102) required for activity. These two sites are located within a degron of SRC-3 and are primary determinants of SRC-3 turnover. Moreover, PP1 regulates the oncogenic cell proliferation and invasion functions of SRC-3 in breast cancer cells.

Retinoic acid enhances differentiation of v-myb-transformed monoblasts induced by okadaic acid

Differentiation of various leukemic cells can be induced by liganded retinoic acid receptors and protein phosphatase inhibitors. In this study, we explored the effects of okadaic acid (OA), the phosphatase inhibitor, and retinoic acid (RA) in v-myb-transformed monoblasts BM2. OA induced differentiation of BM2 monoblasts into macrophage-like cells, as documented by analyses of cell morphology, cell cycle, phagocytic activity, non-specific esterase activity, production of reactive oxygen species and expression of vimentin and Mo-1. In contrast to many other leukemic cell lines, BM2 cells do not respond to retinoic acid. However, once exposed to OA and RA simultaneously, BM2 cells differentiate along monocyte/macrophage pathway more efficiently. We conclude that RA enhances differentiation of v-myb-transformed monoblasts induced by protein phosphorylation.

Phosphorylation by PKA potentiates retinoic acid receptor alpha activity by means of increasing interaction with and phosphorylation by cyclin H/cdk7

Nuclear retinoic acid receptors (RARs) work as ligand-dependent heterodimeric RAR/retinoid X receptor transcription activators, which are targets for phosphorylations. The N-terminal activation function (AF)-1 domain of RARalpha is phosphorylated by the cyclin-dependent kinase (cdk) 7/cyclin H complex of the general transcription factor TFIIH and the C-terminal AF-2 domain by the cAMP-dependent protein kinase A (PKA). Here, we report the identification of a molecular pathway by which phosphorylation by PKA propagates cAMP signaling from the AF-2 domain to the AF-1 domain. The first step is the phosphorylation of S369, located in loop 9-10 of the AF-2 domain. This signal is transferred to the cyclin H binding domain (at the N terminus of helix 9 and loop 8-9), resulting in enhanced cyclin H interaction and, thereby, greater amounts of RARalpha phosphorylated at S77 located in the AF-1 domain by the cdk7/cyclin H complex. This molecular mechanism relies on the integrity of the ligand-binding domain and the cyclin H binding surface. Finally, it results in higher DNA-binding efficiency, providing an explanation for how cAMP synergizes with retinoic acid for transcription.

Nuclear export of retinoid X receptor alpha in response to interleukin-1beta-mediated cell signaling: roles for JNK and SER260

As the obligate heterodimer partner to class II nuclear receptors, the retinoid X receptor alpha (RXRalpha) plays a vital physiological role in the regulation of multiple hepatic functions, including bile formation, intermediary metabolism, and endobiotic/xenobiotic detoxification. Many RXRalpha-regulated genes are themselves suppressed in inflamed liver via unknown mechanisms, which constitute a substantial component of the negative hepatic acute phase response. In this study we show that RXRalpha, generally considered a stable nuclear resident protein, undergoes rapid nuclear export in response to signals initiated by the pro-inflammatory cytokine interleukin-1beta (IL-1beta), a central activator of the acute phase response. Within 30 min of exposure to IL-1beta, nuclear levels of RXRalpha are markedly suppressed in human liver-derived HepG2 cells, temporally coinciding with its appearance in the cytoplasm. The nuclear residence of RXRalpha is maintained by inhibiting c-jun N-terminal kinase (JNK, curcumin or SP600125) or CRM-1-mediated nuclear export (Leptomycin B). Pretreatment with the proteasome inhibitor MG132 blocks IL-1beta-mediated reductions in nuclear RXRalpha levels while increasing accumulation in the cytoplasm. Mutational studies identify one residue, serine 260, a JNK phosphoacceptor site whose phosphorylation status had an unknown role in RXRalpha function, as critical for IL-1beta-mediated nuclear export of transfected human RXRalpha-green fluorescent fusion constructs. These findings indicate that inflammation-mediated cell signaling leads to rapid and profound reductions in nuclear RXRalpha levels, via a multistep, JNK-dependent mechanism involving Ser260, nuclear export, and proteasomal degradation. Thus, inflammation-meditated cell signaling targets RXRalpha for nuclear export and degradation; a potential mechanism that explains the broad suppression of RXRalpha-dependent gene expression in the inflamed liver.

New therapeutic target for CNS injury? The role of retinoic acid signaling after nerve lesions

Experiments with sciatic nerve lesions and spinal cord contusion injury demonstrate that the retinoic acid (RA) signaling cascade is activated by these traumatic events. In both cases the RA-synthesizing enzyme is RALDH-2. In the PNS, lesions cause RA-induced gene transcription, intracellular translocation of retinoid receptors, and increased transcription of CRBP-I, CRABP-II, and retinoid receptors. The activation of RARbeta appears to be responsible for neurotrophic and neuritogenic effects of RA on dorsal root ganglia and embryonic spinal cord. While the physiological role of RA in the injured nervous system is still under investigation three domains of functions are suggested: (1) neuroprotection and support of axonal growth, (2) modulation of the inflammatory reaction by microglia/macrophages, and (3) regulation of glial differentiation. Few studies have been performed to support nerve regeneration with RA signals in vivo, but a large number of experiments with neuronal and glial cell cultures and spinal cord explants point to beneficial effects of RA, so that future therapeutic approaches will likely focus on the activation of RA signaling.

Vitamin E activates CRABP-II gene expression in cultured human fibroblasts, role of protein kinase C

The treatment of human fibroblasts with different tocopherols in the presence of retinol caused an increase in cytoplasmic retinoic acid binding protein II (CRABP-II) mRNA and protein. The possibility of an involvement of protein kinase C (PKC) in the response to tocopherols was supported by the results obtained with the PKC-specific inhibitors, calphostin C and bisindolylmaleimide I. The effect of alpha-tocopherol was prevented by okadaic acid, suggesting that a protein phosphatase is responsible for PKC dephosphorylation produced by the presence of tocopherols. The results shown support the hypothesis that phosphorylation/dephosphorylation of RXRalpha via PKC may be involved in the regulation of CRABP-II gene expression.

Growth inhibitory retinoid effects after recruitment of retinoid X receptor beta to the retinoic acid receptor beta promoter

Nuclear retinoid receptors mediate retinoid effects through tissue-specific, ligand-receptor interactions and subsequent transcriptional regulation of secondary target genes. Retinoic acid receptor beta (RARbeta) is itself a retinoid target gene with a retinoic acid response element (betaRARE) in the 5' untranslated region of the RARbeta2 gene. Altered transcriptional regulation of RARbeta may play a role in human carcinogenesis and the retinoid-responsiveness of malignant cells. Here we used retinoid X receptor-specific antibodies in electrophoretic mobility shift assays to show that the retinoid X receptor beta (RXRbeta) protein was recruited to the betaRARE, after retinoid treatment of retinoid-sensitive neuroblastoma (NB), lung and breast cancer cell lines, but not retinoid-resistant lung and breast cancer cell lines. RXRbeta selectively enhanced retinoid-induced transcriptional activation of the betaRARE. Stable overexpression of RXRalpha and RXRbeta in NB cells resulted in marked growth inhibition and cell death, which increased after retinoid treatment. However, only proteins from the RXRbeta transfectants exhibited specific RXRbeta binding to the betaRARE in vitro and in vivo, enhanced histone acetylation and increased endogenous RARbeta expression. These data indicate that recruitment of RXRbeta to the betaRARE, and consequent induction of endogenous RARbeta expression, is an important component in the retinoid anticancer signal. RXRalpha may also participate in the retinoid signal, but through mechanisms that do not involve RARbeta.

Expression of alpha-ENaC2 is dependent on an upstream Sp1 binding motif and is modulated by protein phosphatase 1 in lung epithelial cells

The amiloride-sensitive Na(+) channel ENaC is expressed in lung epithelium and plays a pivotal role in lung fluid clearance in the newborn. Multiple splice variants of the ENaC alpha-subunit have been reported. Among them, alpha-ENaC2 accounts for a considerable portion of alpha-ENaC transcripts in human lung and kidney, possesses channel functions similar to alpha-ENaC1, and is driven by a downstream promoter. In the current study, we examine the regulation of alpha-ENaC2 transcription in lung epithelial cells. We found that transcription factors Sp1 and Sp3 activate alpha-ENaC2 transcription through a GC-rich element (Sp1-binding site) in the promoter. Because alpha-ENaC expression and Sp1 phosphorylation are both significantly up-regulated in the perinatal lung, we then examined the possible connection between Sp1/Sp3 phosphorylation and alpha-ENaC2 expression. We found that protein phosphatase 1 (PP1) dephosphorylates Sp1 and Sp3 in lung epithelial cells, reduces their binding to the alpha-ENaC2 promoter, and decreases Sp1/Sp3-mediated promoter activity. Our results suggest that Sp1 and Sp3 are essential for alpha-ENaC2 transcription in lung epithelial cells and that dephosphorylation of the Sp transcription factors by PP1 suppresses alpha-ENaC2 expression. The significance of these findings in the regulation of gene expression in perinatal lung is discussed.

Interleukin-1 beta-mediated suppression of RXR:RAR transactivation of the Ntcp promoter is JNK-dependent

Bile flow is rapidly and markedly reduced in hepatic inflammation, correlating with suppression of critical hepatic bile acid transporter gene expression, including the principal hepatic bile acid importer, the Na(+)/taurocholate co-transporting polypeptide (Ntcp, Slc10a1). Endotoxin treatment of rats and interleukin-1 beta (IL-1 beta) treatment of liver-derived HepG2 cells leads to a marked decline in the nuclear binding activity of a main Ntcp gene regulator, the nuclear receptor heterodimer retinoid X receptor:retinoic acid receptor (RXR:RAR). How IL-1 beta signaling leads to reduced RXR:RAR nuclear binding activity is unknown, and we sought to determine whether mitogen-activated protein kinase (MAPK) pathways were involved. IL-1 beta treatment of cultured primary rat hepatocytes markedly reduced Ntcp RNA levels and Ntcp promoter activity in transiently transfected HepG2 cells. Pretreatment with inhibitors of extracellular signal-regulated kinase (ERK, PD98059) or p38 MAPK (SB203580) did not affect IL-1 beta-mediated suppression of Ntcp gene expression, whereas curcumin, a derivative of the spice turmeric and a recently described inhibitor of c-Jun N-terminal kinase (JNK), completely ameliorated the effects of IL-1 beta. Co-transfection of a JNK expression plasmid inhibited RXR:RAR-mediated activation of the Ntcp promoter, while a dominant negative JNK expression plasmid completely blocked IL-1 beta-mediated suppression. Curcumin, but not PD98059 or SB203580, inhibited IL-1 beta-mediated suppression of nuclear RXR:RAR binding activity, which correlated with inhibition of JNK phosphorylation and phospho-JNK-mediated phosphorylation of RXR. Taken together, these data provide evidence supporting a novel player (JNK), as well as its inhibitor (curcumin), in inflammation-mediated regulation of hepatobiliary transporters and correlate JNK-dependent RXR phosphorylation with reduced RXR-dependent hepatic gene expression.

Retinoic acid receptors inhibit AP1 activation by regulating extracellular signal-regulated kinase and CBP recruitment to an AP1-responsive promoter

Retinoids exhibit antineoplastic activities that may be linked to retinoid receptor-mediated transrepression of activating protein 1 (AP1), a heterodimeric transcription factor composed of fos- and jun-related proteins. Here we show that transcriptional activation of an AP1-regulated gene through the mitogen-activated protein kinase (MAPK)-extracellular signal-regulated kinase (ERK) pathway (MAPK(ERK)) is characterized, in intact cells, by a switch from a fra2-junD dimer to a junD-fosB dimer loading on its promoter and by simultaneous recruitment of ERKs, CREB-binding protein (CBP), and RNA polymerase II. All-trans-retinoic acid (atRA) receptor (RAR) was tethered constitutively to the AP1 promoter. AP1 transrepression by retinoic acid was concomitant to glycogen synthase kinase 3 activation, negative regulation of junD hyperphosphorylation, and to decreased RNA polymerase II recruitment. Under these conditions, fra1 loading to the AP1 response element was strongly increased. Importantly, CBP and ERKs were excluded from the promoter in the presence of atRA. AP1 transrepression by retinoids was RAR and ligand dependent, but none of the functions required for RAR-mediated transactivation was necessary for AP1 transrepression. These results indicate that transrepressive effects of retinoids are mediated through a mechanism unrelated to transcriptional activation, involving the RAR-dependent control of transcription factors and cofactor assembly on AP1-regulated promoters.

Sp1 transcriptional activity is up-regulated by phosphatase 2A in dividing T lymphocytes

We have followed Sp1 expression in primary human T lymphocytes induced, via CD2 plus CD28 costimulation, to sustained proliferation and subsequent return to quiescence. Binding of Sp1 to wheat germ agglutinin lectin was not modified following activation, indicating that the overall glycosylation of the protein was unchanged. Sp1 underwent, instead, a major dephosphorylation that correlated with cyclin A expression and, thus, with cell cycle progression. A similar change was observed in T cells that re-entered cell cycle following secondary interleukin-2 stimulation, as well as in serum-induced proliferating NIH/3T3 fibroblasts. Phosphatase 2A (PP2A) appears involved because 1) treatment of dividing cells with okadaic acid or cantharidin inhibited Sp1 dephosphorylation and 2) PP2A dephosphorylated Sp1 in vitro and strongly interacted with Sp1 in vivo. Sp1 dephosphorylation is likely to increase its transcriptional activity because PP2A overexpression potentiated Sp1 site-driven chloramphenicol acetyltransferase expression in dividing Kit225 T cells and okadaic acid reversed this effect. This increase might be mediated by a stronger affinity of dephosphorylated Sp1 for DNA, as illustrated by the reduced DNA occupancy by hyperphosphorylated Sp factors from cantharidin- or nocodazole-treated cells. Finally, Sp1 dephosphorylation appears to occur throughout cell cycle except for mitosis, a likely common feature to all cycling cells.

Quantitative axial profiles of retinoic acid in the embryonic mouse spinal cord: 9-cis retinoic acid only detected after all-trans-retinoic acid levels are super-elevated experimentally

Studies using bioassays in normal mice and gene activation in transgenic reporter mice have demonstrated peaks of retinoic acid receptor (RAR) signaling in the brachial and lumbar regions of the spinal cord. Recently, Solomin et al. (Solomin et al. [1998] Nature 395:398-402) detected a retinoid X receptor (RXR) signal in the same region of the developing spinal cord at a slightly later stage than the RAR signal. This finding raises the question of which retinoid ligands underlie RAR and RXR signaling in this part of the embryo. Quantitative measurements of regional differences in retinoid profiles have not been reported previously due to limitation in the sensitivity and specificity of available retinoid detection methods. Here, by using a recently developed ultrasensitive HPLC technique (Sakhi et al. [1998] J. Chromatogr. A 828:451-460), we address this question in an attempt to identify definitively the endogenous retinoids present in different regions of the spinal cord at the stages when regional differences in RAR and RXR signaling have been reported. We find a bimodal distribution of all-trans retinoic acid (at-RA), the ligand for RARs, and relate this to the expression of several retinoid-synthesizing enzymes. However, we do not detect 9-cis-retinoic acid (9-cis-RA), the putative RXR ligand, in any region of the spinal cord unless retinoid levels are massively increased experimentally by gavage feeding pregnant mice with teratogenic doses of at-RA. This study provides for the first time quantitative profiles of endogenous retinoids along the axis of the developing spinal cord, thereby establishing a foundation for more definitive studies of retinoid function in the future. It sets definite limits on how much 9-cis-RA potentially is present and demonstrates that at-RA predominates over 9-cis-RA by at least 30- to 180-fold in different spinal cord regions.

Polyomavirus small t antigen prevents retinoic acid-induced retinoblastoma protein hypophosphorylation and redirects retinoic acid-induced G0 arrest and differentiation to apoptosis

Polyomavirus small t antigen (ST) impedes late features of retinoic acid (RA)-induced HL-60 myeloid differentiation as well as growth arrest, causing apoptosis instead. HL-60 cells were stably transfected with ST. ST slowed the cell cycle, retarding G2/M in particular. Treated with RA, the ST transfectants continued to proliferate and underwent apoptosis. ST also impeded the normally RA-induced hypophosphorylation of the retinoblastoma tumor suppressor protein consistent with failure of the cells to arrest growth. The RA-treated transfectants expressed CD11b, an early cell surface differentiation marker, but inducible oxidative metabolism, a later and more mature functional differentiation marker, was largely inhibited. Instead, the cells underwent apoptosis. ST affected significant known components of RA signaling that result in G0 growth arrest and differentiation in wild-type HL-60. ST increased the basal amount of activated ERK2, which normally increases when wild-type cells are treated with RA. ST caused increased RARalpha expression, which is normally down regulated in RA-treated wild-type cells. The effects of ST on RA-induced myeloid differentiation did not extend to monocytic differentiation and G0 arrest induced by 1,25-dihydroxy vitamin D3, whose receptor is also a member of the steroid-thyroid hormone superfamily. In this case, ST abolished the usually induced G0 arrest and retarded, but did not block, differentiation without inducing apoptosis, thus uncoupling growth arrest and differentiation. In sum, the data show that ST disrupted the normal RA-induced program of G0 arrest and differentiation, causing the cells to abort differentiation and undergo apoptosis.

Control of retinoic acid receptor heterodimerization by ligand-induced structural transitions. A novel mechanism of action for retinoid antagonists

Heterodimerization of retinoic acid receptors (RARs) with 9-cis-retinoic receptors (RXRs) is a prerequisite for binding of RXR.RAR dimers to DNA and for retinoic acid-induced gene regulation. Whether retinoids control RXR/RAR solution interaction remains a debated question, and we have used in vitro and in vivo protein interaction assays to investigate the role of ligand in modulating RXR/RAR interaction in the absence of DNA. Two-hybrid assay in mammalian cells demonstrated that only RAR agonists were able to increase significantly RAR interaction with RXR, whereas RAR antagonists inhibited RXR binding to RAR. Quantitative glutathione S-transferase pull-down assays established that there was a strict correlation between agonist binding affinity for the RAR monomer and the affinity of RXR for liganded RAR, but RAR antagonists were inactive in inducing RXR recruitment to RAR in vitro. Alteration of coactivator- or corepressor-binding interfaces of RXR or RAR did not alter ligand-enhanced dimerization. In contrast, preventing the formation of a stable holoreceptor structure upon agonist binding strongly altered RXR.RAR dimerization. Finally, we observed that RAR interaction with RXR silenced RXR ligand-dependent activation function. We propose that ligand-controlled dimerization of RAR with RXR is an important step in the RXR.RAR activation process. This interaction is dependent upon adequate remodeling of the AF-2 structure and amenable to pharmacological inhibition by structurally modified retinoids.

A functionally active RARalpha nuclear receptor is expressed in retinoic acid non responsive early myeloblastic cell lines

Although all-trans retinoic acid (ATRA) can restore the differentiation capacity of leukemic promyelocytes, early leukemic myeloblasts are conversely not responsive to ATRA induced granulocytic differentiation. To assess whether this resistance to ATRA is related to an impaired function of the Retinoic Acid Receptor alpha (RARalpha), we performed an analysis of RARalpha expression and transactivation activity, in several myeloid leukemic cell lines, representative of different types of spontaneous acute myeloid leukemias. Our results indicate that a functionally active RARalpha nuclear receptor is expressed in all the analyzed cell lines, regardless of their differentiation capacity following exposure to ATRA. The observation that ATRA treatment is able to induce the expression of retinoic acid target genes, in late- but not in early-myeloblastic leukemic cells, raises the possibility that the differentiation block of these cells is achieved through a chromatin mediated mechanism. Acetylation is apparently not involved in this process, since the histone deacetylase inhibitor trichostatin A, is not able to restore the differentiation capacity of early leukemic myeloblasts. Further investigation is needed to clarify whether myeloid transcription factors, distinct to RARalpha, play a role in the resistance of these cells to ATRA treatment.

Serine 27, a human retinoid X receptor alpha residue, phosphorylated by protein kinase A is essential for cyclicAMP-mediated downregulation of RXRalpha function

Retinoid X Receptor alpha (RXRalpha), a member of the steroid-thyroid hormone receptor super family, is phosphorylated in vitro by protein kinase A (PKA) and this phosphorylation is inhibited in presence of PKA inhibitory peptide. Analysis of various deletion mutants of RXRalpha indicate that the amino-terminal A/B domain is the target for PKA phosphorylation. An RXRalpha mutant in which serine residue 27 is mutated to alanine is no longer phosphorylated by PKA. In vivo transfection experiments in COS cells indicate that cyclic AMP represses retinoic acid-mediated transcriptional activation of RXRalpha and this repression is mediated by serine 27. These results indicate that serine 27 of RXRalpha is an unique target for phosphorylation by PKA in vitro and it has an important role in the crosstalk between RXRalpha and cyclic AMP signalling pathways.

Divergence in murine myometrium spontaneous and oxytocin-stimulated contractile responses to serine/threonine protein phosphatase-1 inhibition

Reversible phosphorylation is essential in regulating uterine contractions. Identification, characterization, and functional understanding of myometrium protein phosphatase(s) are lacking. Okadaic acid (OA), which inhibits protein phosphatase-1 (PP1) and PP2A, has been shown to alter uterine contractions. Experiments were conducted to determine the 1) identity of the myometrial OA-sensitive PP, 2) influence of OA on spontaneous and oxytocin (OT)-stimulated myometrial contractions, and 3) expression of uterine PPs during sexual development. Western blot analysis indicated the presence of PP1(alpha) and PP2A in immature and mature mice. As determined by immunohistochemistry, gonadotropin-stimulated adult mouse uteri contain PP1(alpha) in longitudinal and circular myometrial layers and endometrial epithelium. Conversely, PP2A was localized to the endometrial stroma. Cumulative addition of OA (n = 9; 10, 100, 250, 500, 1000 nM) did not significantly alter spontaneous contractions of mouse uterine horns in comparison to vehicle-treated controls (n = 9). By the end of the test period OA- and vehicle-treated uteri displayed a comparable decline in uterine contractions to 79.2% and 63.7%, respectively, of basal contractile activity. Pretreatment of uterine tissue with OA (1 microM; n = 7) significantly reduced contractile response to increasing concentrations of OT (8, 16, 32, 64 nM) in comparison to vehicle pretreatment (dimethyl sulfoxide; n = 7). At the end of the OT-administration period, contractile activity was 160.4% and 67.3% of basal contractile activity for vehicle (no OA) and OA-pretreated groups, respectively. During the early prepubertal period PP1(alpha) was expressed in longitudinal myometrium and absent in circular myometrium; whereas, during the transition to sexual maturity PP1(alpha) was observed in both the longitudinal and circular myometrium. In summary, these studies have indicated 1) that PP1 is the primary myometrial OA-sensitive PP; 2) that inhibition of PP1 had no effect on spontaneous contractions, whereas it markedly inhibited OT-stimulated uterine contractions; and 3) that PP1 is differentially expressed in the circular and longitudinal myometrium in relation to sexual development.

Pathways through which a regimen of melatonin and retinoic acid induces apoptosis in MCF-7 human breast cancer cells

It has been established that melatonin (Mlt) and retinoic acid, individually, inhibit the proliferation of the estrogen receptor-alpha (ER alpha)-positive MCF-7 breast cancer cell line. Our laboratory has previously demonstrated that Mlt and all-trans-retinoic acid (atRA) not only inhibit the proliferation, but also induce apoptosis of MCF-7 cells when used in a sequential regimen of Mlt followed 24 h later by atRA. Using this same MCF-7 breast cancer cell line, we investigated the potential pathways through which apoptosis is being induced. We found that treatment of MCF-7 cells with Mlt for 24 h before the addition of atRA decreased the protein levels of the death suppressor, Bcl-2, and increased, although with different time courses, the levels of the death promoters, Bax and Bak; however, there was no change in the levels of the tumor suppressor gene, p53. MCF-7 cells treated sequentially with Mlt and atRA also demonstrated an enhanced sensitivity to the apoptotic effects of atRA, which did not appear to be due to increased expression of the retinoic acid receptors, RAR alpha or RXR alpha, but rather to enhanced transcriptional activity of the RAR alpha. These data suggest that the sequential treatment regimen of Mlt and atRA may induce apoptosis by modulation of members of the Bcl-2 family of proteins. Thus, this combinatorial regimen, which reduces the concentration of atRA needed for clinical efficacy while enhancing its anti-tumorigenic activity, could be of great therapeutic benefit, and may, in fact, specifically induce the regression of established breast tumors due to its apoptosis-promoting effects.

Transforming growth factor-beta1 (TGF-beta1) and TGF-beta2 decrease expression of CD36, the type B scavenger receptor, through mitogen-activated protein kinase phosphorylation of peroxisome proliferator-activated receptor-gamma

CD36, the macrophage type B scavenger receptor, binds and internalizes oxidized low density lipoprotein, a key event in the development of macrophage foam cells within atherosclerotic lesions. Expression of CD36 in monocyte/macrophages is dependent on differentiation status and exposure to soluble mediators. In this study, we investigated the effect of transforming growth factor-beta1 (TGF-beta1) and TGF-beta2 on the expression of CD36 in macrophages. Treatment of phorbol ester-differentiated THP-1 macrophages with TGF-beta1 or TGF-beta2 significantly decreased expression of CD36 mRNA and surface protein. TGF-beta1/TGF-beta2 also inhibited CD36 mRNA expression induced by oxidized low density lipoprotein and 15-deoxyDelta(12,14) prostaglandin J(2), a peroxisome proliferator-activated receptor (PPAR)-gamma ligand, suggesting that the TGF-beta1/TGF-beta2 down-regulated CD36 expression by inactivating PPAR-gamma-mediated signaling. TGF-beta1/TGF-beta2 increased phosphorylation of both mitogen-activated protein (MAP) kinase and PPAR-gamma, whereas MAP kinase inhibitors reversed suppression of CD36 and inhibited PPAR-gamma phosphorylation induced by TGF-beta1/TGF-beta2. Finally, MAP kinase inhibitors alone increased expression of CD36 mRNA and surface protein but had no effect on PPAR-gamma protein levels. Our data demonstrate for the first time that TGF-beta1 and TGF-beta2 decrease expression of CD36 by a mechanism involving phosphorylation of MAP kinase, subsequent MAP kinase phosphorylation of PPAR-gamma, and a decrease in CD36 gene transcription by phosphorylated PPAR-gamma.

Serine 157, a retinoic acid receptor alpha residue phosphorylated by protein kinase C in vitro, is involved in RXR.RARalpha heterodimerization and transcriptional activity

Retinoic acid (RA) regulation of cellular proliferation and differentiation is mediated, at least in part, through two related nuclear receptors, RAR and RXR. RA-induced modulation of gene expression leads generally to cellular differentiation, whereas stimulation of the protein kinase C (PKC) signaling pathway is associated with cellular proliferation. Pursuant to our discovery that prolonged activation of PKCs induced a strong decrease in RA responsiveness of a retinoid-inducible reporter gene, we have further investigated the connections between these two signaling pathways. We demonstrate that PKC isoforms alpha and gamma are able to phosphorylate human RARalpha (hRARalpha) in vitro on a single serine residue located in the extended DNA binding domain (T box). The introduction of a negative charge at this position (serine 157) strongly decreased hRARalpha transcriptional activity, whereas a similar mutation at other PKC consensus phosphorylation sites had no effect. The effect on transcriptional activation was correlated with a decrease in the capacity of hRARalpha to heterodimerize with hRXRalpha. Thus hRARalpha is a direct target for PKCalpha and gamma, which may control retinoid receptor transcriptional activities during cellular proliferation and differentiation.

Allosteric regulation of the discriminative responsiveness of retinoic acid receptor to natural and synthetic ligands by retinoid X receptor and DNA

Transcriptional activation by retinoids is mediated through two families of nuclear receptors, all-trans-retinoic acid (RARs) and 9-cis retinoic acid receptors (RXRs). Conformationally restricted retinoids are used to achieve selective activation of RAR isotype alpha, beta or gamma, which reduces side effects in therapeutical applications. Synthetic retinoids mimic some of all-trans retinoic acid biological effects in vivo but interact differently with the ligand binding domain of RARalpha and induce distinct structural transitions of the receptor. In this report, we demonstrate that RAR-selective ligands have distinct quantitative activation properties which are reflected by their abilities to promote interaction of DNA-bound human RXRalpha (hRXRalpha)-hRARalpha heterodimers with the nuclear receptor coactivator (NCoA) SRC-1 in vitro. The hormone response element core motifs spacing defined the relative affinity of liganded heterodimers for two NCoAs, SRC-1 and RIP140. hRXRalpha activating function 2 was critical to confer hRARalpha full responsiveness but not differential sensitivity of hRARalpha to natural or synthetic retinoids. We also provide evidence showing that lysines located in helices 3 and 4, which define part of hRARalpha NCoA binding surface, contribute differently to (i) the transcriptional activity and (ii) the interaction of RXR-RAR heterodimers with SRC-1, when challenged by either natural or RAR-selective retinoids. Thus, ligand structure, DNA, and RXR exert allosteric regulations on hRARalpha conformation organized as a DNA-bound heterodimer. We suggest that the use of physically distinct NCoA binding interfaces may be important in controlling specific genes by conformationally restricted ligands.

TAL1 and LIM-only proteins synergistically induce retinaldehyde dehydrogenase 2 expression in T-cell acute lymphoblastic leukemia by acting as cofactors for GATA3

Previously, we have shown that TAL1 and the LIM-only protein gene (LMO) are regularly coactivated in T-cell acute lymphoblastic leukemia (T-ALL). This observation is likely to relate to the findings that TAL1 and LMO are highly synergistic in T-cell tumorigenesis in double-transgenic mice. To understand the molecular mechanisms of functional synergy between TAL1 and LMO in tumorigenesis and transcriptional regulation, we tried to identify downstream target genes regulated by TAL1 and LMO by a subtractive PCR method. One of the isolated genes, that for retinaldehyde dehydrogenase 2 (RALDH2), was regularly expressed in most of the T-ALL cell lines that coexpressed TAL1 and LMO. Exogenously transfected TAL1 and LMO, but not either alone, induced RALDH2 expression in a T-ALL cell line, HPB-ALL, not expressing endogeneous TAL1 or LMO. The RALDH2 transcripts in T-ALL were, however, mostly initiated within the second intron. Promoter analysis revealed that a GATA site in a cryptic promoter in the second intron was essential and sufficient for the TAL1- and LMO-dependent transcriptional activation, and GATA3 binds to this site. In addition, forced expression of GATA3 potentiated the induction of RALDH2 by TAL1 and LMO, and these three factors formed a complex in vivo. Furthermore, a TAL1 mutant not binding to DNA also activated the transcription of RALDH2 in the presence of LMO and GATA3. Collectively, we have identified the RALDH2 gene as a first example of direct transcriptional target genes regulated by TAL1 and LMO in T-ALL. In this case, TAL1 and LMO act as cofactors for GATA3 to activate the transcription of RALDH2.

Retinoids and their receptors in skeletal development

The embryonic vertebrate limb serves as an excellent experimental model system in which to study mechanisms that regulate morphogenesis of the skeleton. The appendicular skeleton arises through the process of endochondral ossification, whereby a cartilage template is initially formed and subsequently replaced by bone. One molecule that has a dramatic effect on these processes is the vitamin-A metabolite, retinoic acid (RA). RA functions through a class of nuclear hormone receptors, the retinoic acid receptors (RARs) and retinoid-X-receptors (RXRs), to regulate gene transcription. Experimental evidence from RA teratogenesis suggests that the presence of ligand-activated RARs and/or inappropriate expression of RARs inhibits chondrogenesis. Conversely, genetic analysis has shown that the absence of the receptors can lead to deficiencies in cartilage formation while also promoting chondrogenesis at ectopic sites. Taken together, these studies suggest that the RARs play a fundamental role in the early stages of skeletal development, specifically those involved in the formation of prechondrogenic condensations and their subsequent differentiation into chondroblasts.

Tuberous sclerosis gene 2 product modulates transcription mediated by steroid hormone receptor family members

Tuberous sclerosis (TSC) is a genetic disorder that results in the development of hamartomatous lesions in a variety of organ systems. Both the prevalence of the disease and the often devastating consequences of these tumors pose a serious health and medical care problem. The disease has been mapped to two distinct genetic loci in humans, and although the genes (TSC1 and TSC2) for both loci have recently been cloned, their function remains an enigma. Data presented here demonstrates that TSC2 protein can bind and selectively modulate transcription mediated by members of the steroid receptor superfamily of genes. These data place TSC2 into a growing list of nuclear receptor coregulators and strengthen the expanding body of evidence that these coregulators may play critical roles in cellular differentiation.

H11-H12 loop retinoic acid receptor mutants exhibit distinct trans-activating and trans-repressing activities in the presence of natural or synthetic retinoids

Retinoids, such as the naturally occurring all-trans-retinoic acid (atRA) and synthetic ligand CD367 modulate ligand-dependent transcription through retinoic acid receptors (RARs). Retinoid binding to RAR is believed to trigger structural transitions in the ligand-binding domain (LBD), leading to helix H1 and helix H12 repositioning and coactivator recruitment and corepressor release. Here, we carried out a detailed mutagenesis analysis of the H11-H12 loop (designated the L box) to study its contribution to hRARalpha activation process. Point mutations that reduced transactivation by atRA also reduced atRA-induced transrepression of AP1 transcription, correlating ligand-induced activation and repression. However, a correlation was not observed with these mutations when tested with another ligand CD367, a synthetic agonist with binding properties identical to those of atRA. Transcription was strongly inhibited in the presence of CD367 for some mutants, thus leading to an inverse agonist activity of this ligand. None of these mutations significantly altered binding affinity for either ligand, indicating that altered transcription was not caused by altered ligand binding by these mutations. Although simple correlations with transcriptional activities were not found, these mutations were also characterized by altered ligand-induced structural transitions, which were distinct for the atRA-hRARalpha or CD367-hRARalpha complexes. These results indicate that amino acids in the L box are involved in specifying trans-repressive and trans-activating properties of the hRARalpha, and support the notion that different agonists induce distinct conformations in the LBD of the receptor.

Binding of retinoic acid receptor heterodimers to DNA. A role for histones NH2 termini

The retinoic acid signaling pathway is controlled essentially through two types of nuclear receptors, RARs and RXRs. Ligand dependent activation or repression of retinoid-regulated genes is dependent on the binding of retinoic acid receptor (RAR)/9-cis-retinoic acid receptor (RXR) heterodimers to retinoic acid response element (RARE). Although unliganded RXR/RAR heterodimers bind constitutively to DNA in vitro, a clear in vivo ligand-dependent occupancy of the RARE present in the RARbeta2 gene promoter has been reported (Dey, A., Minucci, S., and Ozato, K. (1994) Mol. Cell. Biol. 14, 8191-8201). Nucleosomes are viewed as general repressors of the transcriptional machinery, in part by preventing the access of transcription factors to DNA. The ability of hRXRalpha/hRARalpha heterodimers to bind to a nucleosomal template in vitro has therefore been examined. The assembly of a fragment from the RARbeta2 gene promoter, which contains a canonical DR5 RARE, into a nucleosome core prevented hRXRalpha/hRARalpha binding to this DNA, in conditions where a strong interaction is observed with a linear DNA template. However, histone tails removal by limited proteolysis and histone hyperacetylation yielded nucleosomal RAREs able to bind to hRXRalpha/hRARalpha heterodimers. These data establish therefore the role of histones NH2 termini as a major impediment to retinoid receptors access to DNA, and identify histone hyperacetylation as a potential physiological regulator of retinoid-induced transcription.

Activation of the MAP kinase cascade by histone deacetylase inhibitors is required for the stimulation of choline acetyltransferase gene promoter

We previously described that the major promoter (M) of human choline acetyltransferase (ChAT) gene is activated by three inhibitors of histone deacetylase, butyrate, trichostatin and trapoxin, in transfected CHP126 neuroepithelioma cells. We now show that trapoxin and butyrate triggered a rapid and transient phosphorylation of ERK1/2 kinases, that was suppressed by PD98059, a highly specific inhibitor of MAP kinase kinase MEK1. The stimulation of ChAT promoter activity by trapoxin or butyrate did not require ongoing protein synthesis, and was suppressed by PD98059. The overexpression of dominant negative mutants of H-ras or ERK2 proteins depressed ChAT promoter activation by trapoxin in transient transfection assays. Conversely, the overexpression of constitutively active mutants of H-ras or MEK1 proteins had little or no effect on ChAT promoter activity, but strongly synergized with trapoxin. These data thus suggest that the activation of the MEK/ERK kinase cascade plays a necessary, but not sufficient, role in the regulation of ChAT promoter by inhibitors of histone deacetylase.

Distinct modes of interaction of the retinoic acid receptor alpha with natural and synthetic retinoids

Retinoids regulate key cellular processes through their binding to their cognate nuclear receptors, RARs and RXRs. Synthetic ligands mimic most of their biological effects and alteration of their chemical structure confers selectivity for RAR isotypes alpha, beta or gamma. In this study, we have examined the contribution of a domain (L box) of hRARalpha located at the C-terminus of the ligand binding domain (LBD), between helices H11 and H12, to the ligand binding activity of this receptor. By site-directed mutagenesis, we demonstrate that, in the absence of the ligand-dependent activation domain 2 (AF2-AD), the receptor discriminates between classes of structurally distinct retinoids. This property was lost in the presence of the AF2-AD domain, evidencing major structural transitions in this part of the receptor. We propose that ligand binding occurs in two steps: first, the ligand interacts with the LBD in its opened, holo-receptor conformation in which the L box plays a crucial role in defining the ligand binding repertoire of hRARalpha; secondly, the LBD adopts its closed conformation in which the ligand interacts with the receptor mostly through its carboxylic moiety.

Domains of retinoid signalling and neurectodermal expression of zebrafish otx1 and goosecoid are mutually exclusive

Retinoid signalling plays an important role in embryonic pattern formation. Excess of retinoic acid during gastrulation results in axial defects in vertebrate embryos, suggesting that retinoids are involved in early anteroposterior patterning. To study retinoid signalling in zebrafish embryos, we developed a novel method to detect endogenous retinoids in situ in embryos, using a fusion protein of the ligand inducible transactivation domain of a retinoic acid receptor and a heterologous DNA binding domain. Using this method, we show that retinoid signalling is localized in zebrafish embryos in the region of the embryonic shield, and towards the end of gastrulation in a posterior dorsal domain. To investigate the relationships between the spatial distribution of retinoid signalling and the regulation of retinoid target genes, we studied the downregulation by retinoic acid of two genes expressed in anterior regions of the embryo, goosecoid and otx1. These experiments show that expression of both genes is strongly downregulated in the anterior neurectoderm of zebrafish embryos treated with retinoic acid, whereas mesendodermal expression is only mildly affected. Interestingly, a significant downregulation of goosecoid expression by retinoic acid was observed only during midgastrulation but not in earlier stages. In agreement with these results, spatial expression of goosecoid and otx1 does not overlap with the region of retinoid signalling in the late gastrula. Our data support the hypothesis that a localized retinoid signal is involved in axial patterning during early development, at least in part through the repression of anterior genes in posterior regions of the embryo. Furthermore, our data suggest that the action of retinoids is spatially as well as temporally regulated in the developing embryo.

Regulation of peroxisome proliferator-activated receptor gamma activity by mitogen-activated protein kinase

Adipocyte differentiation is regulated both positively and negatively by external growth factors such as insulin, platelet-derived growth factor (PDGF), and epidermal growth factor (EGF). A key component of the adipocyte differentiation process is PPARgamma, peroxisomal proliferator-activated receptor gamma. To determine the relationship between PPARgamma activation and growth factor stimulation in adipogenesis, we investigated the effects of PDGF and EGF on PPARgamma1 activity. PDGF treatment decreased ligand-activated PPARgamma1 transcriptional activity in a transient reporter assay. In vivo [32P]orthophosphate labeling experiments demonstrated that PPARgamma1 is a phosphoprotein that undergoes EGF-stimulated MEK/mitogen-activated protein (MAP) kinase-dependent phosphorylation. Purified PPARgamma1 protein was phosphorylated in vitro by recombinant activated MAP kinase. Examination of the PPARgamma1 sequence revealed a single MAP kinase consensus recognition site at Ser82. Mutation of Ser82 to Ala inhibited both in vitro and in vivo phosphorylation and growth factor-mediated transcriptional repression. Therefore, phosphorylation of PPARgamma1 by MAP kinase contributes to the reduction of PPARgamma1 transcriptional activity by growth factor treatment.

One-step immunoaffinity purification of recombinant human retinoic acid receptor gamma

Retinoic acid receptors (RAR) are members of the steroid/thyroid hormone receptor superfamily and serve as ligand-activated transcription factors. In order to facilitate studies of receptor protein, we have generated a monoclonal antibody to the human RAR gamma, and have developed a procedure to purify the full-length receptor expressed in insect cells. The monoclonal antibody (A10) was developed using as antigen a carboxy-terminal fragment of the human RAR gamma expressed as a bacterial fusion protein. The A10 monoclonal antibody binds to both native and denatured forms of the human RAR gamma. This antibody was immobilized on a resin and used to purify full-length, baculovirus-expressed human RAR gamma to near homogeneity. The immunoaffinity-purified receptor is > 90-95% pure as revealed by silver-stained gels. The identity of the single protein band as RAR gamma was verified by immunoblotting using a polyclonal antibody to an epitope distinct from that recognized by the A10 antibody. The pure human RAR gamma is functional with respect to both ligand and DNA binding. Scatchard analysis of 3H-labeled all-trans retinoic acid binding to purified human RAR gamma revealed a single, high-affinity binding site with a Kd of approximately 2 nM. Binding of the pure RAR gamma to a DR5-type retinoic acid response element was also studied. Response element binding by RAR gamma required the presence of the retinoid X receptor, but did not require the presence of additional proteins. Human RAR gamma protein purified in this fashion will be useful in future structural and functional studies.

Steroid hormone receptors and their regulation by phosphorylation

The steroid/thyroid hormone receptor superfamily of ligand-activated transcription factors encompasses not only the receptors for steroids, thyroid hormone, retinoids and vitamin D, but also a large number of proteins whose functions and/or ligands are unknown and which are thus termed orphan receptors. Recent studies have highlighted the importance of phosphorylation in receptor function. Although most of the phosphorylation sites are serine and threonine residues, a few of the family members are also phosphorylated on tyrosine. Those steroid receptor family members that are bound to heat-shock proteins in the absence of ligand typically are basally phosphorylated and exhibit increases in phosphorylation upon ligand binding. Most of these sites contain Ser-Pro motifs, and there is evidence that cyclin-dependent kinases and MAP kinases (mitogen-activated protein kinases) phosphorylate subsets of these sites. In contrast, phosphorylation sites identified thus far in members of the family that bind to DNA in the absence of hormone typically do not contain Ser-Pro motifs and are frequently casein kinase II or protein kinase A sites. Phosphorylation has been implicated in DNA binding, transcriptional activation and stability of the receptors. The finding that some of the steroid receptor family members can be activated in the absence of ligand by growth factors or neurotransmitters that modulate kinase and/or phosphatase pathways underscores the role of phosphorylation in receptor function. Hence this family of transcription factors integrates signals from ligands as well as from signal transduction pathways, resulting in alterations in mRNA and protein expression that are unique to the complex signals received.

Activation and repression by nuclear hormone receptors: hormone modulates an equilibrium between active and repressive states

Transactivation-defective retinoid X and thyroid hormone receptors have been used to examine mechanisms of hormonal activation. Activation and repression of transcription by retinoid X and thyroid hormone receptors are shown to be mediated by physically distinct and functionally independent regions of the hormone binding domain. Nevertheless, the ability of receptors to respond to hormone requires communication between both functional domains. Deletion of the hormone-dependent transactivation function of the retinoid X receptor, the common subunit of heterodimeric nuclear receptors, significantly impairs hormone-dependent transcription by retinoic acid, thyroid hormone, and vitamin D receptors. The results indicate that receptors do not exist in static off and on conformations but that hormone alters an equilibrium between inactive and active states.

Characterization of the AB (AF-1) region in the muscle-specific retinoid X receptor-gamma: evidence that the AF-1 region functions in a cell-specific manner

The retinoid X receptors alpha, beta and gamma (RXRs) share a highly conserved 'C' region or DNA binding domain (DBD). The conserved 'DE' region or ligand binding domain (LBD) of the RXRs is functionally complex, mediating dimerization and a ligand-dependent activation function (AF-2). The AB or N-terminal region of the RXRs is poorly conserved and encodes a ligand-independent activation function (AF-1). RXR gamma mRNA is preferentially expressed in skeletal and cardiac muscle, however, cell-specific steroid receptor-mediated trans-activation is a poorly understood phenomenon. We utilized the GAL4 hybrid assay system and have demonstrated that RXR gamma contains two functional domains in the AB and DE regions that activate transcription in a ligand-independent and -dependent manner respectively. The functions of the AB (AF-1) and DE (AF-2) domains were regulated by cAMP-dependent protein kinases, furthermore, the function of AF-2 in the LBD was activated by 8-Br-cAMP, independent of 9-cis-retinoic acid treatment. Deletion analysis demonstrated that the AF-1 of RXR gamma, is located between amino acids 1 and 103 and contained multiple motifs that were targets of cAMP-dependent protein kinases. Transfection analyses in non-muscle and myogenic cells clearly demonstrated that: (i) the AF-1 of RXR gamma functions in a muscle-specific manner and is required for optimal ligand-dependent trans-activation from an RXRE; (ii) RXR gamma trans-activates more efficiently in a myogenic background.