Physical interaction between retinoic acid receptor and the oncoprotein myb inhibits retinoic acid-dependent transactivation

BPDCN MYB fusions regulate cell cycle genes, impair differentiation, and induce myeloid-dendritic cell leukemia

MYB fusions are recurrently found in select cancers, including blastic plasmacytoid DC neoplasm (BPDCN), an acute leukemia with poor prognosis. They are markedly enriched in BPDCN compared with other blood cancers and, in some patients, are the only obvious somatic mutation detected. This suggests that they may alone be sufficient to drive DC transformation. MYB fusions are hypothesized to alter the normal transcription factor activity of MYB, but, mechanistically, how they promote leukemogenesis is poorly understood. Using CUT&RUN chromatin profiling, we found that, in BPDCN leukemogenesis, MYB switches from being a regulator of DC lineage genes to aberrantly regulating G2/M cell cycle control genes. MYB fusions found in patients with BPDCN increased the magnitude of DNA binding at these locations, and this was linked to BPDCN-associated gene expression changes. Furthermore, expression of MYB fusions in vivo impaired DC differentiation and induced transformation to generate a mouse model of myeloid-dendritic acute leukemia. Therapeutically, we present evidence that all-trans retinoic acid (ATRA) may cause loss of MYB protein and cell death in BPDCN.

Rapid-kinetics degron benchmarking reveals off-target activities and mixed agonism-antagonism of MYB inhibitors

Attenuating aberrant transcriptional circuits holds great promise for the treatment of numerous diseases, including cancer. However, development of transcriptional inhibitors is hampered by the lack of a generally accepted functional cellular readout to characterize their target specificity and on-target activity. We benchmarked the direct gene-regulatory signatures of six agents reported as inhibitors of the oncogenic transcription factor MYB against targeted MYB degradation in a nascent transcriptomics assay. The inhibitors demonstrated partial specificity for MYB target genes but displayed significant off-target activity. Unexpectedly, the inhibitors displayed bimodal on-target effects, acting as mixed agonists-antagonists. Our data uncover unforeseen agonist effects of small molecules originally developed as TF inhibitors and argue that rapid-kinetics benchmarking against degron models should be used for functional characterization of transcriptional modulators.

Zebrafish blastomere screen identifies retinoic acid suppression of MYB in adenoid cystic carcinoma

Pluripotent cells have been used to probe developmental pathways that are involved in genetic diseases and oncogenic events. To find new therapies that would target MYB-driven tumors, we developed a pluripotent zebrafish blastomere culture system. We performed a chemical genetic screen and identified retinoic acid agonists as suppressors of c-myb expression. Retinoic acid treatment also decreased c-myb gene expression in human leukemia cells. Translocations that drive overexpression of the oncogenic transcription factor MYB are molecular hallmarks of adenoid cystic carcinoma (ACC), a malignant salivary gland tumor with no effective therapy. Retinoic acid agonists inhibited tumor growth in vivo in ACC patient-derived xenograft models and decreased MYB binding at translocated enhancers, thereby potentially diminishing the MYB positive feedback loop driving ACC. Our findings establish the zebrafish pluripotent cell culture system as a method to identify modulators of tumor formation, particularly establishing retinoic acid as a potential new effective therapy for ACC.

THRAP3 interacts with HELZ2 and plays a novel role in adipocyte differentiation

Using yeast two-hybrid screen, we previously isolated HELZ2 (helicase with zinc finger 2, transcriptional coactivator) that functions as a coregulator of peroxisome proliferator-activated receptorγ (PPARγ). To further delineate its molecular function, we here identified thyroid hormone receptor-associated protein3 (THRAP3), a putative component of the Mediator complex, as a protein stably associating with HELZ2 using immunoprecipitation coupled with mass spectrometry analyses. In immunoprecipitation assays, Thrap3 could associate with endogenous Helz2 as well as Pparg in differentiated 3T3-L1 cells. HELZ2 interacts with the serine/arginine-rich domain and Bcl2 associated transcription factor1-homologous region in THRAP3, whereas THRAP3 directly binds 2 helicase motifs in HELZ2. HELZ2 and THRAP3 synergistically augment transcriptional activation mediated by PPARγ, whereas knockdown of endogenous THRAP3 abolished the enhancement by HELZ2 in reporter assays. Thrap3, similar to Helz2, is evenly expressed in the process of adipogenic differentiation in 3T3-L1 cells. Knockdown of Thrap3 in 3T3-L1 preadipocytes using short-interfering RNA did not influence the expression of Krox20, Klf5, Cebpb, or Cebpd during early stages of adipocyte differentiation, but significantly attenuated the expression of Pparg, Cebpa, and Fabp4/aP2 and accumulation of lipid droplets. Pharmacologic activation of Pparg by troglitazone could not fully restore the differentiation of Thrap3-knockdown adipocytes. In chromatin immunoprecipitation assays, endogenous Helz2 and Thrap3 could be co-recruited, in a ligand-dependent manner, to the PPARγ-response elements in Fabp4/aP2 and Adipoq gene enhancers in differentiated 3T3-L1 cells. These findings collectively suggest that Thrap3 could play indispensable roles in terminal differentiation of adipocytes by enhancing PPARγ-mediated gene activation cooperatively with Helz2.

The ETS family member TEL binds to nuclear receptors RAR and RXR and represses gene activation

Retinoic acid receptor (RAR) signaling is important for regulating transcriptional activity of genes involved in growth, differentiation, metabolism and reproduction. Defects in RAR signaling have been implicated in cancer. TEL, a member of the ETS family of transcription factors, is a DNA-binding transcriptional repressor. Here, we identify TEL as a transcriptional repressor of RAR signaling by its direct binding to both RAR and its dimerisation partner, the retinoid x receptor (RXR) in a ligand-independent fashion. TEL is found in two isoforms, created by the use of an alternative startcodon at amino acid 43. Although both isoforms bind to RAR and RXR in vitro and in vivo, the shorter form of TEL represses RAR signaling much more efficiently. Binding studies revealed that TEL binds closely to the DNA binding domain of RAR and that both Helix Loop Helix (HLH) and DNA binding domains of TEL are mandatory for interaction. We have shown that repression by TEL does not involve recruitment of histone deacetylases and suggest that polycomb group proteins participate in the process.

Synthetic retinoid Am80 reduces scavenger receptor expression and atherosclerosis in mice by inhibiting IL-6

BACKGROUND

Macrophage scavenger receptors facilitate the uptake of modified low-density lipoprotein (LDL), formation of foam cells, and development of atherosclerosis. Given that proinflammatory cytokines, including IL-6, can modulate the macrophage foaming process, the aim of the present study was to determine whether the synthetic retinoic acid receptor-alpha/beta-specific agonist Am80, which is also an IL-6 inhibitor, can modulate macrophage lipid accumulation and foam cell formation.

METHODS AND RESULTS

Am80 suppressed IL-6 production induced by 12-myristate 13-acetate (PMA) or angiotensin II in mouse Raw264 macrophages. It also suppressed expression of the 2 major scavenger receptors (scavenger receptor-A [SR-A] and CD36), in part by inhibiting IL-6, and inhibited macrophage foam cell formation. Systemic administration of Am80 led to reductions in the areas of atherosclerotic lesions and foam cell accumulation in the aortas of apolipoprotein E (apoE)-deficient mice and reduced serum concentrations of IL-6 and IL-1beta without affecting body weights, serum lipid profiles or IL-10 levels.

CONCLUSIONS

Am80 suppresses scavenger receptor expression and macrophage foam cell formation in vitro and prevents atherogenesis in apoE-deficient mice in vivo. This suggests Am80 is a novel candidate agent that could be highly useful in the prevention and treatment of atherosclerosis.

Structure of the Unphosphorylated STAT5a Dimer*

STAT proteins have the function of signaling from the cell membrane into the nucleus, where they regulate gene transcription. Latent mammalian STAT proteins can form dimers in the cytoplasm even before receptor-mediated activation by specific tyrosine phosphorylation. Here we describe the 3.21-A crystal structure of an unphosphorylated STAT5a homodimer lacking the N-terminal domain as well as the C-terminal transactivation domain. The overall structure of this fragment is very similar to phosphorylated STATs. However, important differences exist in the dimerization mode. Although the interface between phosphorylated STATs is mediated by their Src-homology 2 domains, the unphosphorylated STAT5a fragment dimerizes in a completely different manner via interactions between their beta-barrel and four-helix bundle domains. The STAT4 N-terminal domain dimer can be docked onto this STAT5a core fragment dimer based on shape and charge complementarities. The separation of the dimeric arrangement, taking place upon activation and nuclear translocation of STAT5a, is demonstrated by fluorescence resonance energy transfer experiments in living cells.

The GRIMs: a new interface between cell death regulation and interferon/retinoid induced growth suppression

Cytokines and vitamins play a central role in controlling neoplastic cell growth. The interferon (IFN) family of cytokines regulates antiviral, anti-tumor, antimicrobial, differentiation, and immune responses in mammals. Significant advances have been made with respect to IFN-induced signal transduction pathways and antiviral responses. However, the IFN-induced anti-tumor actions are poorly defined. Although IFNs themselves inhibit tumor growth, combination of IFNs with retinoids (a class of Vitamin A related compounds) strongly potentiates the IFN-regulated anti-tumor action in a number of cell types. To define the molecular mechanisms involved in IFN/retinoid (RA)-induced apoptosis we have employed a genetic approach and identified several critical genes. In this review, I provide the current picture of IFN- RA- and IFN/RA-regulated growth suppressive pathways. In particular, I focus on a novel set of genes, the genes-associated with retinoid-interferon induced mortality (GRIM). GRIMs may be novel types of tumor suppressors, useful as biological response markers and potentially novel targets for drug development.

Enhancement of B-MYB transcriptional activity by ZPR9, a novel zinc finger protein

By using the yeast two-hybrid system, the zinc finger protein ZPR9 was identified as one of the B-MYB interacting proteins that associates with the carboxyl-terminal conserved region of B-MYB. ZPR9 was found to form in vivo complexes with B-MYB, as demonstrated by in vivo binding assay and coimmunoprecipitation experiments of the endogenously and exogenously expressed proteins. Deletion analysis revealed that this binding was mediated by all three functional domains, an amino-terminal DNA-binding domain, a transactivation domain, and a carboxyl-terminal conserved region of B-MYB. We show that the interaction of ZPR9 with B-MYB is functional because cotransfection of ZPR9 significantly up-regulates B-MYB transcriptional activity in a dose-dependent manner. In addition, coexpression of ZPR9 with B-MYB caused the accumulation of B-MYB, as well as ZPR9, in the nucleus. Furthermore, constitutive expression of ZPR9 in human neuroblastoma cells induces apoptosis in the presence of retinoic acid. These results strongly suggest that ZPR9 plays an important role in modulation of the transactivation by B-MYB and cellular growth of neuroblastoma cells.

Synergistic activation of the human choline acetyltransferase gene by c-Myb and C/EBPbeta

To elucidate regulatory mechanisms at the transcriptional level of the human choline acetyltransferase gene (hChAT) we performed cotransfections assays in NG108-15 and SN56 cells using ChAT-CAT reporter plasmids with c-Myb and C/EBPbeta expression plasmids. The hChAT gene has several promoters, one of which (promoter P2 or M-type) is both c-Myb and C/EBPbeta inducible as 3-4-fold trans-activation was obtained in both cell lines when using either c-Myb or C/EBPbeta expression vectors alone. The simultaneous expression of c-Myb and C/EBPbeta in the absence or presence of NGFI-C (egr4) leads respectively to a 15-fold and 32-fold synergistic transcriptional activation of promoter P2. In the region upstream of exon M (P2) we identified a functional composite element including a c-Myb next to a C/EBP binding site. An oligonucleotide containing the composite element confers c-Myb and C/EBPbeta responsiveness to a heterologous promoter which is reduced after mutation of the c-Myb binding site. We also show that the coactivators CBP/p300 are required for c-Myb and C/EBPbeta trans-activation function and that RARalpha, RXRalpha and T3R have an inhibitory action on the synergistic transcriptional activity of c-Myb and C/EBPbeta and propose a model to explain the phenomena. Taken together, the results suggest that the synergistic effect of c-Myb and C/EBPbeta, previously observed in the hematopoietic system, functions equally in the neuronal system.

Characterization of monoclonal antibodies against GRIM-19, a novel IFN-beta and retinoic acid-activated regulator of cell death

A combination of interferon-beta (IFN-beta) and all-trans retinoic acid (IFN/RA) induces tumor cell apoptosis via some unknown mechanisms. Apoptosis is a gene-directed process that limits the proliferation of undesired cells. Several genes are required to regulate cell death in the higher-order animals. Earlier, we employed a gene expression knockout technique to isolate cell death-related genes. A novel gene, the gene associated with retinoid-interferon-induced mortality-19 (GRIM-19), was found to be essential for tumor cell death induced by IFN/RA. Here, we describe the development and characterization of three monoclonal antibodies (mAbs) against GRIM-19. GRIM-19 is present in the nucleus and cytoplasm. Its expression is induced by the IFN/RA combination. We also show that GRIM-19 inhibits the cell-transforming property of viral oncogenic protein viral IFN regulatory factor-1 (vIRF-1) via a physical interaction. mAbs developed in this study should be useful for studying the other physiologic roles of GRIM-19 and serve as a potent tool for studying tumor responses to IFN/RA therapy.

An LXXLL motif in the transactivation domain of STAT6 mediates recruitment of NCoA-1/SRC-1

Signal transducer and activator of transcription 6 (STAT6) regulates transcriptional activation in response to interleukin-4 (IL-4)-induced tyrosine phosphorylation by direct interaction with coactivators. The CREB-binding protein and the nuclear coactivator 1 (NCoA-1), a member of the p160/steroid receptor coactivator family, bind independently to specific regions of STAT6 and act as coactivators. In this study we show that an LXXLL motif in the STAT6 transactivation domain mediates the interaction with NCoA-1. Peptides representing this motif as well as antibodies generated against this motif inhibited STAT6/NCoA-1 interaction in glutathione S-transferase pulldown assays. Peptides derived from the STAT6 transactivation domain adjacent to the LXXLL motif as well as antibodies against these peptides showed no inhibitory effect. Mutagenesis of the LXXLL motif eliminated the STAT6/NCoA-1 interaction in vitro and in vivo, supporting the specific role of this motif in NCoA-1 binding. Importantly, mutagenesis of the STAT-LXXLL motif strongly diminished the IL-4-regulated activation of the endogenous STAT6 target gene eotaxin-3. Taken together, these results indicate that the STAT6-LXXLL-binding motif mediates the interaction with NCoA-1 in transcriptional activation and represents a new potential drug target for the inhibition of the STAT6 transactivation function in allergic diseases.

Retinoids induce the PAI-1 gene expression through tyrosine kinase-dependent pathways in vascular smooth muscle cells

Retinoids exert their pleiotropic effects on several pathophysiologic processes, including neointima formation after experimental vascular injury. Plasminogen activator inhibitor-1 (PAI-1) has been proposed to play an inhibitory role in arterial neointima formation after injury. We examined whether retinoids regulate PAI-1 expression in cultured vascular smooth muscle cells (SMCs). Northern blot analysis showed that all-trans retinoic acid (atRA) and 9-cis retinoic acid (9cRA) increased PAI-1 mRNA levels in a dose-dependent manner. These responses were completely inhibited by tyrosine kinase inhibitors. The half-life of PAI-1 was not affected by atRA, suggesting that induction of PAI-1 mRNA was mainly regulated at the transcriptional levels. Stable and transient transfection assays of the human PAI-1 promoter-luciferase constructs indicate that DNA sequence responsive to either ligand-stimulated or overexpressed retinoic acid receptor-alpha expression vector lies downstream of -363 relative to the transcription start site, where no putative retinoic acid response element is found. These results indicate that atRA and 9cRA increase PAI-1 gene transcription through pathways involving tyrosine kinases in SMCs. Because PAI-1 inhibits the production of fibrinolytic protein plasmin that facilitates SMC migration, induction of the PAI-1 gene expression by atRA may at least partly account for the role of atRA as an important inhibitor of neointima formation.

Human immunodeficiency virus type 1 Tat binding protein-1 is a transcriptional coactivator specific for TR

The DNA-binding domain of nuclear hormone receptors functions as an interaction interface for other transcription factors. Using the DNA-binding domain of TRbeta1 as bait in the yeast two-hybrid system, we cloned the Tat binding protein-1 that was originally isolated as a protein binding to the human immunodeficiency virus type 1 Tat transactivator. Tat binding protein-1 has subsequently been identified as a member of the ATPase family and a component of the 26S proteasome. Tat binding protein-1 interacted with the DNA-binding domain but not with the ligand binding domain of TR in vivo and in vitro. TR bound to the amino-terminal portion of Tat binding protein-1 that contains a leucine zipper-like structure. In mammalian cells, Tat binding protein-1 potentiated the ligand-dependent transactivation by TRbeta1 and TRalpha1 via thyroid hormone response elements. Both the intact DNA-binding domain and activation function-2 of the TR were required for the transcriptional enhancement in the presence of Tat binding protein-1. Tat binding protein-1 did not augment the transactivation function of the RAR, RXR, PPARgamma, or ER. The intrinsic activation domain in Tat binding protein-1 resided within the carboxyl-terminal conserved ATPase domain, and a mutation of a putative ATP binding motif but not a helicase motif in the carboxyl-terminal conserved ATPase domain abolished the activation function. Tat binding protein-1 synergistically activated the TR-mediated transcription with the steroid receptor coactivator 1, p120, and cAMP response element-binding protein, although Tat binding protein-1 did not directly interact with these coactivators in vitro. In contrast, the N-terminal portion of Tat binding protein-1 directly interacted in vitro and in vivo with the TR-interacting protein 1 possessing an ATPase activity that interacts with the activation function-2 of liganded TR. Collectively, Tat binding protein-1 might function as a novel DNA-binding domain-binding transcriptional coactivator specific for the TR probably in cooperation with other activation function-2-interacting cofactors such as TR-interacting protein 1.

Myeloblastin is an Myb target gene: mechanisms of regulation in myeloid leukemia cells growth-arrested by retinoic acid

A pivotal role has been assigned to Myb in the control of myeloid cell growth. Although Myb is a target of retinoic acid, little is known about the mechanisms by which it may contribute to induced growth arrest in leukemia cells. Indeed, few Myb target genes are known to be linked to proliferation. Myeloblastin is involved in the control of proliferation in myeloid leukemia cells. It is expressed early during hematopoiesis and is a granulocyte colony-stimulating factor-responsive gene. Myeloblastin can confer factor-independent growth to hematopoietic cells, an early step in leukemia transformation. The myeloblastin promoter contains PU.1, C/EBP, and Myb binding sites, each of which are critical for constitutive expression in myeloid cells. Inhibition of myeloblastin expression in leukemia cells growth-arrested by retinoic acid is demonstrated to depend on Myb down-regulation. Myb is shown to induce myeloblastin expression and abolish its down-regulation by retinoic acid. Altogether, the data offer a clue as to how a myeloid-specific transcriptional machinery can be accessible to regulation by retinoic acid and point to myeloblastin as a novel target of Myb. This link between Myb and myeloblastin suggests a previously nonidentified Myb pathway through which growth arrest is induced by retinoic acid in myeloid leukemia cells.

The effects of RARalpha and RXRalpha proteins on growth, viability, and differentiation of v-myb-transformed monoblasts

Retinoids are important agents which regulate differentiation and proliferation processes in various cell types, including cancer cells. Growth arrest and induction of terminal differentiation demonstrate the tumor-suppressive effects of retinoids on leukemic cells. We studied differentiation, proliferation, and death processes in the cell line of v-myb-transformed monoblasts BM2 and their retinoic acid receptor (RAR) alpha- and retinoid X receptor (RXR) alpha-expressing derivatives after exposure to four different retinoids: all-trans retinoic acid, 9-cis retinoic acid, TTNPB, and LG1000153. The effects of retinoids on the phenotype of BM2, BM2RAR, and BM2RXR cells were correlated with the transcription activation function of the v-Myb oncoprotein of avian myeloblastosis virus. We found that the efficiency of terminal differentiation of BM2RAR and BM2RXR cells induced by retinoids is indirectly proportional to the v-Myb transcription activation activity. In contrast, the effects of liganded retinoid receptors on growth of BM2 cells are more complex. Activated RAR protein induces growth inhibition of BM2 cells by suppression of v-Myb function. However, liganded RXR protein is less efficient in cell cycle arrest and rather decreases cellular viability. This process can occur in the presence of active v-Myb protein. These results suggest that ligand-activated RARalpha protein is primarily engaged in control of proliferation and differentiation of v-myb-transformed monoblasts, while activated RXRalpha protein controls their differentiation and death.

Modulation of CRX transactivation activity by phosducin isoforms

Phosducin (Phd) and Phd-like proteins (PhLPs) selectively bind guanine nucleotide protein (G protein) betagamma subunits (Gbetagamma), while Phd-like orphan proteins (PhLOPs) lack the major functional domain for the binding of Gbetagamma. A retina- and pineal gland-specific transcription factor, cone-rod homeobox (CRX), was identified by a yeast two-hybrid screen using PhLOP1 as the bait. Direct protein-protein interactions between Phd or PhLOP1 and CRX were demonstrated using a beta-galactosidase quantitative assay in the yeast two-hybrid system and were confirmed by an in vitro binding assay and a glutathione S-transferase (GST) pull-down assay. To determine if the interaction with Phd or PhLOP1 affected CRX transactivation, a 120-bp interphotoreceptor retinoid binding protein (IRBP) promoter-luciferase reporter construct containing a CRX consensus element (GATTAA) was cotransfected into either COS-7 or retinoblastoma Weri-Rb-1 cells with expression constructs for CRX and either Phd or PhLOP1. Phd and PhLOP1 inhibited the transcriptional activation activity of CRX by 50% during transient cotransfection in COS-7 cells and by 70% in Weri-Rb-1 cells and COS-7 cells stably transfected with CRX. Phd inhibited CRX transactivation in a dose-dependent manner. Whereas Phd is a cytoplasmic phosphoprotein, coexpression of Phd with CRX results in Phd being localized both in the cytoplasm and nucleus. By contrast, PhLOP1 is found in the nucleus even without CRX coexpression. To address the physiological relevance of these potential protein interacting partners, we identified immunoreactive proteins for Phd and CRX in retinal cytosolic and nuclear fractions. Immunohistochemical analysis of bovine retinas reveals colocalization of Phd isoforms with CRX predominantly in the inner segment of cone cells, with additional costaining in the outer nuclear layer and the synaptic region. Our findings demonstrate that both Phd and PhLOP1 interact directly with CRX and that each diminishes the transactivation activity of CRX on the IRBP promoter. A domain that interacts with CRX is found in the carboxyl terminus of the Phd isoforms. Phd antibody-immunoreactive peptides are seen in light-adapted mouse retinal cytosolic and nuclear extracts. Neither Phd nor PhLOP1 affected CRX binding to its consensus DNA element in electrophoretic mobility shift assays. A model that illustrates separate functional roles for interactions between Phd and either SUG1 or CRX is proposed. The model suggests further a mechanism by which Phd isoforms could inhibit CRX transcriptional activation.

Poly(ADP-ribose) polymerase is a B-MYB coactivator

B-MYB is implicated in cell growth control, differentiation, and cancer and belongs to the MYB family of nuclear transcription factors. Evidence exists that cellular proteins bind directly to B-MYB, and it has been hypothesized that B-MYB transcriptional activity may be modulated by specific cofactors. In an attempt to isolate proteins that interact with the B-MYB DNA-binding domain, a modular domain that has the potential to mediate protein-protein interaction, we performed pull-down experiments with a glutathione S-transferase-B-MYB protein and mammalian protein extracts. We isolated a 110-kDa protein associated endogenously with B-MYB in the nuclei of HL60 cells. Microsequence analysis and immunoprecipitation experiments determined that the bound protein was poly(ADP-ribose) polymerase (PARP). Transient transfection assays showed that PARP enhanced B-MYB transactivation and that PARP enzymatic activity is not required for B-MYB-dependent transactivation. These results suggest that PARP, as a transcriptional cofactor of a potentially oncogenic protein, may play a role in growth control and cancer.

Regulation of the mouse preprothyrotropin-releasing hormone gene by retinoic acid receptor

Retinoic acid (RA) has been reported to inhibit the secretion and synthesis of the pituitary TSH in vivo and in vitro. However, little is known about the influence of RA on the expression of the prepro-TRH gene. We therefore investigated whether the promoter activity of the mouse TRH gene is directly regulated by RA using a transient transfection assay into CV-1 cells. In the absence of cotransfected RA receptor (RAR), all-trans-RA did not affect the promoter activity. In contrast, the cotransfected RARalpha significantly stimulated promoter activity in the absence of ligand, and all-trans-RA reversed basal promoter activation. The cotransfected thyroid hormone receptor-beta (TRbeta), but not 9-cis-RA receptor (RXR), had an additive effect on the RAR-dependent stimulation. TR and RAR can similarly interact with the corepressor proteins, and the cotransfected nuclear receptor corepressor (N-CoR) has been demonstrated to augment the transcriptional stimulation of the TRH gene by unliganded TR. As observed with TR, the coexpression of a N-CoR variant significantly enhanced the ligand-independent stimulation by RAR. A mutant RAR (RAR403) lacking the C-terminal activation function-2 (AF-2) activation domain that was essential for ligand-induced corepressor release constitutively stimulated the promoter activity. The constitutive stimulation by RAR403 was augmented by the cotransfected N-CoR variant. A deletion analysis of the 5'-flanking region of the TRH gene revealed that the minimal promoter region for the regulation by RAR was -83 to +53, with a consensus half-site motif for the thyroid hormone response element at -57. In contrast to the strong binding of TR to the thyroid hormone response element half-site in gel retardation assays, no binding of RAR homodimer, RAR/ RXR heterodimer, or RAR/TR heterodimer was observed to the minimal promoter region. These results collectively suggest that RAR without heterodimerization with RXR and TR regulates transcription of the mouse TRH gene in cooperation with the corepressor, and that the DNA binding of RAR appeared to be unnecessary for regulation of the TRH gene promoter.

Role of secondary structure in discrimination between constitutive and inducible activators

We have examined structural differences between the proto-oncogene c-Myb and the cyclic AMP-responsive factor CREB that underlie their constitutive or signal-dependent activation properties. Both proteins stimulate gene expression via activating regions that articulate with a shallow hydrophobic groove in the KIX domain of the coactivator CREB-binding protein (CBP). Three hydrophobic residues in c-Myb that are conserved in CREB function importantly in cellular gene activation and in complex formation with KIX. These hydrophobic residues are assembled on one face of an amphipathic helix in both proteins, and mutations that disrupt c-Myb or CREB helicity in this region block interaction of either factor with KIX. Binding of the helical c-Myb domain to KIX is accompanied by a substantial increase in entropy that compensates for the comparatively low enthalpy of complex formation. By contrast, binding of CREB to KIX entails a large entropy cost due to a random coil-to-helix transition in CREB that accompanies complex formation. These results indicate that the constitutive and inducible activation properties of c-Myb and CREB reflect secondary structural characteristics of their corresponding activating regions that influence the thermodynamics of formation of a complex with CBP.

Physical Interaction Between Retinoic Acid Receptor and Sp1: Mechanism for Induction of Urokinase by Retinoic Acid

Induction of urokinase plasminogen activator (uPA) by retinoic acid (RA) is the initial event preceding certain subsequent biological changes in vascular endothelial cells. We investigated the molecular mechanism by which RA stimulates the expression of uPA, which lacks a canonical RA receptor (RAR)-responsive element, in bovine and human aortic endothelial cells. Upon stimulation with RA, mRNA levels of RAR and β transiently increased in parallel with the induction of uPA, and this increase was inhibited by cycloheximide. Results of transient transfection of RAR/RXR cDNAs and experiments using specific agonists and antagonists suggested that uPA induction is dependent upon RAR (initially, RAR) with the help of RXR. Deletion analysis of the uPA promoter suggested that RAR/RXR acts on GC box region within the uPA promoter. This was further supported by inhibition of Sp1 binding to this region. Coimmunoprecipitation studies, glutathioneS-transferase pull-down experiment, and mammalian two-hybrid assays suggested a physical interaction between RAR/RXR and Sp1. Furthermore, gel shift studies showed that the binding of Sp1 to the uPA GC box is significantly potentiated in the presence of RARs/RXRs. Finally, Sp1 and RAR/RXR synergistically enhanced the transactivation activity of the uPA promoter. These results suggest that (1) RA induces RARs mainly via RAR and that (2) RAR/RXR physically and functionally interact with Sp1, resulting in a potentiation of uPA transcription.

Physical Interaction Between Retinoic Acid Receptor and Sp1: Mechanism for Induction of Urokinase by Retinoic Acid

Induction of urokinase plasminogen activator (uPA) by retinoic acid (RA) is the initial event preceding certain subsequent biological changes in vascular endothelial cells. We investigated the molecular mechanism by which RA stimulates the expression of uPA, which lacks a canonical RA receptor (RAR)-responsive element, in bovine and human aortic endothelial cells. Upon stimulation with RA, mRNA levels of RAR and β transiently increased in parallel with the induction of uPA, and this increase was inhibited by cycloheximide. Results of transient transfection of RAR/RXR cDNAs and experiments using specific agonists and antagonists suggested that uPA induction is dependent upon RAR (initially, RAR) with the help of RXR. Deletion analysis of the uPA promoter suggested that RAR/RXR acts on GC box region within the uPA promoter. This was further supported by inhibition of Sp1 binding to this region. Coimmunoprecipitation studies, glutathioneS-transferase pull-down experiment, and mammalian two-hybrid assays suggested a physical interaction between RAR/RXR and Sp1. Furthermore, gel shift studies showed that the binding of Sp1 to the uPA GC box is significantly potentiated in the presence of RARs/RXRs. Finally, Sp1 and RAR/RXR synergistically enhanced the transactivation activity of the uPA promoter. These results suggest that (1) RA induces RARs mainly via RAR and that (2) RAR/RXR physically and functionally interact with Sp1, resulting in a potentiation of uPA transcription.

B-Myb protein in cellular proliferation, transcription control, and cancer: latest developments

Since its isolation exactly a decade ago, B-Myb has intrigued a growing number of scientists interested in understanding the mechanisms of cell proliferation. In many aspects the B-Myb story resembles that of a fashionable transcription factor involved in cell cycle control: E2F-1. Similar to E2F-1, B-Myb is a transcription factor whose expression is regulated at the G1/S border of the cell cycle. Given the ubiquitous expression of B-Myb within different cell types, its link with the cell cycle, and augmented expression in transformed cells, studies are in progress to define the potential role of B-Myb in human cancer. The purpose of this review is not to provide an extensive background to the B-Myb field but rather to describe the latest developments. A comprehensive outline of B-Myb structure and function can be found in the review by Saville and Watson (1998a, Adv. Cancer Res., 72:109-140).

Myb-interacting protein, ATBF1, represses transcriptional activity of Myb oncoprotein

Using the yeast two-hybrid system, the transcription factor ATBF1 was identified as v-Myb- and c-Myb-binding protein. Deletion mutagenesis revealed amino acids 2484-2520 in human ATBF1 and 279-300 in v-Myb as regions required for in vitro binding of both proteins. Further experiments identified leucines Leu325 and Leu332 of the Myb leucine zipper motif as additional amino acid residues important for efficient ATBF1-Myb interaction in vitro. In co-transfection experiments, the full-length ATBF1 was found to form in vivo complexes with v-Myb and inhibit v-Myb transcriptional activity. Both ATBF1 2484-2520 and Myb 279-300 regions were required for the inhibitory effect. Finally, the chicken ATBF1 was identified, showing high degree of amino acid sequence homology with human and murine proteins. Our data reveal Myb proteins as the first ATBF1 partners detected so far and identify amino acids 279-300 in v-Myb as a novel protein-protein interaction interface through which Myb transcriptional activity can be regulated.

Myb binding proteins: regulators and cohorts in transformation

The c-Myb and v-Myb proteins are transcription factors that regulate cell proliferation and differentiation. Both Myb proteins have been shown to interact with a number of cellular proteins, some of which are transcription factors that cooperate to activate specific promoters, while others regulate the transcriptional activity of Myb in specific contexts. By comparing and analysing the types of proteins that bind Myb, and the conserved domains of Myb that interact with other proteins, conclusions can be drawn regarding the role of specific partner proteins in the regulation of gene expression, cell proliferation and disease.