Two domains unique to osteoblast-specific transcription factor Osf2/Cbfa1 contribute to its transactivation function and its inability to heterodimerize with Cbfbeta

MAPK pathways activate and phosphorylate the osteoblast-specific transcription factor, Cbfa1

The bone-specific transcription factor, Cbfa1, regulates expression of the osteocalcin (OCN) gene and is essential for bone formation. However, little is known about the mechanisms regulating Cbfa1 activity. This work examines the role of the MAPK pathway in regulating Cbfa1-dependent transcription. Stimulation of MAPK by transfecting a constitutively active form of MEK1, MEK(SP), into MC3T3-E1 preosteoblast cells increased endogenous OCN mRNA, while a dominant negative mutant, MEK(DN), was inhibitory. MEK(SP) also stimulated activity of a 147-base pair minimal OCN promoter, and this stimulation required an intact copy of OSE2, the DNA binding site for Cbfa1. Effects of MEK(SP) were specific to Cbfa1-positive osteoblast-like cells. A purified His-tagged Cbfa1 fusion protein was directly phosphorylated by activated recombinant MAPK in vitro. Furthermore, (32)P metabolic labeling studies demonstrated that MEK(SP) clearly enhanced phosphorylation of Cbfa1 in intact cells, while MEK(DN) decreased phosphorylation. The specific MEK1/MEK2 inhibitor, PD98059, inhibited extracellular matrix-dependent up-regulation of the OCN promoter, indicating that the MAPK pathway and, presumably, Cbfa1 phosphorylation are also required for responsiveness of osteoblasts to extracellular matrix signals. This study is the first demonstration that Cbfa1 is controlled by MAPKs and suggests that this pathway has an important role in the control of osteoblast-specific gene expression.

The mammalian basic helix loop helix protein HES-1 binds to and modulates the transactivating function of the runt-related factor Cbfa1

Drosophila Runt is the founding member of a family of related transcription factors involved in the regulation of a variety of cell-differentiation events in invertebrates and vertebrates. Runt-related proteins act as both transactivators and transcriptional repressors, suggesting that context-dependent mechanisms modulate their transcriptional properties. The aim of this study was to elucidate the molecular mechanisms that contribute to the regulation of the functions of the mammalian Runt-related protein, Cbfa1. Here we provide the first demonstration that Cbfa1 (as well as the related protein, Cbfa2/AML1) physically interacts with the basic helix loop helix transcription factor, HES-1, a mammalian counterpart of the Drosophila Hairy and Enhancer of split proteins. This interaction is mediated by the carboxyl-terminal domains of Cbfa1 and HES-1, but does not require their respective tetrapeptide motifs, WRPY and WRPW. Our studies also show that HES-1 can antagonize the binding of Cbfa1 to mammalian transcriptional corepressors of the Groucho family. Moreover, HES-1 can potentiate Cbfa1-mediated transactivation in transfected cells. Taken together, these findings implicate HES-1 in the transcriptional functions of Cbfa1 and suggest that the concerted activities of Groucho and HES proteins modulate the functions of mammalian Runt-related proteins.

Auto-inhibition and partner proteins, core-binding factor beta (CBFbeta) and Ets-1, modulate DNA binding by CBFalpha2 (AML1)

Core-binding factor alpha2 (CBFalpha2; otherwise known as AML1 or PEBP2alphaB) is a DNA-binding subunit in the family of core-binding factors (CBFs), heterodimeric transcription factors that play pivotal roles in multiple developmental processes in mammals, including hematopoiesis and bone development. The Runt domain in CBFalpha2 (amino acids 51 to 178) mediates DNA binding and heterodimerization with the non-DNA-binding CBFbeta subunit. Both the CBFbeta subunit and the DNA-binding protein Ets-1 stimulate DNA binding by the CBFalpha2 protein. Here we quantify and compare the extent of cooperativity between CBFalpha2, CBFbeta, and Ets-1. We also identify auto-inhibitory sequences within CBFalpha2 and sequences that modulate its interactions with CBFbeta and Ets-1. We show that sequences in the CBFalpha2 Runt domain and sequences C terminal to amino acid 214 inhibit DNA binding. Sequences C terminal to amino acid 214 also inhibit heterodimerization with the non-DNA-binding CBFbeta subunit, particularly heterodimerization off DNA. CBFbeta rescinds the intramolecular inhibition of CBFalpha2, stimulating DNA binding approximately 40-fold. In comparison, Ets-1 stimulates CBFalpha2 DNA binding 7- to 10-fold. Although the Runt domain alone is sufficient for heterodimerization with CBFbeta, sequences N terminal to amino acid 41 and between amino acids 190 and 214 are required for cooperative DNA binding with Ets-1. Cooperative DNA binding with Ets-1 is less pronounced with the CBFalpha2-CBFbeta heterodimer than with CBFalpha2 alone. These analyses demonstrate that CBFalpha2 is subject to both negative regulation by intramolecular interactions, and positive regulation by two alternative partnerships.

A full-length Cbfa1 gene product perturbs T-cell development and promotes lymphomagenesis in synergy with myc

The Cbfa1/PEBP2 alpha A/AML3 gene plays an essential role in osteogenesis but is also expressed in the T-cell lineage where it has been implicated in lymphoma development as a target for retroviral insertional mutagenesis. As lymphoma cells with til-1 insertion express at least five distinct Cbfa1 isoforms, it is important to establish which, if any, have intrinsic oncogenic potential. We have generated transgenic mice in which the most abundant lymphoma isoform (G1/p57) is expressed under the control of the CD2 locus control region. Co-precipitation analysis of transgenic thymus revealed high levels of Cbfa1 protein in an abundant complex containing the binding cofactor Cbfb. CD2-Cbfa1-G1 mice displayed abnormal T-cell development, with a pronounced skew towards CD8 SP cells in the thymus and developed a low incidence of spontaneous lymphomas (6% at 12 months) with cells of similar phenotype. Strongly synergistic tumour development was seen when CD2-Cbfa1-G1 mice were crossed with lines carrying myc transgenes (CD2-myc or tamoxifen-regulatable CD2-mycER) and Cbfa1 was found to rescue expression of the CD2-myc transgene in pre-leukaemic mice. However, synergy did not appear to be due to a dominant block of myc-induced apoptosis by Cbfa1 as explanted primary tumours and cell lines from CD2-Cbfa1-G1/CD2-mycER mice showed accelerated death on induction with tamoxifen at similar rates to CD2-mycER controls. Moreover, thymocytes from preleukaemic CD2-Cbfa1-G1 mice showed reduced survival in vitro and increased sensitivity to the inhibitory effects of TGF-beta. This study demonstrates that a full-length Cbf alpha-chain gene can act as an oncogene without fusion to a heterologous protein.

CBFA1 mutation analysis and functional correlation with phenotypic variability in cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is a dominantly inherited skeletal dysplasia caused by mutations in the osteoblast-specific transcription factor CBFA1. To correlate CBFA1 mutations in different functional domains with the CCD clinical spectrum, we studied 26 independent cases of CCD and a total of 16 new mutations were identified in 17 families. The majority of mutations were de novo missense mutations that affected conserved residues in the runt domain and completely abolished both DNA binding and transactivation of a reporter gene. These, and mutations which result in premature termination in the runt domain, produced a classic CCD phenotype by abolishing transactivation of the mutant protein with consequent haploinsufficiency. We further identified three putative hypomorphic mutations (R391X, T200A and 90insC) which result in a clinical spectrum including classic and mild CCD, as well as an isolated dental phenotype characterized by delayed eruption of permanent teeth. Functional studies show that two of the three mutations were hypomorphic in nature and two were associated with significant intrafamilial variable expressivity, including isolated dental anomalies without the skeletal features of CCD. Together these data show that variable loss of function due to alterations in the runt and PST domains of CBFA1 may give rise to clinical variability, including classic CCD, mild CCD and isolated primary dental anomalies.

Mutation analysis of core binding factor A1 in patients with cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is a dominantly inherited disorder characterized by patent fontanelles, wide cranial sutures, hypoplasia of clavicles, short stature, supernumerary teeth, and other skeletal anomalies. We recently demonstrated that mutations in the transcription factor CBFA1, on chromosome 6p21, are associated with CCD. We have now analyzed the CBFA1 gene in 42 unrelated patients with CCD. In 18 patients, mutations were detected in the coding region of the CBFA1 gene, including 8 frameshift, 2 nonsense, and 9 missense mutations, as well as 2 novel polymorphisms. A cluster of missense mutations at arginine 225 (R225) identifies this residue as crucial for CBFA1 function. In vitro green fluorescent protein fusion studies show that R225 mutations interfere with nuclear accumulation of CBFA1 protein. There is no phenotypic difference between patients with deletions or frameshifts and those with other intragenic mutations, suggesting that CCD is generally caused by haploinsufficiency. However, we were able to extend the CCD phenotypic spectrum. A missense mutation identified in one family with supernumerary teeth and a radiologically normal skeleton indicates that mutations in CBFA1 can be associated exclusively with a dental phenotype. In addition, one patient with severe CCD and a frameshift mutation in codon 402 had osteoporosis leading to recurrent bone fractures and scoliosis, providing first evidence that CBFA1 may help maintain adult bone, in addition to its function in bone development.

Multiple Cbfa/AML sites in the rat osteocalcin promoter are required for basal and vitamin D-responsive transcription and contribute to chromatin organization

Three Cbfa motifs are strategically positioned in the bone-specific rat osteocalcin (rOC) promoter. Sites A and B flank the vitamin D response element in the distal promoter and sites B and C flank a positioned nucleosome in the proximal promoter. The functional significance of each Cbfa element was addressed by mutating individual or multiple Cbfa sites within the context of the -1.1-kb rOC promoter fused to a chloramphenicol acetyltransferase reporter gene. Promoter activity was assayed following transient transfection and after stable genomic integration in ROS 17/2.8 osteoblastic cell lines. We show that all three Cbfa sites are required for maximal basal expression of the rOC promoter. However, the distal sites A and B each contribute significantly more (P < 0.001) to promoter activity than site C. In a genomic context, sites A and B can largely compensate for a mutation at the proximal site C, and paired mutations involving site A (mAB or mAC) result in a far greater loss of activity than the mBC mutation. Strikingly, mutation of the three Cbfa sites leads to abrogation of responsiveness to vitamin D. Vitamin D-enhanced activity is also not observed when sites A and B are mutated. Significantly, related to these losses in transcriptional activity, mutation of the three Cbfa sites results in altered chromatin structure as reflected by loss of DNase I-hypersensitive sites at the vitamin D response element and over the proximal tissue-specific basal promoter. These findings strongly support a multifunctional role for Cbfa factors in regulating gene expression, not only as simple transcriptional transactivators but also by facilitating modifications in promoter architecture and chromatin organization.

AML3/CBFalpha1 is required for androgen-specific activation of the enhancer of the mouse sex-limited protein (Slp) gene

A complex 120-base pair enhancer, derived from the mouse sex-limited protein (Slp) gene, is activated solely by the androgen receptor (AR) in specific tissues, although it contains a hormone response element recognized by several steroid receptors. The generation of this transcriptional specificity has been ascribed to the interactions of the receptor with tissue-specific nonreceptor factors bound to accessory sites within the enhancer. Protein-DNA interaction assays revealed two factors binding the 5' part of the enhancer that differ widely in abundance between cells showing AR-specific activation of the Slp element compared with those that also permit activation by glucocorticoid receptor (GR). The factor designated B formed a complex centered on the sequence TGTGGT, a core motif recognized by members of the AML/CBFalpha transcription factor family. This complex was competed by a high affinity binding site specific for AML/CBFalpha and was specifically supershifted by an antibody to AML3/CBFalpha1, placing factor B within the AML3/CBFalpha1 subclass. Interestingly, this factor was shown to bind to a second site in the 3' part of the enhancer, positioned between the two critical AR binding sites. Transfection studies revealed that AML1-ETO, a dominant-negative AML/CBFalpha construct, abrogated AR induction of the enhancer, but not of simple hormone response elements. Furthermore, overexpression of AML3/CBFalpha1 could rescue the AML1-ETO repression. Finally, glutathione S-transferase-AML/CBFalpha fusion proteins demonstrated direct interaction between AML/CBFalpha and steroid receptors. Although this interaction was equivalent between AML1/CBFalpha2 and AR or GR, AML3/CBFalpha1 showed stronger interaction with AR than with GR. These data demonstrate that AML3/CBFalpha1 is functionally required for hormonal induction of the Slp enhancer and that direct, preferential protein-protein interactions may contribute to AR-specific activation. These results demonstrate an intriguing role of AML3/CBFalpha1 in steroid- as well as tissue-specific activation of target genes.

Inhibition of osteoblast-specific transcription factor Cbfa1 by the cAMP pathway in osteoblastic cells. Ubiquitin/proteasome-dependent regulation

The cAMP pathway, a major intracellular pathway mediating parathyroid hormone signal, regulates osteoblastic function. Parathyroid hormone (through activation of protein kinase A) has also been shown to stimulate ubiquitin/proteasome activity in osteoblasts. Since the osteoblast-specific transcription factor Osf2/Cbfa1 is important for differentiation of osteoblastic cells, we examined the roles of the cAMP and ubiquitin/proteasome pathways in regulation of Cbfa1. In the osteoblastic cell line, MC3T3-E1, continuous treatment with cAMP elevating agents inhibited both osteoblastic differentiation based on alkaline phosphatase assay and DNA binding ability of Cbfa1 based on a gel retardation assay. Cbfa1 inhibition was paralleled by an inhibitory effect of forskolin on Cbfa1-regulated genes. Northern and Western blot analyses suggested that the inhibition of Cbfa1 by forskolin was mainly at the protein level. Pretreatment with proteasome inhibitors prior to forskolin treatment reversed the effect of forskolin. Furthermore, addition of proteasome inhibitors to forskolin-pretreated samples resulted in recovery of Cbfa1 protein levels and accumulation of polyubiquitinated forms of Cbfa1, indicating a role for the proteasome pathway in the degradation of Cbfa1. These results suggest that suppression of osteoblastic function by the cAMP pathway is through proteolytic degradation of Cbfa1 involving a ubiquitin/proteasome-dependent mechanism.

Isolation and characterization of the distal promoter region of mouse Cbfa1

Cbfa1 is an essential transcription factor for bone formation, and as such little is known about the region responsible for the transcriptional regulation of this gene. Here we report the determination of the transcription start sites, isolation and partial characterization of distal promoter region of this gene. Three transcription start sites were identified by the 5'-Cap site method, recently invented for rapid examination of the 5'-end of genes of interest. A reporter construct containing 1.8 kb of 5' of transcription start sites had approximately 25-fold more luciferase activity than the promoter-less vector in osteoblastic cell lines. Deletion analysis of the reporter construct demonstrated that the minimal region to express promoter activity lies between bp -168 and -99, taking the most downstream transcription start site as +1. By Northern blot analysis, mRNA expression from the distal promoter was detected in the differentiated osteoblastic cell lines, UMR-106, ROS17/2.8 and MC3T3-E1, but not in cell lines of immature phenotype or originated from other organs. Luciferase activity was strongest in UMR-106 and ROS17/2.8, and weakest in COS-1 and HepG2, which are cell lines originating from other organs, corresponding to the level of mRNA expression. These results demonstrated that the distal promoter region examined here is important for tissue- and cell-type-specific gene expression of Cbfa1.

Collagenase 3 is a target of Cbfa1, a transcription factor of the runt gene family involved in bone formation

Collagenase 3 (MMP-13) is a recently identified member of the matrix metalloproteinase (MMP) gene family that is expressed at high levels in diverse human carcinomas and in articular cartilage from arthritic patients. In addition to its expression in pathological conditions, collagenase 3 has been detected in osteoblasts and hypertrophic chondrocytes during fetal ossification. In this work, we have evaluated the possibility that Cbfa1 (core binding factor 1), a transcription factor playing a major role in the expression of osteoblastic specific genes, is involved in the expression of collagenase 3 during bone formation. We have functionally characterized a Cbfa motif present in the promoter region of collagenase 3 gene and demonstrated, by cotransfection experiments and gel mobility shift assays, that this element is involved in the inducibility of the collagenase 3 promoter by Cbfa1 in osteoblastic and chondrocytic cells. Furthermore, overexpression of Cbfa1 in osteoblastic cells unable to produce collagenase 3 leads to the expression of this gene after stimulation with transforming growth factor beta. Finally, we show that mutant mice deficient in Cbfa1, lacking mature osteoblasts but containing hypertrophic chondrocytes which are also a major source of collagenase 3, do not express this protease during fetal development. These results provide in vivo evidence that collagenase 3 is a target of the transcriptional activator Cbfa1 in these cells. On the basis of these transcriptional regulation studies, together with the potent proteolytic activity of collagenase 3 on diverse collagenous and noncollagenous bone and cartilage components, we proposed that this enzyme may play a key role in the process of bone formation and remodeling.

A Cbfa1-dependent genetic pathway controls bone formation beyond embryonic development

The molecular mechanisms controlling bone extracellular matrix (ECM) deposition by differentiated osteoblasts in postnatal life, called hereafter bone formation, are unknown. This contrasts with the growing knowledge about the genetic control of osteoblast differentiation during embryonic development. Cbfa1, a transcriptional activator of osteoblast differentiation during embryonic development, is also expressed in differentiated osteoblasts postnatally. The perinatal lethality occurring in Cbfa1-deficient mice has prevented so far the study of its function after birth. To determine if Cbfa1 plays a role during bone formation we generated transgenic mice overexpressing Cbfa1 DNA-binding domain (DeltaCbfa1) in differentiated osteoblasts only postnatally. DeltaCbfa1 has a higher affinity for DNA than Cbfa1 itself, has no transcriptional activity on its own, and can act in a dominant-negative manner in DNA cotransfection assays. DeltaCbfa1-expressing mice have a normal skeleton at birth but develop an osteopenic phenotype thereafter. Dynamic histomorphometric studies show that this phenotype is caused by a major decrease in the bone formation rate in the face of a normal number of osteoblasts thus indicating that once osteoblasts are differentiated Cbfa1 regulates their function. Molecular analyses reveal that the expression of the genes expressed in osteoblasts and encoding bone ECM proteins is nearly abolished in transgenic mice, and ex vivo assays demonstrated that DeltaCbfa1-expressing osteoblasts were less active than wild-type osteoblasts. We also show that Cbfa1 regulates positively the activity of its own promoter, which has the highest affinity Cbfa1-binding sites characterized. This study demonstrates that beyond its differentiation function Cbfa1 is the first transcriptional activator of bone formation identified to date and illustrates that developmentally important genes control physiological processes postnatally.

Cbfa1 isoforms exert functional differences in osteoblast differentiation

Cbfa1 is an essential transcription factor for osteoblast differentiation and bone formation. We investigated functional differences among three isoforms of Cbfa1: Type I (originally reported as Pebp2alphaA by Ogawa et al. (Ogawa, E., Maruyama, M., Kagoshima, H., Inuzuka, M., Lu, J., Satake, M., Shigesada, K., and Ito, Y. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 6859-6863), Type II (originally reported as til-1 by Stewart et al. (Stewart, M., Terry, A., Hu, M., O'Hara, M., Blyth, K., Baxter, E., Cameron, E., Onions, D. E., and Neil, J. C. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 8646-8651), and Type III (originally reported as Osf2/Cbfa1 by Ducy et al. (Ducy, P., Zhang, R., Geoffroy, V., Ridall, A. L., and Karsenty, G. (1997) Cell 89, 747-754). A reverse transcriptase-polymerase chain reaction analysis demonstrated that these isoforms were expressed in adult mouse bones. The transient transfection of Type I or Type II Cbfa1 in a mouse fibroblastic cell line, C3H10T1/2, induced the expression of alkaline phosphatase (ALP) activity. This induction was synergistically enhanced by the co-introduction of Xenopus BMP-4 cDNA. In contrast, the transient transfection of Type III cDNA induced no ALP activity. In C3H10T1/2 cells stably transfected with each isoform of Cbfa1, the gene expression of ALP was also strongly induced in cells transfected with Type I and Type II Cbfa1 but not in cells with Type III Cbfa1. Osteocalcin, osteopontin,and type I collagen gene expressions were induced or up-regulated in all of the cells stably transfected with each isoform of Cbfa1, and Type II transfected cells exhibited the highest expression level of osteocalcin gene. A luciferase reporter gene assay using a 6XOSE2-SV40 promoter (6 tandem binding elements for Cbfa1 ligated in front of the SV40 promoter sequence), a mouse osteocalcin promoter, and a mouse osteopontin promoter revealed the differences in the transcriptional induction of target genes by each Cbfa1 isoform with or without its beta-subunit. These results suggest that all three of the Cbfa1 isoforms used in the present study are involved in the stimulatory action of osteoblast differentiation, but they exert different functions in the process of osteoblast differentiation.

Groucho/transducin-like enhancer of split (TLE) family members interact with the yeast transcriptional co-repressor SSN6 and mammalian SSN6-related proteins: implications for evolutionary conservation of transcription repression mechanisms

The yeast proteins TUP1 and SSN6 form a transcription repressor complex that is recruited to different promoters via pathway-specific DNA-binding proteins and regulates the expression of a variety of genes. TUP1 is functionally related to invertebrate and vertebrate transcriptional repressors of the Groucho/transducin-like Enhancer of split (TLE) family. The aim was to examine whether similar mechanisms underlie the transcription repression functions of TUP1 and Groucho/TLEs by determining whether TLE family members can interact with yeast SSN6 and mammalian SSN6-like proteins. It is shown in the present work that SSN6 binds to TLE1 and mediates transcriptional repression when expressed in mammalian cells. Moreover, TLE1 and TLE2 interact with two mammalian proteins related to SSN6, designated as the products of the ubiquitously transcribed tetratricopeptide-repeat genes on the Y (or X) chromosomes (UTY/X). These findings suggest that mammalian TLE and UTY/X proteins may mediate repression mechanisms similar to those performed by TUP1-SSN6 in yeast.

Transcriptional repression by AML1 and LEF-1 is mediated by the TLE/Groucho corepressors

The mammalian AML/CBFalpha runt domain (RD) transcription factors regulate hematopoiesis and osteoblast differentiation. Like their Drosophila counterparts, most mammalian RD proteins terminate in a common pentapeptide, VWRPY, which serves to recruit the corepressor Groucho (Gro). Using a yeast two-hybrid assay, in vitro association and pull-down experiments, we demonstrate that Gro and its mammalian homolog TLE1 specifically interact with AML1 and AML2. In addition to the VWRPY motif, other C-terminal sequences are required for these interactions with Gro/TLE1. TLE1 inhibits AML1-dependent transactivation of the T cell receptor (TCR) enhancers alpha and beta, which contain functional AML binding sites, in transfected Jurkat T cells. LEF-1 is an additional transcription factor that mediates transactivation of TCR enhancers. LEF-1 and its Drosophila homolog Pangolin (Pan) are involved in the Wnt/Wg signaling pathway through interactions with the coactivator beta-catenin and its highly conserved fly homolog Armadillo (Arm). We show that TLE/Gro interacts with LEF-1 and Pan, and inhibits LEF-1:beta-catenin-dependent transcription. These data indicate that, in addition to their activity as transcriptional activators, AML1 and LEF-1 can act, through recruitment of the corepressor TLE1, as transcriptional repressors in TCR regulation and Wnt/Wg signaling.