Identification of functional domains involved in BTG1 cell localization

BTG1 mutation yields supercompetitive B cells primed for malignant transformation

Multicellular life requires altruistic cooperation between cells. The adaptive immune system is a notable exception, wherein germinal center B cells compete vigorously for limiting positive selection signals. Studying primary human lymphomas and developing new mouse models, we found that mutations affecting BTG1 disrupt a critical immune gatekeeper mechanism that strictly limits B cell fitness during antibody affinity maturation. This mechanism converted germinal center B cells into supercompetitors that rapidly outstrip their normal counterparts. This effect was conferred by a small shift in MYC protein induction kinetics but resulted in aggressive invasive lymphomas, which in humans are linked to dire clinical outcomes. Our findings reveal a delicate evolutionary trade-off between natural selection of B cells to provide immunity and potentially dangerous features that recall the more competitive nature of unicellular organisms.

Conformational transitions in BTG1 antiproliferative protein and their modulation by disease mutants

B cell translocation gene 1 (BTG1) protein belongs to the BTG/transducer of ERBB2 (TOB) family of antiproliferative proteins whose members regulate various key cellular processes such as cell cycle progression, apoptosis, and differentiation. Somatic missense mutations in BTG1 are found in ∼70% of a particularly malignant and disseminated subtype of diffuse large B cell lymphoma (DLBCL). Antiproliferative activity of BTG1 has been linked to its ability to associate with transcriptional cofactors and various enzymes. However, molecular mechanisms underlying these functional interactions and how the disease-linked mutations in BTG1 affect these mechanisms are currently unknown. To start filling these knowledge gaps, here, using atomistic molecular dynamics (MD) simulations, we explored structural, dynamic, and kinetic characteristics of BTG1 protein, and studied how various DLBCL mutations affect these characteristics. We focused on the protein region formed by α2 and α4 helices, as this interface has been reported not only to serve as a binding hotspot for several cellular partners but also to harbor sites for the majority of known DLBCL mutations. Markov state modeling analysis of extensive MD simulations revealed that the α2-α4 interface in the wild-type (WT) BTG1 undergoes conformational transitions between closed and open metastable states. Importantly, we show that some of the mutations in this region that are observed in DLBCL, such as Q36H, F40C, Q45P, E50K (in α2), and A83T and A84E (in α4), either overstabilize one of these two metastable states or give rise to new conformations in which these helices are distorted (i.e., kinked or unfolded). Based on these results, we conclude that the rapid interconversion between the closed and open conformations of the α2-α4 interface is an essential component of the BTG1 functional dynamics that can prime the protein for functional associations with its binding partners. Disruption of the native dynamic equilibrium by DLBCL mutants leads to the ensemble of conformations in BTG1 that are unlikely structurally and/or kinetically to enable productive functional interactions with the binding proteins.

Key Genes Regulating Skeletal Muscle Development and Growth in Farm Animals

Simple Summary

Skeletal muscle mass is an important economic trait, and muscle development and growth is a crucial factor to supply enough meat for human consumption. Thus, understanding (candidate) genes regulating skeletal muscle development is crucial for understanding molecular genetic regulation of muscle growth and can be benefit the meat industry toward the goal of increasing meat yields. During the past years, significant progress has been made for understanding these mechanisms, and thus, we decided to write a comprehensive review covering regulators and (candidate) genes crucial for muscle development and growth in farm animals. Detection of these genes and factors increases our understanding of muscle growth and development and is a great help for breeders to satisfy demands for meat production on a global scale.

Abstract

Farm-animal species play crucial roles in satisfying demands for meat on a global scale, and they are genetically being developed to enhance the efficiency of meat production. In particular, one of the important breeders’ aims is to increase skeletal muscle growth in farm animals. The enhancement of muscle development and growth is crucial to meet consumers’ demands regarding meat quality. Fetal skeletal muscle development involves myogenesis (with myoblast proliferation, differentiation, and fusion), fibrogenesis, and adipogenesis. Typically, myogenesis is regulated by a convoluted network of intrinsic and extrinsic factors monitored by myogenic regulatory factor genes in two or three phases, as well as genes that code for kinases. Marker-assisted selection relies on candidate genes related positively or negatively to muscle development and can be a strong supplement to classical selection strategies in farm animals. This comprehensive review covers important (candidate) genes that regulate muscle development and growth in farm animals (cattle, sheep, chicken, and pig). The identification of these genes is an important step toward the goal of increasing meat yields and improves meat quality.

Genomic sequencing analysis reveals copy number variations and their associations with economically important traits in beef cattle

Copy number variation (CNV) represents a major source of genetic variation, which may have potentially large effects, including alternating gene regulation and dosage, as well as contributing to gene expression and risk for normal phenotypic variability. We carried out a comprehensive analysis of CNV based on whole genome sequencing in Chinese Simmental beef cattle. Totally, we found 9313 deletion and 234 duplication events, covering 147.5 Mb autosomal regions. Within them, 257 deletion events of high frequency overlapped with 193 known RefGenes. Among these genes, we observed several genes were related to economically important traits, like residual feed intake, immune responding, pregnancy rate and muscle differentiation. Using a locus-based analysis, we identified 11 deletions and 1 duplication, which were significantly associated with three traits including carcass weight, tenderloin and longissimus muscle area. Our sequencing-based study provided important insights into investigating the association of CNVs with important traits in beef cattle.

Tumor suppressors BTG1 and BTG2: Beyond growth control

Since the identification of B‐cell translocation gene 1 (BTG1) and BTG2 as antiproliferation genes more than two decades ago, their protein products have been implicated in a variety of cellular processes including cell division, DNA repair, transcriptional regulation and messenger RNA stability. In addition to affecting differentiation during development and in the adult, BTG proteins play an important role in maintaining homeostasis under conditions of cellular stress. Genomic profiling of B‐cell leukemia and lymphoma has put BTG1 and BTG2 in the spotlight, since both genes are frequently deleted or mutated in these malignancies, pointing towards a role as tumor suppressors. Moreover, in solid tumors, reduced expression of BTG1 or BTG2 is often correlated with malignant cell behavior and poor treatment outcome. Recent studies have uncovered novel roles for BTG1 and BTG2 in genotoxic and integrated stress responses, as well as during hematopoiesis. This review summarizes what is currently known about the roles of BTG1 and BTG2 in these and other cellular processes. In addition, we will highlight the molecular mechanisms and biological consequences of BTG1 and BTG2 deregulation during cancer progression and elaborate on the potential clinical implications of these findings.

Genomic regions underlying uniformity of yearling weight in Nellore cattle evaluated under different response variables

BACKGROUND

In livestock, residual variance has been studied because of the interest to improve uniformity of production. Several studies have provided evidence that residual variance is partially under genetic control; however, few investigations have elucidated genes that control it. The aim of this study was to identify genomic regions associated with within-family residual variance of yearling weight (YW; N = 423) in Nellore bulls with high density SNP data, using different response variables. For this, solutions from double hierarchical generalized linear models (DHGLM) were used to provide the response variables, as follows: a DGHLM assuming non-null genetic correlation between mean and residual variance (rmv ≠ 0) to obtain deregressed EBV for mean (dEBVm) and residual variance (dEBVv); and a DHGLM assuming rmv = 0 to obtain two alternative response variables for residual variance, dEBVv_r0 and log-transformed variance of estimated residuals (ln_[Formula: see text]).

RESULTS

The dEBVm and dEBVv were highly correlated, resulting in common regions associated with mean and residual variance of YW. However, higher effects on variance than the mean showed that these regions had effects on the variance beyond scale effects. More independent association results between mean and residual variance were obtained when null rmv was assumed. While 13 and 4 single nucleotide polymorphisms (SNPs) showed a strong association (Bayes Factor > 20) with dEBVv and ln_[Formula: see text], respectively, only suggestive signals were found for dEBVv_r0. All overlapping 1-Mb windows among top 20 between dEBVm and dEBVv were previously associated with growth traits. The potential candidate genes for uniformity are involved in metabolism, stress, inflammatory and immune responses, mineralization, neuronal activity and bone formation.

CONCLUSIONS

It is necessary to use a strategy like assuming null rmv to obtain genomic regions associated with uniformity that are not associated with the mean. Genes involved not only in metabolism, but also stress, inflammatory and immune responses, mineralization, neuronal activity and bone formation were the most promising biological candidates for uniformity of YW. Although no clear evidence of using a specific response variable was found, we recommend consider different response variables to study uniformity to increase evidence on candidate regions and biological mechanisms behind it.

BTG1 might be employed as a biomarker for carcinogenesis and a target for gene therapy in colorectal cancers

Here, BTG1 overexpression inhibited proliferation, induced differentiation, autophagy, and apoptosis in colorectal cancer cells (p₂ arrest might be related to Cyclin B1 and Cdc25B hypoexpression in HCT-15 transfectants, while G₁ arrest in HCT-116 transfectants overexpressing p21 and p27. BTG1 overexpression decreased the expression of Bcl-2, Bcl-xL, XIAP, Akt1 or survivin and increased the expression of Bax or p53 in colorectal cancer cells. BTG1-induced autophagy was dependent on Beclin-1 expression. BTG1 overexpression might weaken β-catenin pathway in colorectal cancer cells. The chemosensitivity of BTG1 transfectants to paclitaxel, cisplatin, MG132 or SAHA was positively correlated with its apoptotic induction. There was a lower expression level of BTG1 in cancer than matched non-neoplastic mucosa by RT-PCR (p

The B-cell translocation gene 1 (CgBTG1) identified in oyster Crassostrea gigas exhibit multiple functions in immune response

B-cell translocation gene 1 (BTG1) is a member of the anti-proliferative gene family, which plays important roles in regulation of cell cycle. In the present study, a B-cell translocation gene 1 molecule homologue (designed CgBTG1) are identified and characterized in oyster Crassostrea gigas. CgBTG1 contains a conserved BTG domain with Box A and Box B motifs, and it shares high similarities with both BTG1 and BTG2 proteins in vertebrates. CgBTG1 mRNA is predominantly expressed in hemocytes, and its expression level in hemocytes is significantly up-regulated at 6 h (5.40-fold, p < 0.01) post Vibrio splendidus stimulation. The apoptosis rate of oyster hemocytes is significantly decreased (p < 0.05) after CgBTG1 interfered by dsRNA (dsCgBTG1). This is indicated that CgBTG1 participated in the regulation of oyster hemocytes apoptosis. Furthermore, CgBTG1 could also induce the apoptosis of cancer cells (HeLa, A549 and BEL7402) in vitro. Compared with normal oysters, both vessel-like structures and muscle fibers in CgBTG1 interfered oysters are severely damaged after V. splendidus challenge in paraffin section, considering that CgBTG1 possessed an analogous feature of angiogenesis for maintenance of vessel-like structures in adductor muscle of oyster. The results suggests that CgBTG1 is a multi-functional molecule involved in the immune response of C. gigas against pathogen infection, which provides more clues for intensive studies of BTG family proteins in invertebrates.

Multi-Trait GWAS and New Candidate Genes Annotation for Growth Curve Parameters in Brahman Cattle

Understanding the genetic architecture of beef cattle growth cannot be limited simply to the genome-wide association study (GWAS) for body weight at any specific ages, but should be extended to a more general purpose by considering the whole growth trajectory over time using a growth curve approach. For such an approach, the parameters that are used to describe growth curves were treated as phenotypes under a GWAS model. Data from 1,255 Brahman cattle that were weighed at birth, 6, 12, 15, 18, and 24 months of age were analyzed. Parameter estimates, such as mature weight (A) and maturity rate (K) from nonlinear models are utilized as substitutes for the original body weights for the GWAS analysis. We chose the best nonlinear model to describe the weight-age data, and the estimated parameters were used as phenotypes in a multi-trait GWAS. Our aims were to identify and characterize associated SNP markers to indicate SNP-derived candidate genes and annotate their function as related to growth processes in beef cattle. The Brody model presented the best goodness of fit, and the heritability values for the parameter estimates for mature weight (A) and maturity rate (K) were 0.23 and 0.32, respectively, proving that these traits can be a feasible alternative when the objective is to change the shape of growth curves within genetic improvement programs. The genetic correlation between A and K was -0.84, indicating that animals with lower mature body weights reached that weight at younger ages. One hundred and sixty seven (167) and two hundred and sixty two (262) significant SNPs were associated with A and K, respectively. The annotated genes closest to the most significant SNPs for A had direct biological functions related to muscle development (RAB28), myogenic induction (BTG1), fetal growth (IL2), and body weights (APEX2); K genes were functionally associated with body weight, body height, average daily gain (TMEM18), and skeletal muscle development (SMN1). Candidate genes emerging from this GWAS may inform the search for causative mutations that could underpin genomic breeding for improved growth rates.

Diversity of clinical implication of B-cell translocation gene 1 expression by histopathologic and anatomic subtypes of gastric cancer

BACKGROUND

Genetic signatures may differ by histopathologic and anatomic subtypes of gastric cancer (GC). B-cell translocation gene 1 (BTG1) was identified as one of genes downregulated in GC tissues from our microarray data.

AIMS

To evaluate the clinical implications of BTG1 expression in GC and the genetic diversity among GC subtypes.

METHODS

BTG1 mRNA expression was analyzed in GC cell lines and 233 pairs of surgical specimens. The mutational and methylation status of BTG1 in GC cell lines was analyzed, and immunohistochemistry was conducted to determine the distribution of BTG1. The pattern and prognostic significance of BTG1 expression were correlated with the three proposed GC subtypes.

RESULTS

BTG1 mRNA was downregulated in 82 % of GC cell lines and in 88 % of clinical GC tissues. Promoter hypermethylation events or sequence mutations were not detected in GC cell lines. The pattern of BTG1 expression as observed by immunohistochemistry was consistent with that of its mRNA. Downregulation of BTG1 mRNA in GCs was significantly associated with shorter disease-specific and recurrence-free survival. Multivariate analysis of disease-specific survival identified downregulation of BTG1 transcription as an independent prognostic factor. BTG1 mRNA expression was more strongly suppressed in proximal nondiffuse and diffuse GC compared with distal nondiffuse GC, and subgroup analysis revealed that BTG1 downregulation led to adverse prognosis, specifically in patients with proximal nondiffuse and diffuse GC.

CONCLUSIONS

Altered expression of BTG1 is a potential biomarker for carcinogenesis and progression of GC, particularly for proximal nondiffuse and diffuse GC.

Molecular cloning, sequence analysis, and cadmium stress-rated expression changes of BTG1 in freshwater pearl mussel (Hyriopsis schlegelii)

The B cells translocation gene 1 (BTG1) is a member of the BTG/TOB family of anti-proliferative genes, which have recently emerged as important regulators of cell growth and differentiation among verteates. Here, for the first time we cloned the full-length cDNA sequence of Hyriopsis schlegelii (Hs-BTG1), an economically important freshwater shellfish and potential indicator of environmental heavy metal pollution, for the first time. Using rapid amplification of cDNA ends (RACE) together with splicing the EST sequence from a haemocyte cDNA liary, we found that Hs-BTG1 contains a 525 bp open reading frame (ORF) encoding a 174 amino-acid polypeptide, a 306 bp 5' untranslated region (5' UTR), and a 571 bp 3' UTR with a Poly(A) tail as well as a transcription termination signal (AATAAA). Homologue searching against GenBank revealed that Hs-BTG1 was closest to Crassostrea gigas BTG1, sharing 50.57% of protein identities. Hs-BTG1 also shares some typical features of the BTG/TOB family, possessing two well-conserved A and B boxes. Clustering analysis of Hs-BTG1 and other known BTGs showed that Hs-BTG1 was also closely related to BTG1 of C. gigas from the inverteate BTG1 clade. Function prediction via homology modeling showed that both Hs-BTG1 and C. gigas BTG1 share a similar three-dimensional structure with Homo sapiens BTG1. Tissue-specific expression analysis of the Hs-BTG1 via real-time PCR showed that the transcripts were constitutively expressed, with the highest levels in the hepatopancreas and gills, and the lowest in both haemocyte and muscle tissue. Expression levels of Hs-BTG1 in hepatopancreas (2.03-fold), mantle (2.07-fold), kidney (2.2-fold) and haemocyte (2.5-fold) were enhanced by cadmium (Cd²⁺) stress, suggesting that Hs-BTG1 may have played a significant role in H. schlegelii adaptation to adverse environmental conditions.

B‑cell translocation gene 1 serves as a novel prognostic indicator of hepatocellular carcinoma

Although the B‑cell translocation gene 1 (BTG1) plays an important role in apoptosis and negatively regulates cell proliferation, BTG1 expression in hepatocellular carcinoma (HCC) has not been evaluated. In this study expression analysis of BTG1 was conducted to clarify the role of BTG1 in the initiation of HCC carcinogenesis and progression. BTG1 mRNA expression levels were determined for HCC cell lines and 151 surgical specimen pairs using quantitative real‑time reverse transcription polymerase chain reaction (RT‑qPCR) assay. The mutational and methylation status of HCC cell lines were analyzed via high resolution melting (HRM) analysis and direct sequencing analysis to elucidate the regulatory mechanisms of BTG1 expression. The expression and distribution of the BTG1 protein in liver tissues were evaluated using immunohistochemistry (IHC). Decreased expression of BTG1 mRNA was confirmed in the majority of HCC cell lines (89%) and clinical HCC tissues (85%) compared with non‑cancerous liver tissues. Mutations or promoter hypermethylation were not identified in HCC cell lines. BTG1 mRNA expression levels were not influenced by background liver status. The pattern of BTG1 protein expression was consistent with that of BTG1 mRNA. Downregulation of BTG1 mRNA in HCC was significantly associated with shorter disease‑specific and recurrence‑free survival rates. Multivariate analysis of disease‑specific survival rates identified BTG1 mRNA downregulation as an independent prognostic factor for HCC (hazard ratio 2.12, 95% confidence interval 1.12‑4.04, P=0.022). Our results indicate that altered BTG1 expression might affect hepatocarcinogenesis and may represent a novel biomarker for HCC carcinogenesis and progression.

BTG1 expression correlates with the pathogenesis and progression of ovarian carcinomas

BTG (B-cell translocation gene) can inhibit cell proliferation, metastasis, and angiogenesis and regulate cell cycle progression and differentiation in a variety of cell types. We aimed to clarify the role of BTG1 in ovarian carcinogenesis and progression. A BTG1-expressing plasmid was transfected into ovarian carcinoma cells and their phenotypes and related proteins were examined. BTG1 mRNA expression was detected in ovarian normal tissue (n = 17), ovarian benign tumors (n = 12), and ovarian carcinoma (n = 64) using real-time RT-PCR. Ectopic BTG1 expression resulted in lower growth rate, high cisplatin sensitivity, G₁ arrest, apoptosis, and decreased migration and invasion. Phosphoinositide 3-kinase, protein kinase B, Bcl-xL, survivin, vascular endothelial growth factor, and matrix metalloproteinase-2 mRNA and protein expression was reduced in transfectants as compared to control cells. There was higher expression of BTG1 mRNA in normal tissue than in carcinoma tissue (p = 0.001) and in benign tumors than in carcinoma tissue (p = 0.027). BTG1 mRNA expression in International Federation of Gynecology and Obstetrics (FIGO) stage I/II ovarian carcinomas was higher than that in FIGO stage III/IV ovarian carcinomas (p = 0.038). Altered BTG1 expression might play a role in the pathogenesis and progression of ovarian carcinoma by modulating proliferation, migration, invasion, the cell cycle, and apoptosis.

C. elegans FOG-3/Tob can either promote or inhibit germline proliferation, depending on gene dosage and genetic context

Vertebrate Tob/BTG proteins inhibit cell proliferation when overexpressed in tissue-culture cells, and they can function as tumor suppressors in mice. The single Caenorhabditis elegans Tob/BTG ortholog, FOG-3, by contrast, was identified from its loss-of-function phenotype as a regulator of sperm fate specification. Here we report that FOG-3 also regulates proliferation in the germline tissue. We first demonstrate that FOG-3 is a positive regulator of germline proliferation. Thus, fog-3 null mutants possess fewer germ cells than normal, a modest but reproducible decrease observed for each of two distinct fog-3 null alleles. A similar decrease also occurred in fog-3/+ heterozygotes, again for both fog-3 alleles, revealing a haplo-insufficient effect on proliferation. Therefore, FOG-3 normally promotes proliferation, and two copies of the fog-3 gene are required for this function. We next overexpressed FOG-3 by removal of FBF, the collective term for FBF-1 and FBF-2, two nearly identical PUF RNA-binding proteins. We find that overexpressed FOG-3 blocks proliferation in fbf-1 fbf-2 mutants; whereas germ cells stop dividing and instead differentiate in fbf-1 fbf-2 double mutants, they continue to proliferate in fog-3; fbf-1 fbf-2 triple mutants. Therefore, like its vertebrate Tob/BTG cousins, overexpressed FOG-3 is 'antiproliferative'. Indeed, some fog-3; fbf-1 fbf-2 mutants possess small tumors, suggesting that FOG-3 can act as a tumor suppressor. Finally, we show that FOG-3 and FBF work together to promote tumor formation in animals carrying oncogenic Notch mutations. A similar effect was not observed when germline tumors were induced by manipulation of other regulators; therefore, this FOG-3 tumor-promoting effect is context dependent. We conclude that FOG-3 can either promote or inhibit proliferation in a manner that is sensitive to both genetic context and gene dosage. The discovery of these FOG-3 effects on proliferation has implications for our understanding of vertebrate Tob/BTG proteins and their influence on normal development and tumorigenesis.

PC3 is involved in the shift from proliferation to differentiation and maturation in spiral ganglion neurons

PC3 is a member of the BTG/Tob family of antiproliferative genes. Here, we report the results of an analysis of PC3 protein expression in spiral ganglion neurons of the rat cochlea at embryonic days 16 (E16) and 20 (E20), and postnatal days 4 (P4) and 7 (P7). PC3 expression was observed in the cytoplasm of ganglion neurons at E16 and E20, and this protein had translocated to the nucleus by P4. The expression of Ki-67, a nuclear antigen expressed by dividing cells, was detected in ganglion neurons at E16 and E20, but not at P4 or P7. These results suggest that PC3 is involved in the shift from proliferation to differentiation and maturation in the ganglion neurons of the rat cochlea.

Molecular characterization of the BTG2 and BTG3 genes in fetal muscle development of pigs

BTG2 and BTG3 are two members of the B-cell translocation gene family with anti-proliferative properties. BTG1 gene in this gene family has been reported to play a key role in muscle growth. In this study, we identified and characterized the porcine BTG2 and BTG3 genes, mapped the two genes to porcine chromosomes, and analyzed their expression differences in the longissimus dorci muscle of 33 dpc (day postconception), 65 dpc and 90 dpc in the lean Landrace and fatty Chinese Tongcheng pig breeds. Expression changes in differentiated C2C12 cells were also investigated with myogenin as internal control. The results showed that the porcine BTG2 and BTG3 genes were mapped on SSC9q21-25 and SSC13q47, respectively. BTG2 gene expressed at high levels in skeletal muscle and heart in both Tongcheng and Landrace pigs whereas BTG3 gene expressed at lower levels in skeletal muscle and heart than in other tissues. Furthermore, BTG3 expressed at higher levels in skeletal muscle of Tonghceng compared with Landrace pig. The expression of BTG2 and BTG3 was significantly different in skeletal muscle among different developmental stages and between the two breeds. Expression analysis in murine myoblast cells showed that both genes were induced in differentiated C2C12 cells, suggesting a role of them in myogenic differentiation. Our study indicated that BTG2 and BTG3, especially BTG3 gene, may be important genes for skeletal muscle growth.

B cell translocation gene 1 contributes to antisense Bcl-2-mediated apoptosis in breast cancer cells

The antiapoptotic protein Bcl-2 is overexpressed in a majority of breast cancers, and is associated with a diminished apoptotic response and resistance to various antitumor agents. Bcl-2 inhibition is currently being explored as a possible strategy for sensitizing breast cancer cells to standard chemotherapeutic agents. Antisense Bcl-2 oligonucleotides represent one method for blocking the antiapoptotic effects of Bcl-2. In this study, we show that antisense Bcl-2 efficiently blocks Bcl-2 expression, resulting in the apoptosis of breast cancer cells. Antisense Bcl-2-mediated cytotoxicity was associated with the induction of the B cell translocation gene 1 (BTG1). Importantly, knockdown of BTG1 reduced antisense Bcl-2-mediated cytotoxicity in breast cancer cells. Furthermore, BTG1 expression seems to be negatively regulated by Bcl-2, and exogenous expression of BTG1 induced apoptosis. These results suggest that BTG1 is a Bcl-2-regulated mediator of apoptosis in breast cancer cells, and that its induction contributes to antisense Bcl-2-mediated cytotoxic effects.

Loss of B-cell translocation gene-2 in estrogen receptor-positive breast carcinoma is associated with tumor grade and overexpression of cyclin d1 protein

The B-cell translocation gene-2 (BTG2) is present in the nuclei of epithelial cells in many tissues, including the mammary gland where its expression is regulated during glandular proliferation and differentiation in pregnancy. In immortalized mammary epithelial cells and breast cancer cells, BTG2 protein localized predominantly to the nucleus and cytoplasm, respectively. The highly conserved domains (BTG boxes A, B, and C) were required for regulating localization, suppression of cyclin D1 and growth inhibitory function of BTG2. Expression analysis of BTG2 protein in human breast carcinoma (n = 148) revealed the loss of nuclear expression in 46% of tumors, whereas it was readily detectable in the nuclei of adjacent normal glands. Loss of nuclear BTG2 expression in estrogen receptor-alpha (ERalpha)-positive breast tumors correlated significantly with increased histologic grade and tumor size. Consistent with its ability to suppress cyclin D1 transcription, loss of nuclear BTG2 expression in ER-positive breast carcinomas showed a significant correlation with cyclin D1 protein overexpression, suggesting that loss of BTG2 may be a factor involved in deregulating cyclin D1 expression in human breast cancer.

Btg2 enhances retinoic acid-induced differentiation by modulating histone H4 methylation and acetylation

Retinoic acid controls hematopoietic differentiation through the transcription factor activity of its receptors. They act on specific target genes by recruiting protein complexes that deacetylate or acetylate histones and modify chromatin status. The regulation of this process is affected by histone methyltransferases, which can inhibit or activate transcription depending on their amino acid target. We show here that retinoic acid treatment of hematopoietic cells induces the expression of BTG2. Overexpression of this protein increases RARalpha transcriptional activity and the differentiation response to retinoic acid of myeloid leukemia cells and CD34+ hematopoietic progenitors. In the absence of retinoic acid, BTG2 is present in the RARalpha transcriptional complex, together with the arginine methyltransferase PRMT1 and Sin3A. Overexpressed BTG2 increases PRMT1 participation in the RARalpha protein complex on the RARbeta promoter, a target gene model, and enhances gene-specific histone H4 arginine methylation. Upon RA treatment Sin3A, BTG2, and PRMT1 detach from RARalpha and thereafter BGT2 and PRMT1 are driven to the cytoplasm. These events prime histone H4 demethylation and acetylation. Overall, our data show that BTG2 contributes to retinoic acid activity by favoring differentiation through a gene-specific modification of histone H4 arginine methylation and acetylation levels.

Coactivation of nuclear receptors and myogenic factors induces the major BTG1 influence on muscle differentiation

The btg1 (B-cell translocation gene 1) gene coding sequence was isolated from a translocation break point in a case of B-cell chronic lymphocytic leukaemia. We have already shown that BTG1, considered as an antiproliferative protein, strongly stimulates myoblast differentiation. However, the mechanisms involved in this influence remained unknown. In cultured myoblasts, we found that BTG1 stimulates the transcriptional activity of nuclear receptors (T3 and all-trans retinoic acid receptors but not RXRalpha and PPARgamma), c-Jun and myogenic factors (CMD1, Myf5, myogenin). Immunoprecipitation experiments performed in cells or using in vitro-synthesized proteins and GST pull-down assays established that BTG1 directly interacts with T3 and all-trans retinoic acid receptors and with avian MyoD (CMD1). These interactions are mediated by the transactivation domain of each transcription factor and the A box and C-terminal part of BTG1. NCoR presence induces the ligand dependency of the interaction with nuclear receptors. Lastly, deletion of BTG1 interacting domains abrogates its ability to stimulate nuclear receptors and CMD1 activity, and its myogenic influence. In conclusion, BTG1 is a novel important coactivator involved in the regulation of myoblast differentiation. It not only stimulates the activity of myogenic factors, but also of nuclear receptors already known as positive myogenic regulators.

Nuclear localization of Tob is important for regulation of its antiproliferative activity

TOB: is a member of an antiproliferative gene family that includes btg1, pc3/tis21/btg2, pc3b, ana/btg3, and tob2. Exogenous overexpression of the family proteins suppresses cell proliferation. These proteins participate in transcriptional regulation of several genes. Here, we show that Tob is a nuclear protein that is imported into the nucleus through a nuclear localization signal (NLS)-mediated mechanism. Mutation in the NLS sequence of Tob affects its nuclear localization and impairs antiproliferative activity. Additionally, Tob contains a nuclear export signal (NES). In oncogenic ErbB2-transformed cells, nuclear export of Tob is facilitated by NES-mediated mechanism, resulting in decrease of its antiproliferative activity. These results indicate that regulation of nuclear localization of Tob is important for its antiproliferative activity.

FoxO3a regulates erythroid differentiation and induces BTG1, an activator of protein arginine methyl transferase 1

Erythropoiesis requires tight control of expansion, maturation, and survival of erythroid progenitors. Because activation of phosphatidylinositol-3-kinase (PI3K) is required for erythropoietin/stem cell factor-induced expansion of erythroid progenitors, we examined the role of the PI3K-controlled Forkhead box, class O (FoxO) subfamily of Forkhead transcription factors. FoxO3a expression and nuclear accumulation increased during erythroid differentiation, whereas untimely induction of FoxO3a activity accelerated differentiation of erythroid progenitors to erythrocytes. We identified B cell translocation gene 1 (BTG1)/antiproliferative protein 2 as a FoxO3a target gene in erythroid progenitors. Promoter studies indicated BTG1 as a direct target of FoxO3a. Expression of BTG1 in primary mouse bone marrow cells blocked the outgrowth of erythroid colonies, which required a domain of BTG1 that binds protein arginine methyl transferase 1. During erythroid differentiation, increased arginine methylation coincided with BTG1 expression. Concordantly, inhibition of methyl transferase activity blocked erythroid maturation without affecting expansion of progenitor cells. We propose FoxO3a-controlled expression of BTG1 and subsequent regulation of protein arginine methyl transferase activity as a novel mechanism controlling erythroid expansion and differentiation.