Regulation of differentiation by HBP1, a target of the retinoblastoma protein

Longitudinal Analysis of Human Pancreatic Adenocarcinoma Development Reveals Transient Gene Expression Signatures

Previous transcriptome studies of human pancreatic ductal adenocarcinoma (PDAC) compare non-cancerous pancreatic intraepithelial neoplasias (PanIN) with late-stage PDAC obtained from different patients, thus have limited ability to discern network dynamics that contribute to the disease progression. We demonstrated previously that the 10-22 cell line, an induced pluripotent stem cell-like line reprogrammed from late-stage human PDAC cells, recapitulated the progression from PanINs to PDAC upon transplantation into NOD/LtSz-scid/IL2R-gamma^null mice. Herein, we investigated the transition from precursor to PDAC using the isogenic model. We analyzed transcriptomes of genetically tagged 10-22 cells progressing from PanINs to PDAC in mice and validated the results using The Cancer Genome Atlas PDAC dataset, human clinical PanIN and PDAC tissues, and a well-established murine PDAC model. We functionally studied candidate proteins using human normal (H6C7) and cancerous (Miapaca2, Aspc1) pancreatic ductal epithelial cell lines. 10-22 cell-derived PDAC displayed the molecular signature of clinical human PDAC. Expression changes of many genes were transient during PDAC progression. Pathways for extracellular vesicle transport and neuronal cell differentiation were derepressed in the progression of PanINs to PDAC. HMG-box transcription factor 1 (HBP1) and BTB domain and CNC homolog 1 (BACH1) were implicated in regulating dynamically expressed genes during PDAC progression, and their expressions inversely correlated with PDAC patients' prognosis. Ectopic expression of HBP1 increased proliferation and migration of normal and cancerous pancreatic cells, indicating that HBP1 may confer the cell dissemination capacity in early PDAC progression. This unique longitudinal analysis provides insights into networks underlying human PDAC progression and pathogenesis. IMPLICATIONS: Manipulation of HBP1, BACH1, and RUN3 networks during PDAC progression can be harnessed to develop new targets for treating PDAC.

Regulatory role of transcription factor HBP1 in anticancer efficacy of EGFR inhibitor erlotinib in HNSCC

BACKGROUND

Epidermal growth factor receptor (EGFR) is often hyperactivated in head and neck squamous cell carcinoma (HNSCC); however, its downstream mediators are not fully identified. Here, we investigate the role of transcription factor HBP1 in the anticancer efficacy of EGFR inhibitor erlotinib in HNSCC.

METHODS

The effect of erlotinib and HBP1 on cell proliferation and invasion was examined by flow cytometric analysis and a Matrigel invasion assay, respectively. Oral tumor specimens were used to evaluate the association between the expression level of EGFR and HBP1, and metastatic potential.

RESULTS

Erlotinib caused cell growth arrest in the G1 phase and sluggish invasion with a concomitant increase in HBP1 and p27 expression. The erlotinib effect was attenuated upon HBP1 knockdown. Analysis of oral tumor specimens revealed that the low HBP1/high EGFR status can predict metastatic potential.

CONCLUSIONS

Our data support HBP1 as a crucial mediator of EGFR-targeting inhibitors in HNSCC.

The HMG box transcription factor HBP1: a cell cycle inhibitor at the crossroads of cancer signaling pathways

HMG box protein 1 (HBP1) is a transcription factor and a potent cell cycle inhibitor in normal and cancer cells. HBP1 activates or represses the expression of different cell cycle genes (such as CDKN2A, CDKN1A, and CCND1) through direct DNA binding, cofactor recruitment, chromatin remodeling, or neutralization of other transcription factors. Among these are LEF1, TCF4, and MYC in the WNT/beta-catenin pathway. HBP1 also contributes to oncogenic RAS-induced senescence and terminal cell differentiation. Collectively, these activities suggest a tumor suppressor function. However, HBP1 is not listed among frequently mutated cancer driver genes. Nevertheless, HBP1 expression is lower in several tumor types relative to matched normal tissues. Several micro-RNAs, such as miR-155, miR-17-92, and miR-29a, dampen HBP1 expression in cancer cells of various origins. The phosphatidylinositol-3 kinase (PI3K)/AKT pathway also inhibits HBP1 transcription by preventing FOXO binding to the HBP1 promoter. In addition, AKT directly phosphorylates HBP1, thereby inhibiting its transcriptional activity. Taken together, these findings place HBP1 at the center of a network of micro-RNAs and oncoproteins that control cell proliferation. In this review, we discuss our current understanding of HBP1 function in human physiology and diseases.

Matrix metalloproteinase-13 is a target gene of high-mobility group box-containing protein 1 in modulating oral cancer cell invasion

Transcription factor high-mobility group box-containing protein 1 (HBP1) may function as a tumor suppressor in various types of cancer. In a previous study, we demonstrated that HBP1 suppressed cell invasion in oral cancer. To further understand the underlying mechanism, the current study is aimed at investigating how HBP1 exerts its antimetastatic potential in oral cancer. In a cell model, ectopic expression of HBP1 potently suppressed epithelial-mesenchymal transition, cellular migration, and invasion; conversely, HBP1 knockdown promoted these malignant phenotypes. The matrix metalloproteinase (MMP) family is highly implicated in tumor metastasis. Therefore, we examined the effect of HBP1 on the activation of the MMP members, MMP-2, -9, and -13 that are highly associated with the aggressiveness of oral cancer. Ectopic expression of HBP1 resulted in a mild reduction in the expression and activity of MMP-2 and -9, yet it had a potent inhibitory effect on MMP-13. In contrast, HBP1 knockdown strongly enhanced the activation of MMP-13. Further, we demonstrated that MMP-13 is a target of HBP1 transcription repression as evidenced by the identification of an HBP1 binding site in the cis proximal region of the MMP-13 promoter. More important, MMP-13 knockdown significantly alleviated HBP1 small interfering RNA-mediated promotion in cell invasion. Analysis of oral tumor specimens revealed that the low HBP1 (<0.3-fold)/high MMP-13 (>3-fold) status was associated with metastatic potential. All told, our study provides evidence supporting the idea that the HBP1-MMP-13 axis is a key regulator of the aggressiveness in oral cancer.

Transcriptional regulation of metabolism in disease: From transcription factors to epigenetics

Every cell in an individual has largely the same genomic sequence and yet cells in different tissues can present widely different phenotypes. This variation arises because each cell expresses a specific subset of genomic instructions. Control over which instructions, or genes, are expressed is largely controlled by transcriptional regulatory pathways. Each cell must assimilate a huge amount of environmental input, and thus it is of no surprise that transcription is regulated by many intertwining mechanisms. This large regulatory landscape means there are ample possibilities for problems to arise, which in a medical context means the development of disease states. Metabolism within the cell, and more broadly, affects and is affected by transcriptional regulation. Metabolism can therefore contribute to improper transcriptional programming, or pathogenic metabolism can be the result of transcriptional dysregulation. Here, we discuss the established and emerging mechanisms for controling transcription and how they affect metabolism in the context of pathogenesis. Cis- and trans-regulatory elements, microRNA and epigenetic mechanisms such as DNA and histone methylation, all have input into what genes are transcribed. Each has also been implicated in diseases such as metabolic syndrome, various forms of diabetes, and cancer. In this review, we discuss the current understanding of these areas and highlight some natural models that may inspire future therapeutics.

High mobility group box transcription factor 1 (HBP1) from Lampetra japonica affects cell cycle regulation

High mobility group (HMG) box-containing protein 1 (HBP1) is a member of the HMG family of chromosomal proteins. Previous studies have shown that human HBP1 exhibits tumor-suppressor activity. Here, we identified a homologue of HBP1, L-hbp1, in Lampetra japonica. The L-hbp1 gene shared high sequence similarity with its homologues in jawed vertebrates, as shown by bioinformatics analyses. L-hbp1 contains a 1,584-bp open reading frame that encodes 527 amino acids. A pAdenox-L-HBP1 plasmid was constructed and transfected successfully in Raji cells, as revealed by real-time PCR. The overexpression of L-HBP1 reduced cell growth rates, inhibited G1 phase progression, decreased cyclin D1 and c-Myc protein expression, and increased p53 protein expression. Western blot and immunohistochemical assays showed that L-HBP1 was primarily distributed in the heart, kidney, gill and liver of lamprey. Cell cycle analysis revealed that decreased L-HBP1 expression in HBP1 morpholino oligonucleotide-transfected lamprey cells resulted in a decreased fraction of cells in the G1 phase and corresponding increases in the S and G2/M phases. Additionally, treatment of lamprey cardiac cells with pharmacological inhibitors of p38 MAP kinase released the cells from G1 arrest. Together, these results indicated that HBP1 expression in lamprey was correlated with the onset of mitotic arrest in these cells, which have implications for cell cycle regulation.

HBP1 phosphorylation by AKT regulates its transcriptional activity and glioblastoma cell proliferation

The HMG-box protein 1 (HBP1) is a transcriptional regulator and a potential tumor suppressor that controls cell proliferation, differentiation and oncogene-mediated senescence. In a previous study, we showed that AKT activation through the PI3K/AKT/FOXO pathway represses HBP1 expression at the transcriptional level in human fibroblasts as well as in cancer cell lines. In the present study, we investigated whether AKT could also regulate HBP1 directly. First, AKT1 phosphorylated recombinant human HBP1 in vitro on three conserved sites, Ser380, Thr484 and Ser509. In living cells, we confirmed the phosphorylation of HBP1 on residues 380 and 509 using phospho-specific antibodies. HBP1 phosphorylation was induced by growth factors, such as EGF or IGF-1, which activated AKT. Conversely, it was blocked by treatment of cells with an AKT inhibitor (MK-2206) or by AKT knockdown. Next, we observed that HBP1 transcriptional activity was strongly modified by mutating its phosphorylation sites. The regulation of target genes such as DNMT1, P47phox, p16^INK4A and cyclin D1 was also affected. HBP1 had previously been shown to limit glioma cell growth. Accordingly, HBP1 silencing by small-hairpin RNA increased human glioblastoma cell proliferation. Conversely, HBP1 overexpression decreased cell growth and foci formation. This effect was amplified by mutations that prevented phosphorylation by AKT, and blunted by mutations that mimicked phosphorylation. In conclusion, our results suggest that HBP1 phosphorylation by AKT blocks its functions as transcriptional regulator and tumor suppressor.

HMG-box transcription factor 1: a positive regulator of the G1/S transition through the Cyclin-CDK-CDKI molecular network in nasopharyngeal carcinoma

HMG-box transcription factor 1 (HBP1) has been reported to be a tumor suppressor in diverse malignant carcinomas. However, our findings provide a conclusion that HBP1 plays a novel role in facilitating nasopharyngeal carcinoma (NPC) growth. The Kaplan-Meier analysis indicates that high expression HBP1 and low miR-29c expression both are negatively correlated with the overall survival rates of NPC patients. HBP1 knockdown inhibits cellular proliferation and growth, and arrested cells in G1 phase rather than affected cell apoptosis via flow cytometry (FCM) analysis. Mechanistically, HBP1 induces the expression of CCND1 and CCND3 levels by binding to their promoters, and binds to CDK4, CDK6 and p16^INK4A promoters while not affects their expression levels. CCND1 and CCND3 promote CCND1-CDK4, CCND3-CDK6, and CDK2-CCNE1 complex formation, thus, E2F-1 and DP-1 are activated to accelerate the G1/S transition in the cell cycle. MiR-29c is down-regulated and correlated with NPC tumorigenesis and progression. Luciferase assays confirms that miR-29c binds to the 3' untranslated region (3'-UTR) of HBP1. Introduction of pre-miR-29c decreased HBP1 mRNA and protein levels. Therefore, the high endogenous HBP1 expression might be attributed to the low levels of endogenous miR-29c in NPC. In addition, HBP1 knockdown and miR-29c agomir administration both decrease xenograft growth in nude mice in vivo. It is firstly reported that HBP1 knockdown inhibited the proliferation and metastasis of NPC, which indicates that HBP1 functions as a non-tumor suppressor gene in NPC. This study provides a novel potential target for the prevention of and therapies for NPC.

Transcription factor HMG box-containing protein 1 (HBP1) modulates mitotic clonal expansion (MCE) during adipocyte differentiation

Transcription factor HMG box-containing protein 1 (HBP1) has been found to be up-regulated in rat adipose tissue and differentiated preadipocyte; however, how HBP1 is involved in adipocyte formation remains unclear. In the present study, we demonstrated that under a standard differentiation protocol HBP1 expression fluctuates with down-regulation in the mitotic clonal expansion (MCE) stage followed by up-regulation in the terminal differentiation stage in both 3T3-L1 and MEF cell models. Also, HBP1 knockdown accelerated cell cycle progression in the MCE stage, but it impaired final adipogenesis. To gain further insight into the role of HBP1 in the MCE stage, we found that the HBP1 expression pattern is reciprocal to that of C/EBPβ, and ectopic expression of HBP1suppresses C/EBPβ expression. These data indicate that HBP1 functions as a negative regulator of MCE. In contrast, when HBP1 expression was gradually elevated along with a concomitant induction of C/EBPα at the end of the MCE, HBP1 knockdown leads to a significant reduction of C/EBPα expression, suggesting that HBP1-mediated C/EBPα expression may be needed for the termination of the cell cycle at the end of MCE for terminal differentiation. All told, our findings show that HBP1 is a key transcription factor in the already complicated regulatory cascade during adipocyte differentiation.

Transcription factor HBP1 is a direct anti-cancer target of transcription factor FOXO1 in invasive oral cancer

Either FOXO1 or HBP1 transcription factor is a downstream effector of the PI3K/Akt pathway and associated with tumorigenesis. However, the relationship between FOXO1 and HBP1 in oral cancer remains unclear. Analysis of 30 oral tumor specimens revealed that mean mRNA levels of both FOXO1 and HBP1 in non-invasive and invasive oral tumors were found to be significantly lower than that of the control tissues, and the status of low FOXO1 and HBP1 (< 0.3 fold of the control) was associated with invasiveness of oral tumors. To investigate if HBP1 is a direct transcription target of FOXO1, we searched potential FOXO1 binding sites in the HBP1 promoter using the MAPPER Search Engine, and two putative FOXO1 binding sites located in the HBP1 promoter –132 to –125 bp and –343 to –336 bp were predicted. These binding sites were then confirmed by both reporter gene assays and the in cellulo ChIP assay. In addition, Akt activity manipulated by PI3K inhibitor LY294002 or Akt mutants was shown to negatively affect FOXO1-mediated HBP1 promoter activation and gene expression. Last, the biological significance of the FOXO1-HBP1 axis in oral cancer malignancy was evaluated in cell growth, colony formation, and invasiveness. The results indicated that HBP1 knockdown potently promoted malignant phenotypes of oral cancer and the suppressive effect of FOXO1 on cell growth, colony formation, and invasion was alleviated upon HBP1 knockdown in invasive oral cancer cells. Taken together, our data provide evidence for HBP1 as a direct downstream target of FOXO1 in oral cancer malignancy.

Identification of transcription factors potentially involved in human adipogenesis in vitro

BACKGROUND

Increased adiposity in humans leads to obesity, which is a major risk factor for cardiovascular disease, type 2 diabetes, and cancer. We previously conducted an extensive unbiased in vitro transcriptomic analysis of adipogenesis, using human adipose-derived stromal cells (ASCs). Here, we have applied computational methods to these data to identify transcription factors (TFs) that constitute the upstream gene regulatory networks potentially, driving adipocyte formation in human ASCs.

METHODS

We used Affymetrix Transcription Analysis Console™ v3.0 for calculating differentially expressed genes. MATCH™ and F-MATCH™ algorithms for TF identification. STRING v10 to predict protein-protein interactions between TFs.

RESULTS

A number of TFs that were reported to have a significant role in adipogenesis, as well as novel TFs that have not previously been described in this context, were identified. Thus, 32 upstream TFs were identified, with most belonging to the C2H2-type zinc finger and HOX families, which are potentially involved in regulating most of the differentially expressed genes observed during adipocyte differentiation. Furthermore, 17 important upstream TFs were found to have increased regulatory effects on their downstream target genes and were consistently up-regulated during the differentiation process. A strong hypothetical functional interaction was observed among these TFs, which supports their common role in the downstream regulation of gene expression during adipogenesis.

CONCLUSION

Our results support several previous findings on TFs involved in adipogenesis and thereby validate the comprehensive and systematic in silico approach described in this study. In silico analysis also allowed for the identification of novel regulators of adipocyte differentiation.

Mice Lacking Hbp1 Function Are Viable and Fertile

Fetal germ cell development is tightly regulated by the somatic cell environment, and is characterised by cell cycle states that differ between XY and XX gonads. In the testis, gonocytes enter G1/G0 arrest from 12.5 days post coitum (dpc) in mice and maintain cell cycle arrest until after birth. Failure to correctly maintain G1/G0 arrest can result in loss of germ cells or, conversely, germ cell tumours. High mobility group box containing transcription factor 1 (HBP1) is a transcription factor that was previously identified in fetal male germ cells at the time of embryonic cell cycle arrest. In somatic cells, HBP1 is classified as a tumour suppressor protein, known to regulate proliferation and senescence. We therefore investigated the possible role of HBP1 in the initiation and maintenance of fetal germ cell G1/G0 arrest using the mouse model. We identified two splice variants of Hbp1, both of which are expressed in XY and XX fetal gonads, but only one of which is localised to the nucleus in in vitro assays. To investigate Hbp1 loss of function, we used embryonic stem (ES) cells carrying a Genetrap mutation for Hbp1 to generate mice lacking Hbp1 function. We found that Hbp1-genetrap mouse mutant germ cells proliferated correctly throughout development, and adult males were viable and fertile. Multiple Hbp1-LacZ reporter mouse lines were generated, unexpectedly revealing Hbp1 embryonic expression in hair follicles, eye and limbs. Lastly, in a model of defective germ cell G1/G0 arrest, the Rb1-knockout model, we found no evidence for Hbp1 mis-regulation, suggesting that the reported RB1-HBP1 interaction is not critical in the germline, despite co-expression.

Hbp1 regulates the timing of neuronal differentiation during cortical development by controlling cell cycle progression

In the developing mammalian brain, neural stem cells (NSCs) initially expand the progenitor pool by symmetric divisions. NSCs then shift from symmetric to asymmetric division and commence neurogenesis. Although the precise mechanisms regulating the developmental timing of this transition have not been fully elucidated, gradual elongation in the length of the cell cycle and coinciding accumulation of determinants that promote neuronal differentiation may function as a biological clock that regulates the onset of asymmetric division and neurogenesis. We conducted gene expression profiling of embryonic NSCs in the cortical regions and found that expression of high mobility group box transcription factor 1 (Hbp1) was upregulated during neurogenic stages. Induced conditional knockout mice of Hbp1 generated by crossing with Nestin-CreERT2 mice exhibited a remarkable dilatation of the telencephalic vesicles with a tangentially expanded ventricular zone and a thinner cortical plate containing reduced numbers of neurons. In these Hbp1-deficient mouse embryos, neural stem/progenitor cells continued to divide with a shorter cell cycle length. And downstream target genes of the Wnt signaling, such as cyclin D1 and c-jun, were upregulated in the germinal zone of the cortical regions. These results indicate that Hbp1 plays a critical role in regulating the timing of cortical neurogenesis by elongating the cell cycle and is essential for normal cortical development.

The expression of the tumour suppressor HBP1 is down-regulated by growth factors via the PI3K/PKB/FOXO pathway

Growth factors inactivate the FOXO (forkhead box O) transcription factors through PI3K (phosphoinositide 3-kinase) and PKB (protein kinase B). By comparing microarray data from multiple model systems, we identified HBP1 (high-mobility group-box protein 1) as a novel downstream target of this pathway. HBP1 mRNA was down-regulated by PDGF (platelet-derived growth factor), FGF (fibroblast growth factor), PI3K and PKB, whereas it was up-regulated by FOXO factors. This observation was confirmed in human and murine fibroblasts as well as in cell lines derived from leukaemia, breast adenocarcinoma and colon carcinoma. Bioinformatics analysis led to the identification of a conserved consensus FOXO-binding site in the HBP1 promoter. By luciferase activity assay and ChIP, we demonstrated that FOXO bound to this site and regulated the HBP1 promoter activity in a PI3K-dependent manner. Silencing of HBP1 by shRNA increased the proliferation of human fibroblasts in response to growth factors, suggesting that HBP1 limits cell growth. Finally, by analysing a transcriptomics dataset from The Cancer Genome Atlas, we observed that HBP1 expression was lower in breast tumours that had lost FOXO expression. In conclusion, HBP1 is a novel target of the PI3K/FOXO pathway and controls cell proliferation in response to growth factors.

Mitogen-activated protein kinase p38 and retinoblastoma protein signalling is required for DNA damage-mediated formation of senescence-associated heterochromatic foci in tumour cells

DNA-damaging agents are able to induce irreversible cell growth arrest and senescence in some types of tumour cells, thus contributing to the static feature of cancer. However, senescent tumour cells may re-enter the cell cycle, leading to tumour relapse. Understanding the mechanisms that control the viability of senescent cells may be critical for tumour suppression. Primary human fibroblasts undergoing oncogene-induced or replicative senescence are known to form senescence-associated heterochromatin foci (SAHF), which contribute to the stability of the senescent state. However, it is unclear whether SAHF formation is universal in tumour cells. We report that the DNA-damaging agents doxorubicin and 7-ethyl-10-hydroxycamptothecin were able to induce the formation of SAHF in some tumour cell types, and this induction was accompanied by activation of the retinoblastoma protein pathway. By contrast, tumour cells in which the retinoblastoma protein pathway could not be activated by doxorubicin or 7-ethyl-10-hydroxycamptothecin failed to form SAHF. In parallel, tumour cells with deficient retinoblastoma protein were also unable to form SAHF. In addition, we show that the mitogen-activated protein kinase p38 pathway was involved in tumour cell SAHF formation in response to doxorubicin and 7-ethyl-10-hydroxycamptothecin. Furthermore, HMG box transcription factor 1 (HBP1), a downstream target of the mitogen-activated protein kinase p38-mediated senescence pathway, was required for SAHF formation. Taken together, the results of the present study highlight the roles of the mitogen-activated protein kinase p38/retinoblastoma protein pathway in tumour cell SAHF formation in response to DNA-damaging agents, and provide new insights into the mechanisms of DNA damage-mediated tumour suppression.

HBP1-mediated transcriptional regulation of DNA methyltransferase 1 and its impact on cell senescence

The activity of DNA methyltransferase 1 (DNMT1) is associated with diverse biological activities, including cell proliferation, senescence, and cancer development. In this study, we demonstrated that the HMG box-containing protein 1 (HBP1) transcription factor is a new repressor of DNMT1 in a complex mechanism during senescence. The DNMT1 gene contains an HBP1-binding site at bp -115 to -134 from the transcriptional start site. HBP1 repressed the endogenous DNMT1 gene through sequence-specific binding, resulting in both gene-specific (e.g., p16(INK4)) and global DNA hypomethylation changes. The HBP1-mediated repression by DNMT1 contributed to replicative and premature senescence, the latter of which could be induced by Ras and HBP1 itself. A detailed investigation unexpectedly revealed that HBP1 has dual and complex transcriptional functions, both of which contribute to premature senescence. HBP1 both repressed the DNMT1 gene and activated the p16 gene in premature senescence. The opposite transcriptional functions proceeded through different DNA sequences and differential protein acetylation. While intricate, the reciprocal partnership between HBP1 and DNMT1 has exceptional importance, since its abrogation compromises senescence and promotes tumorigenesis. Together, our results suggest that the HBP1 transcription factor orchestrates a complex regulation of key genes during cellular senescence, with an impact on overall DNA methylation state.

N-acetylcysteine (NAC) inhibits cell growth by mediating the EGFR/Akt/HMG box-containing protein 1 (HBP1) signaling pathway in invasive oral cancer

OBJECTIVES

Overexpression of the epidermal growth factor (EGF) receptor (EGFR) gene in the squamous cell carcinomas of the head and neck (SCCHN) is often associated with inauspicious prognosis and poor survival. N-acetylcysteine (NAC), a compound from some vegetables and allium species, appears anti-tumorigenesis, but the underlying mechanism is unclear. The objective of this study is to investigate the role of NAC in EGFR-overexpressing oral cancer.

MATERIALS AND METHODS

Both HSC-3 and SCC-4 human tongue squamous carcinoma cell lines and an HSC-3 xenograft mouse model were used to test the anti-growth efficacy of NAC in vitro and in vivo, respectively.

RESULTS

NAC treatment suppressed cell growth, with concomitantly increased expression of HMG box-containing protein 1 (HBP1), a transcription suppressor, and decreased EGFR/Akt activation, in EGFR-overexpressing HSC-3 oral cancer cells. HBP1 knockdown attenuated the growth arrest and apoptosis induced by NAC. Lastly, NAC and AG1478, an EGFR inhibitor, additively suppressed colony formation in HSC-3 cells.

CONCLUSION

Taken together, our data indicate that NAC exerts its growth-inhibitory function through modulating EGFR/Akt signaling and HBP1 expression in EGFR-overexpressing oral cancer.

The acetylation of transcription factor HBP1 by p300/CBP enhances p16INK4A expression

HBP1 is a sequence-specific DNA-binding transcription factor with many important biological roles. It activates or represses the expression of some specific genes during cell growth and differentiation. Previous studies have exhibited that HBP1 binds to p16^INK4A promoter and activates p16^INK4A expression. We found that trichostatin A (TSA), an inhibitor of HDAC (histone deacetylase), induces p16^INK4A expression in an HBP1-dependent manner. This result was drawn from a transactivation experiment by measuring relative luciferase activities of p16^INK4A promoter with HBP1-binding site in comparison with that of the wild-type p16^INK4A promoter by transient cotransfection with HBP1 into HEK293T cells and 2BS cells. HBP1 acetylation after TSA treatment was confirmed by immunoprecipitation assay. Our data showed that HBP1 interacted with histone acetyltransferase p300 and CREB-binding protein (CBP) and also recruited p300/CBP to p16^INK4A promoter. HBP1 was acetylated by p300/CBP in two regions: repression domain (K297/305/307) and P domain (K171/419). Acetylation of Repression domain was not required for HBP1 transactivation on p16^INK4A. However, luciferase assay and western blotting results indicate that acetylation of P domain, especially K419 acetylation is essential for HBP1 transactivation on p16^INK4A. As assayed by SA-beta-gal staining, the acetylation of HBP1 at K419 enhanced HBP1-induced premature senescence in 2BS cells. In addition, HDAC4 repressed HBP1-induced premature senescence through permanently deacetylating HBP1. We conclude that our data suggest that HBP1 acetylation at K419 plays an important role in HBP1-induced p16^INK4A expression.

Mouse germ cell development: from specification to sex determination

Primordial germ cells (PGCs) are embryonic progenitors for the gametes. In the gastrulating mouse embryo, a small group of cells begin expressing a unique set of genes and so commit to the germline. Over the next 3-5 days, these PGCs migrate anteriorly and increase rapidly in number via mitotic division before colonizing the newly formed gonads. PGCs then express a different set of unique genes, their inherited epigenetic imprint is erased and an individual methylation imprint is established, and for female PGCs, the silent X chromosome is reactivated. At this point, germ cells (GCs) commit to either a female or male sexual lineage, denoted by meiosis entry and mitotic arrest, respectively. This developmental program is determined by cues emanating from the somatic environment.

Transcriptional factor HBP1 targets P16(INK4A), upregulating its expression and consequently is involved in Ras-induced premature senescence

Oncogene-mediated premature senescence has emerged as a potential tumor-suppressive mechanism in early cancer transitions. Many studies showed that Ras and p38 mitogen-activated protein kinase (MAPK) participate in premature senescence. Our previous work indicated that the HMG box-containing protein 1 (HBP1) transcription factor is involved in Ras- and p38 MAPK-induced premature senescence, but the mechanism of which has not yet been identified. Here, we showed that the p16(INK4A) cyclin-dependent kinase inhibitor is a novel target of HBP1 participating in Ras-induced premature senescence. The promoter of the p16(INK4A) gene contains an HBP1-binding site at position -426 to -433 bp from the transcriptional start site. HBP1 regulates the expression of the endogenous p16(INK4A) gene through direct sequence-specific binding. With HBP1 expression and the subsequent increase of p16(INK4A) gene expression, Ras induces premature senescence in primary cells. The data suggest a model in which Ras and p38 MAPK signaling engage HBP1 and p16(INK4A) to trigger premature senescence. In addition, we report that HBP1 knockdown is also required for Ras-induced transformation. All the data indicate that the mechanism of HBP1-mediated transcriptional regulation is important for not only premature senescence but also tumorigenesis.

The retinoblastoma protein modulates Tbx2 functional specificity

This study demonstrates that Tbx2 binds Rb1. The interaction with Rb1 increases Tbx2 DNA-binding activity and enhances the ability of Tbx2 to repress transcription. The results show that Tbx2 regulates the expression of genes involved in cell division and DNA replication and that Rb1 modulates Tbx2 target gene recognition and specificity.

Tbx2 is a member of a large family of transcription factors defined by homology to the T-box DNA-binding domain. Tbx2 plays a key role in embryonic development, and in cancer through its capacity to suppress senescence and promote invasiveness. Despite its importance, little is known of how Tbx2 is regulated or how it achieves target gene specificity. Here we show that Tbx2 specifically associates with active hypophosphorylated retinoblastoma protein (Rb1), a known regulator of many transcription factors involved in cell cycle progression and cellular differentiation, but not with the Rb1-related proteins p107 or p130. The interaction with Rb1 maps to a domain immediately carboxy-terminal to the T-box and enhances Tbx2 DNA binding and transcriptional repression. Microarray analysis of melanoma cells expressing inducible dominant-negative Tbx2, comprising the T-box and either an intact or mutated Rb1 interaction domain, shows that Tbx2 regulates the expression of many genes involved in cell cycle control and that a mutation which disrupts the Rb1-Tbx2 interaction also affects Tbx2 target gene selectivity. Taken together, the data show that Rb1 is an important determinant of Tbx2 functional specificity.

Macrophage migration inhibitory factor is a direct target of HBP1-mediated transcriptional repression that is overexpressed in prostate cancer

Macrophage migration inhibitory factor (MIF) is a well-described proinflammatory mediator. MIF overexpression has been observed in many tumors and is implicated in oncogenic transformation and tumor progression. However, the molecular mechanisms responsible for regulating MIF expression remain poorly understood. In this study, we showed that the transcriptional repressor HBP1 (HMG box-containing protein 1) negatively regulates MIF expression. We first identified a large high-affinity HBP1 DNA-binding element at positions -811 to -792 from the transcriptional start site within the MIF promoter by computer analysis. Reporter analyses showed that this element was required for HBP1-mediated transcriptional repression. Furthermore, HBP1 associated with the MIF promoter in vivo and repressed endogenous MIF gene expression. Consistent with HBP1-mediated repression of MIF, low levels of HBP1 expression were associated with high levels of MIF expression in prostate cancer samples. Importantly, HBP1-mediated repression of MIF inhibited tumorigenic growth and invasion, and the repressive effect of HBP1 on tumorigenic growth and invasion could be partially rescued by the addition of recombinant MIF to the culture medium. Finally, prostate tumor samples with low HBP1 and high MIF expression were associated with a significant decrease in relapse-free survival. Taken together, these results indicated that HBP1 directly inhibited MIF gene transcription, and suggested that the loss of HBP1 expression or activity may contribute to the upregulation of MIF expression in prostate tumor tissue.

microRNA-29a induces aberrant self-renewal capacity in hematopoietic progenitors, biased myeloid development, and acute myeloid leukemia

The function of microRNAs (miRNAs) in hematopoietic stem cells (HSCs), committed progenitors, and leukemia stem cells (LSCs) is poorly understood. We show that miR-29a is highly expressed in HSC and down-regulated in hematopoietic progenitors. Ectopic expression of miR-29a in mouse HSC/progenitors results in acquisition of self-renewal capacity by myeloid progenitors, biased myeloid differentiation, and the development of a myeloproliferative disorder that progresses to acute myeloid leukemia (AML). miR-29a promotes progenitor proliferation by expediting G1 to S/G2 cell cycle transitions. miR-29a is overexpressed in human AML and, like human LSC, miR-29a-expressing myeloid progenitors serially transplant AML. Our data indicate that miR-29a regulates early hematopoiesis and suggest that miR-29a initiates AML by converting myeloid progenitors into self-renewing LSC.

The tumor suppressor protein HBP1 is a novel c-myc-binding protein that negatively regulates c-myc transcriptional activity

c-Myc is an important transcription factor that regulates cellular proliferation, cell growth, and differentiation. A number of transcriptional co-factors for c-Myc have been described that have binding sites within highly conserved regions of the c-Myc transactivational domain (TAD). Given the importance of the c-Myc TAD, we set out to identify new proteins that interact with this region using a yeast two-hybrid assay. HBP1 was identified in our screen as a protein that interacts with full-length c-Myc but not a c-Myc mutant lacking the TAD. HBP1 is a transcriptional repressor and has been shown to negatively regulate the cell cycle. A correlation between HBP1 under-expression and breast cancer relapse has been described, suggesting that HBP1 may be an important tumor suppressor protein. We have found that HBP1 binds c-Myc in cells, and expression of HBP1 inhibits c-Myc transactivational activity at least partly by preventing c-Myc binding to target gene promoters. c-Myc binds to the C terminus of HBP1, a region lost in some breast tumors, and some HBP1 mutants found in breast cancer weakly interact with and/or no longer negatively regulate c-Myc. This work adds to our understanding of c-Myc regulation and mechanisms of tumor suppression by HBP1.

Cyclin D1 downregulation is important for permanent cell cycle exit and initiation of differentiation induced by anchorage-deprivation in human keratinocytes

To understand the relationship between permanent cell cycle exit and differentiation the immortalized keratinocyte cell line, SIK and the squamous cell carcinoma, SCC9 were compared during differentiation induced by anchorage-deprivation. The SIK cells when placed in suspension culture promptly lost almost all ability to reinitiate growth by 2 days concomitantly expressing the differentiation specific proteins, transglutaminase (TGK) and involucrin. These cells rapidly underwent G1 cell cycle arrest with complete disappearance of phosphorylated RB. In contrast SCC9 cells neither showed TGK expression nor increase in involucrin. They decreased their colony-forming ability much more slowly, which coordinated well with a gradual decrease in phosphorylated RB, demonstrating the significant resistance to loss of colony-forming ability and cell cycle exit. In accordance, cyclin D1, a positive regulator of cyclin-dependent kinase (CDK) 4/6 which phosphorylates RB decreased drastically in anchorage deprived SIK but not in SCC9 cells. Endogenous cyclin D1 knockdown in SCC9 cells by siRNA enhanced loss of the colony-forming ability during anchorage-deprivation. Conversely enforced expression of cyclin D1 in SIK cells and in another immortalized keratinocyte cell line, HaCaT, partly prevented loss of their colony-forming abilities. Cyclin D1 overexpression antagonized Keratin 10 expression in suspended HaCaT cells. The result demonstrates the importance of cyclin D1 down regulation for proper initiation of keratinocyte differentiation.

Estradiol and the estradiol metabolite, 2-hydroxyestradiol, activate AMP-activated protein kinase in C2C12 myotubes

OBJECTIVE

Systemic loss of estradiol (E2) during menopause is associated with increased adiposity which can be prevented with E2 replacement. Rodent studies suggest that E2, or lack of, is a key mediator in menopause-related metabolic changes. We have previously demonstrated that E2 treatment produces a rapid, dose-dependent activation of AMP-activated protein kinase (AMPK) in murine skeletal muscle. Activation of AMPK is implicated in the therapeutic benefits of many insulin sensitizing agents including metformin and thiazolidinediones. Here, we expand our observations and provide novel data which demonstrate that in addition to E2, its metabolite 2-hydroxyestradiol (2-HE2), activate AMPK in C2C12 myotubes.

METHODS AND PROCEDURES

C2C12 myotubes were used to examine the effects on E2 and the by-products of its metabolism on AMPK activation.

RESULTS

Low concentrations of E2 (10 and 100 nmol/l) were found to increase AMPK phosphorylation by approximately 1.6-fold, while a higher concentration (10 micromol/l) resulted in a approximately 3.0-fold increase. In comparison to E2 treatment alone, incubation of myotubes with E2 and 1-aminobenzotriazole (ABT) (a CYP450 inhibitor that blocks metabolism of E2) caused AMPK activation to be enhanced at low E2 concentrations, but attenuated at higher concentrations. The effects of ABT suggested that one or more E2 metabolites contribute to the maximal activation of AMPK at high E2 concentrations. Indeed, the estrogen metabolite 2-HE2, but not 2-methoxyestradiol (2-ME2), directly activated AMPK in C2C12 myotubes.

DISCUSSION

We propose a model where E2, acting through its metabolite 2-HE2 and the estrogen receptors (ERs), activates AMPK in myotubes. Finally, activation is abolished when all E2 is metabolized to 2-ME2.

Alterations of the HBP1 transcriptional repressor are associated with invasive breast cancer

Invasive breast cancer has a high risk of recurrence to incurable disease and needs improved prognostic and therapeutic tools. Our work combines clinical and molecular analyses to show that the transcriptional repressor HBP1 may be a new target for invasive breast cancer. Previous work indicated that HBP1 regulated proliferation and senescence and inhibited Wnt signaling. Two of these functions have been associated with invasive breast cancer. In 76 breast tumors, we identified 10 HBP1 mutations/variants that were associated with fully invasive breast cancer. In a separate analysis, we found that a subset of invasive breast cancer specimens also had reduced HBP1 mRNA levels. These clinical correlations suggested that mutation or reduction of HBP1 occurs in invasive breast cancer and that HBP1 might regulate the proliferation and invasiveness of this breast cancer type. Analysis of the HBP1 mutants showed they were functionally defective for suppressing Wnt signaling. To test the consequences of reduced HBP1 levels, we used RNA interference to knock down HBP1 and observed increased Wnt signaling, tumorigenic proliferation, and invasiveness in cell and animal breast cancer models. Lastly, statistical analysis of a breast cancer patient database linked reduced HBP1 expression to breast cancer recurrence. In considering two-gene criteria for relapse potential, reduced expression of HBP1 and SFRP1, which is another Wnt inhibitor that was recently linked to invasive breast cancer, strikingly correlated with recurrence. Together, these data indicate that HBP1 may be a molecularly and clinically relevant regulator of breast cancer transitions that eventually lead to poor prognosis.

The HBP1 transcriptional repressor participates in RAS-induced premature senescence

Oncogene-mediated premature senescence has emerged as a potential tumor-suppressive mechanism in early cancer transitions. Previous work shows that RAS and p38 MAPK participate in premature senescence, but transcriptional effectors have not been identified. Here, we demonstrate that the HBP1 transcriptional repressor participates in RAS- and p38 MAPK-induced premature senescence. In cell lines, we had previously isolated HBP1 as a retinoblastoma (RB) target but have determined that it functions as a proliferation regulator by inhibiting oncogenic pathways as a transcriptional repressor. In primary cells, the results indicate that HBP1 is a necessary component of premature senescence by RAS and p38 MAPK. Similarly, a knockdown of WIP1 (a p38 MAPK phosphatase) induced premature senescence that also required HBP1. Furthermore, HBP1 requires regulation by RB, in which few transcriptional regulators for premature senescence have been shown. Together, the data suggest a model in which RAS and p38 MAPK signaling engage HBP1 and RB to trigger premature senescence. As an initial step toward clinical relevance, a bioinformatics approach shows that the relative expression levels of HBP1 and WIP1 correlated with decreased relapse-free survival in breast cancer patients. Together, these studies highlight p38 MAPK, HBP1, and RB as important components for a premature-senescence pathway with possible clinical relevance to breast cancer.

pRb: master of differentiation. Coupling irreversible cell cycle withdrawal with induction of muscle-specific transcription

The protein product of the retinoblastoma (RB) gene is necessary for the completion of the muscle differentiation program and for myogenic basic helix-loop-helix-dependent transcription. In fact, in addition to induction and maintenance of permanent cell cycle withdrawal through negative regulation of E2F-responsive genes involved in proliferation, pRb also plays a positive role in the activation of muscle-specific genes. In pRb-/- myocytes, the expression of late myogenic markers is defective and myoblast fusion into myotubes occurs without irreversible cell cycle exit. This evidence demonstrates only a partial functional redundancy between pRb and its relatives p107 and pRb2/p130, as these pRb-/- multinucleated cells, which display p107 levels higher than normal myotubes, respond to mitogens with cell cycle re-entry and DNA synthesis. At the molecular level, pRb myogenic functions are mediated by cooperation with MyoD, Myocyte enhancer factor 2 (MEF2), High mobility group box protein-1 (HBP1) and histone deacetylase1, affecting chromatin configuration and tissue-specific transcription, and by post-translational modification in response to intracellular signaling cascades.

Pocket protein function in melanocyte homeostasis and neoplasia

Melanoma is the most lethal of human skin cancers and its incidence is increasing worldwide [L.K. Dennis (1999). Arch. Dermatol. 135, 275; C. Garbe et al. (2000). Cancer 89, 1269]. Melanomas often metastasize early during the course of the disease and are then highly intractable to current therapeutic regimens [M.F. Demierre and G. Merlino (2004). Curr. Oncol. Rep. 6, 406]. Consequently, understanding the factors that maintain melanocyte homeostasis and prevent their neoplastic transformation into melanoma is of utmost interest from the perspective of therapeutic interdiction. This review will focus on the role of the pocket proteins (PPs), Rb1 (retinoblastoma protein), retinoblastoma-like 1 (Rbl1 also known as p107) and retinoblastoma-like 2 (Rbl2 also known as p130), in melanocyte homeostasis, with particular emphasis on their functions in the cell cycle and the DNA damage repair response. The potential mechanisms of PP deregulation in melanoma and the possibility of PP-independent pathways to melanoma development will also be considered. Finally, the role of the PP family in ultraviolet radiation (UVR)-induced melanoma and the precise contribution that each PP family member makes to melanocyte homeostasis will be discussed in the context of a number of genetically engineered mouse models.

Suppression of Wnt signaling by the green tea compound (-)-epigallocatechin 3-gallate (EGCG) in invasive breast cancer cells. Requirement of the transcriptional repressor HBP1

Genetic and biochemical de-regulation of Wnt signaling is correlated with breast and other cancers. Our goal was to identify compounds that block Wnt signaling as a first step toward investigating new strategies for suppression of invasive and other breast cancers. In a limited phytonutrient screen, EGCG ((-)-epigallocatechin 3-gallate), the major phytochemical in green tea, emerged as an intriguing candidate. Epidemiological studies have associated green tea consumption with reduced recurrence of invasive and other breast cancers. Wnt signaling was inhibited by EGCG in a dose-dependent manner in breast cancer cells. The apparent mechanism targeted the HBP1 transcriptional repressor, which we had previously characterized as a suppressor of Wnt signaling. EGCG treatment induced HBP1 transcriptional repressor levels through an increase in HBP1 mRNA stability, but not transcriptional initiation. To test functionality, DNA-based short hairpin RNA (shRNA) was used to knockdown the endogenous HBP1 gene. Consistently, the HBP1 knockdown lines had reduced sensitivity to EGCG in the suppression of Wnt signaling and of a target gene (c-MYC). Because our ongoing studies clinically link abrogation of HBP1 with invasive breast cancer, we tested if EGCG also regulated biological functions associated with de-regulated Wnt signaling and with invasive breast cancer. EGCG reduced both breast cancer cell tumorigenic proliferation and invasiveness in an HBP1-dependent manner. Together, the emerging mechanism is that EGCG blocks Wnt signaling by inducing the HBP1 transcriptional repressor and inhibits aspects of invasive breast cancer. These studies provide a framework for considering future studies in breast cancer treatment and prevention.

LEK1 protein expression in normal and dysregulated cardiomyocyte mitosis

A defining characteristic of embryonic cells is their ability to divide rapidly, even in tissues such as cardiac muscle, which cannot divide once fully differentiated. This suggests that regulators of cell division differ in embryonic and differentiated cells. LEK1 is a member of an emerging family of proteins with diverse functions but shared structural domains, including numerous leucine zippers, a nuclear localization site, and a functional Rb-binding domain. LEK1 is expressed ubiquitously in the developing mouse embryo from the earliest stages of differentiation through birth. It is absent in adult tissues, even those that maintain active cell division. We hypothesize that LEK1 is a regulator of mitosis restricted to the developing embryo and early neonate. Here, using BrdU incorporation, we show that LEK1 protein downregulation in cardiac myocytes correlates directly with cessation of DNA synthesis between neonatal days 6 and 10. In contrast, in an immortalized cardiac cell line (HL1 cells), both BrdU incorporation and LEK1 protein expression persist, and actively dividing cells express LEK1. However, BrdU incorporation can be decreased in these cells by treatment with a morpholino targeting LEK1 mRNA. These data suggest a role for LEK1 in regulating the normal embryonic cardiomyocyte cell cycle and in promoting continued mitosis in transformed, abnormally dividing cardiomyocytes.

Effects of overexpression of HBP1 upon growth and differentiation of leukemic myeloid cells

HMG-box containing protein 1 (HBP1) is a member of the high mobility group (HMG) of chromosomal proteins. Since HBP1 exhibits tumor-suppressor activity in nonmyeloid tissues, we examined the effects of ectopic overexpression of HBP1 upon the growth and differentiation of myeloid cells. We prepared transient and stable transfectants of the myeloblast cell line K562, which overexpress HBP1 mRNA and protein. HBP1 transfectants displayed slower growth in cell culture and reduced colony formation in soft agar, retardation of S-phase progression, reduced expression of cyclin D1 and D3 mRNAs and increased expression of p21 mRNA. HBP1 transfectants also underwent increased apoptosis, as demonstrated by morphology and binding of Annexin V. Fas ligand mRNA levels were increased in HBP1 transfectants, suggesting involvement of the Fas/Fas ligand pathway. HBP1 overexpression enhanced differentiation of K562 cells towards erythroid and megakaryocyte lineages, as evidenced by increased hemoglobin and CD41a expression. Overexpression of HBP1 modulated mRNA levels for myeloid-specific transcription factors C/EBPalpha, c-Myb, c-Myc, and JunB, as well as lineage-specific transcription factors PU.1, GATA-1, and RUNX1. These findings suggest that in myeloid cells HBP1 may serve as a tumor suppressor and a general differentiation inducer and may synergize with chemical differentiating agents to enhance lineage-specific differentiation.

Estrogen regulation of adiposity and fuel partitioning. Evidence of genomic and non-genomic regulation of lipogenic and oxidative pathways

Menopause is associated with increased adiposity and greater risk of metabolic disease. In the ovariectomized (OVX) rodent model of menopause, increased adiposity is prevented by estrogen (E2) replacement, reflecting both anorexigenic and potentially metabolic actions of E2. To elucidate metabolic and molecular mechanisms by which E2 regulates fat storage and fat mobilization independently of reduced energy intake, C57 BL/6 mice were ovariectomized, randomized to estrogen (OVX-E2) or control pellet implants (OVX-C), and pairfed for 40 days. E2 treatment was associated with reduced adipose mass and adipocyte size and down-regulation of lipogenic genes in adipocytes under the control of sterol-regulatory element-binding protein 1c. Adipocytes of OVX-E2 mice contained >3-fold more perilipin protein than adipocytes of pairfed control (OVX) mice, and this difference was associated with enhanced ex vivo lipolytic response to catecholamines and with greater levels of serum-free fatty acids following fasting. As in adipose tissue, E2 decreased the expression of lipogenic genes in liver and skeletal muscle. In the latter, E2 appears to promote the partitioning of free fatty acids toward oxidation and away from triglyceride storage by up-regulating the expression of peroxisome proliferation activator receptor-delta and its downstream targets and also by directly and rapidly activating AMP-activated protein kinase. Thus, novel genomic and non-genomic actions of E2 promote leanness in OVX mice independently of reduced energy intake.

The HBP1 transcriptional repressor and the p38 MAP kinase: unlikely partners in G1 regulation and tumor suppression

Mechanisms that inhibit cell cycle progression and establish growth arrest are fundamental to tumor suppression and to normal cell differentiation. A complete understanding of these mechanisms should provide new diagnostic and therapeutic targets for future clinical applications related to cancer-specific pathways. This review will focus on the HMG-box protein 1 (HBP1) transcriptional repressor and its roles in cell cycle progression and tumor suppression. The work of several labs now suggests a new pathway for inhibiting G1 progression with exciting possible implications for tumor suppression. Our recent work suggests that the two previously unassociated proteins-the HBP1 transcription factor and the p38 MAP kinase pathway-may now participate together in a G1 regulatory network. Several recent papers collectively highlight an unexpected role and connection of the p38 MAP kinase-signaling pathway in cell cycle control, senescence, and tumor suppression. Together, these initially divergent observations may provide clues into a new tumor suppressive network and spur further investigations that may contribute to new diagnostic and therapeutic targets for cancer.

HBP1 and Mad1 repressors bind the Sin3 corepressor PAH2 domain with opposite helical orientations

Recruitment of the histone deacetylase (HDAC)-associated Sin3 corepressor is an obligatory step in many eukaryotic gene silencing pathways. Here we show that HBP1, a cell cycle inhibitor and regulator of differentiation, represses transcription in a HDAC/Sin3-dependent manner by targeting the mammalian Sin3A (mSin3A) PAH2 domain. HBP1 is unrelated to the Mad1 repressor for which high-resolution structures in complex with PAH2 have been described. We show that like Mad1, the HBP1 transrepression domain binds through a helical structure to the hydrophobic cleft of mSin3A PAH2. Notably, the HBP1 helix binds PAH2 in a reversed orientation relative to Mad1 and, equally unexpectedly, this is correlated with a chain reversal of the minimal Sin3 interaction motifs. These results not only provide insights into how multiple, unrelated transcription factors recruit the same coregulator, but also have implications for how sequence similarity searches are conducted.

Retinoblastoma protein and CCAAT/enhancer-binding protein beta are required for 1,25-dihydroxyvitamin D3-induced monocytic differentiation of HL60 cells

Derivatives of vitamin D (deltanoids) are well known to have the ability to induce differentiation of a variety of malignant cells, including human leukemia cells, but the signaling pathways that lead to such an outcome are unclear. In this study we investigated the role of the retinoblastoma protein (pRb) and the CCAAT/enhancer-binding protein (C/EBP) beta in 1,25-dihydroxyvitamin D(3) (1,25D(3))-induced monocytic differentiation of human leukemia HL60 cells. It was found that in this system, pRb is up-regulated within 12 h of exposure to the inducer, and the kinetics of its increase parallel the appearance of the early markers of differentiation, CD14 and monocyte-specific esterase. The increase in pRb expression was accompanied by a similar increase in C/EBPbeta protein, and these two proteins coimmunoprecipitated, suggesting formation of a complex. Oligonucleotides antisense to pRb or C/EBPbeta (but not to C/EBPalpha) or containing the C/EBP-binding sequence ("decoys"), all inhibited 1,25D(3)-induced differentiation. Inhibition of signaling by vitamin D receptor or by mitogen-activated protein kinase (MAPK) extracellular signal-regulated kinase and c-Jun-NH(2)-terminal kinase pathways using pharmacological inhibitors ZK159222, PD98059, or SP600125, respectively, inhibited pRb and C/EBPbeta expression and differentiation in a coordinate manner. In contrast, inhibition of the p38MAPK pathway by SB202190 potentiated differentiation and the up-regulation of pRb and C/EBPbeta. We suggest that 1,25D(3) may signal monocytic differentiation of HL60 cells in a vitamin D receptor-dependent manner that includes activation of extracellular signal-regulated kinase and c-Jun-NH(2)-terminal kinase MAPK pathways, which then up-regulate pRb and C/EBPbeta expression and in turn initiate the differentiation process.

HBP1 repression of the p47phox gene: cell cycle regulation via the NADPH oxidase

Several studies have linked the production of reactive oxygen species (ROS) by the NADPH oxidase to cellular growth control. In many cases, activation of the NADPH oxidase and subsequent ROS generation is required for growth factor signaling and mitogenesis in nonimmune cells. In this study, we demonstrate that the transcriptional repressor HBP1 (HMG box-containing protein 1) regulates the gene for the p47phox regulatory subunit of the NADPH oxidase. HBP1 represses growth regulatory genes (e.g., N-Myc, c-Myc, and cyclin D1) and is an inhibitor of G(1) progression. The promoter of the p47phox gene contains six tandem high-affinity HBP1 DNA-binding elements at positions -1243 to -1318 bp from the transcriptional start site which were required for repression. Furthermore, HBP1 repressed the expression of the endogenous p47phox gene through sequence-specific binding. With HBP1 expression and the subsequent reduction in p47phox gene expression, intracellular superoxide production was correspondingly reduced. Using both the wild type and a dominant-negative mutant of HBP1, we demonstrated that the repression of superoxide production through the NADPH oxidase contributed to the observed cell cycle inhibition by HBP1. Together, these results indicate that HBP1 may contribute to the regulation of NADPH oxidase-dependent superoxide production through transcriptional repression of the p47phox gene. This study defines a transcriptional mechanism for regulating intracellular ROS levels and has implications in cell cycle regulation.

Inhibition of Lens Fiber Cell Morphogenesis by Expression of a Mutant SV40 Large T Antigen That Binds CREB-binding Protein/p300 but Not pRb

Simian virus (SV) 40 large T antigen can both induce tumors and inhibit cellular differentiation. It is not clear whether these cellular changes are synonymous, sequential, or distinct responses to the protein. T antigen is known to bind to p53, to the retinoblastoma (Rb) family of tumor suppressor proteins, and to other cellular proteins such as p300 family members. To test whether SV40 large T antigen inhibits cellular differentiation in vivo in the absence of cell cycle induction, we generated transgenic mice that express in the lens a mutant version of the early region of SV40. This mutant, which we term E107KDelta, has a deletion that eliminates synthesis of small t antigen and a point mutation (E107K) that results in loss of the ability to bind to Rb family members. At embryonic day 15.5 (E15.5), the transgenic lenses show dramatic defects in lens fiber cell differentiation. The fiber cells become post-mitotic, but do not elongate properly. The cells show a dramatic reduction in expression of their beta- and gamma-crystallins. Because CBP and p300 are co-activators for crystallin gene expression, we assayed for interactions between E107KDelta and CBP/p300. Our studies demonstrate that cellular differentiation can be inhibited by SV40 large T antigen in the absence of pRb inactivation, and that interaction of large T antigen with CBP/p300 may be enhanced by a mutation that eliminates the binding to pRb.

The transcriptional repressor HBP1 is a target of the p38 mitogen-activated protein kinase pathway in cell cycle regulation

The p38 mitogen-activated protein (MAP) kinase signaling pathway participates in both apoptosis and G1 arrest. In contrast to the established role in apoptosis, the documented induction of G1 arrest by activation of the p38 MAP kinase pathway has attracted recent attention with reports of substrates that are linked to cell cycle regulation. Here, we identify the high-mobility group box protein HBP1 transcriptional repressor as a new substrate for p38 MAP kinase. Our previous work had shown that HBP1 inhibits G1 progression in cell and animal models, and thus indicated that HBP1 could be a relevant substrate for p38 MAP kinase in cell cycle regulation. In the present work, a p38 MAP kinase docking site (amino acids [aa] 81 to 125) and a p38 MAP kinase phosphorylation site (serine 401) were identified in the HBP1 protein. Furthermore, the docking and phosphorylation sites on HBP1 were specific for p38 MAP kinase. In defining the role of p38 MAP kinase regulation, the inhibition of p38 MAP kinase activity was shown to decrease HBP1 protein levels by triggering protein instability, as manifested by a decrease in protein half-life. Consistently, a decrease in protein levels was accompanied by a decrease in overall DNA binding activity. A mutation of the p38 MAP kinase phosphorylation site at aa 401 [(S-A)401HBP1] also triggered HBP1 protein instability. While protein stability was compromised by mutation, the specific activities of (S-A)401HBP1 and of wild-type HBP1 appeared comparable for transcriptional repression. This comparison of transcription-specific activity highlighted that p38 MAP kinase regulated HBP1 protein levels but not the intrinsic activity for DNA binding or for transcriptional repression. Finally, p38 MAP kinase-mediated regulation of the HBP1 protein also contributed to the regulation of G1 progression. Together, our work supports a molecular framework in which p38 MAP kinase activity contributes to cell cycle inhibition by increasing HBP1 and other G1 inhibitory factors by regulating protein stability.

lin-35/Rb and ubc-18, an E2 ubiquitin-conjugating enzyme, function redundantly to control pharyngeal morphogenesis in C. elegans

The retinoblastoma gene product has been implicated in the regulation of multiple cellular and developmental processes, including a well-defined role in the control of cell cycle progression. The Caenorhabditis elegans retinoblastoma protein homolog, LIN-35, is also a key regulator of cell cycle entry and, as shown by studies of synthetic multivulval genes, plays an important role in the determination of vulval cell fates. We demonstrate an additional and unexpected function for lin-35 in organ morphogenesis. Using a genetic approach to isolate lin-35 synthetic-lethal mutations, we have identified redundant roles for lin-35 and ubc-18, a gene that encodes an E2 ubiquitin-conjugating enzyme closely related to human UBCH7. lin-35 and ubc-18 cooperate to control one or more steps during pharyngeal morphogenesis. Based on genetic and phenotypic analyses, this role for lin-35 in pharyngeal morphogenesis appears to be distinct from its cell cycle-related functions. lin-35 and ubc-18 may act in concert to regulate the levels of one or more critical targets during C. elegans development.

The role of retinoblastoma protein family in the control of germ cell proliferation, differentiation and survival

Retinoblastoma family proteins pRb, p107 and p130 are differentially expressed in the rat testis. They function in specific cell types during testicular development and spermatogenesis, participating in the control of proliferation, differentiation, and survival. Their expression levels and phosphorylation status are modulated during germ cell cycle progression and apoptosis. Hyperphosphorylated states and elevated levels of p107 are correlated with cell cycle progression, whereas hypophosphorylated states and reduced levels are associated with suppression of proliferation and apoptosis in germ cells and Leydig cells. These proteins may also serve as markers of cell cycle status of germ cells during spermatogenesis.

Cell-nonautonomous function of the retinoblastoma tumour suppressor protein: new interpretations of old phenotypes

Loss of the retinoblastoma protein (pRb) induces a cell-nonautonomous defect in both erythroid and neuronal differentiation. It has previously been thought that this reflects a requirement for pRb function in cells that normally support erythropoiesis and neurogenesis, rather than in the erythrocytes or neurons themselves. However, recent studies have challenged this interpretation, and it appears that erythrocytes and neurons themselves have the intrinsic requirement for pRb function. This requirement can be bypassed by signals supplied by wild-type erythroid or neuronal cells. The existence of such a signalling mechanism has implications not only in understanding pRb function but also in the interpretation of other cell-nonautonomous phenotypes.

Retinoblastoma protein partners

Studies of the retinoblastoma gene (Rb) have shown that its protein product (pRb) acts to restrict cell proliferation, inhibit apoptosis, and promote cell differentiation. The frequent mutation of the Rb gene, and the functional inactivation of pRb in tumor cells, have spurred interest in the mechanism of pRb action. Recently, much attention has focused on pRb's role in the regulation of the E2F transcription factor. However, biochemical studies have suggested that E2F is only one of many pRb-targets and, to date, at least 110 cellular proteins have been reported to associate with pRb. The plethora of pRb-binding proteins raises several important questions. How many functions does pRb possess, which of these functions are important for development, and which contribute to tumor suppression? The goal of this review is to summarize the current literature of pRb-associated proteins.

HMG box transcriptional repressor HBP1 maintains a proliferation barrier in differentiated liver tissue

We previously isolated HBP1 as a target of the retinoblastoma (RB) and p130 family members and as the first of the HMG box transcriptional repressors. Our subsequent work demonstrated that HBP1 coordinates differentiation in cell culture models. In the present study, we show that HBP1 regulates proliferation in a differentiated tissue of an animal. Using transgenic mice in which HBP1 expression was specifically increased in hepatocytes under control of the transthyretin promoter, we determined the impact of HBP1 on synchronous cell cycle reentry following partial hepatectomy. Modest overexpression of HBP1 yielded a detectable cell cycle phenotype. Following a mitogenic stimulus induced by two-thirds partial hepatectomy, mice expressing the HBP1 transgene showed a 10- to 12-h delay in progression through G(1) to the peak of S phase. There was a concomitant delay in mid-G(1) events, such as the induction of cyclin E. While the delay in G(1) and S phases correlated with the slight overexpression of transgenic HBP1, the level of the endogenous HBP1 protein itself declined in S phase. In contrast, the onset of the immediate-early response following partial hepatectomy was unchanged in HBP1 transgenic mice. This observation indicated that the observed delay in S phase did not result from changes in signaling pathways leading into the G(0)-to-G(1) transition. Finally, transgenic mice expressing a mutant HBP1 lacking the N-terminal RB interacting domain showed a stronger S-phase response following partial hepatectomy. These results provide the first evidence that HBP1 can regulate cell cycle progression in differentiated tissues.

Negative regulation of the Wnt-beta-catenin pathway by the transcriptional repressor HBP1

In certain cancers, constitutive Wnt signaling results from mutation in one or more pathway components. The result is the accumulation and nuclear localization of beta-catenin, which interacts with the lymphoid enhancer factor-1 (LEF)/T-cell factor (TCF) family of HMG-box transcription factors, which activate important growth regulatory genes, including cyclin D1 and c-myc. As exemplified by APC and axin, the negative regulation of beta-catenin is important for tumor suppression. Another potential mode of negative regulation is transcriptional repression of cyclin D1 and other Wnt target genes. In mammals, the transcriptional repressors in the Wnt pathway are not well defined. We have previously identified HBP1 as an HMG-box repressor and a cell cycle inhibitor. Here, we show that HBP1 is a repressor of the cyclin D1 gene and inhibits the Wnt signaling pathway. The inhibition of Wnt signaling and growth requires a common domain of HBP1. The apparent mechanism is an inhibition of TCF/LEF DNA binding through a physical interaction with HBP1. These data suggest that the suppression of Wnt signaling by HBP1 may be a mechanism to prevent inappropriate proliferation.

Histone H1 diversity: bridging regulatory signals to linker histone function

Genes encoding linker histone variants have evolved to link their expression to signals controlling the proliferative capacities of cells, i.e. cycling and growth-arrested cells express distinct and specific H1 subtypes. In metazoan, these variants show a tripartite structure, with considerably divergent sequences in their amino and carboxyl terminus domains. The aim of this review is to show how specific regulatory signals control the expression of an individual H1 and to discuss the functional significance of the two variables associated with a linker histone: its primary sequence and the timing of its expression.

Cloning and expression of human HBP1, a high mobility group protein that enhances myeloperoxidase (MPO) promoter activity

Factors which regulate transcription in immature myeloid cells are of great current interest for the light they may shed upon myeloid differentiation. In the course of screening for transcription factors which interact with the human myeloperoxidase (MPO) promoter we, for the first time, identified and cloned the cDNA and genomic DNA for human HBP1 (HMG-Box containing protein 1), a member of the high mobility group of non-histone chromosomal proteins. HBP1 cDNA was initially cloned from rat brain in 1994, but its presence in human cells or in myeloid tissue had not been described previously. The sequence of human HBP1 cDNA shows 84% overall homology with the rat HBP1 cDNA sequence. We have subsequently cloned the gene, which is present as a single copy, 25 kbp in length. Northern blotting reveals a single 2.6 kb mRNA transcript which is expressed at higher levels in human myeloid and B lymphoid cell lines than in T cell lines tested and is present in several non-myeloid human cell lines. Comparison of the mRNA and genomic sequences reveals the gene to contain 10 exons and 9 introns. The sequence of human HBP1 mRNA contains a single open reading frame, which codes for a protein 514 amino acids in length. The amino acid sequence specified by the coding region shows 95% homology with the rat HBP1 protein. The human protein sequence exhibits a putative DNA-binding domain similar to that seen in rat HBP1 and shows homology with the activation and repressor domains previously demonstrated in the rat protein. We have expressed human HBP1 protein both in vitro and in prokaryotic and eukaryotic cells. The expressed fusion protein binds to a sequence in a functionally important region within the basal human MPO promoter. In transient co-transfection experiments HBP1 enhances MPO promoter activity. Human HBP1 appears to be a novel transcription factor which is likely to play an important role in regulating transcription in developing myeloid cells.