Differential regulation of a multipromoter gene. Selective 12-O-tetradecanoylphorbol-13-acetate induction of a single transcription start site in the HMG-I/Y gene

High Mobility Group A (HMGA): Chromatin Nodes Controlled by a Knotty miRNA Network

High mobility group A (HMGA) proteins are oncofoetal chromatin architectural factors that are widely involved in regulating gene expression. These proteins are unique, because they are highly expressed in embryonic and cancer cells, where they play a relevant role in cell proliferation, stemness, and the acquisition of aggressive tumour traits, i.e., motility, invasiveness, and metastatic properties. The HMGA protein expression levels and activities are controlled by a connected set of events at the transcriptional, post-transcriptional, and post-translational levels. In fact, microRNA (miRNA)-mediated RNA stability is the most-studied mechanism of HMGA protein expression modulation. In this review, we contribute to a comprehensive overview of HMGA-targeting miRNAs; we provide detailed information regarding HMGA gene structural organization and a comprehensive evaluation and description of HMGA-targeting miRNAs, while focusing on those that are widely involved in HMGA regulation; and, we aim to offer insights into HMGA-miRNA mutual cross-talk from a functional and cancer-related perspective, highlighting possible clinical implications.

AP-1 Signaling by Fra-1 Directly Regulates HMGA1 Oncogene Transcription in Triple-Negative Breast Cancers

The architectural chromatin protein HMGA1 and the transcription factor Fra-1 are both overexpressed in aggressive triple-negative breast cancers (TNBC), where they both favor epithelial-to-mesenchymal transition, invasion, and metastasis. We therefore explored the possibility that Fra-1 might be involved in enhanced transcription of the HMGA1 gene in TNBCs by exploiting cancer transcriptome datasets and resorting to functional studies combining RNA interference, mRNA and transcriptional run-on assays, chromatin immunoprecipitation, and chromosome conformation capture approaches in TNBC model cell lines. Our bioinformatic analysis indicated that Fra-1 and HMGA1 expressions positively correlate in primary samples of patients with TNBC. Our functional studies showed that Fra-1 regulates HMGA1 mRNA expression at the transcriptional level via binding to enhancer elements located in the last two introns of the gene. Although Fra-1 binding is required for p300/CBP recruitment at the enhancer domain, this recruitment did not appear essential for Fra-1-stimulated HMGA1 gene expression. Strikingly, Fra-1 binding is required for efficient recruitment of RNA Polymerase II at the HMGA1 promoter. This is permitted owing to chromatin interactions bringing about the intragenic Fra-1-binding enhancers and the gene promoter region. Fra-1 is, however, not instrumental for chromatin loop formation at the HMGA1 locus but rather exerts its transcriptional activity by exploiting chromatin interactions preexisting to its binding. IMPLICATIONS: We demonstrate that Fra-1 bound to an intragenic enhancer region is required for RNA Pol II recruitement at the HMGA1 promoter. Thereby, we provide novel insights into the mechanisms whereby Fra-1 exerts its prooncogenic transcriptional actions in the TNBC pathologic context.

Transcriptional Regulation of Glucose Metabolism: The Emerging Role of the HMGA1 Chromatin Factor

HMGA1 (high mobility group A1) is a nonhistone architectural chromosomal protein that functions mainly as a dynamic regulator of chromatin structure and gene transcription. As such, HMGA1 is involved in a variety of fundamental cellular processes, including gene expression, epigenetic regulation, cell differentiation and proliferation, as well as DNA repair. In the last years, many reports have demonstrated a role of HMGA1 in the transcriptional regulation of several genes implicated in glucose homeostasis. Initially, it was proved that HMGA1 is essential for normal expression of the insulin receptor (INSR), a critical link in insulin action and glucose homeostasis. Later, it was demonstrated that HMGA1 is also a downstream nuclear target of the INSR signaling pathway, representing a novel mediator of insulin action and function at this level. Moreover, other observations have indicated the role of HMGA1 as a positive modulator of the Forkhead box protein O1 (FoxO1), a master regulatory factor for gluconeogenesis and glycogenolysis, as well as a positive regulator of the expression of insulin and of a series of circulating proteins that are involved in glucose counterregulation, such as the insulin growth factor binding protein 1 (IGFBP1), and the retinol binding protein 4 (RBP4). Thus, several lines of evidence underscore the importance of HMGA1 in the regulation of glucose production and disposal. Consistently, lack of HMGA1 causes insulin resistance and diabetes in humans and mice, while variations in the HMGA1 gene are associated with the risk of type 2 diabetes and metabolic syndrome, two highly prevalent diseases that share insulin resistance as a common pathogenetic mechanism. This review intends to give an overview about our current knowledge on the role of HMGA1 in glucose metabolism. Although research in this field is ongoing, many aspects still remain elusive. Future directions to improve our insights into the pathophysiology of glucose homeostasis may include epigenetic studies and the use of "omics" strategies. We believe that a more comprehensive understanding of HMGA1 and its networks may reveal interesting molecular links between glucose metabolism and other biological processes, such as cell proliferation and differentiation.

Transcriptional regulation of the HMGA1 gene by octamer-binding proteins Oct-1 and Oct-2

The High-Mobility Group AT-Hook 1 (HMGA1) protein is an architectural transcription factor that binds to AT-rich sequences in the promoter region of DNA and functions as a specific cofactor for gene activation. Previously, we demonstrated that HMGA1 is a key regulator of the insulin receptor (INSR) gene and an important downstream target of the INSR signaling cascade. Moreover, from a pathogenic point of view, overexpression of HMGA1 has been associated with human cancer, whereas functional variants of the HMGA1 gene have been recently linked to type 2 diabetes mellitus and metabolic syndrome. However, despite of this biological and pathological relevance, the mechanisms that control HMGA1 gene expression remain unknown. In this study, to define the molecular mechanism(s) that regulate HMGA1 gene expression, the HMGA1 gene promoter was investigated by transient transfection of different cell lines, either before or after DNA and siRNA cotransfections. An octamer motif was identified as an important element of transcriptional regulation of this gene, the interaction of which with the octamer transcription factors Oct-1 and Oct-2 is crucial in modulating HMGA1 gene and protein expression. Additionally, we demonstrate that HMGA1 binds its own promoter and contributes to its transactivation by Oct-2 (but not Oct-1), supporting its role in an auto-regulatory circuit. Overall, our results provide insight into the transcriptional regulation of the HMGA1 gene, revealing a differential control exerted by both Oct-1 and Oct-2. Furthermore, they consistently support the hypothesis that a putative defect in Oct-1 and/or Oct-2, by affecting HMGA1 expression, may cause INSR dysfunction, leading to defects of the INSR signaling pathway.

The HMGA1 protoncogene frequently deregulated in cancer is a transcriptional target of E2F1

Reactivation of the HMGA1 protoncogene is very frequent in human cancer, but still very little is known on the molecular mechanisms leading to this event. Prompted by the finding of putative E2F binding sites in the human HMGA1 promoter and by the frequent deregulation of the RB/E2F1 pathway in human carcinogenesis, we investigated whether E2F1 might contribute to the regulation of HMGA1 gene expression. Here we report that E2F1 induces HMGA1 by interacting with a 193 bp region of the HMGA1 promoter containing an E2F binding site surrounded by three putative Sp1 binding sites. Both gain and loss of function experiments indicate that Sp1 functionally interacts with E2F1 to promote HMGA1 expression. However, while Sp1 constitutively binds HMGA1 promoter, it is the balance between different E2F family members that tunes the levels of HMGA1 expression between quiescence and proliferation. Finally, we found increased HMGA1 expression in pituitary and thyroid tumors developed in Rb(+/-) mice, supporting the hypothesis that E2F1 is a novel important regulator of HMGA1 expression and that deregulation of the RB/E2F1 path might significantly contribute to HMGA1 deregulation in cancer.

Nuclear functions of the HMG proteins

Although the three families of mammalian HMG proteins (HMGA, HMGB and HMGN) participate in many of the same nuclear processes, each family plays its own unique role in modulating chromatin structure and regulating genomic function. This review focuses on the similarities and differences in the mechanisms by which the different HMG families impact chromatin structure and influence cellular phenotype. The biological implications of having three architectural transcription factor families with complementary, but partially overlapping, nuclear functions are discussed.

Transcriptional Control of the HumanHigh Mobility Group A1Gene: Basal and Oncogenic Ras-Regulated Expression

Several studies have already shown that the high mobility group A1 (HMGA1) gene is up-regulated in most common types of cancer and immortalized tissue culture cell lines. HMGA1 expression is also much higher during embryonic development than in adult life. The elevated expression of HMGA1 in cancer thus likely occurs through oncofetal transcriptional mechanisms, which to date have not been well characterized. In the present study, we have cloned and functionally analyzed the TATA-less 5'-flanking regulatory region of human HMGA1. We identified two proximal regulatory regions that are important for basal transcription and in which specificity protein 1 (SP1) and activator protein 1 (AP1) transcription factors seem to be the regulating elements. In addition, we showed that the HMGA1 promoter is strongly inducible by oncogenic Ras, via a distal regulatory region. An AP1 site and three SP1-like sites are responsible for this inducible activity. An even more convincing finding for a role of oncogenic Ras in the regulation of HMGA1 in cancers is the discovery that HMGA1 up-regulation in the HCT116 colon cancer cell line is abolished when the mutated Ras allele is removed from these cells. Our data constitute the first extensive study of the regulation of basal and Ras-induced human HMGA1 gene expression and suggest that the elevated expression of HMGA1 in cancer cells requires, among others, a complex cooperation between SP1 family members and AP1 factors by the activation of Ras GTPase signaling.

An association of a simultaneous nuclear and cytoplasmic localization of Fra-1 with breast malignancy

Background

Overexpression of Fra-1 in fibroblasts causes anchorage-independent cell growth and oncogenic transformation. A high level of Fra-1 expression is found in various tumors and tumorigenic cell lines, suggesting that Fra-1 may be involved in malignant progression. This study aimed to investigate the significance of Fra-1 expression in breast carcinogenesis.

Methods

The expression of Fra-1 was investigated by immunohistochemistry in neoplastic breast diseases ranging from benign fibroadenoma to very aggressive undifferentiated carcinoma. The correlations of Fra-1 expression with other indicators of breast carcinoma prognosis (ER, PR and ErbB2 receptors) were analyzed.

Results

All neoplastic breast tissues, either benign or malignant breast tissues, were nuclear immunoreactive for Fra-1-recognizing antibody. The pattern of Fra-1 expression by benign neoplastic cells was predominantly nuclear. However, the nuclear/cytoplasmic concomitant immunoreactivity was observed in all types of breast carcinomas. A clear shift in Fra-1 immunoreactivity, from an exclusively nuclear to a simultaneous nuclear and cytoplasmic localization was noticed in ~90% of breast carcinomas.

Conclusion

The overall expression, pattern and intensity of Fra-1 proteins were correlated with breast oncogenesis. Overexpression of Fra-1, leading to a persistent high cytoplasmic accumulation, may play a role in the process of breast carcinogenesis.

Molecular mechanism of HMGA1 deregulation in human neuroblastoma

Very soon after their original identification in HeLa cells in 1983, HMGA proteins appeared as interesting cancer-related molecules. Indeed, they were immediately noted as a sub-class of High Mobility Group proteins induced in fibroblast or epithelial cells transformed with sarcoma viruses. After more than 20 years, the association between HMGA protein expressions and cellular transformation has been largely confirmed and HMGA are among the most widely expressed cancer-associated proteins. Nevertheless, their functional contribution to tumour development and progression is far from being completely understood. Furthermore, although HMGA1 expression has been reported to be inducible by a number of factors and circumstances, the question of how their expression is deregulated in cancer is even less clear and somehow has been ignored from most researchers. An active AP1 site is the only characterized element of the HMGA1 human promoter, that remains a rather complicated and unexplored source of information to answer this question. Following the indication that c-Myc might bind and activate the mouse HMGA1 gene promoter, we have demonstrated that HMGA1 is a new target for MYCN in human neuroblastomas. In this report, we overview part of the current information on HMGA1 and focus our attention on the analysis of its human promoter.

High Mobility Group A1 Is a Molecular Target for MYCN in Human Neuroblastoma

High mobility group A1 (HMGA1) is an architectural transcription factor and a putative protoncogene. Deregulation of its expression has been shown in most human cancers. We have previously shown that the expression of the HMGA family members is deregulated in neuroblastoma cell lines and primary tumors. On retinoic acid (RA) treatment of MYCN-amplified neuroblastoma cell lines, HMGA1 decreases with a kinetics that strictly follows MYCN repression. In addition, MYCN constitutive expression abolishes HMGA1 repression by RA. Here we explored the possibility that HMGA1 expression might be sustained by MYCN in amplified cells. Indeed, MYCN transfection induced HMGA1 expression in several neuroblastoma cell lines. HMGA1 expression increased in a transgene dose-dependent fashion in neuroblastoma-like tumors of MYCN transgenic mice. In addition, it was significantly more expressed in MYCN-amplified compared with MYCN single-copy primary human neuroblastomas. MYCN cotransfection activated a promoter/luciferase reporter containing a 1,600 bp region surrounding the first three transcription start sites of the human HMGA1 and eight imperfect E-boxes. By heterodimerizing with its partner MAX, MYCN could bind to multiple DNA fragments within the 1,600 bp. Either 5' or 3' deletion variants of the 1,600 bp promoter/luciferase reporter strongly decreased luciferase activity, suggesting that, more than a single site, the cooperative function of multiple cis-acting elements mediates direct HMGA1 transactivation by MYCN. Finally, HMGA1 repression by RNA interference reduced neuroblastoma cell proliferation, indicating that HMGA1 is a novel MYCN target gene relevant for neuroblastoma tumorigenesis.

Dominant-negative c-Jun (TAM67) target genes: HMGA1 is required for tumor promoter-induced transformation

Activation of the transcription factor AP-1 (activator protein-1) is required for tumor promotion and maintenance of malignant phenotype. A number of AP-1-regulated genes that play a role in tumor progression have been identified. However, AP-1-regulated genes driving tumor induction are yet to be defined. Previous studies have established that expression of a dominant-negative c-Jun (TAM67) inhibits phorbol 12-tetradecanoyl-13-acetate (TPA)-induced AP-1 transactivation as well as transformation in mouse epidermal JB6/P+ cells and tumor promotion in mouse skin carcinogenesis. In this study, we utilized the tumor promotion-sensitive JB6/P+ cells to identify AP-1-regulated TAM67 target genes and to establish causal significance in transformation for one target gene. A 2700 cDNA microarray was queried with RNA from TPA-treated P+ cells with or without TAM67 expression. Under conditions in which TAM expression inhibited TPA-induced transformation, microarray analysis identified a subset of six genes induced by TPA and suppressed by TAM67. One of the identified genes, the high-mobility group protein A1 (Hmga1) is induced by TPA in P+, but not in transformation-resistant P cells. We show that TPA induction of the architectural transcription factor HMGA1 is inhibited by TAM67, is extracellular-signal-regulated kinase (ERK)-activation dependent, and is mediated by AP-1. HMGA1 antisense construct transfected into P+ cells blocked HMGA1 protein expression and inhibited TPA-induced transformation indicating that HMGA1 is required for transformation. HMGA1 is not however sufficient as HMGA1a or HMGA1b overexpression did not confer transformation sensitivity on P- cells. Although HMGA1 expression is ERK dependent, it is not the only ERK-dependent event required for transformation because it does not suffice to rescue ERK-deficient P- cells. Our study shows (a) TAM 67 when it inhibits AP-1 and transformation, targets a relatively small number of genes; (b) HMGA1, a TAM67 target gene, is causally related to transformation and therefore a potentially important target for cancer prevention.

HMG-I/Y Is a c-Jun/Activator Protein-1 Target Gene and Is Necessary for c-Jun–Induced Anchorage-Independent Growth in Rat1a Cells

The transcription complex activator protein-1 (AP-1) plays a role in a diverse number of cellular processes including proliferation, differentiation, and apoptosis. To identify AP-1–responsive target genes, we used a doxycycline-inducible c-Jun system in Rat1a cells. The HMG-I/Y chromatin binding protein was found to be up-regulated by c-Jun. Following induction of c-Jun expression, Rat1a cells under nonadherent growth conditions have sustained HMG-I/Y mRNA expression and 2-fold higher protein than uninduced cells. HMG-I/Y promoter reporter assays show that HMG-I/Y promoter activity increases in the presence of c-Jun expression, and gel mobility shift assays demonstrate that induced c-Jun binds to an AP-1 consensus site at position −1,091 in the HMG-I/Y promoter. Suppression of HMG-I/Y expression by its antisense sequence significantly reduces the ability of c-Jun–overexpressing Rat1a cells to grow in an anchorage-independent fashion. HMG-I/Y transforms Rat1a cells (although the colonies are smaller than that observed for the cells overexpressing c-Jun). Taken together, these results suggest that HMG-I/Y is a direct transcriptional target of c-Jun necessary for c-Jun–induced anchorage-independent growth in Rat1a cells.

HMGI(Y) gene expression as a potential marker of thyroid follicular carcinoma

BACKGROUND

We assessed HMGI(Y) gene expression in thyroid tumors, control thyroid tissue and in the blood of patients diagnosed with papillary and follicular thyroid cancers to try to differentiate between malignant and benign disease.

METHODS

HMGI(Y) gene expression was analyzed by reverse transcriptase-polymerase chain reaction (RT-PCR) in 60 cases of thyroid tumors. Among this number 11 were diagnosed as papillary carcinoma, 37 as follicular carcinoma, and 12 as follicular adenoma. All carcinoma cases selected for this study were classified according to the tumor, lymph node metastases, distant metastases (TNM) classification.

RESULTS

HMGI(Y) gene expression was detected only in follicular carcinomas, whereas in papillary carcinomas, follicular adenomas and control tissues there was no positive reaction. In follicular carcinomas the percentage of positive cases (number of samples with presence of HMGI(Y) gene transcript) was the highest and reached approximately 84. There was no statistical dependence between the presence of HMGI(Y) gene expression and tumor size or the presence of lymph node and distant metastases. HMGI(Y) gene expression was also analyzed in whole blood taken from a selected group of patients diagnosed with papillary or follicular carcinomas. Among follicular carcinomas there were 83% of positive cases, whereas among papillary carcinomas there were only 6%.

CONCLUSIONS

On the basis of our study, we conclude that HMGI(Y) gene expression analysis could be helpful in differentiation between follicular carcinoma and adenoma.

High-mobility group A1a protein regulates Ras/ERK signaling in MCF-7 human breast cancer cells

High-mobility group (HMG) A1 proteins are gene regulatory factors whose overexpression is frequently observed in naturally occurring human cancers. The overexpression of transgenic HMGA1 proteins in cells results in neoplastic transformation and promotes progression to malignant cellular phenotypes. To understand the underlying molecular and biological events involved in these phenomena, we used oligonucleotide microarray analyses to generate an HMGA1a-induced expression profile for approximately 22,000 genes. This gene expression profile was generated using a well-characterized transgenic human MCF-7 mammary adenocarcinoma cell line in which overexpression of transgenic HMGA1 promotes a transition to a more malignant and metastatic phenotype. Microarray expression analyses, together with independent quantitative real-time reverse transcriptase polymerase chain reaction results, indicate that HMGA1a regulates genes involved in the Ras-extracellular signal-related kinase (Ras/ERK) mitogenic signaling pathway, including KIT ligand and caveolins 1 and 2. We also found that many cholesterol biosynthesis genes were decreased in cells overexpressing HMGA1a. Cholesterol depletion, decreased caveolin, and increased KIT ligand expression, are all independently associated with the activation of Ras/ERK signaling. Upon further analysis, we found that sensitivity to epidermal growth factor activation of ERK phosphorylation was significantly higher, and that cholesterol was significantly depleted, in cells overexpressing HMGA1a. The cumulative evidence indicates that one likely mechanism by which the HMGA1a protein promotes malignant changes in cells is through increased sensitivity to the activation of the Ras/ERK signaling pathway.

High-mobility group protein 1(Y): Metastasis-associated or metastasis-inducing?

Metastasis is the major cause of mortality and morbidity for patients with cancer. The high-mobility group protein 1(Y) [HMG-1(Y)] has a role in the transcription of many genes involved at different steps in the metastatic cascade and has been linked with cancer in human and animal models. This may represent a potential therapeutic target for patients. The following review summarizes and critically appraises the evidence for the role of HMG-1(Y) in metastasis.

Histone deacetylase inhibition is associated with transcriptional repression of the Hmga2 gene

The high-mobility-group A2 protein (HMGA2) plays important functional roles in transcriptional regulation, DNA replication and chromatin structure. In this study, the effect of histone deacetylase inhibition on the transcriptional activity of the Hmga2 gene was investigated in vivo both at the endogenous gene level and in a variety of cell lines using transiently transfected promoter constructs. Trichostatin A (TSA) repressed both transfected murine and human Hmga2 promoter constructs 3-8-fold in NIH3T3, F9 and HeLa cells. Steady-state Hmga2 mRNA levels in NIH3T3 cells decreased 4-5-fold following TSA treatment, while pre- treatment of NIH3T3 cells with the transcriptional inhibitor, actinomycin D, completely blocked TSA mediated repression of the Hmga2 gene. Cross-linked chromatin immunoprecipitation (X-ChIP) analysis revealed a 5-6-fold decrease in endogenous Hmga2 promoter bound Sp1 and Sp3 proteins following TSA treatment in parallel with observed loss of acetylated histone H3 and H4. In addition, the poly-pyrimidine-tract-binding protein (PTB) was observed to bind to the Hmga2 promoter in both TSA treated and untreated NIH3T3 cells. Together, these results suggest TSA treatment leads to a decrease in Hmga2 gene transcription, and a significant decrease in promoter bound Sp1, Sp3 and acetylated histones H3 and H4.

Localization, genomic organization, and alternative transcription of a novel human SAM-dependent methyltransferase gene on chromosome 2p22-->p21

As part of our studies to identify the gene responsible for hereditary gingival fibromatosis, GINGF (OMIM 135300), we have identified and cloned a novel human gene that contains the highly conserved methyltransferase domain characteristic of S-adenosylmethionine-dependent methyltransferases. We localized this gene (C2orf8 encoding 288L6 SAM-methyltransferase) to chromosome 2p22-->p21 by FISH, and sublocalized it to BAC RP11 288L6 flanked by D2S2238 and D2S2331. Computational analysis of aligned ESTs identified ten exons in the hypothetical C2orf8 gene. Results of RACE analyses in placenta identified multiple transcripts of this gene with heterogeneity at the 5'-UTR. Alternative transcription and tissue specific expression of C2orf8 were detected by RT-PCR and Northern blot analyses. C2orf8 is expressed in a variety of tissues including brain, colon, gingiva, heart, kidney, liver, lung, placenta, small intestine, spleen, and thymus. Open reading frame analysis of the alternative transcripts identified a shared coding region spanning exons 6-10. This ORF consists of 732 nucleotides encoding a putative 244 amino acid protein. Bioinformational searches of both C2orf8 and the putative protein product identified three methyltransferase motifs conserved across many prokaryotic and eukaryotic species. Sequence analyses of C2orf8 excluded coding region mutations as causative of GINGF.

Molecular biology of HMGA proteins: hubs of nuclear function

Members of the HMGA (a.k.a. HMGI/Y) family of 'high mobility group' (HMG) proteins participate in a wide variety of nuclear processes ranging from chromosome and chromatin mechanics to acting as architectural transcription factors that regulate the expression of numerous genes in vivo. As a consequence, they function in the cell as highly connected 'nodes' of protein-DNA and protein-protein interactions that influence a diverse array of normal biological processes including growth, proliferation, differentiation and death. The HMGA proteins, likewise, participate in pathological processes by, for example, acting as regulators of viral gene transcription and by serving as host-supplied proteins that facilitate retroviral integration. HMGA genes are bona fide proto-oncogenes that promote tumor progression and metastasis when overexpressed in cells. High constitutive HMGA protein levels are among the most consistent feature observed in all types of cancers with increasing concentrations being correlated with increasing malignancy. The intrinsic attributes that endow the HMGA proteins with these remarkable abilities are a combination of structural, biochemical and biological characteristics that are unique to these proteins. HMGA proteins have little, if any, secondary structure while free in solution but undergo disordered-to-ordered structural transitions when bound to substrates such as DNA or other proteins. Each protein contains three copies of a conserved DNA-binding peptide motif called the 'AT-hook' that preferentially binds to the minor groove of stretches of AT-rich sequence. In vivo HMGA proteins specifically interact with a large number of other proteins, most of which are transcription factors. They are also subject to many types of in vivo biochemical modifications that markedly influence their ability to interact with DNA substrates, other proteins and chromatin. And, most importantly, both the transcription of HMGA genes and the biochemical modifications of HMGA proteins are direct downstream targets of numerous signal transduction pathways making them exquisitely responsive to various environmental influences. This review covers recent advances that have contributed to our understanding of how this constellation of structural and biological features allows the HMGA proteins to serve as central 'hubs' of nuclear function.

Sequence and analysis of the murine Hmgiy (Hmga1) gene locus

The HMGIY non-histone proteins play important roles as architectural transcription factors that regulate gene transcription in mammalian cells and also act as host-supplied cofactors necessary for retroviral integration. The genes coding for the HMGIY proteins are proto-oncogenes, and their aberrant or over-expression is correlated with both neoplastic transformation and metastatic progression in a wide variety of tumors. Here, we report the first complete sequence of the murine Hmgiy (a.k.a. Hmga1) gene and provide a detailed comparison of this with the sequence and organization of the human HMGIY gene, including an analysis of its promoter region with the previously unreported 5' upstream region of the human gene. These analyses reveal a remarkable degree of overall sequence conservation in both the protein coding and promoter regions of the murine and human genes, including conservation of the c-Myc binding site that has been demonstrated to regulate murine Hmgiy transcription (Wood et al., 2000. Mol. Cell. Biol. 20, 5490-5502). The promoters of both genes contain other conserved transcription factor binding sites that may also represent important cis-regulatory elements. Two exons present in the 5' untranslated region of the human gene, however, are missing from the murine gene, suggesting that these two closely related mammalian species regulate transcription of their Hmgiy genes in an individualistic manner.

HMGI/Y proteins: flexible regulators of transcription and chromatin structure

The mammalian HMGI/Y (HMGA) non-histone proteins participate in a wide variety of cellular processes including regulation of inducible gene transcription, integration of retroviruses into chromosomes and the induction of neoplastic transformation and promotion of metastatic progression of cancer cells. Recent advances have contributed greatly to our understanding of how the HMGI/Y proteins participate in the molecular mechanisms underlying these biological events. All members of the HMGI/Y family of 'high mobility group' proteins are characterized by the presence of multiple copies of a conserved DNA-binding peptide motif called the 'AT hook' that preferentially binds to the narrow minor groove of stretches of AT-rich sequence. The mammalian HMGI/Y proteins have little, if any, secondary structure in solution but assume distinct conformations when bound to substrates such as DNA or other proteins. Their intrinsic flexibility allows the HMGI/Y proteins to participate in specific protein-DNA and protein-protein interactions that induce both structural changes in chromatin substrates and the formation of stereospecific complexes called 'enhanceosomes' on the promoter/enhancer regions of genes whose transcription they regulate. The formation of such regulatory complexes is characterized by reciprocal inductions of conformational changes in both the HMGI/Y proteins themselves and in their interacting substrates. It may well be that the inherent flexibility of the HMGI/Y proteins, combined with their ability to undergo reversible disordered-to-ordered structural transitions, has been a significant factor in the evolutionary selection of these proteins for their functional role(s) in cells.

Down-regulation of high mobility group-I(Y) protein contributes to the inhibition of nitric-oxide synthase 2 by transforming growth factor-beta1

The inducible isoform of nitric-oxide synthase (NOS2) catalyzes the production of nitric oxide (NO), which participates in the pathophysiology of systemic inflammatory diseases such as sepsis. NOS2 is transcriptionally up-regulated by endotoxin and inflammatory cytokines, and down-regulated by transforming growth factor (TGF)-beta1. Recently we have shown that high mobility group (HMG)-I(Y) protein, an architectural transcription factor, contributes to NOS2 gene transactivation by inflammatory mediators. The aim of the present study was to determine whether regulation of HMG-I(Y) by TGF-beta1 contributes to the TGF-beta1-mediated suppression of NOS2. By Northern blot analysis, we show that TGF-beta1 decreased cytokine-induced HMG-I(Y) mRNA levels in vascular smooth muscle cells and macrophages in vitro and in vivo. Western analysis confirmed the down-regulation of HMG-I(Y) protein by TGF-beta1. To determine whether the down-regulation of HMG-I(Y) contributed to a decrease in NOS2 gene transactivation by TGF-beta1, we performed cotransfection experiments. Overexpression of HMG-I(Y) was able to restore cytokine inducibility of the NOS2 promoter that was suppressed by TGF-beta1. The effect of TGF-beta1 on NOS2 gene transactivation was not related to a decrease in binding of HMG-I(Y) to the promoter of the NOS2 gene, but due to a decrease in endogenous HMG-I(Y) protein. These data provide the first evidence that cytokine-induced HMG-I(Y) can be down-regulated by TGF-beta1. This down-regulation of HMG-I(Y) contributes to the TGF-beta1-mediated decrease in NOS2 gene transactivation by proinflammatory stimuli.

Architectural transcription factor HMGI(Y) promotes tumor progression and mesenchymal transition of human epithelial cells

Numerous studies have demonstrated that overexpression or aberrant expression of the HMGI(Y) family of architectural transcription factors is frequently associated with both neoplastic transformation of cells and metastatic tumor progression. Little is known, however, about the molecular roles played by the HMGI(Y) proteins in these events. Here we report that human breast epithelial cells harboring tetracycline-regulated HMGI(Y) transgenes acquire the ability to form both primary and metastatic tumors in nude mice only when the transgenes are actively expressed. Unexpectedly, the HMG-Y, rather than the HMG-I, isoform of these proteins is the most effective elicitor of both neoplastic transformation and metastatic progression in vivo. Furthermore, expression of either antisense or dominant-negative HMGI(Y) constructs inhibits both the rate of proliferation of tumor cells and their ability to grow anchorage independently in soft agar. Array analysis of transcription profiles demonstrates that the HMG-I and HMG-Y isoform proteins each modulate the expression of distinctive constellations of genes known to be involved in signal transduction, cell proliferation, tumor initiation, invasion, migration, induction of angiogenesis, and colonization. Immunohistochemical analyses of tumors formed in nude mice indicate that many have undergone an epithelial-mesenchymal transition in vivo. Together, these findings demonstrate that overexpression of the HMGI(Y) proteins, more specifically, the HMG-Y isoform protein, is causally associated with both neoplastic transformation and metastatic progression and suggest that induction of integrins and their signaling pathways may play significant molecular roles in these biological events.

HMG-I/Y, a new c-Myc target gene and potential oncogene

The HMG-I/Y gene encodes the HMG-I and HMG-Y proteins, which function as architectural chromatin binding proteins important in the transcriptional regulation of several genes. Although increased expression of the HMG-I/Y proteins is associated with cellular proliferation, neoplastic transformation, and several human cancers, the role of these proteins in the pathogenesis of malignancy remains unclear. To better understand the role of these proteins in cell growth and transformation, we have been studying the regulation and function of HMG-I/Y. The HMG-I/Y promoter was cloned, sequenced, and subjected to mutagenesis analysis. A c-Myc-Max consensus DNA binding site was identified as an element important in the serum stimulation of HMG-I/Y. The oncoprotein c-Myc and its protein partner Max bind to this site in vitro and activate transcription in transfection experiments. HMG-I/Y expression is stimulated by c-Myc in a Myc-estradiol receptor cell line in the presence of the protein synthesis inhibitor cycloheximide, indicating that HMG-I/Y is a direct c-Myc target gene. HMG-I/Y induction is decreased in Myc-deficient fibroblasts. HMG-I/Y protein expression is also increased in Burkitt's lymphoma cell lines, which are known to have increased c-Myc protein. Like Myc, increased expression of HMG-I protein leads to the neoplastic transformation of both Rat 1a fibroblasts and CB33 cells. In addition, Rat 1a cells overexpressing HMG-I protein form tumors in nude mice. Decreasing HMG-I/Y proteins using an antisense construct abrogates transformation in Burkitt's lymphoma cells. These findings indicate that HMG-I/Y is a c-Myc target gene involved in neoplastic transformation and a member of a new class of potential oncogenes.

Differential in vivo modifications of the HMGI(Y) nonhistone chromatin proteins modulate nucleosome and DNA interactions

The HMGI(Y) family of "high mobility group" nonhistone proteins are architectural transcription factors whose overexpression is highly correlated with both cancerous transformation and increased malignancy and metastatic potential of tumors in vivo. Here we report on the types of posttranslational modifications found in vivo on the HMG-I and HMG-Y proteins isolated from two human breast epithelial cell lines, MCF-7 and MCF-7/PKC-alpha, that represent different stages of neoplastic progression. The MCF-7 cell line exhibits many characteristics of normal breast epithelial cells and does not form tumors when injected into nude mice, whereas the MCF-7/PKC-alpha cell line, a derivative of MCF-7 that expresses a transgene coding for the enzyme protein kinase C-alpha (PKC-alpha), is both malignant and highly metastatic. Using MALDI mass spectrometry, we show that the HMG-Y protein is more highly modified than the HMG-I protein in both the MCF-7 and the MCF-7/PKC-alpha cells. Significantly, the HMG-Y protein isolated from the highly metastatic MCF-7/PKC-alpha cells possesses a unique constellation of phosphorylations, methylations, and acetylations not found on the HMG-I protein isolated from either the MCF-7 or MCF-7/PKC-alpha cells. We further demonstrate that some of the same amino acid residues phosphorylated on recombinant HMGI(Y) proteins by purified PKC in vitro are also phosphorylated on the HMG-I(Y) proteins isolated from MCF-7/PKC-alpha cells, suggesting that PKC phosphorylates these proteins in vivo. Quantitative substrate binding analyses indicate that the biochemical modifications present on the HMG-I and HMG-Y proteins differentially influence the ability of these proteins to interact with both A.T-rich DNA substrates and nucleosome core particles in vitro, suggesting a similar modulation of such binding affinities in vivo. To our knowledge, this is the first demonstration of differences in the types of in vivo biochemical modifications found on the HMG-I and HMG-Y proteins in cells and also the first experimental evidence suggesting a possible linkage between such posttranslational modifications and the neoplastic potential of cells.

Activator protein 1 (AP-1)- and nuclear factor kappaB (NF-kappaB)-dependent transcriptional events in carcinogenesis

Generation of reactive oxygen species (ROS) during metabolic conversion of molecular oxygen imposes a constant threat to aerobic organisms. Other than the cytotoxic effects, many ROS and oxidants are also potent tumor promoters linking oxidative stress to carcinogenesis. Clonal variants of mouse epidermal JB6 cells originally identified for their differential susceptibility to tumor promoters also show differential reduction-oxidation (redox) responses providing a unique model to study oxidative events in tumor promotion. AP-1 and NF-kappaB, inducible by tumor promoters or oxidative stimuli, show differential protein levels or activation in response to tumor promoters in JB6 cells. We further demonstrated that AP-1 and NF-kappaB are both required for maintaining the transformed phenotypes where inhibition of either activity suppresses transformation response in JB6 cells as well as human keratinocytes and transgenic mouse. NF-kappaB proteins or extracellular signal-regulated kinase (ERK) but not AP-1 proteins are shown to be sufficient for conversion from transformation-resistant to transformation-susceptible phenotype. Insofar as oxidative events regulate AP-1 and NF-kappaB transactivation, these oxidative events can be important molecular targets for cancer prevention.

The role of high-mobility group I(Y) proteins in expression of IL-2 and T cell proliferation

The high-mobility group I(Y) (HMGI(Y)) family of proteins plays an important architectural role in chromatin and have been implicated in the control of inducible gene expression. We have previously shown that expression of HMGI antisense RNA in Jurkat T cells inhibits the activity of the IL-2 promoter. Here we have investigated the role of HMGI(Y) in controlling IL-2 promoter-reporter constructs as well as the endogenous IL-2 gene in both Jurkat T cells and human PBL. We found that the IL-2 promoter has numerous binding sites for HMGI(Y), which overlap or are adjacent to the known transcription factor binding sites. HMGI(Y) modulates binding to the IL-2 promoter of at least three transcription factor families, AP-1, NF-AT and NF-kappaB. By using a mutant HMGI that cannot bind to DNA but can still interact with the transcription factors, we found that DNA binding by HMGI was not essential for the promotion of transcription factor binding. However, the non-DNA binding mutant acts as a dominant negative protein in transfection assays, suggesting that the formation of functional HMGI(Y)-containing complexes requires DNA binding as well as protein:protein interactions. The alteration of HMGI(Y) levels affects IL-2 promoter activity not only in Jurkat T cells but also in PBL. Importantly, we also show here that expression of the endogenous IL-2 gene as well as proliferation of PBL are affected by changes in HMGI(Y) levels. These results demonstrate a major role for HMGI(Y) in IL-2 expression and hence T cell proliferation.

Induction of high mobility group-I(Y) protein by endotoxin and interleukin-1beta in vascular smooth muscle cells. Role in activation of inducible nitric oxide synthase

Nonhistone chromosomal proteins of the high mobility group (HMG) affect the transcriptional regulation of certain mammalian genes. For example, HMG-I(Y) controls cytokine-mediated promoters that require transcription factors, such as nuclear factor-kappaB, for maximal expression. Even though a great deal is known about how HMG-I(Y) facilitates expression of other genes, less is known about the regulation of HMG-I(Y) itself, especially in cells in primary culture. Therefore we investigated the effect of endotoxin and the cytokine interleukin-1beta on HMG-I(Y) expression in vascular smooth muscle cells. Induction of HMG-I(Y) peaked after 48 h of interleukin-1beta stimulation (6.2-fold) in cells in primary culture, and this increase in mRNA corresponded to an increase in HMG-I(Y) protein. Moreover, immunohistochemical staining revealed a dramatic increase in HMG-I(Y) protein expression in vascular smooth muscle cells after endotoxin stimulation in vivo. This increase in HMG-I(Y) expression (both in vitro and in vivo) mirrored an up-regulation of inducible nitric oxide synthase, a cytokine-responsive gene. The functional significance of this coinduction is underscored by our finding that HMG-I(Y) potentiated the response of inducible nitric oxide synthase to nuclear factor-kappaB transactivation. Taken together, these studies suggest that induction of HMG-I(Y), and subsequent transactivation of iNOS, may contribute to a reduction in vascular tone during endotoxemia and other systemic inflammatory processes.

Effects of epidermal growth factor and estrogen on the regulation of the HMG-I/Y gene in human mammary epithelial cell lines

Members of the HMG-I/Y family of high-mobility-group chromatin proteins have been demonstrated to regulate gene expression in human cells in vivo. They are thought to function as gene regulatory molecules by acting as architectural transcription factors that modulate DNA and/or chromatin structure. Numerous studies have indicated that elevated HMG-I/Y gene expression is directly correlated with more advanced cancers and with increased metastatic potential. The inducible expression of the HMG-I/Y gene was studied in two human mammary epithelial cell lines, MCF7 and Hs578T, in the presence, or absence, of either 17 beta-estradiol or epidermal growth factor (EGF). Northern blot analysis indicated that there was no increase in HMG-I/Y mRNA in the nonmetastatic MCF7 cells when they were treated with either 17 beta-estradiol or EGF. In contrast, in the highly metastatic Hs578T cell line, there is a dramatic induction of HMG-I/Y mRNA expression of up to 23-fold when the cells are treated with EGF. mRNA primer extension analysis indicated that only two (of the possible four different) transcription initiation start sites in the HMG-I/Y gene are induced by EGF treatment of the Hs578T cells. Additional experiments demonstrated that in both epithelial cell types HMG-I/Y mRNAs are very stable (tl/2 of approximately 30 hr) and that in the Hs578T cells treated with EGF the cellular concentrations of the HMG-I/Y proteins increase concurrently with the induced mRNA levels. Given that HMG-I/Y proteins are regulators of gene activity whose elevated in vivo concentrations are known to be correlated with increased metastatic potential, these data demonstrating an EGF-induced over-expression of HMG-I/Y in the highly metastatic Hs578T, but not in the nonmetastatic MCF7cells, may have important implications concerning the cellular mechanisms involved in the progression of mammary epithelial tumors.

High-mobility-group proteins and cancer--an emerging link

In the last few years, considerable interest has been generated in the role of high-mobility-group (HMG) proteins, and HMG box proteins generally, in cancer development and therapy. These proteins were discovered in the early 1970s (Goodwin et al. 1973) as a group of nonhistone proteins. Some members of the HMG protein family (i) constitute a class of important architectural proteins involved in transcriptional regulation of genes, (ii) are frequently expressed in transformed cells at levels that correlate with the degree of neoplastic cell transformation, (iii) participate in gene rearrangements, which are linked to the emergence of benign solid tumors, (iv) confer the ability to recognize DNA-cisplatin adducts selectively, and (v) provide a new delivery system for efficient gene transfer. It should be considered that some HMG proteins, acting as architectural proteins that bring many of the transcription factors into precise three-dimensional shapes, may have a similar critical role in neoplastic transformation to that of some transcription factors themselves.

Human papillomavirus type 16 E6 protein up-regulates the expression of the high mobility group protein HMG-I(Y) gene in mouse 10T1/2 cells

Using a differential hybridization technique, we have identified a mouse cellular gene, high mobility group protein HMG-I(Y), whose expression is up-regulated by the E6 protein of human papillomavirus (HPV) type 16. This gene was overexpressed in E6-expressing mouse 10T1/2 cells, but not in G418-resistant 10T1/2 cells. The expression of the HMG-I(Y) gene was up-regulated by the transient expression of E6 from a zinc-inducible human metallothionein-IIA gene promoter. Expression was found to be more efficient at a confluent cell density than at a subconfluent cell density. The up-regulation of HMG-I(Y) gene expression by E6, in particular at a confluent cell density, may be part of an altered genetic program in host cells infected with HPV-16.