Interaction between HMGA1 and retinoblastoma protein is required for adipocyte differentiation

Overexpression of HMGA1 confers radioresistance by transactivating RAD51 in cholangiocarcinoma

Cholangiocarcinomas (CCAs) are rare but aggressive tumors of the bile ducts. CCAs are often diagnosed at an advanced stage and respond poorly to current conventional radiotherapy and chemotherapy. High mobility group A1 (HMGA1) is an architectural transcription factor that is overexpressed in multiple malignant tumors. In this study, we showed that the expression of HMGA1 is frequently elevated in CCAs and that the high expression of this gene is associated with a poor prognosis. Functionally, HMGA1 promotes CCA cell proliferation/invasion and xenograft tumor growth. Furthermore, HMGA1 transcriptionally activates RAD51 by binding to its promoter through two HMGA1 response elements. Notably, overexpression of HMGA1 promotes radioresistance whereas its knockdown causes radiosensitivity of CCA cells to X-ray irradiation. Moreover, rescue experiments reveal that inhibition of RAD51 reverses the effect of HMGA1 on radioresistance and proliferation/invasion. These findings suggest that HMGA1 functions as a novel regulator of RAD51 and confers radioresistance in cholangiocarcinoma.

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.

Genome-Wide Overexpression Screen Identifies Genes Able to Bypass p16-Mediated Senescence in Melanoma

Malignant melanomas often arise from nevi, which result from initial oncogene-induced hyperproliferation of melanocytes that are maintained in a CDKN2A/p16-mediated senescent state. Thus, genes that can bypass this senescence barrier are likely to contribute to melanoma development. We have performed a gain-of-function screen of 17,030 lentivirally expressed human open reading frames (ORFs) in a melanoma cell line containing an inducible p16 construct to identify such genes. Genes known to bypass p16-induced senescence arrest, including the human papilloma virus 18 E7 gene ( HPV18E7), and genes such as the p16-binding CDK6 with expected functions, as well as panel of novel genes, were identified, including high-mobility group box (HMGB) proteins. A number of these were further validated in two other models of p16-induced senescence. Tissue immunohistochemistry demonstrated higher levels of CDK6 in primary melanomas compared with normal skin and nevi. Reduction of CDK6 levels drove melanoma cells expressing functional p16 into senescence, demonstrating its contribution to bypass senescence.

The high mobility group A1 molecular switch: turning on cancer - can we turn it off?

INTRODUCTION

Emerging evidence demonstrates that the high mobility group A1 (HMGA1) chromatin remodeling protein is a key molecular switch required by cancer cells for tumor progression and a poorly differentiated, stem-like state. Because the HMGA1 gene and proteins are expressed at high levels in all aggressive tumors studied to date, research is needed to determine how to 'turn off' this master regulatory switch in cancer.

AREAS COVERED

In this review, we describe prior studies that underscore the central role of HMGA1 in refractory cancers and we discuss approaches to target HMGA1 in cancer therapy.

EXPERT OPINION

Given the widespread overexpression of HMGA1 in diverse, aggressive tumors, further research to develop technology to target HMGA1 holds immense promise as potent anticancer therapy. Previous work in preclinical models indicates that delivery of short hairpin RNA or interfering RNA molecules to 'switch off' HMGA1 expression dramatically impairs cancer cell growth and tumor progression. The advent of nanoparticle technology to systemically deliver DNA or RNA molecules to tumors brings this approach even closer to clinical applications, although further efforts are needed to translate these advances into therapies for cancer patients.

Reduced UCP-1 content in in vitro differentiated beige/brite adipocytes derived from preadipocytes of human subcutaneous white adipose tissues in obesity

Introduction

Brown adipose tissue (BAT) is a potential therapeutic target to reverse obesity. The purpose of this study was to determine whether primary precursor cells isolated from human adult subcutaneous white adipose tissue (WAT) can be induced to differentiate in-vitro into adipocytes that express key markers of brown or beige adipose, and whether the expression level of such markers differs between lean and obese young adult males.

Methods

Adipogenic precursor cells were isolated from lean and obese individuals from subcutaneous abdominal WAT biopsies. Cells were grown to confluence, differentiated for 2.5 weeks then harvested for measurement of gene expression and UCP1 protein.

Results

There was no difference between groups with respect to differentiation into adipocytes, as indicated by oil red-O staining, rates of lipolysis, and expression of adipogenic genes (FABP4, PPARG). WAT genes (HOXC9, RB1) were expressed equally in the two groups. Post differentiation, the beige adipose specific genes CITED1 and CD137 were significantly increased in both groups, but classic BAT markers ZIC1 and LHX8 decreased significantly. Cell lines from both groups also equally increased post-differentiation expression of the thermogenic-responsive gene PPARGC1A (PGC-1α). UCP1 gene expression was undetectable prior to differentiation, however after differentiation both gene expression and protein content were increased in both groups and were significantly greater in cultures from lean compared with obese individuals (p<0.05).

Conclusion

Human subcutaneous WAT cells can be induced to attain BAT characteristics, but this capacity is reduced in WAT cells from obese individuals.

HMGA proteins as modulators of chromatin structure during transcriptional activation

High mobility group (HMG) proteins are the most abundant non-histone chromatin associated proteins. HMG proteins bind to DNA and nucleosome and alter the structure of chromatin locally and globally. Accessibility to DNA within chromatin is a central factor that affects DNA-dependent nuclear processes, such as transcription, replication, recombination, and repair. HMG proteins associate with different multi-protein complexes to regulate these processes by mediating accessibility to DNA. HMG proteins can be subdivided into three families: HMGA, HMGB, and HMGN. In this review, we will focus on recent advances in understanding the function of HMGA family members, specifically their role in gene transcription regulation during development and cancer.

A Novel pro-adipogenesis factor abundant in adipose tissues and over-expressed in obesity acts upstream of PPARγ and C/EBPα

An important question about adipogenesis is how master adipogenesis factors (defined as being able to initiate adipogenesis when expressed alone) peroxisome proliferator-activated receptor (PPAR) initiate adipogenesis only in differentiating preadipocytes. The objective of our research was to find previously unidentified factors that are unique or highly enriched in cells of the adipocyte lineage during adipogenesis that may provide functional tissue specificity to preadipocytes. We reasoned that such factors may alter expression profile specifically in obese individuals. Omental adipose tissues were obtained from obese and non-obese male patients undergoing emergency abdominal surgery. mRNAs extracted from either group were used for suppression subtraction hybridization (SSH). Genes corresponding to mRNAs enriched in obese versus non-obese patients were identified through sequencing and further analyzed for tissue distribution. Out of ~20 genes, we found several that showed clear fat cell specific expression patterns. In this study, we functionally studied one of these genes, previously designated as open reading frame C10orf116. Our data demonstrated that C10orf116 is highly expressed in adipose tissue and is localized primarily within the nucleus. Over-expression studies in 3T3-L1 cells indicated that it up-regulates the levels of CCAAT/enhancer binding protein α (C/EBPα) and PPARγ and promotes adipogenic differentiation starting from the early stage of adipogenesis. Over-expressed in omental tissues from obese patients, C10orf16 manifested the characteristics of an adipocyte lineage-specific nuclear factor that can modulate the master adipogenesis transcription factors early during differentiation. Further studies of this factor should help reveal tissue-specific events leading to fat cell development at the transcriptional level.

High mobility group A1 protein acts as a new target of Notch1 signaling and regulates cell proliferation in T leukemia cells

Active mutations of Notch1 play pivotal roles during leukemogenesis, but the downstream targets and molecular mechanisms of activated Notch1 signaling have not yet been fully clarified. In this study, we detected the overexpression of the high mobility group A1 (HMGA1) and activation of Notch1 signaling in mouse thymic lymphomas. A direct regulation of Notch1 on HMGA1 transcription was demonstrated and two Notch1/RBPJ cobinding sites of T/CTCCCACA were found in HMGA1 promoter regions. It was the first time demonstrated that HMGA1 was the downstream target of Notch1 signaling. Moreover, knockdown of HMGA1 resulted in significantly impaired cell growth and decreased expressions of cyclin D and cyclin E in human T leukemia cells. The formation of complexes was also observed between HMGA1 and retinoblastoma (RB) protein indicating a mechanism of cell cycle regulation. These findings suggest that activated HMGA1 regulates cell proliferation through the Notch1 signaling pathway, which represents an important molecular pathway leading to leukemogenesis.

Overexpression of HMGA1 deregulates tumor growth via cdc25A and alters migration/invasion through a cdc25A-independent pathway in medulloblastoma

Overexpression of high mobility group AT-hook 1 (HMGA1) is common in human cancers. Little is known about the mechanisms underlying its deregulation and downstream targets, and information about its clinical and biological significance in medulloblastoma (MB) is lacking. Here, we demonstrated frequent genomic gain at 6p21.33-6p21.31 with copy number increase leading to overexpression of HMGA1 in MB. The overexpression correlated with a high proliferation index and poor prognosis. Moreover, we found that hsa-miR-124a targeted 3'UTR of HMGA1 and negatively modulated the expression in MB cells, indicating that loss/downregulation of hsa-miR-124a reported in our previous study could contribute to the overexpression. Regarding the biological significance of HMGA1, siRNA knockdown and ectopic expression studies revealed the crucial roles of HMGA1 in controlling MB cell growth and migration/invasion through modulation of apoptosis and formation of filopodia and stress fibers, respectively. Furthermore, we identified cdc25A as a target of HMGA1 and showed that physical interaction between HMGA1 and the cdc25A promoter is required for transcriptional upregulation. In clinical samples, HMGA1 and cdc25A were concordantly overexpressed. Functionally, cdc25A is involved in the HMGA1-mediated control of MB cell growth. Finally, netropsin, which competes with HMGA1 in DNA binding, reduced the expression of cdc25A by suppression of its promoter activity and inhibited in vitro and in vivo intracranial MB cell growth. In conclusion, our results delineate the mechanisms underlying the deregulation and reveal the functional significance of HMGA1 in controlling MB cell growth and migration/invasion. Importantly, the results highlight the therapeutic potential of targeting HMGA1 in MB patients.

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.

Expression of a truncated Hmga1b gene induces gigantism, lipomatosis and B-cell lymphomas in mice

HMGA1 gene rearrangements have been frequently described in human lipomas. In vitro studies suggest that HMGA1 proteins have a negative role in the control of adipocyte cell growth, and that HMGA1 gene truncation acts in a dominant-negative fashion. Therefore, to define better the role of the HMGA1 alterations in the generation of human lipomas, we generated mice carrying an Hmga1b truncated (Hmga1b/T) gene. These mice develop a giant phenotype together with a drastic expansion of the retroperitoneal and subcutaneous white adipose tissue. We show that the activation of the E2F pathway likely accounts, at least in part, for this phenotype. Interestingly, the Hmga1b/T mice also develop B-cell lymphomas similar to that occurring in Hmga1-knockout mice, supporting a dominant-negative role of the Hmga1b/T mutant also in vivo.