Dynamic mitochondrial localization of nuclear transcription factor HMGA1

HMGA2 promotes cancer metastasis by regulating epithelial-mesenchymal transition

Epithelial-mesenchymal transition (EMT) is a complex physiological process that transforms polarized epithelial cells into moving mesenchymal cells. Dysfunction of EMT promotes the invasion and metastasis of cancer. The architectural transcription factor high mobility group AT-hook 2 (HMGA2) is highly overexpressed in various types of cancer (e.g., colorectal cancer, liver cancer, breast cancer, uterine leiomyomas) and significantly correlated with poor survival rates. Evidence indicated that HMGA2 overexpression markedly decreased the expression of epithelial marker E-cadherin (CDH1) and increased that of vimentin (VIM), Snail, N-cadherin (CDH2), and zinc finger E-box binding homeobox 1 (ZEB1) by targeting the transforming growth factor beta/SMAD (TGFβ/SMAD), mitogen-activated protein kinase (MAPK), and WNT/beta-catenin (WNT/β-catenin) signaling pathways. Furthermore, a new class of non-coding RNAs (miRNAs, circular RNAs, and long non-coding RNAs) plays an essential role in the process of HMGA2-induced metastasis and invasion of cancer by accelerating the EMT process. In this review, we discuss alterations in the expression of HMGA2 in various types of cancer. Furthermore, we highlight the role of HMGA2-induced EMT in promoting tumor growth, migration, and invasion. More importantly, we discuss extensively the mechanism through which HMGA2 regulates the EMT process and invasion in most cancers, including signaling pathways and the interacting RNA signaling axis. Thus, the elucidation of molecular mechanisms that underlie the effects of HMGA2 on cancer invasion and patient survival by mediating EMT may offer new therapeutic methods for preventing cancer progression.

HMGA2 mediates Cr (VI)-induced metabolic reprogramming through binding to mitochondrial D-Loop region

Hexavalent chromium [Cr (VI)] exists environmentally and occupationally. It has been shown to pose a carcinogenic hazard in certain occupations. This study was to investigate the role of high mobility group A2 (HMGA2) in Cr (VI)-induced metabolism reprogramming from oxidative phosphorylation (OXPHOS) to glycolysis in A549 and HELF cells. First, knockdown of HMGA2 by siHMGA2 significantly attenuated Cr (VI)-reduced expression of OXPHOS-related proteins (COX IV and ND1) and mitochondrial mass, indicating that HMGA2 was involved in Cr (VI)-reduced OXPHOS. Overexpression of HMGA2 by transfection of HMGA2-DNA plasmids reduced the expression of COX IV, ND1 and mitochondrial mass, suggesting the negative role of HMGA2 in OXPHOS. Secondly, both CCCP, the inhibitor of mitochondrial function, and the ER stress inhibitor, 4-phenylbutyric acid (4-PBA), decreased the level of HMGA2, indicating that the interaction of mitochondrial dysfunction and ER stress resulted in Cr (VI)-induced HMGA2 expression. Further study demonstrated that ER stress/HMGA2 axis mediated the metabolism rewiring from OXPHOS to aerobic glycolysis. Notably, Cr (VI) induced the accumulation of HMGA2 proteins in mitochondria and ChIP assay demonstrated that HMGA2 proteins could bind to D-loop region of mitochondrial DNA (mtDNA), which provided the proof for HMGA2-modulating OXPHOS. Taken together, our results suggested that the interaction of mitochondria and ER stress-enhanced HMGA2 played an important role in Cr (VI)-induced metabolic reprogramming from OXPHOS to glycolysis by binding directly to D-loop region of mtDNA. This work informs on the potential mode of action for Cr (VI)-induced tumors and builds on growing evidence regarding the contribution of cellular metabolic disruption contributing to carcinogenicity.

A requirement for autophagy in HMGA2-induced metabolic reprogramming to support Cd-induced migration

High mobility group A2 (HMGA2) is closely related to the occurrence, development and prognosis of tumors. But the mechanism is unclear. Metabolic reprogramming is a dominant way to meet anabolic and energy requirements of tumor cells for their survival, growth and proliferation. Here, we investigated the role of metabolic reprogramming from oxidative phosphorylation (OXPHOS) to glycolysis mediated by HMGA2/autophagy axis in cadmium (Cd, CdCl₂)-induced migration. First, we found that Cd induced glycolysis and reduced OXPHOS in vivo (0.5 and 1 mg/kg, i.p. or 0.8 and 1.6 μM, i.t.) and in vitro (2 μM in A549 cells and 0.05 μM in HELF cells). Then, genetic knockdown of HMGA2 restored Cd-reduced mitochondrial mass and OXPHOS and inhibited Cd-increased glycolysis, indicating that HMGA2 was involved in Cd-induced metabolic reprogramming. 2-Deoxy-d-glucose (2DG, 5 mM), the inhibitor of glycolysis decreased Cd/HMGA2-induced cell migration and restored Cd/HMGA2-decreased OXPHOS and mitochondrial mass. Inhibition of autophagy by 3-Methyladenine (3MA, 3 mM) elucidated an essential role of autophagy in HMGA2-induced glycolysis, migration, and HMGA2-reduced OXPHOS. Overall, our study demonstrated that autophagy was required for HMGA2-mediated metabolic reprogramming, which was critical for Cd-induced migration. Targeting HMGA2 and autophagy-dependent reprogrammed metabolism may be an effective way to inhibit Cd-induced cell migration.

Prognostic significance of high mobility group A2 (HMGA2) in pancreatic ductal adenocarcinoma: malignant functions of cytoplasmic HMGA2 expression

PURPOSE

HMGA2 has frequently been found in benign as well as malignant tumors and a significant association between HMGA2 overexpression and poor survival in different malignancies was described. In pancreatic ductal adenocarcinoma (PDAC), nuclear HMGA2 expression is associated with tumor dedifferentiation and presence of lymph node metastasis. Nevertheless, the impact of HMGA2 occurrence in other cell compartments is unknown.

METHODS

Intracellular distribution of HMGA2 was analyzed in PDAC (n = 106) and peritumoral, non-malignant ducts (n = 28) by immunohistochemistry. Findings were correlated with clinico-pathological data. Additionally, intracellular HMGA2 presence was studied by Western blotting of cytoplasmic and nuclear fractions of cultured cells.

RESULTS

HMGA2 was found in the cytoplasm and in the nucleus of cultured cells. In human tumor tissue, HMGA2 was also frequently found in the cytoplasm and the nucleus of tumor cells, however, nuclear staining was generally stronger. Direct comparison from tumor tissue with corresponding non-neoplastic peritumoral tissue revealed significantly stronger expression in tumors (p = 0.003). Of note, the nuclear staining was significantly stronger in lymph node metastatic cell nuclei compared to primary tumor cell nuclei (p = 0.049). Interestingly, cytoplasmic staining positively correlated with lymph vessel (p = 0.004) and venous invasion (p = 0.046).

CONCLUSION

HMGA2 is a prognostic marker in PDAC. Firstly, we found a positive correlation for cytoplasmic HMGA2 expression with lympho-vascular invasion and, secondly, we found a significantly stronger nuclear expression of HMGA2 in cancer-positive lymph node nuclei compared to primary tumor cell nuclei. So far, the role of cytoplasmic HMGA2 is nearly unknown, however, our data lend support to the hypothesis that cytoplasmic HMGA2 expression is involved in nodal spread.

Cr (VI) induced mitophagy via the interaction of HMGA2 and PARK2

Chromium (Cr) (VI) is a proven toxin, mutagen and carcinogen. Here, the role of high mobility group A2 (HMGA2) mediating Cr (VI)-induced mitophagy was investigated. Cr (VI)-treatment caused the formation of double membrane autophagic vesicles (AVs) engulfing mitochondria and increased the expression of PINK1, PARK2, LC3 as well as HMGA2 particularly in mitochondria in A549 cells. Silencing of HMGA2 by siRNA decreased expression of PINK1, PARK2 and LC3 II especially in mitochondria, while over-expression of HMGA2 increased the expression of them in A549 cells. It indicated that HMGA2 played a critical role in Cr (VI)-induced mitophagy. Most importantly, the results of co-immunoprecipitation showed for the first time that HMGA2 could bind to PARK2 in mitochondria to activate the mitophagy pathway. In BALB/c mice, Cr (VI) increased the expression of PINK1 and PARK2 in lung tissues. Furthermore, over-expression of HMGA2 in BALB/c mice by transfection of plasmid HMGA2 significantly increased the levels of PINK1, PARK2 and LC3 II in lung tissues. Collectively, our data demonstrated that HMGA2 plays an important role in Cr (VI)-induced mitophagy through direct interaction with PARK2 in A549 cells and lung tissue.

Cytoplasmic levels of high mobility group A2 determine survival prognoses in breast cancer patients

BACKGROUND

High mobility group A proteins are involved in chromatin remodeling, thereby influencing multiple fundamental biological processes. HMGA2 has been linked to oncogenic traits among a variety of malignancies.

OBJECTIVE

To determine the prognostic implications of subcellular distribution patterns of HMGA2 in breast cancer.

METHODS

Nuclear and cytoplasmic HMGA2 was evaluated in 342 breast cancer specimens and matched with clinico-pathological parameters.

RESULTS

Overall and cytoplasmic, but not nuclear, levels of HMGA2 correlated with better survival prognoses in our collective (hazard ratio (HR) 0.34, P = 0.001 and HR 0.34, P < 0.001, respectively). The protective effect of cytoplasmic HMGA2 persisted in the Luminal A and triple negative breast cancer subgroups. Evaluating Luminal A and B subgroups jointly, only cytoplasmic, but not overall or nuclear HMGA2 levels were associated with better survival (HR 0.42, 95% confidence interval 0.21, 0.86, P = 0.017), irrespective of tumor size and node status. The addition of HMGA2 overall and cytoplasmic scores strengthened the prognostic selectivity in a model of conventional breast cancer risk factors. No predictive significance with regard to endocrine or chemoendocrine therapies was observed.

CONCLUSION

Unexpectedly, we found a favorable survival probability upon overall levels of HMGA2 in our breast cancer collective, which was predominantly determined by the presence of HMGA2 in the cytoplasm.

Clinical Implications of Extracellular HMGA1 in Breast Cancer

The unconventional secretion of proteins is generally caused by cellular stress. During the tumorigenesis, tumor cells experience high levels of stress, and the secretion of some theoretically intracellular proteins is activated. Once in the extracellular space, these proteins play different paracrine and autocrine roles and could represent a vulnerability of cancer. One of these proteins is the high mobility group A1 (HMGA1), which is frequently overexpressed in tumors and presents a low expression in normal adult tissues. We have recently described that HMGA1 establishes an autocrine loop in invasive triple-negative breast cancer (TNBC) cells. The secretion of HMGA1 and its binding to the receptor for advanced glycation end products (RAGE) mediates the migration, invasion, and metastasis of TNBC cells and predicts the onset of metastasis in these patients. In this review, we summarized different strategies to exploit the novel tumorigenic phenotype mediated by extracellular HMGA1. We envisioned future clinical applications where the association between its change in subcellular localization and breast cancer progression could be used to predict tumor aggressiveness and guide treatment decisions. Furthermore, we proposed that targeting extracellular HMGA1 as monotherapy using monoclonal antibodies, or in combination with chemotherapy and other targeted therapies, could bring new therapeutic options for TNBC patients.

HMGA1 promoting gastric cancer oncogenic and glycolytic phenotypes by regulating c-myc expression

The high mobility group A1 (HMGA1) protein, an architectural transcription factor, is profoundly implicated in the pathogenesis and progression of multiple malignant tumors. Reprogrammed energy metabolism is a hallmark of diverse types of cancer cells. However, little is known about the regulatory role of HMGA1 in aerobic glycolysis. In this study, we found that HMGA1 was highly expressed in many types of human cancers including gastric cancer and predicted a poor prognosis. However, high HMGA1 expression was not correlated with TNM stages. Gene set enrichment analysis result suggested a link between HMGA1 expression and glycolytic phenotype in gastric cancer. Genetic silencing of HMGA1 significantly inhibited gastric cancer glycolytic activity as revealed by reduced glucose uptake, lactate release, and extracellular acidification ratio. In addition, cell proliferation and invasive capacity of gastric cancer cells were also suppressed by HMGA1 knockdown. Mechanistically, the key glycolysis regulator c-Myc was identified as a downstream target of HMGA1. In gastric cancer patients, HMGA1 and c-Myc expression were closely associated with the glycolysis gene signature. Taken together, our findings identify a novel function of HMGA1 in regulating aerobic glycolysis in gastric cancer.

IL‑17 induces NSCLC A549 cell proliferation via the upregulation of HMGA1, resulting in an increased cyclin D1 expression

Non-small cell lung cancer (NSCLC) is considered to be an inflammation-associated carcinoma. Although interleukin‑17 (IL‑17) production contributes to the proliferation and growth of NSCLC, the mechanisms underlying IL‑17-induced NSCLC cell proliferation have not been fully elucidated. In the present study, by using ELISA and immunohistochemical analyses, we first found that the expression levels of IL‑17, IL‑17 receptor (IL‑17R), high-mobility group A1 (HMGA1) and cyclin D1 were elevated in the samples of patients with NSCLC. Subsequently, by RT-qPCR, western blot analysis and cell proliferation assay in vitro, we revealed that stimulation with recombinant human IL‑17 (namely IL‑17A) markedly induced the expression of HMGA1 and cyclin D1 in the A549 cells (a human lung adenocarcinoma cell line) and promoted cell proliferation. Furthermore, luciferase reporter and ChIP assays confirmed that upregulated HMGA1 directly bound to the cyclin D1 gene promoter and activated its transcription. Notably, the response element of HMGA1 binding to the cyclin D1 promoter was disclosed for the first time, at least to the best of our knowledge. Taken together, our findings indicate that the IL‑17/HMGA1/cyclin D1 axis plays an important role in NSCLC cell proliferation and may provide new insight into NSCLC pathogenesis and may thus aid in the development of novel therapeutic targets for NSCLC.

The High Mobility Group A1 (HMGA1) Transcriptome in Cancer and Development

BACKGROUND & OBJECTIVES

Chromatin structure is the single most important feature that distinguishes a cancer cell from a normal cell histologically. Chromatin remodeling proteins regulate chromatin structure and high mobility group A (HMGA1) proteins are among the most abundant, nonhistone chromatin remodeling proteins found in cancer cells. These proteins include HMGA1a/HMGA1b isoforms, which result from alternatively spliced mRNA. The HMGA1 gene is overexpressed in cancer and high levels portend a poor prognosis in diverse tumors. HMGA1 is also highly expressed during embryogenesis and postnatally in adult stem cells. Overexpression of HMGA1 drives neoplastic transformation in cultured cells, while inhibiting HMGA1 blocks oncogenic and cancer stem cell properties. Hmga1 transgenic mice succumb to aggressive tumors, demonstrating that dysregulated expression of HMGA1 causes cancer in vivo. HMGA1 is also required for reprogramming somatic cells into induced pluripotent stem cells. HMGA1 proteins function as ancillary transcription factors that bend chromatin and recruit other transcription factors to DNA. They induce oncogenic transformation by activating or repressing specific genes involved in this process and an HMGA1 "transcriptome" is emerging. Although prior studies reveal potent oncogenic properties of HMGA1, we are only beginning to understand the molecular mechanisms through which HMGA1 functions. In this review, we summarize the list of putative downstream transcriptional targets regulated by HMGA1. We also briefly discuss studies linking HMGA1 to Alzheimer's disease and type-2 diabetes.

CONCLUSION

Further elucidation of HMGA1 function should lead to novel therapeutic strategies for cancer and possibly for other diseases associated with aberrant HMGA1 expression.

Analysis of the ways and methods of signaling pathways in regulating cell cycle of NIH3T3 at transcriptional level

BACKGROUND

To analyze the ways and methods of signaling pathways in regulating cell cycle progression of NIH3T3 at transcriptional level, we modeled cell cycle of NIH3T3 and found that G1 phase of NIH3T3 cell cycle was at 5-15 h after synchronization, S phase at 15-21 h, G2 phase at 21-22 h, M phase at 22-25 h.

RESULTS

Mouse Genome 430 2.0 microarray was used to detect the gene expression profiles of the model, and results showed remarkable changes in the expressions of 64 cell cycle genes and 960 genes associated with other physiological activity during the cell cycle of NIH3T3. For the next step, IPA software was used to analyze the physiological activities, cell cycle genes-associated signal transduction activities and their regulatory roles of these genes in cell cycle progression, and our results indicated that the reported genes were involved in 17 signaling pathways in the regulation of cell cycle progression. Newfound genes such as PKC, RAS, PP2A, NGR and PI3K etc. belong to the functional category of molecular mechanism of cancer, cyclins and cell cycle regulation HER-2 signaling in breast cancer signaling pathways. These newfound genes could promote DNA damage repairment and DNA replication progress, regulate the metabolism of protein, and maintain the cell cycle progression of NIH3T3 modulating the reported genes CCND1 and C-FOS.

CONCLUSION

All of the aforementioned signaling pathways interacted with the cell cycle network, indicating that NIH3T3 cell cycle was regulated by a number of signaling pathways.

High mobility group (HMG) proteins: Modulators of chromatin structure and DNA repair in mammalian cells

It has been almost a decade since the last review appeared comparing and contrasting the influences that the different families of High Mobility Group proteins (HMGA, HMGB and HMGN) have on the various DNA repair pathways in mammalian cells. During that time considerable progress has been made in our understanding of how these non-histone proteins modulate the efficiency of DNA repair by all of the major cellular pathways: nucleotide excision repair, base excision repair, double-stand break repair and mismatch repair. Although there are often similar and over-lapping biological activities shared by all HMG proteins, members of each of the different families appear to have a somewhat 'individualistic' impact on various DNA repair pathways. This review will focus on what is currently known about the roles that different HMG proteins play in DNA repair processes and discuss possible future research areas in this rapidly evolving field.

Nrf2 protects mitochondrial decay by oxidative stress

Sublethal levels of oxidative stress are commonly associated with various pathophysiological conditions. Cardiomyocytes have the highest content of mitochondria among all cell types, allowing the study of mitochondria in cells surviving oxidative stress and address whether nuclear factor-erythroid-derived 2-related factor 2 (Nrf2) can reverse these changes. Mitochondria normally exist in elaborated networks, which were replaced by predominately individual punctuate mitochondria 24 h after exposure to a nonlethal dose of H2O2. Electron microscopy revealed that cells surviving H2O2 show swelling of mitochondria with disorganized cristae and areas of condensation. Measurements of functional mitochondria showed a H2O2 dose-dependent decrease over a course of 5 d. At the protein and mRNA levels, cells surviving H2O2 treatment show a reduction of mitochondrial components, cytochrome c, and cytochrome b. Nrf2 overexpression prevented H2O2 from inducing mitochondria morphologic changes and reduction of cytochrome b/c. Although Nrf2 is known as a transcription factor regulating antioxidant and detoxification genes, Nrf2 overexpression did not significantly reduce the level of protein oxidation. Instead, Nrf2 was found to associate with the outer mitochondrial membrane. Mitochondria prepared from the myocardium of Nrf2 knockout mice are more sensitive to permeability transition. Our data suggest that Nrf2 protects mitochondria from oxidant injury likely through direct interaction with mitochondria.

HMGB1 in the pathogenesis of ultraviolet-induced ocular surface inflammation

High-mobility group box 1 (HMGB1) functions as a transcription-enhancing nuclear protein as well as a crucial cytokine that regulates inflammation. This study demonstrated that secretion of HMGB1 due to ultraviolet (UV) radiation inducing ocular surface inflammation-mediated reactive oxygen species (ROS) production. After treating conjunctival epithelial cells with UV radiation, HMGB1 was translocated from the nucleus to the cytoplasm and then eventually to the extracellular space. HMGB1 played a crucial role in UV-induced conjunctival neutrophil infiltration, which subsided when mice were pretreated with the HMGB1 inhibitors soluble receptor for advanced glycation endproducts (sRAGEs) and HMGB1 A box protein. In case of using ROS quencher, there was decrease in UV-induced HMGB1 secretion in conjunctival epithelial cells and mice. Considering that UV-induced chronic inflammation causes ocular surface change as pterygium, we have confirmed high HMGB1 translocation and ROS expression in human pterygium. Our findings therefore revealed a previously unknown mechanism of UV-induced ocular inflammation related to ROS and HMGB1 suggesting a new medical therapeutic target.

Molecular and cellular characterization of an AT-hook protein from Leishmania

AT-rich DNA, and the proteins that bind it (AT-hook proteins), modulate chromosome structure and function in most eukaryotes. Unlike other trypanosomatids, the genome of Leishmania species is unusually GC-rich, and the regulation of Leishmania chromosome structure, replication, partitioning is not fully understood. Because AT-hook proteins modulate these functions in other eukaryotes, we examined whether AT-hook proteins are encoded in the Leishmania genome, to test their potential functions. Several Leishmania ORFs predicted to be AT-hook proteins were identified using in silico approaches based on sequences shared between eukaryotic AT-hook proteins. We have used biochemical, molecular and cellular techniques to characterize the L. amazonensis ortholog of the L. major protein LmjF06.0720, a potential AT-hook protein that is highly conserved in Leishmania species. Using a novel fusion between the AT-hook domain encoded by LmjF06.0720 and a herpesviral protein, we have demonstrated that LmjF06.0720 functions as an AT-hook protein in mammalian cells. Further, as observed for mammalian and viral AT-hook proteins, the AT-hook domains of LmjF06.0720 bind specific regions of condensed mammalian metaphase chromosomes, and support the licensed replication of DNA in mammalian cells. LmjF06.0720 is nuclear in Leishmania, and this localization is disrupted upon exposure to drugs that displace AT-hook proteins from AT-rich DNA. Coincidentally, these drugs dramatically alter the cellular physiology of Leishmania promastigotes. Finally, we have devised a novel peptido-mimetic agent derived from the sequence of LmjF06.0720 that blocks the proliferation of Leishmania promastigotes, and lowers amastigote parasitic burden in infected macrophages. Our results indicate that AT-hook proteins are critical for the normal biology of Leishmania. In addition, we have described a simple technique to examine the function of Leishmania chromatin-binding proteins in a eukaryotic context amenable to studying chromosome structure and function. Lastly, we demonstrate the therapeutic potential of compounds directed against AT-hook proteins in Leishmania.

Role of Meis1 in mitochondrial gene transcription of pancreatic cancer cells

Pre-B-cell leukemia transcription factor (PBX)/three-amino-acid loop extension (TALE) class transcription factors [PBX1-4, Meis homeobox (Meis) 1-3, pbx/knotted 1 homeobox (Prep) 1, 2] are involved in tumorigenesis and metastasis. To investigate further the function of PBX/TALE class transcription factors, mRNA expression profile after downregulation of each mRNA expression by siRNA transfection in pancreatic cancer cell line, Panc-1, was examined. Downregulation of Meis1 resulted in downregulation of mitochondrial genes, but those of PBX1 and PBX2 did not. Quantitative reverse transcription polymerase chain reaction confirmed downregulation of mitochondrial genes by Meis1 siRNA transfection. Chromatin immunoprecipitation assay revealed the binding of Meis1 to the mitochondrial promoter region that contained the putative Meis1 binding site. Luciferase reporter assay showed the increase of luciferase activity of a construct containing the Meis1 binding site compared with that with shorter fragment without Meis1 binding region. These findings indicate that Meis1 works as a transcription factor for mitochondrial genes in pancreatic cancer cells.

A compendium of human mitochondrial gene expression machinery with links to disease

Mammalian mitochondrial DNA encodes 37 essential genes required for ATP production via oxidative phosphorylation, instability or misregulation of which is associated with human diseases and aging. Other than the mtDNA-encoded RNA species (13 mRNAs, 12S and 16S rRNAs, and 22 tRNAs), the remaining factors needed for mitochondrial gene expression (i.e., transcription, RNA processing/modification, and translation), including a dedicated set of mitochondrial ribosomal proteins, are products of nuclear genes that are imported into the mitochondrial matrix. Herein, we inventory the human mitochondrial gene expression machinery, and, while doing so, we highlight specific associations of these regulatory factors with human disease. Major new breakthroughs have been made recently in this burgeoning area that set the stage for exciting future studies on the key outstanding issue of how mitochondrial gene expression is regulated differentially in vivo. This should promote a greater understanding of why mtDNA mutations and dysfunction cause the complex and tissue-specific pathology characteristic of mitochondrial disease states and how mitochondrial dysfunction contributes to more common human pathology and aging.

The nuclear architectural protein HMGA1a triggers receptor-mediated endocytosis

High mobility group proteins A (HMGA), nuclear architectural factors, locate in the cell nuclei and mostly execute gene-regulation function. However, our results reveal that a HMGA member (HMGA1a) has a unique plasma membrane receptor; this receptor specifically binds to HMGA-decorated species, effectively mediates endocytosis, and internalizes extracellular HMGA-functionalized cargoes. Indeed, dyes or nanoparticles labeled with HMGA1a protein readily enter Hela cells. Using a stratagem chemical cross-linker, we covalently bonded the HMGA receptor to the HMGA1a-GFP fusion protein, thus capturing the plasma membrane receptor. Subsequent Western blots and SDS-PAGE gel revealed that the HMGA receptor is a 26-kDa protein. Confocal live-cell microscopic imaging was used to monitor the whole endocytic process, in which the internalized HMGA1a-decorated species are transported by motor proteins on microtubules and eventually arrive at the late endosomes/lysosomes. Cell viability assays also suggested that extracellular HMGA1a protein directly influences the survival ability of Hela cells in a dose-dependent manner, implying versatility of HMGA1a protein and its potent role to suppress cancer cell survivability and to regulate growth.

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.

HMGA1 levels influence mitochondrial function and mitochondrial DNA repair efficiency

HMGA chromatin proteins, a family of gene regulatory factors found at only low concentrations in normal cells, are almost universally overexpressed in cancer cells. HMGA proteins are located in the nuclei of normal cells except during the late S/G(2) phases of the cell cycle, when HMGA1, one of the members of the family, reversibly migrates to the mitochondria, where it binds to mitochondrial DNA (mtDNA). In many cancer cells, this controlled shuttling is lost and HMGA1 is found in mitochondria throughout the cell cycle. To investigate the effects of HMGA1 on mitochondria, we employed a genetically engineered line of human MCF-7 cells in which the levels of transgenic HMGA1 protein could be reversibly controlled. "Turn-ON" and "turn-OFF" time course experiments were performed with these cells to either increase or decrease intracellular HMGA1 levels, and various mitochondrial changes were monitored. Results demonstrated that changes in both mtDNA levels and mitochondrial mass inversely paralleled changes in HMGA1 concentrations, strongly implicating HMGA1 in the regulation of these parameters. Additionally, the level of cellular reactive oxygen species (ROS) increased and the efficiency of repair of oxidatively damaged mtDNA decreased as consequences of elevated HMGA1 expression. Increased ROS levels and reduced repair efficiency in HMGA1-overexpressing cells likely contribute to the increased occurrence of mutations in mtDNA frequently observed in cancer cells.

Nuclear receptors and other nuclear transcription factors in mitochondria: regulatory molecules in a new environment

The mitochondrion is the major energy generating organelle of the cell and the site of other basic processes, including apoptosis. The mitochondrial functions are performed in concert with other cell compartments and are regulated by various extracellular and intracellular signals. Several nuclear receptors and other nuclear transcription factors, such as NF-kappa B, AP-1, CREB and p53, involved in growth, metabolic and developmental processes, have been detected in mitochondria. This finding raises the question as to the role of these regulatory molecules in their "new" environment. Experimental evidence supports the action of the mitochondrially localized transcription factors on mitochondrial transcription, energy yield and apoptosis, extending the known nuclear role of these molecules outside the nucleus. A principle of coordination of nuclear and mitochondrial gene transcription has been ascertained as regards the regulatory action of steroid and thyroid hormones on energy yield. Accordingly, the same nuclear receptors, localized in the two compartments-nuclei and mitochondria-regulate transcription of genes serving a common function by way of interaction with common binding sites in the two genomes. This principle is now expanding to encompass other nuclearly and mitochondrially localized transcription factors.

The DNA binding parvulin Par17 is targeted to the mitochondrial matrix by a recently evolved prepeptide uniquely present in Hominidae

BACKGROUND

The parvulin-type peptidyl prolyl cis/trans isomerase Par14 is highly conserved in all metazoans. The recently identified parvulin Par17 contains an additional N-terminal domain whose occurrence and function was the focus of the present study.

RESULTS

Based on the observation that the human genome encodes Par17, but bovine and rodent genomes do not, Par17 exon sequences from 10 different primate species were cloned and sequenced. Par17 is encoded in the genomes of Hominidae species including humans, but is absent from other mammalian species. In contrast to Par14, endogenous Par17 was found in mitochondrial and membrane fractions of human cell lysates. Fluorescence of EGFP fusions of Par17, but not Par14, co-localized with mitochondrial staining. Par14 and Par17 associated with isolated human, rat and yeast mitochondria at low salt concentrations, but only the Par17 mitochondrial association was resistant to higher salt concentrations. Par17 was imported into mitochondria in a time and membrane potential-dependent manner, where it reached the mitochondrial matrix. Moreover, Par17 was shown to bind to double-stranded DNA under physiological salt conditions.

CONCLUSION

Taken together, the DNA binding parvulin Par17 is targeted to the mitochondrial matrix by the most recently evolved mitochondrial prepeptide known to date, thus adding a novel protein constituent to the mitochondrial proteome of Hominidae.

Mitochondrial localization of non-histone protein HMGB1 during human endothelial cell-Toxoplasma gondii infection

Toxoplasma gondii is an obligate intracellular pathogen, replicating only within a specialized membrane-bounded cytoplasmic vacuole, the parasitophorous vacuole (PV), which interacts with host cell mitochondria. High mobility group box 1 (HMGB1), a known nuclear transcription factor, also may be involved in pathological conditions, whose function is to signal tissue damage. Using confocal microscopy, we have investigated the localization of HMGB1 and the mitochondria performance during interaction between human umbilical vein endothelial cells (HUVEC) and Toxoplasma. Immunofluorescence showed HMGB1 localization in HUVEC tubular mitochondria stained with Mito Tracker (MT). At 2h post-infection, MT labeled spherical structures scattered throughout the cytoplasm and HMGB1 were still present. After 24h of infection, long and tubular structures were localized around PVs and were double labeled by MT and HMGB1, suggesting a structural reorganization of the mitochondria over a long period of infection. For the first time, these results show there is HMGB1 in HUVEC mitochondria and that this protein could be playing a part in mitochondrial DNA events which are important for fission and fusion processes reported here during HUVEC-T. gondii infection.

LaRbp38: a Leishmania amazonensis protein that binds nuclear and kinetoplast DNAs

Leishmania amazonensis causes a wide spectrum of leishmaniasis. There are no vaccines or adequate treatment for leishmaniasis, therefore there is considerable interest in the identification of new targets for anti-leishmania drugs. The central role of telomere-binding proteins in cell maintenance makes these proteins potential targets for new drugs. In this work, we used a combination of purification chromatographies to screen L. amazonensis proteins for molecules capable of binding double-stranded telomeric DNA. This approach resulted in the purification of a 38kDa polypeptide that was identified by mass spectrometry as Rbp38, a trypanosomatid protein previously shown to stabilize mitochondrial RNA and to associate with nuclear and kinetoplast DNAs. Western blotting and supershift assays confirmed the identity of the protein as LaRbp38. Competition and chromatin immunoprecipitation assays confirmed that LaRbp38 interacted with kinetoplast and nuclear DNAs in vivo and suggested that LaRbp38 may have dual cellular localization and more than one function.

Nuclear HMGA1 nonhistone chromatin proteins directly influence mitochondrial transcription, maintenance, and function

We have previously demonstrated that HMGA1 proteins translocate from the nucleus to mitochondria and bind to mitochondrial DNA (mtDNA) at the D-loop control region [G.A. Dement, N.R. Treff, N.S. Magnuson, V. Franceschi, R. Reeves, Dynamic mitochondrial localization of nuclear transcription factor HMGA1, Exp. Cell Res. 307 (2005) 388-401.] [11]. To elucidate possible physiological roles for such binding, we employed methods to analyze mtDNA transcription, mitochondrial maintenance, and other organelle functions in transgenic human MCF-7 cells (HA7C) induced to over-express an HA-tagged HMGA1 protein and control (parental) MCF-7 cells. Quantitative real-time (RT) PCR analyses demonstrated that mtDNA levels were reduced approximately 2-fold in HMGA1 over-expressing HA7C cells and flow cytometric analyses further revealed that mitochondrial mass was significantly reduced in these cells. Cellular ATP levels were also reduced in HA7C cells and survival studies showed an increased sensitivity to killing by 2-deoxy-D-glucose, a glycolysis-specific inhibitor. Flow cytometric analyses revealed additional mitochondrial abnormalities in HA7C cells that are consistent with a cancerous phenotype: namely, increased reactive oxygen species (ROS) and increased mitochondrial membrane potential (Delta Psi(m)). Additional RT-PCR analyses demonstrated that gene transcripts from both the heavy (ND2, COXI, ATP6) and light (ND6) strands of mtDNA were up-regulated approximately 3-fold in HA7C cells. Together, these mitochondrial changes are consistent with many previous reports and reveal several possible mechanisms by which HMGA1 over-expression, a common feature of naturally occurring cancers, may affect tumor progression.