A poly(dA-dT) upstream activating sequence binds high-mobility group I protein and contributes to lymphotoxin (tumor necrosis factor-beta) gene regulation

Regulation of Tumor Progression by Programmed Necrosis

Rapidly growing malignant tumors frequently encounter hypoxia and nutrient (e.g., glucose) deprivation, which occurs because of insufficient blood supply. This results in necrotic cell death in the core region of solid tumors. Necrotic cells release their cellular cytoplasmic contents into the extracellular space, such as high mobility group box 1 (HMGB1), which is a nonhistone nuclear protein, but acts as a proinflammatory and tumor-promoting cytokine when released by necrotic cells. These released molecules recruit immune and inflammatory cells, which exert tumor-promoting activity by inducing angiogenesis, proliferation, and invasion. Development of a necrotic core in cancer patients is also associated with poor prognosis. Conventionally, necrosis has been thought of as an unregulated process, unlike programmed cell death processes like apoptosis and autophagy. Recently, necrosis has been recognized as a programmed cell death, encompassing processes such as oncosis, necroptosis, and others. Metabolic stress-induced necrosis and its regulatory mechanisms have been poorly investigated until recently. Snail and Dlx-2, EMT-inducing transcription factors, are responsible for metabolic stress-induced necrosis in tumors. Snail and Dlx-2 contribute to tumor progression by promoting necrosis and inducing EMT and oncogenic metabolism. Oncogenic metabolism has been shown to play a role(s) in initiating necrosis. Here, we discuss the molecular mechanisms underlying metabolic stress-induced programmed necrosis that promote tumor progression and aggressiveness.

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.

HMGB1 overexpression as a prognostic factor for survival in cancer: a meta-analysis and systematic review

As there are millions of cancer deaths every year, it is of great value to identify applicable prognostic biomarkers. As an important alarm, the prognostic role of high mobility group box 1 (HMGB1) in cancer remains controversial. We aim to assess the association of HMGB1 expression with prognosis in cancer patients. Systematic literature searches of PubMed, Embase and Web of Science databases were performed for eligible studies of HMGB1 as prognostic factor in cancer. Pooled hazard ratios (HRs) and 95% confidence intervals (CIs) were calculated to evaluate the influence of HMGB1 expression on overall survival (OS) and progression-free survival (PFS) in cancer patients. 18 studies involving 11 different tumor types were included in meta-analysis. HMGB1 overexpression was significantly associated with poorer OS (HR: 1.99; 95% CI, 1.71-2.31) and PFS (HR: 2.26; 95% CI, 1.65-3.10) irrespective of cancer types including gastric cancer, colorectal cancer, hepatocellular carcinoma, pancreatic cancer, nasopharyngeal carcinoma, head and neck squamous-cell carcinoma, esophageal cancer, malignant pleural mesothelioma, bladder cancer, prostate cancer, and cervical carcinoma. Subgroup analyses indicated geographical area and size of studies did not affect the prognostic effects of HMGB1 for OS. Morever, HMGB1 overexpression had a consistent correlation with poorer OS when detected by immunohistochemistry in tissues and enzyme-linked immunosorbent assay in serum, whereas the correlation did not exist by quantitative real-time reverse-transcription polymerase chain reaction in tissues. HMGB1 overexpression is associated with poorer prognosis in patients with various types of cancer, suggesting that it is a prognostic factor and potential biomarker for survival in cancer.

Relationship between high-mobility group box 1 overexpression in ovarian cancer tissue and serum: a meta-analysis

OBJECTIVE

To implement a meta-analysis to investigate the relationship between high-mobility group box 1 (HMGB1) overexpression in the tissue and serum of ovarian cancer patients, and to evaluate its prognostic significance.

METHODS

Searches were made of China National Knowledge Infrastructure, EMBASE, WanFang, PubMed, MEDLINE, and Web of Science databases up to August 2015, with no language or style restrictions. Reference lists of related studies were also carefully reviewed to identify additional articles.

RESULTS

The literature search identified a total of 12 relevant studies on HMGB1 expression for inclusion in the meta-analysis: seven in ovarian tumor tissue, four in ovarian tumor patient serum, and one in both tissue and serum. HMGB1 protein levels in ovarian cancer tissues were notably higher than those in normal ovarian tissues with no evidence of heterogeneity between studies (RD=0.64, 95% confidence interval (CI): 0.57-0.70, Z=18.70, P<0.00001, I (2)=15%), and also higher than those in benign tumor tissues with no evidence of heterogeneity between studies (RD=0.52, 95% CI: 0.43-0.61, Z=11.14, P<0.00001, I (2)=0). Serum HMGB1 levels were similarly significantly higher in ovarian cancer patients than those with benign tumors or normal ovaries. Pooled mean differences of HMGB1 in ovarian cancer patients compared with patients with benign tumors or normal ovaries were 99.32 with 95% CI: 67.82-130.81, Z=6.18, P<0.00001, and 95.34 with 95% CI: 62.11-128.57, Z=5.62, P<0.0001. The pooled relative risk of ovarian cancer with high vs low HMGB1 expression levels was 1.40 with 95% CI: 1.09-1.79, Z=2.66, P=0.008, heterogeneity I (2)=50%.

CONCLUSION

This meta-analysis suggested that HMGB1 levels in both tissue and serum of ovarian cancer patients were significantly higher than those of benign tumor and normal ovarian samples. High serum or tissue HMGB1 expression may therefore be an effective molecular marker for ovarian benign or malignant tumor diagnosis and patient prognosis.

Eukaryotic genomes may exhibit up to 10 generic classes of gene promoters

BACKGROUND

The main function of gene promoters appears to be the integration of different gene products in their biological pathways in order to maintain homeostasis. Generally, promoters have been classified in two major classes, namely TATA and CpG. Nevertheless, many genes using the same combinatorial formation of transcription factors have different gene expression patterns. Accordingly, we tried to ask ourselves some fundamental questions: Why certain genes have an overall predisposition for higher gene expression levels than others? What causes such a predisposition? Is there a structural relationship of these sequences in different tissues? Is there a strong phylogenetic relationship between promoters of closely related species?

RESULTS

In order to gain valuable insights into different promoter regions, we obtained a series of image-based patterns which allowed us to identify 10 generic classes of promoters. A comprehensive analysis was undertaken for promoter sequences from Arabidopsis thaliana, Drosophila melanogaster, Homo sapiens and Oryza sativa, and a more extensive analysis of tissue-specific promoters in humans. We observed a clear preference for these species to use certain classes of promoters for specific biological processes. Moreover, in humans, we found that different tissues use distinct classes of promoters, reflecting an emerging promoter network. Depending on the tissue type, comparisons made between these classes of promoters reveal a complementarity between their patterns whereas some other classes of promoters have been observed to occur in competition. Furthermore, we also noticed the existence of some transitional states between these classes of promoters that may explain certain evolutionary mechanisms, which suggest a possible predisposition for specific levels of gene expression and perhaps for a different number of factors responsible for triggering gene expression. Our conclusions are based on comprehensive data from three different databases and a new computer model whose core is using Kappa index of coincidence.

CONCLUSIONS

To fully understand the connections between gene promoters and gene expression, we analyzed thousands of promoter sequences using our Kappa Index of Coincidence method and a specialized Optical Character Recognition (OCR) neural network. Under our criteria, 10 classes of promoters were detected. In addition, the existence of "transitional" promoters suggests that there is an evolutionary weighted continuum between classes, depending perhaps upon changes in their gene products.

Diagnostic significance of serum HMGB1 in colorectal carcinomas

High mobility group box 1 protein (HMGB1), a nuclear protein, can be translocated to the cytoplasm and secreted in colon cancer cells. However, the diagnostic significance of HMGB1 has not been evaluated in colorectal carcinomas. For this purpose, we have screened the expression and secretion of HMGB1 in 10 colon cancer cell lines and 1 control cell line and found that HMGB1 was detected in the culture medium. To evaluate the diagnostic value of HMGB1, we performed an enzyme-linked immunosorbent assay to measure HMGB1 levels and compared them to carcinoembryonic antigen (CEA) levels in the serum samples of 219 colorectal carcinoma patients and 75 healthy control subjects. We found that the serum HMGB1 level was increased by 1.5-fold in patients with colorectal carcinoma compared to those in healthy controls. When HMGB1 and CEA levels were compared, HMGB1 had similar efficacy as CEA regarding cancer detection (the sensitivity was 20.1% for HMGB1 vs. 25.6% for CEA, and the specificity was 96% for HMGB1 vs. 90.7% for CEA). Moreover, the diagnostic accuracy of HMGB1 for stage I cancer was significantly higher than that of CEA (sensitivity: 41.2% vs. 5.9%; specificity: 96% vs. 90.7). When we combined HMGB1 and CEA, the overall diagnostic sensitivity was higher than that of CEA alone (42% vs. 25.6%), and the diagnostic sensitivity for stage I was also elevated (47% vs. 5.9%). However, the prognosis of patients was not related with serum HMGB1 concentrations. Our findings indicate that serum HMGB1 levels are increased in a subset of colorectal carcinomas, suggesting their potential utility as a supportive diagnostic marker for colorectal carcinomas.

Hmga1 is differentially expressed and mediates silencing of the CD4/CD8 loci in T cell lineages and leukemic cells

High-mobility group A1 (Hmga1) protein is an architectural chromatin factor, and aberrant Hmga1 expression in mice causes hematopoietic malignancies with defects in cellular differentiation. However, the functional involvement of Hmga1 in hematopoietic development and leukemic cells remains to be elucidated. Using Hmga1-green fluorescent protein (GFP) knock-in mice that endogenously express an Hmga1-GFP fusion protein, we examined Hmga1 expression in undifferentiated and differentiated populations of hematopoietic cells. During early T cell development in the thymus, Hmga1 is highly expressed in CD4/CD8-double negative (DN) cells and is transiently downregulated in CD4/CD8-double positive (DP) cells. Consistently, Hmga1 directly binds to cis-regulatory elements in the CD4/CD8 loci and the heterochromatin foci in DN-stage cells, but not in DP cells. Interestingly, CD4/CD8 expression in DN-stage leukemic cells is induced by inhibition of Hmga1 binding to nuclear DNA or RNA interference-mediated Hmga1 knockdown. In addition, Hmga1-depleted leukemic T cells markedly diminish proliferation, with transcriptional activation of cyclin-dependent kinase inhibitor genes as a direct target of Hmga1. The data in the present study reveal a role of Hmga1 in transcriptional silencing in T cell lineages and leukemic cells.

Polyetheylenimine-polyplexes of Spiegelmer NOX-A50 directed against intracellular high mobility group protein A1 (HMGA1) reduce tumor growth in vivo

High mobility group A1 (HMGA1) proteins belong to a group of architectural transcription factors that are overexpressed in a range of human malignancies, including pancreatic adenocarcinoma. They promote anchorage-independent growth and epithelial-mesenchymal transition and are therefore suggested as potential therapeutic targets. Employing in vitro selection techniques against a chosen fragment of HMGA1, we have generated biostable l-RNA oligonucleotides, so-called Spiegelmers, that specifically bind HMGA1b with low nanomolar affinity. We demonstrate that the best binding Spiegelmers, NOX-A50 and NOX-f33, compete HMGA1b from binding to its natural binding partner, AT-rich double-stranded DNA. We describe a formulation method based on polyplex formation with branched polyethylenimine for efficient delivery of polyethylene glycol-modified Spiegelmers and show improved tissue distribution and persistence in mice. In a xenograft mouse study using the pancreatic cancer cell line PSN-1, subcutaneous administration of 2 mg/kg per day NOX-A50 formulated in polyplexes showed an enhanced delivery of NOX-A50 to the tumor and a significant reduction of tumor volume. Our results demonstrate that intracellular targets can be successfully addressed with a Spiegelmer using polyethylenimine-based delivery and underline the importance of HMGA1 as a therapeutic target in pancreatic 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.

HMGB1: endogenous danger signaling

While foreign pathogens and their products have long been known to activate the innate immune system, the recent recognition of a group of endogenous molecules that serve a similar function has provided a framework for understanding the overlap between the inflammatory responses activated by pathogens and injury. These endogenous molecules, termed alarmins, are normal cell constituents that can be released into the extracellular milieu during states of cellular stress or damage and subsequently activate the immune system. One nuclear protein, High mobility group box-1 (HMGB1), has received particular attention as fulfilling the functions of an alarmin by being involved in both infectious and non-infectious inflammatory conditions. Once released, HMGB1 signals through various receptors to activate immune cells involved in the immune process. Although initial studies demonstrated HMGB1 as a late mediator of sepsis, recent findings indicate HMGB1 to have an important role in models of non-infectious inflammation, such as autoimmunity, cancer, trauma, and ischemia reperfusion injury. Furthermore, in contrast to its pro-inflammatory functions, there is evidence that HMGB1 also has restorative effects leading to tissue repair and regeneration. The complex functions of HMGB1 as an archetypical alarmin are outlined here to review our current understanding of a molecule that holds the potential for treatment in many important human conditions.

Chromatin profiling across the human tumour necrosis factor gene locus reveals a complex, cell type-specific landscape with novel regulatory elements

The TNF locus on chromosome 6p21 encodes a family of proteins with key roles in the immune response whose dysregulation leads to severe disease. Transcriptional regulation is important, with cell type and stimulus-specific enhancer complexes involving the proximal TNF promoter. We show how quantitative chromatin profiling across a 34 kb region spanning the TNF locus has allowed us to identify a number of novel DNase hypersensitive sites and characterize more distant regulatory elements. We demonstrate DNase hypersensitive sites corresponding to the lymphotoxin alpha (LTA) and tumour necrosis factor (TNF) promoter regions, a CpG island in exon 4 of lymphotoxin beta (LTB), the 3′ end of nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor-like 1 (NFKBIL1) and 3.4 kb upstream of LTA. These sites co-localize to highly conserved DNA sequences and show evidence of cell type specificity when lymphoblastoid, Jurkat, U937, HeLa and HEK293T cell lines are analysed using Southern blotting. For Jurkat T cells, we define histone modifications across the locus. Peaks of acetylated histone H3 and H4, together with tri-methyl K4 of histone H3, correspond to hypersensitive sites, notably in exon 4 of LTB. We provide evidence of a functional role for an intergenic DNase I hypersensitive site distal to LTA in Jurkat cells based on reporter gene analysis, with evidence of recruitment of upstream stimulatory factors (USF) transcription factors.

Homopolymer tract length dependent enrichments in functional regions of 27 eukaryotes and their novel dependence on the organism DNA (G+C)% composition

Background

DNA homopolymer tracts, poly(dA).poly(dT) and poly(dG).poly(dC), are the simplest of simple sequence repeats. Homopolymer tracts have been systematically examined in the coding, intron and flanking regions of a limited number of eukaryotes. As the number of DNA sequences publicly available increases, the representation (over and under) of homopolymer tracts of different lengths in these regions of different genomes can be compared.

Results

We carried out a survey of the extent of homopolymer tract over-representation (enrichment) and over-proportional length distribution (above expected length) primarily in the single gene documents, but including some whole chromosomes of 27 eukaryotics across the (G+C)% composition range from 20 – 60%. A total of 5.2 × 10⁷ bases from 15,560 cleaned (redundancy removed) sequence documents were analyzed. Calculated frequencies of non-overlapping long homopolymer tracts were found over-represented in non-coding sequences of eukaryotes. Long poly(dA).poly(dT) tracts demonstrated an exponential increase with tract length compared to predicted frequencies. A novel negative slope was observed for all eukaryotes between their (G+C)% composition and the threshold length N where poly(dA).poly(dT) tracts exhibited over-representation and a corresponding positive slope was observed for poly(dG).poly(dC) tracts. Tract size thresholds where over-representation of tracts in different eukaryotes began to occur was between 4 – 11 bp depending upon the organism (G+C)% composition. The higher the GC%, the lower the threshold N value was for poly(dA).poly(dT) tracts, meaning that the over-representation happens at relatively lower tract length in more GC-rich surrounding sequence. We also observed a novel relationship between the highest over-representations, as well as lengths of homopolymer tracts in excess of their random occurrence expected maximum lengths.

Conclusions

We discuss how our novel tract over-representation observations can be accounted for by a few models. A likely model for poly(dA).poly(dT) tract over-representation involves the known insertion into genomes of DNA synthesized from retroviral mRNAs containing 3' polyA tails. A proposed model that can account for a number of our observed results, concerns the origin of the isochore nature of eukaryotic genomes via a non-equilibrium GC% dependent mutation rate mechanism. Our data also suggest that tract lengthening via slip strand replication is not governed by a simple thermodynamic loop energy model.

The HMG I proteins: dynamic roles in gene activation, development, and tumorigenesis

The high mobility group I, Y, and I-C proteins are low-molecular-weight, nonhistone chromosomal proteins that play a general role modulating gene expression during development and the immune response. Consistent with their role in early development, all three proteins are expressed at high levels during embryogenesis, and their expression is markedly diminished in differentiated cells. Exceptions to the general repression of these genes in adult tissues involve (1) A burst of synthesis of the HMG I protein during the immune response (during lymphocyte activation and preceding cytokine/adhesion molecule gene expression), (2) A constitutive expression of the HMG I and Y proteins in photoreceptor cells, and (3) Derepression of HMG I, Y, and often I-C expression in neoplastic cells. Work from several laboratories has now uncovered how these proteins participate in gene activation: (1) By altering the chromatin structure around an inducible gene-and thus influencing accessibility of the locus to regulatory proteins-(2) By facilitating the loading of transcription factors onto the promoters, and (3) By bridging adjacent transcription factors on a promoter via protein/protein interactions. Despite the similar structures and biochemical properties of the three proteins, the work has also provided clues to a division of labor between these proteins. HMG I and Y have demonstrable roles in enhanceosome formation, whereas HMG I-C has a specific role in adipogenesis. C-terminal truncations of HMG I-C and wild-type HMG Y appear to function in a manner analogous to oncogenes, as assessed by cellular transforation assays and transgenic mice. Future work should clearly define the similarities and differences in the biological roles of the three proteins, and should evolve to include attempts at pharmaceutical intervention in disease, based upon structural information concerning HMG I interactions with DNA and with regulatory proteins.

Critical role of the HMGI(Y) proteins in adipocytic cell growth and differentiation

The high-mobility group I (HMGI) nonhistone chromosomal proteins HMGI(Y) and HMGI-C have been implicated in defining chromatin structure and in regulating the transcription of several genes. These proteins have been implicated in adipocyte homeostasis: a severe deficiency of fat tissue is found in mice with targeted disruption of the HMGI-C locus, and lipomagenesis in humans is frequently associated with somatic mutations of HMGI genes. The aim of this study was to examine the role of HMGI(Y) proteins in adipocytic cell growth and differentiation. First, we found that differentiation of the preadipocytic 3T3-L1 cell line caused early induction of HMGI(Y) gene expression. Suppression of HMGI(Y) expression by antisense technology dramatically increased the growth rate and impaired adipocytic differentiation in these cells. The process of adipogenic differentiation involves the interplay of several transcription factors, among which is the CCAAT/enhancer-binding protein (C/EBP) family of proteins. These factors are required for the transcriptional activation of adipocyte-specific genes. We also tested the hypothesis that HMGI(Y) might participate in transcriptional control of adipocyte-specific promoters. We found that HMGI(Y) proteins bind C/EBPbeta in vivo and in vitro. Furthermore, we show that HMGI(Y) strongly potentiates the capacity of C/EBPbeta to transactivate the leptin promoter, an adipose-specific promoter. Taken together, these results indicate that the HMGI(Y) proteins play a critical role in adipocytic cell growth and differentiation.

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.

Adenovirus-mediated suppression of HMGI(Y) protein synthesis as potential therapy of human malignant neoplasias

High mobility group I (HMGI) proteins are overexpressed in several human malignant tumors. We previously demonstrated that inhibition of HMGI synthesis prevents thyroid cell transformation. Here, we report that an adenovirus carrying the HMGI(Y) gene in an antisense orientation (Ad-Yas) induced programmed cell death of two human thyroid anaplastic carcinoma cell lines (ARO and FB-1), but not normal thyroid cells. The Ad-Yas virus led to death of lung, colon, and breast carcinoma cells. A control adenovirus carrying the lacZ gene did not inhibit the growth of either normal or neoplastic cells. Ad-Yas treatment of tumors induced in athymic mice by ARO cells caused a drastic reduction in tumor size. Therefore, suppression of HMGI(Y) protein synthesis by an HMGI(Y) antisense adenoviral vector may be a useful treatment strategy in a variety of human malignant neoplasias, in which HMGI(Y) gene overexpression is a general event.

The expression of the high mobility group I(Y) mRNA in thyroid cancers: useful tool of differential diagnosis of thyroid nodules

OBJECTIVE

Thyroid nodule is frequent and occurs in about 5% of the general population. In contrast, thyroid cancer is much less frequent and occurs in about 5-10% of thyroid nodules. Distinguishing between benign and malignant lesions is an important task that is best accomplished by fine needle aspiration. Recently, Chiappetta et al. reported that the expression of the high mobility group (HMG) I(Y) proteins correlates with the malignant phenotype of human thyroid neoplasia, and suggested that the detection of the HMG I(Y) proteins might be a valid tool for an easy and sensitive discrimination assay between benign and malignant neoplastic thyroid disease.

METHODS

We evaluated the expression of the HMG I(Y) mRNA in 39 frozen thyroid tissues from patients with thyroid nodule by semiquantitative RT-PCR.

RESULTS

The expression of the HMG I(Y) mRNA was low in all of 10 normal thyroid tissues. In all of 3 adenomatous goiters, 6 follicular adenomas and 2 Hürthle cell adenomas, the HMG I(Y) mRNA expression level was low. In 11 of 13 papillary carcinomas and all of 5 follicular carcinomas, the HMG I(Y) mRNA expression level was high.

CONCLUSION

These results indicate that there is a correlation between the expression of HMG I(Y) and the malignant phenotype of thyroid cancer, suggesting that these proteins may be useful as a marker in thyroid cancer.

Genomic structure and expression of the murine Hmgi-c gene

The murine Hmgi-c gene, a member of the Hmgi gene family, contains five exons encompassing >110 kb of genomic DNA at the pygmy locus on mouse chromosome 10. Northern analysis identified a 4.1 kb transcript which contains a 324 bp open reading frame encoding a 12 kDa HMGI-C protein. Further analysis defined both the 5' and 3' untranslated regions of the Hmgi-c mRNA species as 658 and 2967 bp respectively. The HMGI-C protein has three consecutive AT hook DNA binding domains and an acidic domain, each of which are encoded by individual exons; such an organization is conserved among the HMGI gene family members from insects to mammals. Similar to the HMGI/Y proteins, the HMGI-C protein does not function as a typical transcriptional activator. Developmental studies revealed that the Hmgi-c gene is expressed predominantly during mouse embryogenesis. Since the human homolog is disrupted in a number of tumors, HMGI-C could play an important role in cell proliferation and differentiation during mammalian development.

Involvement of a high-mobility-group protein in the transcriptional activity of herpes simplex virus latency-active promoter 2

Latency-active promoter 2 (LAP 2) is a TATA-less promoter in herpes simplex virus type 1 (HSV-1) that can express genes during viral latency. Four regions of LAP2 are protected from DNase I digestion in vitro by either HeLa cell nuclear extracts or purified Sp1. Transient gene expression assays of LAP2 substitution mutants demonstrate that two of the regions protected by Sp1 and three other regions protected by nuclear extract are important for promoter function. The mutation causing the most significant reduction in expression alters a stretch of 23 thymidine residues (T23) that binds a protein with several properties common to high-mobility-group (HMG) proteins. The T23 binding activity is heat stable, can be inhibited by poly(dA-dT).poly(dA-dT), and is inhibited by minor-groove-binding drugs. Antiserum directed against HMG I(Y) blocked the formation of one of the DNA-protein complexes on the T23 oligonucleotide, suggesting that a protein antigenically related to HMG I(Y) binds to LAP2 in vitro. Direct evidence of HMG I(Y) involvement in LAP2 function is provided by the findings that recombinant HMG I(Y) protein facilitates Sp1 binding to LAP2 in mobility shift assays and that antisense HMG I(Y) RNA specifically inhibits LAP2 function in vivo. These results suggest that DNA structure may be an important determinant of the activity of a promoter that is capable of escaping the global shutoff of transcription that occurs during viral latency.

A retrospective study of high mobility group protein I(Y) as progression marker for prostate cancer determined by in situ hybridization

In a previous study using RNA in situ hybridisation (RISH), we found a significant correlation between high mobility group protein I/Y, [HMG-I(Y)] mRNA expression and tumour stage and grade in prostate cancer patients, suggesting that HMG-I(Y) might be a potential prognostic marker in prostate cancer. However, our clinical follow-up was limited because cryopreserved material was used. Assessing the potential prognostic value of this molecule is of importance because the clinical course of prostate cancer patients remains unpredictable. Here we describe our results on paraffin-embedded archival material from a group of 102 patients undergoing radical prostatectomy. These were evaluated for the presence of HMG-I(Y) using RISH, and a follow-up of 12-92 months (average 53 months) was available. In 2 of 14 prostate cancers in which the predominant histological pattern was of Gleason grade 1-2, a high HMG-I(Y) expression was observed, whereas in 19 of 23 Gleason grade 3, and 34 of 35 Gleason grade 4-5 tumours, high HMG-I(Y) mRNA levels were detected (chi-square = 38.78, P < 0.0001). Moreover, of tumours that expressed high HMG-I(Y) levels, 25% were organ confined (T1-2), in contrast to 74.5% of the invading tumours (T3, chi-square = 15.8, P < 0.001). Furthermore, 87% of recurrent tumours showed high HMG-I(Y) expression. However, a multivariate regression analysis including Gleason grade, clinical tumour stage, HMG-I(Y) expression and prostate-specific antigen (PSA) levels showed Gleason grade as the most accurate predictor of progression. High HMG-I(Y) levels measured by RISH were indicative of a worse prognosis, albeit that additional value over the more subjective grading methods was not evident.

Calcium-dependent ADP-ribosylation of high-mobility-group I (HMGI) proteins

Micrococcal nuclease digestion of nuclei from mouse Lewis lung carcinoma cells releases a protein mixture into the supernatant that lacks histone H1 and contains a full complement of high-mobility-group I (HMGI) proteins (i.e. I, Y and I-C). This implies that all three HMGI proteins are localized at the nuclease-sensitive regions of active chromatin. It is also shown that if Ca2+ ions are present in the nuclear incubation buffer (with or without exogenous nuclease), all three HMGI proteins become ADP-ribosylated. We propose that this modification of HMGI family proteins is part of the general poly(ADP-ribosyl)ation that accompanies DNA damage in apoptosis and other processes.

Multivalent DNA-binding properties of the HMG-1 proteins

HMG-I proteins are DNA-binding proteins thought to affect the formation and function of transcription complexes. Each protein contains three DNA-binding motifs, known as AT-hooks, that bind in the minor groove of AT tracts in DNA. Multiple AT-hooks within a polypeptide chain should contact multiple AT tracts, but the rules governing these interactions have not been defined. In this study, we demonstrate that high-affinity binding uses two or three appropriately spaced AT tracts as a single multivalent binding site. These principles have implications for binding to regulatory elements such as the interferon beta enhancer, TATA boxes, and serum response elements.

Chronic inflammation caused by lymphotoxin is lymphoid neogenesis

In presenting a unifying concept for chronic inflammation and lymphoid organogenesis, we suggest that lymphotoxin's (LT, LT-alpha, TNF-beta) crucial role in these processes is pivotal and similar. Chronic inflammatory lesions that developed in the kidney and pancreas at the sites of transgene expression in rat insulin promoter-LT (RIP-LT) mice resembled lymph nodes with regard to cellular composition (T cells, B cells, plasma cells, and antigen-presenting cells), delineated T and B cell areas, primary and secondary follicles, characteristic morphologic and antigenic (ICAM-1, VCAM-1, MAdCAM-1, and PNAd) features of high endothelial venules, and ability to respond to antigen and undergo Ig class switching when obtained from mice immunized with SRBC. The vascular changes, with the exception of PNAd, appear to be the direct consequence of transgene derived LT expression, as they were also observed in RIP-LT mice lacking mature T and B cells. These data show that LT-induced chronic inflammation has the characteristics of organized lymphoid tissue.

AT-rich upstream sequence elements regulate spore germination-specific expression of the Dictyostelium discoideum celA gene

Two members of a family of spore germination-specific cDNAs, celA and celB, are expressed coordinately, exclusively during spore germination. In the present study the regulatory sequence elements responsible for celA germination-specific expression have been identified. The very AT-rich 81 bp sequence between -664 and -584 upstream of the translation initiation site was required for proper temporal transcription of the celA gene. This sequence is comprised of two cis elements, each of which was active by itself in allowing celA expression. Electrophoretic mobility shift assays showed that a factor(s) in an extract prepared from germinating spores bound to the celA regulatory region. One of the three complexes formed was specific for the germinating spore extract. The results are consistent with the notion that the factor(s) that binds to this regulatory region is involved in expression of celA.

Functional roles of the transcription factor Oct-2A and the high mobility group protein I/Y in HLA-DRA gene expression

The class II major histocompatibility complex gene HLA-DRA is expressed in B cells, activated T lymphocytes, and in antigen-presenting cells. In addition, HLA-DRA gene expression is inducible in a variety of cell types by interferon-gamma (IFN-gamma). Here we show that the lymphoid-specific transcription factor Oct-2A plays a critical role in HLA-DRA gene expression in class II-positive B cell lines, and that the high mobility group protein (HMG) I/Y binds to multiple sites within the DRA promoter, including the Oct-2A binding site. Coexpression of HMG I/Y and Oct-2 in cell lines lacking Oct-2 results in high levels of HLA-DRA gene expression, and in vitro DNA-binding studies reveal that HMG I/Y stimulates Oct-2A binding to the HLA-DRA promoter. Thus, Oct-2A and HMG I/Y may synergize to activate HLA-DRA expression in B cells. By contrast, Oct-2A is not involved in the IFN-gamma induction of the HLA-DRA gene in HeLa cells, but antisense HMG I/Y dramatically decreases the level of induction. We conclude that distinct sets of transcription factors are involved in the two modes of HLA-DRA expression, and that HMG I/Y may be important for B cell-specific expression, and is essential for IFN-gamma induction.

Functional interaction between the POU domain protein Tst-1/Oct-6 and the high-mobility-group protein HMG-I/Y

The POU domain protein Tst-1/Oct-6 is a transcriptional activator of human papovavirus JC virus in transient transfections. Because of its endogenous expression in myelinating glia, Tst-1/Oct-6 might also be an important determinant for the glia specificity of JC virus in vivo. Activation of viral early and late genes depends on the ability of Tst-1/Oct-6 to interact with an AT-rich element within the viral regulatory region. Here, we show that this element not only is bound by Tst-1/Oct-6 but, in addition, serves as a binding site for the high-mobility-group protein HMG-I/Y. In the presence of HMG-I/Y, Tst-1/Oct-6 exhibited an increased affinity for this AT-rich element. The specificity of this effect was evident from the fact that no stimulation of Tst-1/Oct-6 binding was observed on a site that did not allow binding of HMG-I/Y. In addition, both proteins interacted with each other in solution. Direct contacts were identified between the POU domain of Tst-1/Oct-6 and a short stretch of 10 amino acids in the central portion of HMG-I/Y. These results point to an accessory role for HMG-I/Y in the activation of JC viral gene expression by the POU domain protein Tst-1/Oct-6. In agreement with such a role, HMG-Y synergistically supported the function of Tst-1/Oct-6 in transient transfections, measured on the early promoter of JC virus or on an artificial promoter consisting of only a TATA box and the common binding element for Tst-1 and HMG-I/Y.

Regulation of cell-type-specific interleukin-2 receptor alpha-chain gene expression: potential role of physical interactions between Elf-1, HMG-I(Y), and NF-kappa B family proteins

The interleukin 2 receptor alpha-chain (IL-2R alpha) gene is rapidly and potently induced in T cells in response to mitogenic stimuli. Previously, an inducible enhancer between nucleotides -299 and -228 that contains NF-kappa B and CArG motifs was identified. We now report the characterization of a second essential positive regulatory element located between nucleotides -137 and -64 that binds Elf-1 and HMG-I(Y). This element had maximal activity in lymphoid cells, paralleling the cell type specificity of Elf-1 expression. Transcription from the IL-2R alpha promoter was inhibited when either the Elf-1 or the HMG-I(Y) binding site was mutated. Coexpression of both proteins activated transcription of the -137 to -64 element in COS-7 cells. Elf-1 physically associated with HMG-I and with NF-kappa B p50 and c-Rel in vitro, suggesting that protein-protein interactions might functionally coordinate the actions of the upstream and downstream positive regulatory elements. This is the first report of a physical interaction between an Ets family member and NF-kappa B family proteins. These findings provide significant new insights into the protein-protein and protein-DNA interactions that regulate cell-type-specific and inducible IL-2R alpha gene expression and also have implications for other genes regulated by Elf-1 and NF-kappa B family proteins.

Inhibition of HMGI-C protein synthesis suppresses retrovirally induced neoplastic transformation of rat thyroid cells

Elevated expression of the three high-mobility group I (HMGI) proteins (HMGI, HMGY, and HMGI-C) has previously been correlated with the presence of a highly malignant phenotype in epithelial and fibroblastic rat thyroid cells and in experimental thyroid, lung, mammary, and skin carcinomas. Northern (RNA) blot and run-on analyses demonstrated that the induction of HMGI genes in transformed thyroid cells occurs at the transcriptional level. An antisense methodology to block HMGI-C protein synthesis was then used to analyze the role of this protein in the process of thyroid cell transformation. Transfection of an antisense construct for the HMGI-C cDNA into normal thyroid cells, followed by infection with transforming myeloproliferative sarcoma virus or Kirsten murine sarcoma virus, generated cell lines that expressed significant levels of the retroviral transforming oncogenes v-mos or v-ras-Ki and removed the dependency on thyroid-stimulating hormones. However, in contrast with untransfected cells or cells transfected with the sense construct, those containing the antisense construct did not demonstrate the appearance of any malignant phenotypic markers (growth in soft agar and tumorigenicity in athymic mice). A great reduction of the HMGI-C protein levels and the absence of the HMGI(Y) proteins was observed in the HMGI-C antisense-transfected, virally infected cells. Therefore, the HMGI-C protein seems to play a key role in the transformation of these thyroid cells.

A striking similarity in the organization of the E-selectin and beta interferon gene promoters

Transcription of the endothelial leukocyte adhesion molecule 1 (E-selectin or ELAM-1) gene is induced by the inflammatory cytokines interleukin-1 beta and tumor necrosis factor alpha (TNF-alpha). In this report, we identify four positive regulatory domains (PDI to PDIV) in the E-selectin promoter that are required for maximal levels of TNF-alpha induction in endothelial cells. In vitro DNA binding studies reveal that two of the domains contain novel adjacent binding sites for the transcription factor NF-kappa B (PDIII and PDIV), a third corresponds to a recently described CRE/ATF site (PDII), and a fourth is a consensus NF-kappa B site (PDI). Mutations that decrease the binding of NF-kappa B to any one of the NF-kappa B binding sites in vitro abolished cytokine-induced E-selectin gene expression in vivo. Previous studies demonstrated a similar correlation between ATF binding to PDII and E-selectin gene expression. Here we show that the high-mobility-group protein I(Y) [HMG I(Y)] also binds specifically to the E-selectin promoter and thereby enhances the binding of both ATF-2 and NF-kappa B to the E-selectin promoter in vitro. Moreover, mutations that interfere with HMG I(Y) binding decrease the level of cytokine-induced E-selectin expression. The organization of the TNF-alpha-inducible element of the E-selectin promoter is remarkably similar to that of the virus-inducible promoter of the human beta interferon gene in that both promoters require NF-kappa B, ATF-2, and HMG I(Y). We propose that HMG I(Y) functions as a key architectural component in the assembly of inducible transcription activation complexes on both promoters.

A striking similarity in the organization of the E-selectin and beta interferon gene promoters

Transcription of the endothelial leukocyte adhesion molecule 1 (E-selectin or ELAM-1) gene is induced by the inflammatory cytokines interleukin-1 beta and tumor necrosis factor alpha (TNF-alpha). In this report, we identify four positive regulatory domains (PDI to PDIV) in the E-selectin promoter that are required for maximal levels of TNF-alpha induction in endothelial cells. In vitro DNA binding studies reveal that two of the domains contain novel adjacent binding sites for the transcription factor NF-kappa B (PDIII and PDIV), a third corresponds to a recently described CRE/ATF site (PDII), and a fourth is a consensus NF-kappa B site (PDI). Mutations that decrease the binding of NF-kappa B to any one of the NF-kappa B binding sites in vitro abolished cytokine-induced E-selectin gene expression in vivo. Previous studies demonstrated a similar correlation between ATF binding to PDII and E-selectin gene expression. Here we show that the high-mobility-group protein I(Y) [HMG I(Y)] also binds specifically to the E-selectin promoter and thereby enhances the binding of both ATF-2 and NF-kappa B to the E-selectin promoter in vitro. Moreover, mutations that interfere with HMG I(Y) binding decrease the level of cytokine-induced E-selectin expression. The organization of the TNF-alpha-inducible element of the E-selectin promoter is remarkably similar to that of the virus-inducible promoter of the human beta interferon gene in that both promoters require NF-kappa B, ATF-2, and HMG I(Y). We propose that HMG I(Y) functions as a key architectural component in the assembly of inducible transcription activation complexes on both promoters.

Cooperativity between two NF-kappa B complexes, mediated by high-mobility-group protein I(Y), is essential for cytokine-induced expression of the E-selectin promoter

Cytokine-induced expression of the E-selectin gene requires the promoter binding and interaction of the transcription factors NF-kappa B and ATF. Here we have further analyzed the E-selectin promoter and revealed an additional region (nucleotides -140 to -105 [-140/-105]) which is essential in controlling promoter activation by cytokines. We identified high-mobility-group protein I(Y) [HMG-I(Y)] interacting specifically at two sites within this region. We noted that one of the HMG-I(Y)-binding sites overlaps a sequence element (-127/-118) diverging at only one position from the NF-kappa B consensus binding sequence. This led us to ask whether the -127/-118 element represents a second functional NF-kappa B-binding site within the E-selectin promoter. Using specific antisera, we show that p50, p65, and, interestingly, RelB are components of the complex interacting at this site. Mutational analysis of the -127/-118 NF-kappa B site indicates that both NF-kappa B and HMG-I(Y) binding at this site are essential for interleukin-1 induction of the promoter. We demonstrate that the binding affinity of the p50 subunit of NF-kappa B to both NF-kappa B sites within the E-selectin promoter is significantly enhanced by HMG-I(Y). In addition, an essential role for cooperative interaction between the two NF-kappa B complexes is shown by the requirement for both NF-kappa B sites to mediate E-selectin promoter activation by interleukin-1 and p50/p65 expression. We conclude that HMG-I(Y) mediates binding of a distinct NF-kappa B complex at two sites within the E-selectin promoter. Furthermore, a unique cooperativity between these NF-kappa B complexes is essential for induced E-selectin expression. These results suggest mechanisms by which NF-kappa B complexes are involved in specific gene activation.

Cooperativity between two NF-kappa B complexes, mediated by high-mobility-group protein I(Y), is essential for cytokine-induced expression of the E-selectin promoter

Cytokine-induced expression of the E-selectin gene requires the promoter binding and interaction of the transcription factors NF-kappa B and ATF. Here we have further analyzed the E-selectin promoter and revealed an additional region (nucleotides -140 to -105 [-140/-105]) which is essential in controlling promoter activation by cytokines. We identified high-mobility-group protein I(Y) [HMG-I(Y)] interacting specifically at two sites within this region. We noted that one of the HMG-I(Y)-binding sites overlaps a sequence element (-127/-118) diverging at only one position from the NF-kappa B consensus binding sequence. This led us to ask whether the -127/-118 element represents a second functional NF-kappa B-binding site within the E-selectin promoter. Using specific antisera, we show that p50, p65, and, interestingly, RelB are components of the complex interacting at this site. Mutational analysis of the -127/-118 NF-kappa B site indicates that both NF-kappa B and HMG-I(Y) binding at this site are essential for interleukin-1 induction of the promoter. We demonstrate that the binding affinity of the p50 subunit of NF-kappa B to both NF-kappa B sites within the E-selectin promoter is significantly enhanced by HMG-I(Y). In addition, an essential role for cooperative interaction between the two NF-kappa B complexes is shown by the requirement for both NF-kappa B sites to mediate E-selectin promoter activation by interleukin-1 and p50/p65 expression. We conclude that HMG-I(Y) mediates binding of a distinct NF-kappa B complex at two sites within the E-selectin promoter. Furthermore, a unique cooperativity between these NF-kappa B complexes is essential for induced E-selectin expression. These results suggest mechanisms by which NF-kappa B complexes are involved in specific gene activation.

Mitogen-stimulated phosphorylation of histone H3 is targeted to a small hyperacetylation-sensitive fraction

Diverse agents, including growth factors and phorbol esters, induce rapid transcriptional activation of a subset of immediate-early (IE) genes that include the protooncogenes c-fos and c-jun. Among the earliest nuclear signaling events concomitant with IE gene activation is the phosphorylation of nucleosomal histone H3 in its basically charged N-terminal tail. This highly conserved domain is also subject to reversible posttranslational acetylation at specific lysine residues, a process implicated in transcriptional regulation. We show here that H3 phosphorylation associated with G0-G1 transition affects only a small fraction of this histone in the nucleus. Moreover, this fraction is biochemically distinct from bulk H3 in being extremely sensitive to sodium butyrate-induced hyperacetylation. However, acetylation itself does not predispose H3 to phosphorylation, nor does phosphorylation predispose H3 to enhanced acetylation. Further, selectivity is not based on preferential modification of particular histone H3 subtypes. Thus, the mitogen-regulated kinase that phosphorylates histone H3 is restricted to a small subset of nucleosomes that is especially susceptible to hyperacetylation.

Multiple closely-linked NFAT/octamer and HMG I(Y) binding sites are part of the interleukin-4 promoter

We show here that the immediate upstream region (from position -12 to -270) of the murine interleukin 4 (Il-4) gene harbors a strong cell-type specific transcriptional enhancer. In T lymphoma cells, the activity of the Il-4 promoter/enhancer is stimulated by phorbol esters, Ca++ ionophores and agonists of protein kinase A and inhibited by low doses of the immunosuppressant cyclosporin A. The Il-4 promoter/enhancer is transcriptionally inactive in B lymphoma cells and HeLa cells. DNase I footprint protection experiments revealed six sites of the Il-4 promoter/enhancer to be bound by nuclear proteins from lymphoid and myeloid cells. Among them are four purine boxes which have been described to be important sequence motifs of the Il-2 promoter. They contain the motif GGAAA and are recognized by the inducible and cyclosporin A-sensitive transcription factor NFAT-1. Three of the Il-4 NFAT-1 sites are closely linked to weak binding sites of Octamer factors. Several purine boxes and an AT-rich protein-binding site of the Il-4 promoter are also recognized by the high mobility group protein HMG I(Y). Whereas the binding of NFAT-1 and Octamer factors enhance the activity of the Il-4 promoter, the binding of HMG I(Y) suppresses its activity and, therefore, appears to be involved in the suppression of Il-4 transcription in resting T lymphocytes.

Organization, inducible-expression and chromosome localization of the human HMG-I(Y) nonhistone protein gene

Members of the HMG-I(Y) family of mammalian nonhistone proteins are of importance because they have been demonstrated to bind specifically to the minor groove of A.T-rich sequences both in vitro and in vivo and to function as gene transcriptional regulatory proteins in vivo. Here we report the cloning, sequencing, characterization and chromosomal localization of the human HMG-I(Y) gene. The gene has several potential promoter/enhancer regions, a number of different transcription start sites and numerous alternatively spliced exons making it one of the most complex nonhistone chromatin protein-encoding genes so far reported. The putative promoter/enhancer regions each contain a number of conserved nucleotide sequences for potential binding of inducible regulatory transcription factors. Consistent with the presence of these conserved sequences, we found that transcription of the HMG-I(Y) gene is inducible in human lymphoid cells by factors such as phorbol esters and calcium ionophores. Detailed sequence analysis confirms our earlier suggestion that alternative splicing of precursor mRNAs gives rise to the major HMG-I and HMG-Y isoform proteins found in human cells. Furthermore, the gene's exon-intron arrangement fully accounts for all of the previously cloned human HMG-I(Y) cDNAs (1,2). Also of considerable interest is the fact that each of the three different DNA-binding domain peptides present in an individual HMG-I(Y) protein is coded for by sequences present on separate exons thus potentially allowing for exon 'shuffling' of these functional domains during evolution. And, finally, we localized the gene to the short arm of chromosome 6 (6p) in a region that is known to be involved in rearrangements, translocations and other abnormalities correlated with a number of human cancers.

Acute mixed-lineage leukemia t(4;11)(q21;q23) generates an MLL-AF4 fusion product

A chromosomal translocation, t(4;11)-(q21;q23), is associated with an aggressive mixed-lineage leukemia. A yeast artificial chromosome was used to clone the chromosomal breakpoint of this translocation in the RS4;11 cell line. The breakpoint sequences revealed an inverted repeat bordered by a consensus site for topoisomerase II binding and cleavage as well as chi-like elements. The der(11) chromosome encodes a fusion RNA and predicted chimeric protein between the 11q23 gene MLL and a 4q21 gene designated AF4. The sequence of the complete open reading frame for this fusion transcript reveals the MLL protein to have homology with DNA methyltransferase, the Drosophila trithorax gene product, and the "AT-hook" motif of high-mobility-group proteins. An alternative splice that deletes the AT-hook region of MLL was identified. AF4 is a serine- and proline-rich putative transcription factor with a glutamine-rich carboxyl terminus. The composition of the complete MLL-AF4 fusion product argues that it may act through either a gain-of-function or a dominant negative mechanism in leukemogenesis.

Protein phosphatase 2A1 is the major enzyme in vertebrate cell extracts that dephosphorylates several physiological substrates for cyclin-dependent protein kinases

Okadaic acid (2 nM) inhibited by 80-90% the protein phosphatase activities in diluted extracts of rat liver, human fibroblasts, and Xenopus eggs acting on three substrates (high mobility group protein-I(Y), caldesmon and histone H1) phosphorylated by a cyclin-dependent protein kinase (CDK) suggesting that a type-2A phosphatase was responsible for dephosphorylating each protein. This result was confirmed by anion exchange chromatography of rat liver and Xenopus extracts, which demonstrated that the phosphatases acting on these substrates coeluted with the two major species of protein phosphatase 2A, termed PP2A1 and PP2A2. When matched for activity toward glycogen phosphorylase, PP2A1 was five- to sevenfold more active than PP2A2 and 35-fold to 70-fold more active than the free catalytic subunit (PP2Ac) toward the three CDK-labeled substrates. Protein phosphatases 1, 2B, and 2C accounted for a negligible proportion of the activity toward each substrate under the assay conditions examined. The results suggest that PP2A1 is the phosphatase that dephosphorylates a number of CDK substrates in vivo and indicate that the A and B subunits that are associated with PP2Ac in PP2A1 accelerate the dephosphorylation of CDK substrates, while suppressing the dephosphorylation of most other proteins. The possibility that PP2A1 activity is regulated during the cell cycle is discussed.