Characterization of the role of Sp1 and NF-Y in differential regulation of PTTG/securin expression in tumor cells

Hexavalent Chromium Targets Securin to Drive Numerical Chromosome Instability in Human Lung Cells

Hexavalent chromium [Cr(VI)] is a known human lung carcinogen with widespread exposure in environmental and occupational settings. Despite well-known cancer risks, the molecular mechanisms of Cr(VI)-induced carcinogenesis are not well understood, but a major driver of Cr(VI) carcinogenesis is chromosome instability. Previously, we reported Cr(VI) induced numerical chromosome instability, premature centriole disengagement, centrosome amplification, premature centromere division, and spindle assembly checkpoint bypass. A key regulator of these events is securin, which acts by regulating the cleavage ability of separase. Thus, in this study we investigated securin disruption by Cr(VI) exposure. We exposed human lung cells to a particulate Cr(VI) compound, zinc chromate, for acute (24 h) and prolonged (120 h) time points. We found prolonged Cr(VI) exposure caused marked decrease in securin levels and function. After prolonged exposure at the highest concentration, securin protein levels were decreased to 15.3% of control cells, while securin mRNA quantification was 7.9% relative to control cells. Additionally, loss of securin function led to increased separase activity manifested as enhanced cleavage of separase substrates; separase, kendrin, and SCC1. These data show securin is targeted by prolonged Cr(VI) exposure in human lung cells. Thus, a new mechanistic model for Cr(VI)-induced carcinogenesis emerges with centrosome and centromere disruption as key components of numerical chromosome instability, a key driver in Cr(VI) carcinogenesis.

Pituitary Tumor-Transforming Gene 1/Delta like Non-Canonical Notch Ligand 1 Signaling in Chronic Liver Diseases

The management of chronic liver diseases (CLDs) remains a challenge, and identifying effective treatments is a major unmet medical need. In the current review we focus on the pituitary tumor transforming gene (PTTG1)/delta like non-canonical notch ligand 1 (DLK1) axis as a potential therapeutic target to attenuate the progression of these pathological conditions. PTTG1 is a proto-oncogene involved in proliferation and metabolism. PTTG1 expression has been related to inflammation, angiogenesis, and fibrogenesis in cancer and experimental fibrosis. On the other hand, DLK1 has been identified as one of the most abundantly expressed PTTG1 targets in adipose tissue and has shown to contribute to hepatic fibrosis by promoting the activation of hepatic stellate cells. Here, we extensively analyze the increasing amount of information pointing to the PTTG1/DLK1 signaling pathway as an important player in the regulation of these disturbances. These data prompted us to hypothesize that activation of the PTTG1/DLK1 axis is a key factor upregulating the tissue remodeling mechanisms characteristic of CLDs. Therefore, disruption of this signaling pathway could be useful in the therapeutic management of CLDs.

Expression of TMBIM6 in Cancers: The Involvement of Sp1 and PKC

Transmembrane Bax Inhibitor Motif-containing 6 (TMBIM6) is upregulated in several cancer types and involved in the metastasis. Specific downregulation of TMBIM6 results in cancer cell death. However, the TMBIM6 gene transcriptional regulation in normal and cancer cells is least studied. Here, we identified the core promoter region (−133/+30 bp) sufficient for promoter activity of TMBIM6 gene. Reporter gene expression with mutations at transcription factor binding sites, EMSA, supershift, and ChIP assays demonstrated that Sp1 is an essential transcription factor for basal promoter activity of TMBIM6. The TMBIM6 mRNA expression was increased with Sp1 levels in a concentration dependent manner. Ablation of Sp1 through siRNA or inhibition with mithramycin-A reduced the TMBIM6 mRNA expression. We also found that the protein kinase-C activation stimulates promoter activity and endogenous TMBIM6 mRNA by 2- to 2.5-fold. Additionally, overexpression of active mutants of PKCι, PKCε, and PKCδ increased TMBIM6 expression by enhancing nuclear translocation of Sp1. Immunohistochemistry analyses confirmed that the expression levels of PKCι, Sp1, and TMBIM6 were correlated with one another in samples from human breast, prostate, and liver cancer patients. Altogether, this study suggests the involvement of Sp1 in basal transcription and PKC in the enhanced expression of TMBIM6 in cancer.

Pituitary tumor transforming gene: a novel therapeutic target for glioma treatment

Glioma which has strong proliferation and angiogenesis ability is the most common and malignant primary tumor in central nervous system. Pituitary tumor transforming gene (PTTG) is found in pituitary tumor, and plays important role in cell proliferation, cell cycle, cell apoptosis, and angiogenesis. However, the role of PTTG in glioma is still incompletely investigated. Here, we explored the correlation between PTTG and glioma grade, as well as micro-vessel density (MVD). In addition, siRNA was used to silence PTTG expression in glioma cell lines including U87MG, U251, and SHG44. Cell proliferation, apoptosis, invasion, and angiogenesis were studied both in vitro and in vivo. Our results demonstrated that PTTG expression was significantly up-regulated in glioma, and had positive correlation with glioma grade and MVD. Silencing of PTTG inhibited glioma cell proliferation, migration/invasion, and angiogenesis, induced cell apoptosis, suppressed cell invasion, and arrested cell cycle at G0/G1 stage. Silencing of PTTG could also inhibit tumor growth, invasion, and angiogenesis in vivo. Our data indicated that PTTG might be a potential target for glioma treatment.

Transcriptional profiling reveals functional links between RasGrf1 and Pttg1 in pancreatic beta cells

BACKGROUND

Our prior characterization of RasGrf1 deficient mice uncovered significant defects in pancreatic islet count and size as well as beta cell development and signaling function, raising question about the mechanisms linking RasGrf1 to the generation of those "pancreatic" phenotypes.

RESULTS

Here, we compared the transcriptional profile of highly purified pancreatic islets from RasGrf1 KO mice to that of WT control animals using commercial oligonucleotide microarrays. RasGrf1 elimination resulted in differential gene expression of numerous components of MAPK- and Calcium-signaling pathways, suggesting a relevant contribution of this GEF to modulation of cellular signaling in the cell lineages integrating the pancreatic islets. Whereas the overall transcriptional profile of pancreatic islets was highly specific in comparison to other organs of the same KO mice, a significant specific repression of Pttg1 was a common transcriptional alteration shared with other tissues of neuroectodermal origin. This observation, together with the remarkable pancreatic phenotypic similarities between RasGrf1 KO and Pttg1 KO mice suggested the possibility of proximal functional regulatory links between RasGrf1 and Pttg1 in pancreatic cell lineages expressing these proteins.Analysis of the mPttg1 promoter region identified specific recognition sites for numerous transcription factors which were also found to be differentially expressed in RasGrf1 KO pancreatic islets and are known to be relevant for Ras-ERK signaling as well as beta cell function. Reporter luciferase assays in BT3 insulinoma cells demonstrated the ability of RasGrf1 to modulate mPttg1 promoter activity through ERK-mediated signals. Analysis of the phenotypic interplay between RasGrf1 and Pttg1 in double knockout RasGrf1/Pttg1 mice showed that combined elimination of the two loci resulted in dramatically reduced values of islet and beta cell count and glucose homeostasis function which neared those measured in single Pttg1 KO mice and were significantly lower than those observed in individual RasGrf1 KO mice.

CONCLUSIONS

The specific transcriptional profile and signaling behavior of RasgGrf1 KO pancreatic islets, together with the dominance of Pttg1 over RasGrf1 with regards to the generation of these phenotypes in mouse pancreas, suggest that RasGrf1 is an important upstream component of signal transduction pathways regulating Pttg1 expression and controlling beta cell development and physiological responses.

Down-regulation of the oncogene PTTG1 via the KLF6 tumor suppressor during induction of myeloid differentiation

The aberrant expression of proto-oncogenes is involved in processes that are responsible for cellular proliferation and the inhibition of myeloid differentiation in acute myeloid leukemia (AML). Pituitary Tumor-Transforming gene 1 (PTTG1), an oncogenic transcription factor, is abundantly expressed in various human cancers and hematopoietic malignancies. However, its expression in normal leukocytes and most normal tissues is very low or undetectable. The mechanism by which PTTG1 overexpression modifies myeloid cell development and promotes leukemogenesis remain unclear. To investigate the mechanistic links between PTTG1 overexpression and leukemia cell differentiation, we utilized phorbol 12-myristate 13-acetate (PMA), a well-known agent that triggers monocyte/macrophage differentiation, to analyze the expression patterns of PTTG1 in PMA-induced myeloid differentiation. We found that PTTG1 is down-regulated at the transcriptional level in PMA-treated HL-60 and THP1 cells. In addition, we identified a binding site for a tumor suppressor protein, Kruppel-like factor 6 (KLF6), in the PTTG1 promoter. We found that KLF6 could directly bind and repress PTTG1 expression. In HL-60 and THP1 cells, KLF6 mRNA and protein levels are up-regulated with a concordant reduction of PTTG1 expression upon treatment with PMA. Furthermore, KLF6 knockdown by shRNA abolished the suppression of PTTG1 and reduced the activation of the differentiation marker CD11b in PMA-primed cells. The protein kinase C (PKC) inhibitor and the MAPK/ERK kinase (MEK) inhibitor significantly blocked the potentiation of PMA-mediated KLF6 induction and the down-regulation of PTTG1, indicating that PTTG1 is suppressed via the activation of PKC/ERK/KLF6 pathway. Our findings suggest that drugs that increase the KLF6 inhibition of PTTG1 may have a therapeutic application in AML treatment strategies.

The NF-Y/p53 liaison: well beyond repression

NF-Y is a sequence-specific transcription factor - TF - targeting the common CCAAT promoter element. p53 is a master TF controlling the response to stress signals endangering genome integrity, often mutated in human cancers. The NF-Y/p53 - and p63, p73 - interaction results in transcriptional repression of a subset of genes within the vast NF-Y regulome under DNA-damage conditions. Recent data shows that NF-Y is also involved in pro-apoptotic activities, either directly, by mediating p53 transcriptional activation, or indirectly, by being targeted by a non coding RNA, PANDA. The picture is subverted in cells carrying Gain-of-function mutant p53, through interactions with TopBP1, a protein also involved in DNA repair and replication. In summary, the connection between p53 and NF-Y is crucial in determining cell survival or death.

Inhibition of pituitary tumor-transforming gene-1 in thyroid cancer cells by drugs that decrease specificity proteins

Methyl 2-cyano-3,11-dioxo-18β-olean-1,12-dien-30-oate (CDODA-Me) and the corresponding 2-trifluoromethyl analog (CF(3)DODA-Me) are derived synthetically from the triterpenoid glycyrrhetinic acid, a major component of licorice. CDODA-Me and CF(3)DODA-Me inhibited growth of highly invasive ARO, DRO, K-18, and HTh-74 thyroid cancer cells and this was due, in part, to decreased expression of specificity protein (Sp) transcription factors Sp1, Sp3, and Sp4 that are overexpressed in these cells. CDODA-Me and CF(3)DODA-Me also decreased expression of Sp-dependent genes, such as survivin and vascular endothelial growth factor (VEGF), and induced apoptosis. In addition, pituitary tumor-transforming gene-1 (PTTG-1) protein and mRNA levels were also decreased in thyroid cancer cells treated with CDODA-Me or CF(3)DODA-Me and this was accompanied by decreased expression of PTTG-1-dependent c-Myc and fibroblast growth factor-2 (FGF-2) genes. RNA interference studies against Sp1, Sp3, and Sp4 proteins showed that in thyroid cancer cells, PTTG-1 was an Sp-dependent gene. This study demonstrates for the first time that drugs, such as CDODA-Me and CF(3)DODA-Me, that decrease Sp protein expression also downregulate PTTG-1 in thyroid cancer cells and therefore have potential for clinical treatment of thyroid cancer and other endocrine neoplasias where PTTG-1 is a major pro-oncogenic factor.

Pituitary tumor-transforming gene and its binding factor in endocrine cancer

The pituitary tumor-transforming gene (PTTG1) encodes a multifunctional protein (PTTG) that is overexpressed in numerous tumours, including pituitary, thyroid, breast and ovarian carcinomas. PTTG induces cellular transformation in vitro and tumourigenesis in vivo, and several mechanisms by which PTTG contributes to tumourigenesis have been investigated. Also known as the human securin, PTTG is involved in cell cycle regulation, controlling the segregation of sister chromatids during mitosis. This review outlines current information regarding PTTG structure, expression, regulation and function in the pathogenesis of neoplasia. Recent progress concerning the use of PTTG as a prognostic marker or therapeutic target will be considered. In addition, the PTTG binding factor (PBF), identified through its interaction with PTTG, has also been established as a proto-oncogene that is upregulated in several cancers. Current knowledge regarding PBF is outlined and its role both independently and alongside PTTG in endocrine and related cancers is discussed.

Dicoumarol down-regulates human PTTG1/Securin mRNA expression through inhibition of Hsp90

Securin, the natural inhibitor of sister chromatid untimely separation, is a protooncogene overexpressed in tumors. Its protein levels correlate with malignancy and metastatic proneness. Dicoumarol, a long-established oral anticoagulant, is a new Hsp90 inhibitor that represses PTTG1/Securin gene expression and provokes apoptosis through a complex trait involving both intrinsic and extrinsic pathways. Dicoumarol activity as an Hsp90 inhibitor is confirmed by smaller levels of Hsp90 clients in treated cells and inhibition of in vivo heat shock luciferase activity recovery assays. Likewise, established Hsp90 inhibitors (17-allylamino-geldanamycin and novobiocin) repress PTTG1/Securin gene expression. Also, overexpression of human Hsp90 in yeast makes them hypersensitive to dicoumarol. Both apoptosis and PTTG1/Securin gene repression exerted by dicoumarol in cancer cells are independent of three of the most important signaling pathways affected by Hsp90 inhibition: nuclear factor-kappaB, p53, or Akt/protein kinase B signaling pathways. However, effects on PTTG1/Securin could be partially ascribed to inhibition of the Ras/Raf/extracellular signal-regulated kinase pathway. Overall, we show that expression of PTTG1/Securin gene is Hsp90 dependent and that dicoumarol is a bona fide Hsp90 inhibitor. These findings are important to understand the mode of action of Hsp90 inhibitors, mechanisms of action of dicoumarol, and Securin overexpression in tumors.

Thyroid hormone receptors suppress pituitary tumor transforming gene 1 activity in hepatoma

Pituitary tumor transforming gene 1 (PTTG1) is expressed in most tumors. However, whether thyroid hormone (T(3)) and its receptors (TR) regulate PTTG1 in human hepatocellular carcinomas (HCC) remains unclear. Previous cDNA microarrays revealed PTTG1 is down-regulated by T(3)/TR. This study investigated the significance of PTTG1 regulation by T(3) in HCC cells. The PTTG1 mRNA and protein expression were repressed by T(3) in HCC cell lines overexpressing TR. However, after knockdown of TRs expression by RNA interference, PTTG1 repression by T(3) was abolished. Similar results were observed in thyroidectomized rats. To localize the regulatory region in the PTTG1 promoter, serial deletions within the PTTG1 promoter region were constructed. The promoter activity of the PTTG1 gene was repressed (25-51%) by T(3). Additionally, these findings indicate that PTTG1 may be regulated by Sp1. The critical role of the -594 and -520 Sp1 binding sites was confirmed by electrophoretic mobility shift assay. Transfection with Sp1 expression vector enhanced the activity of the PTTG1 promoter fragment reporter. Also, Sp1 was down-regulated in HCC cells and in thyroidectomized rat after T(3) treatment. Additionally, ectopic expression of PTTG1 promotes cell proliferation in Hep3B hepatoma cells. Conversely, knockdown of PTTG1 or Sp1 expression reduced cell proliferation in HepG2 cells. Notably, the expression of PTTG1 and Sp1 was inversely correlated with the expression of TR proteins in HCC. Together, these findings indicate that PTTG1 gene expression is mediated by Sp1 and is indirectly down-regulated by T(3). Finally, overexpression of PTTG1 or SP1 in HCCs is TR-dependent and crucial in the development of HCC.

The important anti-apoptotic role and its regulation mechanism of PTTG1 in UV-induced apoptosis

Pituitary tumor transforming gene (PTTG1) is widely detected in many tumors. Increasing evidence reveals that PTTG1 is associated with cell proliferation, cellular transformation and apoptosis. However, the functions of PTTG1, especially its role in DNA damage-induced apoptosis, remain largely unclear. In this report, we used UV irradiation to induce apoptosis in HeLa cells to examine the role of PTTG1 in UV-induced apoptosis by RNAi-mediated knockdown and overexpression of PTTG1. RNAi-mediated knockdown of PTTG1 expression increased and overexpression of PTTG1 decreased the UV-induced apoptosis. Furthermore, UV irradiation decreased PTTG1 mRNA and protein expression. These effects were found to be mediated by JNK pathway. Therefore, PTTG1 had an important anti-apoptotic role in UV-induced apoptosis and this role was mediated by JNK pathway. These results may provide important information for understanding the exact role and the regulation mechanism of PTTG1 in UV-induced apoptosis.

Pituitary tumor transforming gene interacts with Sp1 to modulate G1/S cell phase transition

Pituitary tumor transforming gene (PTTG1) was isolated from rat pituitary tumor cells, and subsequently identified as a securin protein as well as a transcription factor. We show here a global transcriptional effect of PTTG1 in human choriocarcinoma JEG-3 cells by simultaneously assessing 20,000 gene promoters using chromatin immunoprecipitation (ChIP)-on-Chip experiments. Seven hundred and forty-six gene promoters (P<0.001) were found enriched, with functions relating to cell cycle, metabolic control and signal transduction. Significant interaction between PTTG1 and Sp1 (P<0.000001) was found by transcriptional pattern analysis of ChIP data and further confirmed by immunoprecipitation and pull-down assays. PTTG1 acts coordinately with Sp1 to induce cyclin D3 expression approximately threefold, and promotes G1/S-phase transition independently of p21. PTTG1 deletion was also protective for anchorage-independent cell colony formation. The results show that PTTG1 exhibits properties of a global transcription factor, and specifically modulates the G1/S-phase transition by interacting with Sp1. This novel signaling pathway may be required for PTTG1 transforming activity.

Pituitary tumor-transforming gene: physiology and implications for tumorigenesis

Pituitary tumor-transforming gene-1 (PTTG1) is overexpressed in a variety of endocrine-related tumors, especially pituitary, thyroid, breast, ovarian, and uterine tumors, as well as nonendocrine-related cancers involving the central nervous, pulmonary, and gastrointestinal systems. Forced PTTG1 expression induces cell transformation in vitro and tumor formation in nude mice. In some tumors, high PTTG1 levels correlate with invasiveness, and PTTG1 has been identified as a key signature gene associated with tumor metastasis. Increasing evidence supports a multifunctional role of PTTG1 in cell physiology and tumorigenesis. Physiological PTTG1 properties include securin activity, DNA damage/repair regulation and involvement in organ development and metabolism. Tumorigenic mechanisms for PTTG1 action involve cell transformation and aneuploidy, apoptosis, and tumorigenic microenvironment feedback. This paper reviews recent advances in our understanding of PTTG1 structure and regulation and addresses known mechanisms of PTTG1 action. Recent knowledge gained from PTTG1-null mouse models and transgenic animals and their potential application to subcellular therapeutic targeting PTTG1 are discussed.

NF-Y and Sp1/Sp3 are involved in the transcriptional regulation of the peptidylarginine deiminase type III gene (PADI3) in human keratinocytes

Human peptidylarginine deiminase type III gene (PADI3) encodes a crucial post-translational modification enzyme that converts protein-bound arginine residues into citrulline residues. Its expression is restricted to a few cell types, including keratinocytes in the granular layer of the epidermis and in the inner root sheath of hair follicles. In these cells, the enzyme is involved in terminal processing of intermediate filament-binding proteins such as filaggrin and trichohyalin. To study the molecular mechanisms that control the expression of PADI3 in human keratinocytes at the transcriptional level, we characterized its promoter region using human keratinocytes transfected with variously deleted fragments of the 5'-upstream region of PADI3 coupled to the luciferase gene. We found that as few as 129 bp upstream from the transcription initiation site were sufficient to direct transcription of the reporter gene. Electrophoretic mobility-shift and chromatin immunoprecipitation assays revealed that NF-Y (nuclear factor Y) and Sp1/Sp3 (specificity protein 1/3) bind to this region in vitro and in vivo. Moreover, mutation of the Sp1- or NF-Y-binding motif markedly reduced PADI3 promoter activity. Furthermore, Sp1 or NF-YA (NF-Y subunit) small interfering RNAs effectively diminished PADI3 expression in keratinocytes cultured in both low- and high-calcium medium. These data indicate that PADI3 expression is driven by Sp1/Sp3 and NF-Y binding to the promoter region.

Securin (hPTTG1) expression is regulated by beta-catenin/TCF in human colorectal carcinoma

Overexpression of the transcriptional activator β-catenin, mostly owing to loss-of-function mutations of the adenomatous polyposis coli (APC) tumour suppressor gene, is crucial for the initiation and progression of human colorectal carcinogenesis. Securin is a regulator of chromosome separation and its overexpression has been shown to be involved in different tumour-promoting processes, like transformation, hyperproliferation and angiogenesis, and correlates with tumour cell invasion. However, the molecular mechanism leading to securin overexpression in human colorectal cancer is unknown. Here we show a correlated high expression of β-catenin and securin (hPTTG1) in colorectal adenomas and carcinomas and further demonstrate that securin is a target of β-catenin transcriptional activation. This implies that deregulation of the β-catenin/T-cell factor-signalling pathway leads to overexpression of securin in human colorectal cancer, which subsequently may contribute to tumour progression.

Sp1/Sp3 binding is associated with cell-specific expression of the glucose-dependent insulinotropic polypeptide receptor gene

The physiological effects of glucose-dependent insulinotropic polypeptide (GIP) are mediated through specific receptors expressed on target cells. Because aberrant GIP receptor (GIPR) expression has been implicated in abnormal GIP responses associated with type 2 diabetes mellitus and food-induced Cushing's syndrome, we sought to identify factors that regulate the GIPR. We previously demonstrated that sequences between -1 and -100 of the GIPR gene were sufficient to direct transcription in a rat insulinoma cell line (RIN38). In the present study, we compared the 5'-flanking regions of the rat and human GIPR gene and demonstrated 88% identity within the first 92 bp. Subsequent serial deletion analyses showed that the region between -85 and -40 is essential for maximal promoter activity. Within this region, we identified three putative Sp1 binding motifs, located at positions -77, -60, and -50, that can specifically bind both Sp1 and Sp3. Whereas mutation of the Sp1 sites at -50 and -60 led to 36 and 40% reduction in promoter activity, respectively, mutation of the Sp1 motif at -70 did not affect promoter activity. Cotransfection of S2 Schneider cells with GIPR-luciferase chimeric constructs and either Sp1 or Sp3 expression vectors indicated that both Sp1 and the long form of Sp3 activate transcription through binding to the Sp1 sites located between -100 and -40. Lastly, chromatin immunoprecipitation analyses revealed that both Sp1 and Sp3 bind to the GIPR promoter region in RIN38 cells. These results indicate that cell-specific expression of GIPR is associated with the binding of the transcription factors Sp1 and Sp3 to the GIPR promoter.

Polo-like kinase and survivin are esophageal tumor-specific promoters

For developing successful cancer gene therapy strategies, tumor-specific gene delivery is essential. In this study, we used esophageal cancer (EC) cells to identify and evaluate esophageal tumor-specific gene promoters. Four genes (polo-like kinase-1/PLK, survivin/BIRC5, karyopherin alpha 2/KPNA2, and pituitary tumor transforming gene protein 1/PTTG1) were identified by a microarray analysis as highly expressed in EC cell lines vs. five normal organ tissues (liver, lung, kidney, brain, and heart). By quantitative RT-PCR, the average mRNA expression levels of these four genes in 20 primary ECs were 2.7-fold (PLK), 6.1-fold (survivin), 2.6-fold (KPNA2), and 2.4-fold (PTTG1) higher than that of each gene in 24 different normal organs. By dual luciferase assay, the promoter activity of PLK and survivin in EC cell lines was 18.9-fold and 28.5-fold higher, respectively, than in normal lung and renal cells. The promoters of PLK and survivin could be useful tools for developing EC-specific gene therapy vectors.

Insulin and IGF-1 regulate the expression of the pituitary tumor transforming gene (PTTG) in breast tumor cells

The pituitary tumor transforming Gene (PTTG) is an oncogene that is highly expressed in most tumors analyzed to date. Here, we report the effects of insulin and the insulin like growth factor-1 (IGF-1) on the expression of PTTG. Using MCF-7 cells, a human breast cancer cell line, we observed that both insulin and IGF-1 upregulate the expression of PTTG mRNA by approximately 2.5-fold. Induction of PTTG mRNA expression by insulin or IGF-1 was completely blocked by the specific phosphatidylinositol (PI) 3 kinase inhibitor LY294002, but partially blocked by the MAP kinase inhibitor PD98059. Pretreatment of MCF-7 cells with actinomycin D completely blocked the stimulatory effect of insulin. Transfection of MCF-7 cells with a PTTG promoter-luciferase reporter construct revealed the dose-dependent stimulation of PTTG promoter activity by insulin, suggesting that the increase in PTTG expression by insulin is a result of activation of transcription of the PTTG gene. Taken together, our results suggest that insulin and IGF-1 regulate the expression of PTTG in MCF-7 cells primarily through the activation of PI3K/AKT cascade.

Regulation of the pituitary tumor transforming gene by insulin-like-growth factor-I and insulin differs between malignant and non-neoplastic astrocytes

The reasons for overexpression of the oncogene pituitary tumor transforming gene (PTTG) in tumors are still not fully understood. A possible influence of the insulin-like growth factor I (Igf-I) may be of interest, since enhanced Igf-I signalling was reported in various human tumors. We examined the influence of Igf-I and insulin on PTTG expression in human astrocytoma cells in comparison to proliferating non-neoplastic rat embryonal astrocytes. PTTG mRNA expression and protein levels were increased in malignant astrocytes treated with Igf-I or insulin, whereas in rat embryonic astrocytes PTTG expression and protein levels increased only when cells were exposed to Igf-I. Enhanced transcription did not occur after treatment with inhibitors of phosphoinositol-3-kinase (PI3K) and mitogen-activated protein kinase (MAPK), blocking the two basic signalling pathways of Igf-I and insulin. In addition to this transcriptional regulation, both kinases directly bind to PTTG, suggesting a second regulatory route by phosphorylation. However, the interaction of endogenous PTTG with MAPK and PI3K, as well as PTTG phosphorylation were independent from Igf-I or insulin. The latter results were also found in human testis, which contains high PTTG levels as well as in nonneoplastic astrocytes. This suggest, that PI3K and MAPK signalling is involved in PTTG regulation not only in malignant astrocytomas but also in non-tumorous cells.

The MMA1 gene family of cancer-testis antigens has multiple alternative splice variants: characterization of their expression profile, the genomic organization, and transcript properties

Previously, we reported the identification of MMA1A by screening for differential gene expression in two human melanoma cell lines displaying diverse metastatic behavior after subcutaneous inoculation into nude mice. Splice variant MMA1B, which also was identified through database homology searches, showed a high degree of similarity with the MMA1A for exons 1, 2, and 4, but was missing exon 3. Through extensive expression profiling among normal and tumor samples, both MMA1A and -1B were found to belong to the family of cancer-testis antigens. In this study, we identified four additional alternatively spliced MMA1 variants, named MMA1C, MMA1D, MMA1E, and MMA1F. Generally, these novel MMA1 transcripts differ from MMA1A in that exon 2 or exon 3 is enlarged because of the use of alternative splice sites in intron 2 of the MMA1 gene. Moreover, MMA1E also lacks exon 3, as was previously seen in MMA1B. In screening for expression of the novel MMA1 transcripts in normal and tumor tissues, we demonstrated that MMA1C, -1D, and -1E also are members of the cancer-testis antigen family. MMA1F was found in only one melanoma metastasis sample and therefore is believed to have been expressed incidentally. Furthermore, we comprehensively elucidated the genomic structure of the MMA1 gene and the characteristic features of the alternatively spliced MMA1 transcripts.

Ectopic expression of PTTG1/securin promotes tumorigenesis in human embryonic kidney cells

Background

Pituitary tumor transforming gene1 (PTTG1) is a novel oncogene that is expressed in most tumors. It encodes a protein that is primarily involved in the regulation of sister chromatid separation during cell division. The oncogenic potential of PTTG1 has been well characterized in the mouse, particularly mouse fibroblast (NIH3T3) cells, in which it induces cell proliferation, promotes tumor formation and angiogenesis. Human tumorigenesis is a complex and a multistep process often requiring concordant expression of a number of genes. Also due to differences between rodent and human cell biology it is difficult to extrapolate results from mouse models to humans. To determine if PTTG1 functions similarly as an oncogene in humans, we have characterized its effects on human embryonic kidney (HEK293) cells.

Results

We report that introduction of human PTTG1 into HEK293 cells through transfection with PTTG1 cDNA resulted in increased cell proliferation, anchorage-independent growth in soft agar, and formation of tumors after subcutaneous injection of nu/nu mice. Pathologic analysis revealed that these tumors were poorly differentiated. Both analysis of HEK293 cells transiently transfected with PTTG1 cDNA and analysis of tumors developed on injection of HEK293 cells that had been stably transfected with PTTG1 cDNA indicated significantly higher levels of secretion and expression of bFGF, VEGF and IL-8 compared to HEK293 cells transfected with pcDNA3.1 vector or uninvolved tissues collected from the mice. Mutation of the proline-rich motifs at the C-terminal of PTTG1 abolished its oncogenic properties. Mice injected with this mutated PTTG1 either did not form tumors or formed very small tumors. Taken together our results suggest that PTTG1 is a human oncogene that possesses the ability to promote tumorigenesis in human cells at least in part through the regulation of expression or secretion of bFGF, VEGF and IL-8.

Conclusions

Our results demonstrate that PTTG1 is a potent human oncogene and has the ability to induce cellular transformation of human cells. Overexpression of PTTG1 in HEK293 cells leads to an increase in the secretion and expression of bFGF, VEGF and IL-8. Mutation of C-terminal proline-rich motifs abrogates the oncogenic function of PTTG1. To our knowledge, this is the first study demonstrating the importance of PTTG1 in human tumorigenesis.

PTTG/securin activates expression of p53 and modulates its function

BACKGROUND

Pituitary tumor transforming gene (PTTG) is a novel oncogene that is expressed abundantly in most tumors. Overexpression of PTTG induces cellular transformation and promotes tumor formation in nude mice. PTTG has been implicated in various cellular processes including sister chromatid separation during cell division as well as induction of apoptosis through p53-dependent and p53-independent mechanisms. The relationship between PTTG and p53 remains unclear, however.

RESULTS

Here we report the effects of overexpression of PTTG on the expression and function of p53. Our results indicate that overexpression of PTTG regulates the expression of the p53 gene at both the transcriptional and translational levels and that this ability of PTTG to activate the expression of p53 gene is dependent upon the p53 status of the cell. Deletion analysis of the p53 gene promoter revealed that only a small region of the p53 gene promoter is required for its activation by PTTG and further indicated that the activation of p53 gene by PTTG is an indirect effect that is mediated through the regulation of the expression of c-myc, which then interacts with the p53 gene promoter. Our results also indicate that overexpression of PTTG stimulates expression of the Bax gene, one of the known downstream targets of p53, and induces apoptosis in a human embryonic kidney cell line (HEK293). This stimulation of bax expression by PTTG is indirect and is mediated through modulation of p53 gene expression.

CONCLUSIONS

Overexpression of PTTG activates the expression of p53 and modulates its function, with this action of PTTG being mediated through the regulation of c-myc expression. PTTG also up-regulates the activity of the bax promoter and increases the expression of bax through modulation of p53 expression.