SV40 early region oncoproteins and human cell transformation

We now understand neoplastic transformation to be the consequence of multiple acquired genetic alterations. The combination of these acquired changes confer the various phenotypes that constitute the clinical features of cancer. Although only rare human cancers derive from a viral etiology, the study of DNA tumor viruses that transform rodent and human cells has led to a greater understanding of the molecular events that program the malignant state. In particular, investigation of the viral oncoproteins specified by the Simian Virus 40 Early Region (SV40 ER) has revealed critical host cell pathways, whose perturbation play an essential role in the experimental transformation of mammalian cells. Recent work has re-investigated the roles of two SV40 ER oncoproteins, the large T antigen (LT) and the small t antigen (ST), in human cell transformation. Co-expression of these two oncoproteins, together with the telomerase catalytic subunit, hTERT, and an oncogenic version of the H-Ras oncoprotein, suffices to transform human cells. LT inactivates two key tumor suppressor pathways by binding to the retinoblastoma protein (pRB) and p53. The ability of ST to transform human cells requires interactions with PP2A, an abundant family of serine-threonine phosphatases. Here we review recent developments in our understanding of how these two viral oncoproteins facilitate human cell transformation.     

The EGFR as a target for viral oncoproteins.

The epidermal growth factor receptor (EGFR) is a potent stimulator of the mitogen-activated protein kinase (MAPK) signaling pathway. Chronic stimulation of the EGFR and of multiple steps in the MAPK signaling pathway is involved in the development of cancer. Several tumor viruses encode proteins that induce EGFR expression or stimulate EGFR-mediated signaling and are thus likely to play an important role in the transformation of virus-infected cells.     

Insulin and hypoxia share common target genes but not the hypoxia-inducible factor-1alpha

Both hypoxia and insulin induce common target genes, including vascular endothelial growth factors and several glycolytic enzymes. However, these two signals eventually trigger quite different metabolic pathways. Hypoxia induces glycolysis, resulting in anaerobic ATP production, while insulin increases glycolysis for energy storage. Hypoxia-induced gene expression is mediated by the hypoxia-inducible factor-1 (HIF-1) that consists of HIF-1alpha and the aromatic hydrocarbon nuclear translocator (Arnt). Hypoxia-induced gene expression is initiated by the stabilization of the HIF-1alpha subunit. Here we investigated whether insulin-induced gene expression also requires stabilization of HIF-1alpha. Our results indicate that hypoxia but not insulin stabilizes HIF-1alpha protein levels, whereas both insulin- and hypoxia-induced gene expression require the presence of the Arnt protein. Insulin treatment fails to inactivate proline hydroxylation of HIF-1alpha, which triggers recruitment of the von Hippel-Lindau protein and oxygen-dependent degradation of HIF-1alpha. Insulin-induced gene expression is inhibited by the presence of the phosphoinositide (PI) 3-kinase inhibitor LY294002 and the dominant negative mutant of the p85 subunit of PI 3-kinase, whereas hypoxia-induced gene expression is not. Pyrrolidine dithiocarbamate, a scavenger of H2O2, reduces insulin-induced gene expression but not hypoxia-induced gene expression. Although both hypoxia and insulin induce the expression of common target genes through a hypoxia-responsive element- and Arnt-dependent mechanism, insulin cannot stabilize the HIF-1alpha protein. We believe that insulin activates other putative partner proteins for Arnt in PI 3-kinase- and H2O2-dependent pathways.     

An extracellular matrix-specific microarray allowed the identification of target genes downstream of discoidin domain receptors.

The two discoidin domain receptors, DDR1 and DDR2, are tyrosine kinases that are activated by collagen and are essential regulators of cell-matrix communication. However, the target genes downstream of activated DDRs and their physiological significance are largely unknown. Here, we describe a novel method to dissect signaling pathways induced by extracellular matrix (ECM) receptors. Using the doxycycline-inducible repression system (tet-off), we generated human fibrosarcoma and mouse fibroblast cell lines over-expressing DDR1 or DDR2. These cell lines were employed for gene expression analysis using microarrays specific for human and mouse genes coding for ECM proteins or ECM-interacting factors. We found that approximately 10% of the genes studied were up- or down-regulated more than twofold in response to signals generated by over-expressing DDRs. A common event downstream of DDR1 and DDR2 in human and mouse cells was the up-regulation of P-selectin glycoprotein ligand. Key target genes repressed upon DDR activation were agrin, syndecan-1 and alpha3 integrin. ECM-specific microarrays were found a valuable tool to dissect gene expression changes induced by collagen-receptor signaling pathways.     

联合应用RNA-seq和ATAC-seq寻找FOXQ1转录因子的下游靶基因

结直肠癌(Colorectal cancer, CRC)是一种全球高发的恶性肿瘤,发病原因复杂且预后较差。近年来发现叉头框Q1(Forkhead box Q1,FOXQ1)基因作为一类核转录因子在结直肠癌中高表达,可控制下游基因转录活性。本实验拟探究CRC细胞中FOXQ1的转录调控功能并寻找其下游基因。方法:(1)构建低表达FOXQ1基因的稳定转染CRC细胞株;(2)应用RNA-seq检测FOXQ1敲低前后表达量显著差异的基因;(3)应用转座酶可接近性核染色质区域测序分析(Assay for Transposase-Accessible Chromatin using sequencing, ATAC-seq)检测FOXQ1敲低前后细胞染色质易接近性的变化;(4)进一步对FOXQ1敲低前后的RNA-seq和ATAC-seq数据进行一系列生物信息学分析,寻找CRC中FOXQ1转录调控的潜在下游基因。结果:应用RNA-seq筛选出了敲低FOXQ1后表达显著差异的基因EI24、TLR2、SMAD3,通过联合分析两细胞系的测序结果,发现FOXQ1基因敲低后,在DLD1和SW480两个细胞系中染色质易接近性均增强且表达量均上调的基因有61个,染色质易接近性均减弱且表达量均下调的基因有70个,且EI24、TLR2、SMAD3基因均位于重叠分析结果中,其中TLR2、SMAD3基因的染色质区域有明显变化,而EI24基因的染色质区域变化不明显。通过代谢通路分析找到了EI24、TLR2、SMAD3基因所富集的代谢通路。其中SMAD3、TLR2基因在炎症性肠病(Inflammatory bowel disease, IBD)通路中显著富集。EI24基因在p53信号通路(p53 signaling pathway)通路中显著富集。结论:基于染色质易接近性的变化和转录水平的研究发现:敲低FOXQ1基因对CRC细胞系中染色质的开放情况有较大的影响,且影响FOXQ1转录调控的下游基因的表达。找到了FOXQ1敲低后在SW480、DLD1中均发生变化的基因,为丰富FOXQ1转录因子的下游调控网络提供了研究基础。     

A dominant nuclear streptomycin resistance marker for plant cell transformation

Summary Plant cells in photoheterotrophic culture respond to streptomycin by bleaching and retarded growth but no cell death. A new genetic marker for plant cell transformation has been developed that is based on the expression of the enzyme streptomycin phosphotransferase (SPT), and confers the ability to form green colonies on a selective medium. Coding sequences of SPT from the bacterial transposon Tn5 were placed under the control of gene expression signals derived from the Agrobacterium Ti plasmid Ach5. The 5 end of the SPT gene has been replaced with the promoter region of the gene coding for the first enzyme of agropine biosynthesis, the 3 end with that of the enzyme octopine synthase. The chimeric SPT gene has been linked to a selectable kanamycin resistance gene, and introduced into Nicotiana tabacum and Nicotiana plumbaginifolia by selection for the linked kanamycin resistance marker. Streptomycin resistance was expressed in some but not all of the kanamycin-resistant lines and was transmitted to the seed progeny as a dominant nuclear trait.     

Oncogenic transformation by Jun: role of transactivation and homodimerization.

Jun/JunD and Jun/GCN4 chimeras transform chicken embryo fibroblasts and activate the collagenase promoter in these same cells. Individual constructs differ widely in the two activities, and there is no correlation between transformation and transactivation. These results suggest that oncogenic transformation by Jun is not caused merely by an upregulation of AP-1 activity. Jun constructs with a modified dimerization domain allowing only homodimerization are active in transformation and transactivation in chicken embryo fibroblast cultures. Homodimers of Jun therefore transform and transactivate.     

Cell-mediated and glucocorticoid-mediated target cell lysis do not appear to share common pathways

Target cell lysis by cytolyic lymphocytes follows a sequence of events that culminate in osmotic destruction of the target. Although it is clear that killer cell derived components play a crucial role in target cell lysis it is not clear to what extent the target itself is involved in its destruction. Recent observations have pointed to the possibility that glucocorticoid mediated and cell mediated lysis may utilize common pathways of cell lysis. In analyzing this question we found that cell lines that have nonfunctional glucocorticoid receptors like S49-78 and S49-88 are good targets for both NK and thymus-derived killer (TK) cells. Cell lines that are glucocorticoid sensitive such as Q1(4)6 are sensitive to NK-mediated lysis as its derivative HL4-6-3 which contains glucocorticoid receptors but is glucocorticoid resistant. An intriguing exception to this is the glucocorticoid-resistant mutant S49-4RD which is relatively resistant to both NK and TK lysis compared with parent S49. The resistance of S49-4RD to cell-mediated lysis we show here is most likely due to a defect in the target which results in its failure to trigger the cytolytic machinery in the killer cell rather than in its resistance to lysis per se. In support of this we demonstrate that lysis of S49-4RD by cytolytic granules from TK cells is normal. Moreover TK cells lyse S49-4RD as efficiently as its parent in the presence of the lectin Con A. The conclusion that S49-4RD has a defect in its ability to induce killer cells to initiate the cytolytic reaction is also in agreement with the finding that TK-S49-4RD conjugates show inefficient reorientation of the Golgi apparatus in the effector.     

Akt is a direct target for myricetin to inhibit cell transformation

Akt, a serine/threonine kinase, is a critical regulator in many cellular processes including cell growth, proliferation, and apoptosis. In this study, we found that myricetin, a typical flavonol existing in many fruits and vegetables, could directly target Akt to inhibit cell transformation. Binding assay revealed that myricetin bound to Akt directly by competing with ATP. In vitro and ex vivo data confirmed that myricetin inhibited the phosphorylation and kinase activity of Akt. Molecular modeling suggested that myricetin easily docks to the ATP-binding site of Akt with hydrogen bonds. Signaling analysis data further demonstrated that myricetin inhibited Akt-mediated activator protein-1 (AP-1) transactivation, cyclin D1 expression and cell transformation. Overall, our results indicate that Akt is a direct target for myricetin to inhibit cell transformation.     

Avian reticuloendotheliosis virus: identification of the hematopoietic target cell for transformation

Non-virus-producing hematopoietic cells transformed in vitro by reticuloendotheliosis virus (REV-T) induce lethal "reticuloendotheliosis" when inoculated into histocompatible chickens. This is the first direct demonstration that an in vivo target cell of an avian acute leukemia virus can be transformed in vitro. The tumorigenic, REV-T-transformed non-virus-producing cells fail to express helper-virus-coded proteins. REV-T transformed tumorigenic cells therefore do not require helper-virus functions. Cells transformed in vivo or in vitro by REV-T have lymphoblastoid morphology and express low levels of terminal-deoxynucleotidyl-transferase activity and bursal-cell determinants. One clone synthesized Ig mu. The preferred target cells for REV-T transformation are therefore immature lymphoid cells that express B-cell determinants. We propose that the unique transforming sequence of REV-T be designated rel (lymphoid).     

Ras-mediated cell cycle arrest is altered by nuclear oncogenes to induce Schwann cell transformation.

The cellular responses to ras and nuclear oncogenes were investigated in purified populations of rat Schwann cells. v-Ha-ras and SV40 large T cooperate to transform Schwann cells, inducing growth in soft agar and allowing proliferation in the absence of added mitogens. Expression of large T alone reduces their growth factor requirements but is insufficient to induce full transformation. In contrast, expression of v-Ha-ras leads to proliferation arrest in Schwann cells expressing a temperature-sensitive mutant of large T at the restrictive temperature. Cells arrest in either the G1 or G2/M phases of the cell cycle, and can re-enter cell division at the permissive temperature even after prolonged periods at the restrictive conditions. Oncogenic ras proteins also inhibit DNA synthesis when microinjected into Schwann cells. Adenovirus E1a and c-myc oncogenes behave similarly to SV40 large T. They cooperate with Ha-ras oncogenes to transform Schwann cells, and prevent ras-induced growth arrest. Thus nuclear oncogenes fundamentally alter the response of Schwann cells to a ras oncogene from cell cycle arrest to transformation.