Malignant transformation of murine fibroblasts by a human c-Ha-ras-1 oncogene does not require a functional epidermal growth factor receptor.

Although mutations in ras genes are thought to be important for the development of about 20% of human tumors, almost nothing is known about the way in which these mutations lead to cellular transformation. The known biochemical properties of the 21-kilodalton ras proteins suggest that they may behave as G proteins, regulating the proliferation of cells in response to growth factor stimulation of a receptor. Although the putative receptor(s) has not been identified, several lines of evidence, in particular the fact that rodent cell lines containing ras oncogenes produce transforming growth factor alpha, have suggested that the epidermal growth factor (EGF) receptor is involved in ras transformation. Here we show that murine fibroblasts with no EGF receptors can be transformed to a completely malignant phenotype with a mutated ras gene. It appears, therefore, that the EGF receptor is not required for ras-mediated transformation of these cells.     

Human oncogenes

Summary The information published on human oncogenes up to the fall of 1983 is reviewed. Retroviral oncogenes, protooncogenes, and cellular transforming genes are compared. Transforming genes derived from the ras gene family are described in detail. The different mechanisms of activation of proto-oncogenes are summarized. Finally, the concerted or sequential action of cellular transforming genes in the multistep process of carcinogenesis is discussed.     

C127 cells resistant to transformation by tyrosine protein kinase oncogenes

C127 is a nontumorigenic mouse cell line widely used in in vitro transformation assays due to its normal morphological appearance and its very low levels of spontaneous transformation. We now report that C127 cells are resistant to transformation by tyrosine protein kinase oncogenes derived from growth factor receptors such as the retroviral v-fms and the human trk transforming genes. In contrast, these cells could be efficiently transformed by members of the ras oncogene family and by serine/threonine kinase oncogenes such as v-mos and v-raf. C127 cells were also found to be resistant to transformation by v-src, the prototype of a large family of tyrosine protein kinase oncogenes whose products are associated with the inner side of the plasma membrane. However, morphologically normal C127 cells expressing pp60v-src acquired a transformed phenotype upon continuous passage in vitro. Somatic cell hybrids (neoR, hygroR) obtained by fusion of G418-resistant C127 cells expressing p70trk (neoR) and hygromycin-resistant NIH3T3 cells (hygroR) exhibited transformed properties as determined by their ability to grow in semisolid agar. In contrast, no such growth was observed when these neoR p70trk-containing C127 cells were fused to control hygroR C127 cells. These results indicate that C127 cells may either lack or express insufficient levels of certain critical substrate(s) necessary for the onset of transformation by tyrosine protein kinase oncogenes.     

Co-transfection of normal NIH/3T3 DNA and retroval LTR sequences: a novel strategy for the detection of potential c-onc genes.

Morphologically transformed, tumorigenic cell lines were obtained after co-transfecting normal NIH/3T3 DNA and cloned 3'-long terminal repeat sequences of Moloney leukemia virus (Mo-LTR) onto NIH/3T3 recipient cells. In four such cell lines the malignant phenotype was found to be associated with single and specific Mo-LTR integration sites that were retained after serial passages through NIH/3T3 and rat 208F cells, indicating that Mo-LTR sequences are linked to the activated oncogenes. In one of these clones the activated transforming gene was identified as c-raf, the cellular homologue of a recently described retroviral oncogene. This finding not only demonstrates that the mouse c-raf gene can be activated to exhibit an oncogenic potential but also that the approach chosen in this study is suitable for the detection of potential c-onc genes. In contrast to this clone, the activated transforming genes in other cell lines appear to be different from 19 previously isolated v-onc and c-onc genes. These results demonstrate the potential of the established transformation system for the detection and isolation of previously unidentified c-onc genes.     

Differentiation mechanisms and malignancy

This review considers the relationship between differentiation mechanisms and the genesis and maintenance of tumor phenotype. To a certain extent, carcinomas preserve differentiation markers of normal tissue, and hemoblastoses precisely reflect the direction and differentiation level of their precursor cells. Both tumor types retain the ability to differentiate. Mechanisms of T and B cell differentiation are reviewed considering the activation of protooncogenes by translocation to the region of tissue-specific genes including the immunoglobulin (Ig) and T cell receptor (TCR) genes. Apart from the classical oncogenes (MYC, PRAD, BCL-2), heterologous differentiation of trans-factors can be activated in a similar manner. Their activation at inappropriate time and place induces oncogenic transformation in a number of hemoblastoses. Chimeric genes and fused proteins are analyzed, including their genesis by specific translocation resulting in transformation and their role in differentiation and maintenance of the tumor phenotype. Induction of terminal differentiation in leukemia can have significant therapeutic effect. These hemoblastoses include hairy cell leukemia, promyelocytic leukemia, and in part chronic myeloid leukemia. Specific attention is given to the role of intercellular interactions in the control of tumor growth and maintenance of a differentiated state of the cells. It is suggested that alterations in these interactions during tumor progression simultaneously stimulate malignant growth and decrease differentiation level, thus inducing re-expression of embryonic antigens in the tumors.     

The DNA Damage Signaling Pathway Connects Oncogenic Stress to Cellular Senescence

The mechanisms of tumor suppression must be linked to the oncogenic threats that may affect a normal cell. An important cancer causing mechanism is the accidental activation of genes that stimulate cell proliferation (oncogenes) by a variety of endogenous or environmental mutagens. This event has been experimentally modelled by enforcing the expression of oncogenes in primary cells. The astonishing outcome of these manipulations is that oncogenes trigger antiproliferative responses preventing progression to malignant transformation. These responses bring to an end proliferation due to cell death or a permanent cell cycle arrest called senescence. Here we review evidence indicating that oncogene induced senescence (OIS) involves activation of p53 via the DNA damage response (DDR). These results imply mechanisms of DNA damage in cells expressing oncogenes, that may be secondary to reactive oxygen species and/or some form of “oncogenic stress” that affect normal DNA replication. Interestingly, DNA damage signals persist in cells that escape from senescence. The implications of these signals for tumorigenesis are also discussed. Given that DNA damage signals have now been observed in cells treated with any stimuli known to induce senescence, the process can be redefined as a metabolically viable but permanent cell cycle arrest with persistent DNA damage signaling.     

The role of the IGF-I receptor in the growth and transformation of mammalian cells

Abstract. Recent developments in the molecular biology of the insulin-like growth factor I (IGF-I) receptor have clarified its role in cellular growth and transformation. Although cells homozygous for a targeted disruption of the IGF-I receptor genes can grow in serum-supplemented medium, the IGF-I receptor is required for optimal growth, and is required equally in all phases of the cell cycle. The receptor plays an even more stringent role in cellular transformation and tumorigenicity, which seem to be dependent on its normal expression in several cell types. The expression of both the IGF-I receptor and its ligands is regulated by other growth factors (especially PDGF and EGF), by oncogenes (like SV40 T antigen and c-myb) and by tumour suppressor genes (like WT1 and RB). The picture emerging from these studies is that several transforming agents may exert their growth promoting effects through the direct or indirect activation of the IGF autocrine loop.     

Oncogene expression cloning by retroviral transduction of adenovirus E1A-immortalized rat kidney RK3E cells: transformation of a host with epithelial features by c-MYC and the zinc finger protein GKLF.

The function of several known oncogenes is restricted to specific host cells in vitro, suggesting that new genes may be identified by using alternate hosts. RK3E cells exhibit characteristics of epithelia and are susceptible to transformation by the G protein RAS and the zinc finger protein GLI. Expression cloning identified the major transforming activities in squamous cell carcinoma cell lines as c-MYC and the zinc finger protein gut-enriched Kruppel-like factor (GKLF)/epithelial zinc finger. In oral squamous epithelium, GKLF expression was detected in the upper, differentiating cell layers. In dysplastic epithelium, expression was prominently increased and was detected diffusely throughout the entire epithelium, indicating that GKLF is misexpressed in the basal compartment early during tumor progression. The results demonstrate transformation of epithelioid cells to be a sensitive and specific assay for oncogenes activated during tumorigenesis in vivo, and identify GKLF as an oncogene that may function as a regulator of proliferation or differentiation in epithelia.     

Tumorigenicity of fibroblast lines expressing the adenovirus E1a, cellular p53, or normal c-myc genes.

Cellular and viral oncogenes have been linked to the transformation of established cell lines in vitro, to the induction of tumors in vivo, and to the partial transformation or immortalization of primary cells. Based on the ability to cooperate with mutated ras oncogenes in the transformation of primary cells, the adenovirus E1a and cellular p53 genes have been assigned an immortalizing activity. It is demonstrated in this paper that the adenovirus type 5 E1a gene and simian virus 40 promoter-linked p53 cDNA are able to transform previously immortalized cells to a tumorigenic phenotype without a significant change in cell morphology. It is also shown that, when linked to a constitutive promoter, the normal mouse and human c-myc genes have the same transforming activity. Cells transformed by each of these oncogenes have an increased capacity to grow in the absence of growth factors and a limited anchorage-independent growth capability.     

利用cDNA微阵列-CGH筛选支气管上皮细胞恶性转化中的扩增基因

目的:基因组不稳定是导致肺癌发生与发展的重要分子机理之一。本研究旨在筛选支气管上皮细胞恶性转化过程中拷贝数扩增的基因。方法:利用业已建立的支气管上皮细胞体外恶性转化模型,通过cDNA微阵列-CGH技术对支气管上皮来源的永生化细胞和癌变细胞的基因拷贝数进行了检测,并对部分结果进行了实时PCR验证。结果:永生化BEP2D细胞染色体中的某些区域存在不同程度的扩增,包括5q31.3、9q32-33.1、14q22.2-23.1、19p13.12-13.13、20q13.12-13.31;恶性转化BERP35T2细胞染色体中的扩增区域集中在1p12-13.1、5q33.1、5q31.3、9q32、19p13.12-13.13;5q31.3、9q32、19q13.12-13.13是以上2种细胞系中的共同扩增区域。共检测到201个基因的拷贝数发生扩增,其中PCNA、TP53及GADD45A基因的异常扩增已经实时PCR进一步验证。结论:在支气管上皮细胞恶性转化过程中,病毒与低剂量辐射的双重作用使得某些重要基因的拷贝数发生扩增,因基因剂量增加而导致某些癌基因高表达可能是细胞恶性转化的重要机制之一。     

Complementation by BCL2 and C-HA-RAS oncogenes in malignant transformation of rat embryo fibroblasts.

The BCL2 (B cell lymphoma/leukemia-2) and C-HA-RAS oncogenes encode membrane-associated proteins of 26 and 21 kilodaltons, respectively. Although RAS proteins have long been known for their ability to bind and hydrolyze GTP, recent investigations suggest that BCL2 encodes a novel GTP-binding protein (S. Haldar, C. Beatty, Y. Tsujimoto, and C. M. Croce, Nature [London] 342:195-198, 1989). Cotransfection of BCL2 and HA-RAS oncogenes resulted in morphological transformation of early-passage rodent fibroblasts, rendering these cells tumorigenic in animals and enabling them to grow in semisolid medium. In contrast, cotransfection of BCL2 with oncogenes that encode nuclear proteins (E1A and C-MYC) did not produce malignant transformation, whereas HA-RAS did complement with these genes. These findings suggest that proteins encoded by oncogenes such as BCL2 and HA-RAS, although having similar subcellular locations and perhaps similar biochemical properties, can regulate distinct complementary pathways involved in cellular transformation.     

How do mutated oncogenes and tumor suppressor genes cause cancer?

In recent decades we have been given insight into the process that transforms a normal cell into a malignant cancer cell. It has been recognised that malignant transformation occurs through successive mutations in specific cellular genes, leading to the activation of oncogenes and inactivation of tumor suppressor genes. The further study of these genes has generated much of its excitement from the convergence of experiments addressing the genetic basis of cancer, together with cellular pathways that normally control important cellular regulatory programmes. In the present review the context in which oncogenes such as proliferation, cell death/apoptosis, differentiation and senescence will be described, as well as how these cellular programmes become deregulated in cancer due to mutations.     

Immortalization and malignant transformation of Eukaryotic cells

The process of cellular transformation has been amply studied in vitro using immortalized cell lines. Immortalized cells never have the normal diploid karyotype, nevertheless, they cannot grow over one another in cell culture (contact inhibition), do not form colonies in soft agar (anchorage-dependent growth) and do not form tumors when injected into immunodeficient rodents. All these characteristics can be obtained with additional chromosome changes. Multiple genetic rearrangements, including whole chromosome and gene copy number gains and losses, chromosome translocations, gene mutations are necessary for establishing the malignant cell phenotype. Most of the experiments detecting transforming ability of genes overexpressed and/or mutated in tumors (oncogenes) were performed using mouse embryonic fibroblasts (MEFs), NIH3T3 mouse fibroblast cell line, human embryonic kidney 293 cell line (HEK293), and human mammary epithelial cell lines (mainly HMECs and MCF10A). These cell lines have abnormal karyotypes and are prone to progress to malignantly transformed cells. This review is aimed at understanding the mechanisms of cell immortalization by different “immortalizing agents”, oncogene-induced cell transformation of immortalized cells and moderate response of the advanced tumors to anticancer therapy in the light of tumor “oncogene and chromosome addiction”, intra-/intertumor heterogeneity, and chromosome instability.