Ras-inducible immortalized fibroblasts: focus formation without cell cycle deregulation

The Cell Transformation Assay: A Historical Assessment of Current Knowledge of Applications in an Integrated Approach to Testing and Assessment for Non-Genotoxic Carcinogens

The history of the development of the cell transformation assays (CTAs) is described, providing an overview of in vitro cell transformation from its origin to the new transcriptomic-based CTAs. Application of this knowledge is utilized to address how the different types of CTAs, variously addressing initiation and promotion, can be included on a mechanistic basis within the integrated approach to testing and assessment (IATA) for non-genotoxic carcinogens. Building upon assay assessments targeting the key events in the IATA, we identify how the different CTA models can appropriately fit, following preceding steps in the IATA. The preceding steps are the prescreening transcriptomic approaches, and assessment within the earlier key events of inflammation, immune disruption, mitotic signaling and cell injury. The CTA models address the later key events of (sustained) proliferation and change in morphology leading to tumor formation. The complementary key biomarkers with respect to the precursor key events and respective CTAs are mapped, providing a structured mechanistic approach to represent the complexity of the (non-genotoxic) carcinogenesis process, and specifically their capacity to identify non-genotoxic carcinogenic chemicals in a human relevant IATA.

Krüppel-like factor 6 interferes with cellular transformation induced by the H-ras oncogene

KLF6 is a member of the Krüppel-like factor family of transcription factors, with diverse roles in the regulation of cell physiology, including proliferation, signal transduction, and apoptosis. Mutations or down-regulation of KLF6 have been described in several human cancers. In this work, we found that KLF6-knockdown resulted in the formation of transformed foci and allowed the spontaneous conversion of NIH3T3 cells to a tumorigenic state. We further assessed the role of KLF6 in the context of oncogenic Ras. We showed that KLF6 was up-regulated by H-Ras(G12V) expression in a Jun N-terminal kinase (JNK)-dependent manner, correlated with enhanced klf6 promoter activity. We found that ectopic KLF6 expression induced a G1-phase cell cycle arrest, thereby decreasing the cell proliferation rate. In addition, constitutive KLF6 expression impaired H-Ras(G12V)-mediated loss of density-dependent growth inhibition and anchorage-independent growth. Moreover, growth of H-Ras(G12V)-driven tumors was reduced in mice challenged with cells stably expressing KLF6. KLF6 expression correlated with the up-regulation of p21, whereas neither p53 induction nor apoptotic cell death was detected. Further, p21 knockdown impaired KLF6-induced cell cycle arrest. These findings provide novel evidence highlighting KLF6 function in response to malignant transformation, suggesting the relevance of KLF6 in controlling cell proliferation and hindering tumorigenesis.

The aging of the 2000 and 2011 Hallmarks of Cancer reviews: a critique

Two review articles published in 2000 and 2011 by Hanahan and Weinberg have dominated the discourse about carcinogenesis among researchers in the recent past. The basic tenets of their arguments favour considering cancer as a cell-based, genetic disease whereby DNA mutations cause uncontrolled cell proliferation. Their explanation of cancer phenotypes is based on the premises adopted by the somatic mutation theory (SMT) and its cell-centered variants. From their perspective, eight broad features have been identified as so-called 'Hallmarks of Cancer'. Here, we criticize the value of these features based on the numerous intrinsic inconsistencies in the data and in the rationale behind SMT. An alternative interpretation of the same data plus data mostly ignored by Hanahan and Weinberg is proposed, based instead on evolutionarily relevant premises. From such a perspective, cancer is viewed as a tissue-based disease. This alternative, called the tissue organization field theory, incorporates the premise that proliferation and motility are the default state of all cells, and that carcinogenesis is due to alterations on the reciprocal interactions among cells and between cells and their extracellular matrix. In this view, cancer is development gone awry.

ISGylation governs the oncogenic function of Ki-Ras in breast cancer

Aberrant expression of the oncogenic Kirsten-Ras (Ki-Ras) and interferon-stimulated gene 15 (ISG15) pathways is common in breast and other cancers. However, whether these dysregulated pathways cooperate to promote malignancy is not known. This study links Ki-Ras and ISG15 in a previously unidentified regulatory loop that may underlie malignant transformation of mammary cells. We show that oncogenic Ki-Ras regulates the expression of the ISG15 pathway (free ISG15 and ISG15 conjugates), and ISG15, in turn, stabilizes Ki-Ras protein by inhibiting its targeted degradation via lysosomes in breast cancer cells. Disruption of this loop by silencing either Ki-Ras or the ISG15 pathway restored the disrupted cellular architecture, a hallmark feature of most cancer cells. We also demonstrate that ISG15 and UbcH8 (ISG15-specific conjugating enzyme) shRNAs reversed Ki-Ras mutation-associated phenotypes of cancer cells, such as increased cell proliferation, colony formation, anchorage-independent growth in soft agar, cell migration, and epithelial-mesenchymal transition. As UbcH8-silenced breast cancer cells are devoid of ISG15 conjugates but have free ISG15, our results using UbcH8-silenced cells suggest that ISG15 conjugates, and not free ISG15, contributes to oncogenic Ki-Ras transformation. We have thus identified the conjugated form of ISG15 as a critical downstream mediator of oncogenic Ki-Ras, providing a potential mechanistic link between ISG15 and Ki-Ras-mediated breast tumorigenesis. Our findings, which show that inhibition of the ISGylation reverses the malignant phenotypes of breast cancer cells expressing oncogenic Ki-Ras, support the development of ISG15 conjugation inhibitors for treating breast and also other cancers expressing oncogenic Ki-Ras.

Polyploid cells rewire DNA damage response networks to overcome replication stress-induced barriers for tumour progression

Mutations in genes involved in DNA replication, such as flap endonuclease 1 (FEN1), can cause single-stranded DNA breaks (SSBs) and subsequent collapse of DNA replication forks leading to DNA replication stresses. Persistent replication stresses normally induce p53-mediated senescence or apoptosis to prevent tumour progression. It is unclear how some mutant cells can overcome persistent replication stresses and bypass the p53-mediated pathways to develop malignancy. Here we show that polyploidy, which is often observed in human cancers, leads to overexpression of BRCA1, p19arf and other DNA repair genes in FEN1 mutant cells. This overexpression triggers SSB repair and non-homologous end-joining pathways to increase DNA repair activity, but at the cost of frequent chromosomal translocations. Meanwhile, DNA methylation silences p53 target genes to bypass the p53-mediated senescence and apoptosis. These molecular changes rewire DNA damage response and repair gene networks in polyploid tumour cells, enabling them to escape replication stress-induced senescence barriers.

The myogenic kinome: protein kinases critical to mammalian skeletal myogenesis

Myogenesis is a complex and tightly regulated process, the end result of which is the formation of a multinucleated myofibre with contractile capability. Typically, this process is described as being regulated by a coordinated transcriptional hierarchy. However, like any cellular process, myogenesis is also controlled by members of the protein kinase family, which transmit and execute signals initiated by promyogenic stimuli. In this review, we describe the various kinases involved in mammalian skeletal myogenesis: which step of myogenesis a particular kinase regulates, how it is activated (if known) and what its downstream effects are. We present a scheme of protein kinase activity, similar to that which exists for the myogenic transcription factors, to better clarify the complex signalling that underlies muscle development.

Identification of a c-Jun N-terminal kinase-2-dependent signal amplification cascade that regulates c-Myc levels in ras transformation

Ras is one of the most frequently activated oncogenes in cancer. Two mitogen-activated protein kinases (MAPKs) are important for ras transformation: extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase 2 (JNK2). Here we present a downstream signal amplification cascade that is critical for ras transformation in murine embryonic fibroblasts. This cascade is coordinated by ERK and JNK2 MAPKs, whose Ras-mediated activation leads to the enhanced levels of three oncogenic transcription factors, namely, c-Myc, activating transcription factor 2 (ATF2) and ATF3, all of which are essential for ras transformation. Previous studies show that ERK-mediated serine 62 phosphorylation protects c-Myc from proteasomal degradation. ERK is, however, not alone sufficient to stabilize c-Myc but requires the cooperation of cancerous inhibitor of protein phosphatase 2A (CIP2A), an oncogene that counteracts protein phosphatase 2A-mediated dephosphorylation of c-Myc. Here we show that JNK2 regulates Cip2a transcription via ATF2. ATF2 and c-Myc cooperate to activate the transcription of ATF3. Remarkably, not only ectopic JNK2, but also ectopic ATF2, CIP2A, c-Myc and ATF3 are sufficient to rescue the defective ras transformation of JNK2-deficient cells. Thus, these data identify the key signal converging point of JNK2 and ERK pathways and underline the central role of CIP2A in ras transformation.

c-Jun NH2-terminal kinase 2 is required for Ras transformation independently of activator protein 1

Active Ras oncogene is expressed in approximately 30% of human cancers. Yet, very little is known about the molecular mechanisms responsible for its transforming potential. Here, we show that H-Ras-mediated transformation requires isoform 2 of the c-Jun-NH(2)-terminal kinase (JNK). H-Ras-transduced JNK2-deficient (Jnk2-/-) murine embryonic fibroblasts (MEFs) were severely inhibited in colony formation and growth in soft agar in vitro as well as in tumor formation in immunodeficient mice as compared with corresponding Jnk1-/- and wild-type MEFs. Accordingly, the RNA interference-based depletion of JNK2 form wild-type MEFs also resulted in defective Ras transformation. The extra barrier against H-Ras transformation in Jnk2-/- MEFs was not due to their inability to inactivate p53 signaling because all JNK2-deficient MEF lines had lost p19(Arf). Furthermore, expression of the E6 protein of the human papilloma virus failed to overcome the transformation defect. It could, however, be overcome by coexpression of H-Ras with the SV40 large T antigen or c-Myc. Surprisingly, the H-Ras-transduced JNK2-deficient MEFs exhibited higher activity of activator protein-1 and higher levels of c-Jun expression compared with H-Ras-transduced JNK1-deficient or wild-type cells, indicating that the key target of JNK2 during Ras transformation was divergent from activator protein-1. These results clearly show that a single kinase, JNK2, could control Ras transformation and thus point out a vulnerable control point that may prove important for the tumor development in general.

Akt phosphorylation is not sufficient for insulin-like growth factor-stimulated myogenin expression but must be accompanied by down-regulation of mitogen-activated protein kinase/extracellular signal-regulated kinase phosphorylation

IGF-I has a unique biphasic effect on skeletal muscle differentiation. Initially, IGF-I inhibits expression of myogenin, a skeletal muscle-specific regulatory factor essential for myogenesis. Subsequently, IGF-I switches to stimulating expression of myogenin. The mechanisms that mediate this switch in IGF action are incompletely understood. Several laboratories have demonstrated that the phosphatidylinositol-3-kinase/Akt signaling pathway is essential for myogenic differentiation and have suggested that this pathway mediates IGF-I stimulation of myogenin mRNA expression, an early critical step in the differentiation process. These studies, however, did not address concurrent Akt and MAPK/ERK1/2 phosphorylation, the latter of which is also known to regulate myogenic differentiation. In the present study in rat L6E9 muscle cells, we have manipulated ERK1/2 phosphorylation with either an upstream inhibitor or activator and examined concurrent levels of Akt and ERK1/2 phosphorylation and of myogenin mRNA expression in response to treatment with IGF-I. We find that even in the presence of phosphorylated Akt, it is only when ERK1/2 phosphorylation is inhibited that IGF-I can stimulate myogenin mRNA expression. Thus, although Akt phosphorylation may be necessary, it is not sufficient for induction of myogenic differentiation by IGF-I and must be accompanied by a decrease in ERK1/2 phosphorylation.

Ras oncogene induces β-galactoside α2,6-sialyltransferase (ST6Gal I) via a RalGEF-mediated signal to its housekeeping promoter

Several oncogenic proteins are known to influence cellular glycosylation. In particular, transfection of codon 12 point mutated H-Ras increases CMP-Neu5Ac: Galbeta1,4GlcNAc alpha2,6-sialyltransferase I (ST6Gal I) activity in rodent fibroblasts. Given that Ras mediates its effects through at least three secondary effector pathways (Raf, RalGEFs and PI3K) and that transcriptional control of mouse ST6Gal I is achieved by the selective use of multiple promoters, we attempted to identify which of these parameters are involved in linking the Ras signal to ST6Gal I gene transcription in mouse fibroblasts. Transformation by human K-Ras or H-Ras (S12 and V12 point mutations, respectively) results in a 10-fold increase in ST6Gal I mRNA, but no alteration in the expression of related sialyltransferases. Using an inducible H-RasV12 expression system, a direct causal link between activated H-Ras expression and elevated ST6Gal I mRNA was demonstrated. The accumulation of the ST6Gal I transcript in response to activated Ras was accompanied by an increase of alpha2,6-sialyltransferase activity and of Neu5Acalpha2,6Gal at the cell surface. Results obtained with H-RasV12 partial loss of function mutants H-RasV12S35 (Raf signal only), H-RasV12C40 (PI3-kinase signal only) and H-RasV12G37 (RalGEFs signal only) suggest that the H-Ras induction of the mouse ST6Gal I gene (Siat1) transcription is primarily routed through RalGEFs. 5'-Rapid amplification of cDNA ends analysis demonstrated that the increase in ST6Gal I mRNA upon H-RasV12 or K-RasS12 transfection is mediated by the Siat1 housekeeping promoter P3-associated 5' untranslated exons.

Loss of von Hippel-Lindau protein causes cell density dependent deregulation of CyclinD1 expression through hypoxia-inducible factor

Loss of the von Hippel-Lindau gene (VHL) expression ca-uses deregulation of contact inhibition of cell growth, which might be one of the bases of the tumor suppressor function of VHL. Here we show that this function of the VHL gene product (pVHL) depends on cell autonomous events. To identify the target gene of pVHL, which is directly involved in the contact inhibition, we compared the gene expression profile between VHL-deficient renal carcinoma 786-O cells and those infected with an adenovirus vector encoding VHL. In addition to known pVHL-regulated genes, such as vascular endothelial growth factor and carbonic anhydrase, we found cyclinD1 as a new target of pVHL at a high cell density. In VHL-expressing cells (VHL (+) cells), the cyclinD1 mRNA expression level diminishes at a high cell density, while it remains at a relatively high level in VHL-deficient cells (VHL (-) cells). The cyclinD1 expression level was also abnormally high in VHL (-) cells at a high cell density. Consequently, the phosporylation level of the retinoblastoma (Rb) protein remained high in these cells, whereas there was no phosporylated Rb in VHL (+) cells under the contact inhibition. The abnormal expression of cyclinD1 at a high cell density was observed even in VHL (+) cells under the hypoxic state. Moreover, ectopic expression of a HIF mutant resistant to pVHL-mediated proteolysis causes the abnormal cyclinD1 expression in VHL (+) cells. Taken together, these observations indicate that VHL is required for the downregulation of cyclinD1 at a high cell density through HIF.

Induction of p202, a modulator of apoptosis, during oncogenic transformation of NIH 3T3 cells by activated H-Ras (Q61L) contributes to cell survival

Previous studies have revealed that p202 (52 kDa), an interferon (IFN) and differentiation-inducible protein, negatively regulates cell proliferation and modulates cell survival. However, the role of p202 in transformed cells remains to be investigated. Here we report that constitutive expression of oncogenic H-Ras (Q61L) in NIH 3T3 cells, which resulted in cell transformation, was associated with increases in the steady-state levels of 202 RNA and protein. Interestingly, the increase in p202 levels in transformed cells correlated with increases in the activity of the transcription factor c-Jun/AP-1, which bound to the two potential AP-1 DNA binding sites (the AP-1CS1 and AP-1CS2) in the 5'-regulatory region of the 202 gene in gel mobility shift assays. Furthermore, the site-directed mutagenesis, coupled with promoter-reporter analyses, revealed that these two AP-1 DNA binding sites contribute to the regulation of the 202 gene in Ras transformed cells. Because treatment of transformed cells with a specific inhibitor of MEK (PD 98059) resulted in significant decreases in the levels of p202, these observations raise the possibility that in transformed cells Ras/Raf/MEK pathway regulates the transcriptional activation of the 202 gene. Significantly, decreases in the levels of p202 in Ras transformed NIH 3T3 cells under reduced serum conditions increased the susceptibility to apoptosis. Collectively, our observations support the idea that the transcriptional increases in the levels of p202 by oncogenic H-Ras in NIH 3T3 cells are needed for cell survival.