Epithelial carcinogenesis in the mouse: correlating the genetics and the biology

Can tissue biomarkers be used to determine the prognosis of oral leukoplakia lesions with regards to malignant transformation?

Design Systematic review looking at published studies which aimed to identify human tissue biomarkers which could predict malignant transformation of oral leukoplakia (OL) lesions.Case/control selection Articles were identified from PubMed, EBSCO and Cochrane library databases using combinations of keywords. Randomised clinical trials, case-control studies and prospective and retrospective cohort studies were included. All studies had to include follow-up data. There were no restrictions regarding gender, age groups, geographic origin, or year of publication. The selection process involved two of the researchers independently analysing titles and abstracts to identify articles for full-text reading. The same authors then read the articles identified for full-text reading, applying the eligibility criteria. If there was a disagreement with regards to the selection, a discussion was had until a consensus was reached.Data analysis Qualitative data was presented as reported from the primary studies. Quantitative analysis was not carried out due to the high heterogeneity across the studies.Results Forty-six studies were included within the systematic review. These studies identified a total of 3,783 patients with OL, of which 1,047 went on to develop oral cancer, giving an average malignant transformation incidence of 27.6%. Forty-nine different tissue biomarkers were evaluated among the included studies, with the p53 and podoplanin proteins and loss of heterozygosity the most frequently discussed.Conclusions Of the biomarkers studied, podoplanin and chromosomal loci abnormalities (such as loss of heterozygosity) would appear the most promising in being able to predict malignant transformation of OL lesions; however, further research is required.

Development and validation of nomogram for prediction of malignant transformation in oral leukoplakia: A large-scale cohort study

PURPOSE

Oral leukoplakia (OL) is the well-known disorder of oral mucosa, which has potential to be malignant and can lead to squamous cell carcinoma (OSCC). In the following study, we developed a comprehensive nomogram for predicting the malignant progression of OL, based on analysis of clinicopathological variables.

METHODS

A retrospective analysis of patients diagnosed with OL was performed between 1998 and 2017 at the Peking University School and Hospital of Stomatology. OL was confirmed by pre-treatment biopsy. The candidate risk factors for OL malignant transformation were screened from clinicopathological variables using the Cox proportional hazard regression analysis. The nomogram model was generated based on the COX regression results and was validated through Harrell concordance index (c-index) and calibration plots RESULTS: The incidence of OL malignant transformation (MT) was 12.2% (107/875), and the mean follow-up time was 4.5 years. The risk factors (age, histologic grade, site of lesion and smoking habit) derived from Cox proportional hazard regression analysis were incorporated in a novel nomogram model for prediction of MT severity. The c-index value of the nomogram model was 0.752, which confirmed the prediction ability; and was further confirmed by calibration plots results.

CONCLUSION

Our data suggest that patients with OL who are over 50 years old, non-smokers with dysplasia, and OL lesions involving the lip, the floor of mouth, and tongue have an enhanced risk of MT. The established nomogram model has the predictive value of malignant progression, which is conductive to screen high-risk patients and guide treatment strategy.

Mast cells protect from skin tumor development and limit tumor growth during cutaneous de novo carcinogenesis in a Kit-dependent mouse model

Epidermal tumors belong to the most frequent type of neoplasms, and tumor-associated accumulation of mast cells (MCs) has first been observed more than a century ago. Therefore, MCs have been implicated in tumor development and growth; however, the results regarding the role of MC in cutaneous de novo carcinogenesis are still controversially discussed. Here, we subjected MC-deficient Kit(W) /Kit(W-v) mice to chemical skin carcinogenesis. Tumors were induced using the carcinogen 7,12-dimethylbenz[a]-anthracene and subsequent treatment with the tumor promoter 12-tetradecanoyl-phorbol-13-acetat. The treatment resulted in pronounced inflammatory cell infiltrates that were diminished in MC-deficient animals. Unexpectedly, tumor development and growth was significantly increased in MC-deficient Kit(W) /Kit(W-v) mice. The repair of their MC deficiency by local adoptive transfer of MCs normalized tumor incidence and growth. The recruitment of skin-infiltrating immune cells, particularly of F4/80+ monocytes, Gr-1+ granulocytes, B220+ B cells and CD8+ T lymphocytes, to sites of tumor development was, in part, also controlled by MCs. Recent evidence indicated the importance of local antitumor tissue immunity which prevents tumor development. These findings suggest a critical role for MCs in mediating these host antitumor immune responses in the skin.

Delineating Molecular Mechanisms of Squamous Tissue Homeostasis and Neoplasia: Focus on p63

Mouse models have informed us that p63 is critical for normal epidermal development and homeostasis. The p53/p63/p73 family is expressed as multiple protein isoforms due to a combination of alternative promoter usage and C-terminal alternative splicing. These isoforms can mimic or interfere with one another, and their balance ultimately determines biological outcome in a context-dependent manner. While not frequently mutated, p63, and in particular the ΔNp63 subclass, is commonly overexpressed in human squamous cell cancers. In vitro keratinocytes and murine transgenic and transplantation models have been invaluable in elucidating the contribution of altered p63 levels to cancer development, and studies have identified the roles for ΔNp63 isoforms in keratinocyte survival and malignant progression, likely due in part to their transcriptional regulatory function. These findings can be extended to human cancers; for example, the novel recognition of NFκB/c-Rel as a downstream effector of p63 has identified a role for NFκB/c-Rel in human squamous cell cancers. These models will be critical in enhancing the understanding of the specific molecular mechanisms of cancer development and progression.

Skin squamous cell carcinoma propagating cells increase with tumour progression and invasiveness

Cancer stem cells have been described in various cancers including squamous tumours of the skin by their ability to reform secondary tumours upon transplantation into immunodeficient mice. Here, we used transplantation of limiting dilution of different populations of FACS-isolated tumour cells from four distinct mouse models of squamous skin tumours to investigate the frequency of tumour propagating cells (TPCs) at different stages of tumour progression. We found that benign papillomas, despite growing rapidly in vivo and being clonogenic in vitro, reformed secondary tumours upon transplantation at very low frequency and only when tumour cells were co-transplanted together with tumour-associated fibroblasts or endothelial cells. In two models of skin squamous cell carcinoma (SCC), TPCs increased with tumour invasiveness. Interestingly, the frequency of TPCs increased in CD34(HI) but not in CD34(LO) SCC cells with serial transplantations, while the two populations initially gave rise to secondary tumours with the same frequency. Our results illustrate the progressive increase of squamous skin TPCs with tumour progression and invasiveness and reveal that serial transplantation may be required to define the long-term renewal potential of TPCs.

Establishment and characterization of an osteopontin-null cutaneous squamous cell carcinoma cell line

Osteopontin (OPN) is a secreted glycoprotein implicated to function in cancer development and metastasis. Although elevated expression of OPN are observed in cancer cells of various types, in some cases, only the cells in the stromal region surrounding the tumor express OPN, suggesting distinct functional roles for this protein derived from host cells and from cancer cells. To provide a model for addressing the functions and mechanisms of host-derived OPN in cancer progression and metastasis, a cutaneous squamous cell carcinoma cell line (ONSC) that lacks the OPN gene, Spp1, was established. This line of cells was derived from a squamous cell carcinoma that developed in a female, OPN-null mouse subjected to two-stage skin carcinogenesis. Morphologically, ONSC cells resemble epithelial cells, and they express the epithelial markers, K1, K14, and p63, as confirmed by immunohistochemical analyses. Genomic analyses indicate the presence of mutated H-Ras and p53 genes. ONSC cells form colonies in soft agar and, subcutaneously injected into athymic nude mice, develop into squamous cell carcinomas that metastasize to the lungs. Lacking OPN expression, these squamous cell carcinoma cells provide a model to address the function of host OPN in the context of cancer progression and metastasis.

Effects of MEK and DNMT inhibitors on arsenic-treated human uroepithelial cells in relation to Cyclin-D1 and p16

Arsenic compounds are well-known toxic and carcinogenic agents, and they are widely distributed throughout the earth's crust. These compounds are associated with various human malignancies. It has been reported that there is an elevated risk of bladder cancer in an area highly contaminated with arsenic on the southwest coast of Taiwan. However, the underlying mechanisms of arsenic-associated carcinogenesis are still unclear. The cell cycle regulatory proteins are important indicators in control of cell cycle progression. Moreover, the high expression of Cyclin-D1 and loss of p16 has been associated with a worse prognosis in a variety of human cancers. Therefore, we investigated the effect of arsenic on Cyclin-D1 and p16 expression and evaluated the role of the ERK signaling pathway and DNA methylation in arsenic carcinogenesis. Our study results showed that Cyclin-D1 high expression was found in 56.3% (9/16) of urothelial carcinomas (UC) from a blackfoot disease (BFD) area and 6.3% (1/16) of UC from a non-BFD area (p=0.002). The p16 low expression in 81.2% (13/16) of UC from BFD areas was significantly lower than in non-BFD areas (25.0%; 4/16) (p=0.001). In addition, the Cyclin-D1 increased expression but decreased p16 expression in arsenite-treated SV-HUC-1 cells. However, when cells were pretreated with inhibitors (5-aza-CdR or U0126), the effects of arsenite on Cyclin-D1 and p16 expression were suppressed. Finally, these results indicated that Cyclin-D1 and p16 both might play important roles in carcinogenesis as a result of arsenic.

PAI-1 Regulates the Invasive Phenotype in Human Cutaneous Squamous Cell Carcinoma

The emergence of highly aggressive subtypes of human cutaneous squamous cell carcinoma (SCC) often reflects increased autocrine/paracrine TGF-beta synthesis and epidermal growth factor receptor (EGFR) amplification. Cooperative TGF-beta/EGFR signaling promotes cell migration and induces expression of both proteases and protease inhibitors that regulate stromal remodeling resulting in the acquisition of an invasive phenotype. In one physiologically relevant model of human cutaneous SCC progression, TGF-beta1+EGF stimulation increases the production of several matrix metalloproteinases (MMPs), among the most prominent of which is MMP-10-an MMP known to be elevated in SCC in situ. Activation of stromal plasminogen appears to be critical in triggering downstream MMP activity. Paradoxically, PAI-1, the major physiological inhibitor of plasmin generation, is also upregulated under these conditions and is an early event in progression of incipient epidermal SCC. One testable hypothesis proposes that TGF-beta1+EGF-dependent MMP-10 elevation directs focalized matrix remodeling events that promote epithelial cell plasticity and tissue invasion. Increased PAI-1 expression serves to temporally and spatially modulate plasmin-initiated pericellular proteolysis, further facilitating epithelial invasive potential. Defining the complex signaling and transcriptional mechanisms that maintain this delicate balance is critical to developing targeted therapeutics for the treatment of human cutaneous malignancies.

Targeted disruption of stat3 reveals a major role for follicular stem cells in skin tumor initiation

The initiation stage of mouse skin carcinogenesis involves the induction of mutations in keratinocyte stem cells (KSC), which confers a selective growth advantage allowing clonal expansion during tumor promotion. Targeted disruption of signal transducer and activator of transcription 3 (Stat3) in bulge region KSCs was achieved by treating K15.CrePR1 x Stat3(fl/fl) mice with RU486. Deletion of Stat3 prior to skin tumor initiation with 7,12-dimethylbenz(a)anthracene significantly increased the number of apoptotic KSCs and decreased the frequency of Ha-ras codon 61 A(182)-->T transversion mutations in this cell population compared with wild-type littermates. Targeted disruption of Stat3 in bulge region KSCs at the time of initiation also dramatically reduced the number of skin tumors (by approximately 80%) produced following promotion with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate. These results show that Stat3 is required for the survival of bulge region KSCs during tumor initiation. Furthermore, these data provide direct evidence that bulge region KSCs are the primary targets for the initiation of skin tumors in this model system.

Identification of genes and gene ontology processes critical to skin papilloma development in Tg.AC transgenic mice

This study analyzes gene expression associated with papilloma development in Tg.AC v-Ha-ras transgenic mice and identifies novel genes and biological processes that may be critical to skin carcinogenesis in these mice. Epidermal abrasion was used to synchronously induce epidermal regeneration in FVB/N wild type and transgenic Tg.AC mice. Skin papillomagenesis was uniquely induced in Tg.AC mice, and gene expression profiling was carried out using a 22,000 element mouse DNA microarray. Histological analysis showed that papillomas developed at a high rate by d 30 after abrasion in transgenic animals, while no papilloma developed in wild type mice. Transgene-specific differentially expressed genes were identified at d 30 postabrasion and these genes were annotated using EASE software and literature mining. Annotated and non-annotated genes associated with papilloma development were identified and clustering analysis revealed groups of genes that are coordinately expressed. A number of genes associated with differentiation and development were also physically clustered on mouse chromosome 16, including 16B3 that contains several Stefins and stefin-like genes, and 16A1 containing a number of keratin associated protein genes. Additional analyses presented here yield novel insights into the genes and processes involved in papilloma development in Tg.AC mice.

Chromosomal instability and phenotypic plasticity during the squamous–spindle carcinoma transition: association of a specific T(14;15) with malignant progression

In mouse epidermal carcinogenesis, the latest stage of malignant progression involves the transition from squamous cell carcinoma to a highly aggressive type of tumor with spindle morphology. In this work, we have isolated a minor epithelial cell subpopulation (CarC-R) contained in the highly malignant spindle carcinoma cell line CarC. CarC-R exhibited a drastic reduction in tumorigenicity when compared with CarC, but CarC-R-induced tumors were mainly sarcomatoid, although they subsequently reverted to the epithelial phenotype when tumor explants were recultured in vitro. Several single-cell clones with either stable epithelial or fibroblastic phenotypes were isolated from an explanted CarC-R tumor (CarC-RT). All these cell lines contained the same specific point mutation in H-Ras codon 61, but while CarC spindle cells had lost the normal H-Ras allele, it was retained in CarC-R- and CarC-RT-derived cell lines. Furthermore, CarC cells have inactivated p16INK4a and p19INK4a/ARF transcription, while CarC-R and CarC-RT clones expressed p19 mRNA and protein but not p16. Altogether, these results suggest that CarC-R represents a precursor stage to CarC in malignant progression. Spectral karyotyping analysis revealed that CarC-R was highly aneuploid and contained many chromosomal abnormalities. In contrast, CarC had a diploid or tetraploid modal chromosome number and contained a specific T(14;15) translocation in all of the analysed metaphases. The T(14;15) translocation was present in only a minority (1.9%) of CarC-R cells, but it was widely spread in CarC-RT and its derived cell clones, regardless of their epithelial or fibroblastic phenotype, indicating that T(14;15) segregates with malignancy.

Notch signaling in the integrated control of keratinocyte growth/differentiation and tumor suppression

Oncogenesis is closely linked to abnormalities in cell differentiation. Notch signaling provides an important form of intercellular communication involved in cell fate determination, stem cell potential and differentiation. Here we review the role of this pathway in the integrated growth/differentiation control of the keratinocyte cell type, and the maintenance of normal skin homeostasis. In parallel with the pro-differentiation function of Notch1 in keratinocytes, we discuss recent evidence pointing to a tumor suppressor function of this gene in both mouse skin and human cervical carcinogenesis. The possibility that Notch signaling elicits signals with a duality of growth positive and negative function will be discussed.

A signaling pathway involving TGF-beta2 and snail in hair follicle morphogenesis

In a common theme of organogenesis, certain cells within a multipotent epithelial sheet exchange signals with their neighbors and develop into a bud structure. Using hair bud morphogenesis as a paradigm, we employed mutant mouse models and cultured keratinocytes to dissect the contributions of multiple extracellular cues in orchestrating adhesion dynamics and proliferation to shape the cluster of cells involved. We found that transforming growth factor beta2 signaling is necessary to transiently induce the transcription factor Snail and activate the Ras-mitogen-activated protein kinase (MAPK) pathway in the bud. In the epidermis, Snail misexpression leads to hyperproliferation and a reduction in intercellular adhesion. When E-cadherin is transcriptionally down-regulated, associated adhesion proteins with dual functions in signaling are released from cell-cell contacts, a process which we demonstrate leads to Ras-MAPK activation. These studies provide insights into how multipotent cells within a sheet are stimulated to undergo transcriptional changes that result in proliferation, junctional remodeling, and bud formation. This novel signaling pathway further weaves together the web of different morphogens and downstream transcriptional events that guide hair bud formation within the developing skin.

TNF-alpha regulates epithelial expression of MMP-9 and integrin alphavbeta6 during tumour promotion. A role for TNF-alpha in keratinocyte migration?

Mice deficient in TNF-alpha (TNF-alpha(-/-) mice) are resistant to skin carcinogenesis and expression of MMP-9 is inhibited in TNF-alpha(-/-) mice during skin tumour development. In the early stages of tumour promotion, MMP-9 protein initially localized to the follicular epidermis but subsequently began to accumulate in the interfollicular epidermis of wild-type but not TNF-alpha(-/-) mice. Inhibition of TNF-alpha or MMP-9 function reduced keratinocyte migration in vitro. In addition, a deficiency of TNF-alpha delayed re-epithelialization in vivo and this correlated with reduced MMP-9 expression. Collectively, these data suggest that MMP-9 regulates keratinocyte migration in a TNF-alpha-dependent manner. Expression profiling of genes that control cell adhesion and migration revealed markedly lower levels of the integrin subunits alphav and beta6 in TNF-alpha(-/-) compared with wild-type keratinocytes in vitro. alphavbeta6 expression was upregulated by keratinocytes in vitro and during tumour promotion in vivo in a TNF-alpha-dependent manner. Furthermore, alphavbeta6 blockade significantly inhibited keratinocyte migration and TNF-alpha-stimulated MMP-9 expression in vitro. These data illustrate a novel TNF-alpha-dependent mechanism for the control of alphavbeta6 expression and suggest one pathway for TNF-alpha regulation of MMP-9. Increased MMP-9 and alphavbeta6 expression may stimulate epithelial cell migration during tumour formation and may be one mechanism whereby TNF-alpha acts as an endogenous tumour promoter.

Expression of both TNF-alpha receptor subtypes is essential for optimal skin tumour development

Keratinocyte-derived TNF-alpha acts as an endogenous tumour promoter and can also regulate AP-1 activity in mouse epidermis. To gain further insight into TNF-alpha signalling during skin tumour formation, mice deficient in TNFR1 (TNFR1-/- mice) or TNFR2 (TNFR2-/- mice) were subjected to chemical carcinogenesis. Tumour multiplicity was significantly reduced in TNFR1-/- and TNFR2-/- mice compared to wild-type (wt) mice, suggesting that both receptors have protumour activity. However, TNFR1-/- mice were markedly more resistant to tumour development than TNFR2-/- mice indicating that TNFR1 is the major mediator of TNF-alpha-induced tumour formation. TNFR1 and TNFR2 were both expressed in wt epidermis during tumour promotion and by primary keratinocytes in vitro. TPA-induced c-Jun expression was transient in TNFR1-/- and TNFR2-/- compared to wt epidermis and this was reflected by reduced induction of the AP-1-responsive genes granulocyte/macrophage-colony stimulating factor, matrix metalloproteinase-9 and matrix metalloproteinase-3. These genes were differentially regulated in TNFR1-/- compared to TNFR2-/- epidermis, suggesting that the TNF-alpha receptors act independently via different AP-1 complexes to transduce TNF-alpha signals during tumour promotion. In addition, TNFR2 cooperated with TNFR1 to optimise TNFR1-mediated TNF-alpha bioactivity on keratinocytes in vitro. Our data provide further insight into TNF-alpha signalling in malignancy and provide some rationale for the use of TNF-alpha antagonists in the treatment of cancer.

Cancer selection

Cancers are often thought to be selectively neutral. This is because most of the individuals that they kill are post-reproductive. Some cancers, however, kill the young and so select for anticancer adaptations that reduce the chance of death. These adaptations could reduce the somatic mutation rate or the selective value of a mutant clone of cells, or increase the number of stages required for neoplasia. New theory predicts that cancer selection--selection to prevent or postpone deaths due to cancer--should be especially important as animals evolve new morphologies or larger, longer-lived bodies, and might account for some of the differences in the causes of cancer between mice and men.

High incidence of epithelial cancers in mice deficient for DNA polymerase delta proofreading

Mutations are a hallmark of cancer. Normal cells minimize spontaneous mutations through the combined actions of polymerase base selectivity, 3' --> 5' exonucleolytic proofreading, mismatch correction, and DNA damage repair. To determine the consequences of defective proofreading in mammals, we created mice with a point mutation (D400A) in the proofreading domain of DNA polymerase delta (poldelta, encoded by the Pold1 gene). We show that this mutation inactivates the 3' --> 5' exonuclease of poldelta and causes a mutator and cancer phenotype in a recessive manner. By 18 months of age, 94% of homozygous Pold1(D400A/D400A) mice developed cancer and died (median survival = 10 months). In contrast, only 3-4% of Pold1(+/D400A) and Pold1(+/+) mice developed cancer in this time frame. Of the 66 tumors arising in 49 Pold1(D400A/D400A) mice, 40 were epithelial in origin (carcinomas), 24 were mesenchymal (lymphomas and sarcomas), and two were composite (teratomas); one-third of these animals developed tumors in more than one tissue. Skin squamous cell carcinoma was the most common tumor type, occurring in 60% of all Pold1(D400A/D400A) mice and in 90% of those surviving beyond 8 months of age. These data show that poldelta proofreading suppresses spontaneous tumor development and strongly suggest that unrepaired DNA polymerase errors contribute to carcinogenesis. Mice deficient in poldelta proofreading provide a tractable model to study mechanisms of epithelial tumorigenesis initiated by a mutator phenotype.

Molecular cloning and sequence analysis of the promoter region of mouse cyclin D1 gene: implication in phorbol ester-induced tumour promotion

Cyclin D1 is a cell cycle regulatory protein, which acts as a growth factor sensor to integrate extracellular signals with the cell cycle machinery, particularly during G1 phase of the cell cycle. Previous study using promotion-sensitive JB6 mouse epidermal cells, an in vitro model of the promotion stage of multistage carcinogenesis, showed that the expression of cyclin D1 is stimulated in the presence (but not in the absence) of 12-O-tetradecanoylphorbol-13-acetate (TPA) in these cells maintained under anchorage-independent culture conditions. In the present study, to explore the molecular basis of this observation, the promoter region of mouse cyclin D1 gene was cloned and sequenced (GenBank accession number AF212040). Dot matrix comparison of mouse, human and rat promoter sequences indicated that the mouse promoter is homologous to the human and more so to the rat promoters. The mouse promoter, like human and rat promoters, lacks canonical TATA-box or TATA-like sequence, but it has one or possibly two initiator (Inr) or Inr-like sequences. Energy dot plot analysis predicted that the mouse promoter consists of three domains: (1) the 3' domain contains NF-kappaB response element, cAMP-response element (CRE), Inr or Inr-like elements, Sp1 binding site and Oct 1 (2) the middle domain contains another Sp1 binding site, E-box and E2F binding site and (3) the 5' domain contains TPA-response element (TRE) and a tandem silencer element. The cyclin D1 promoter sequence of either promotion-sensitive or resistant JB6 mouse epidermal cells was, except for a few minor differences, essentially identical to the sequence determined for a mouse genomic clone. Since TPA is capable of stimulating the expression of cyclin D1 not only through TRE but also through CRE and NF-kappaB response element in the promoter, we tentatively propose a sequence of events that possibly leads to TPA-induced, anchorage-independent synthesis of cyclins D1 and A in the promotion-sensitive JB6 mouse epidermal cells.

CCAAT/enhancer binding protein-beta is a mediator of keratinocyte survival and skin tumorigenesis involving oncogenic Ras signaling

The basic leucine zipper transcription factor CCAAT/enhancer binding protein-beta (C/EBPbeta) is expressed in many cell types, including keratinocytes. C/EBPbeta activity can be increased by phosphorylation through pathways stimulated by oncogenic Ras, although the biological implications of Ras-C/EBPbeta signaling are not currently understood. We report here that C/EBPbeta-nullizygous mice are completely refractory to skin tumor development induced by a variety of carcinogens and carcinogenesis protocols, including 7,12-dimethylbenz[a]anthracene-initiation/12-O-tetradecanoylphorbol 13-acetate promotion, that produce tumors containing oncogenic Ras mutations. No significant differences in TPA-induced epidermal keratinocyte proliferation were observed in C/EBPbeta-null versus wild-type mice. However, apoptosis was significantly elevated (17-fold) in the epidermal keratinocytes of 7,12-dimethylbenz[a]anthracene-treated C/EBPbeta-null mice compared with wild-type mice. In v-Ha-ras transgenic mice, C/EBPbeta deficiency also led to greatly reduced skin tumor multiplicity and size, providing additional evidence for a tumorigenesis pathway linking Ras and C/EBPbeta. Oncogenic Ras potently stimulated C/EBPbeta to activate a C/EBP-responsive promoter-reporter in keratinocytes and mutating an ERK1/2 phosphorylation site (T188) in C/EBPbeta abolished this Ras effect. Finally, we observed that C/EBPbeta participates in oncogenic Ras-induced transformation of NIH 3T3 cells. These findings indicate that C/EBPbeta has a critical role in Ras-mediated tumorigenesis and cell survival and implicate C/EBPbeta as a target for tumor inhibition.

Paired human fibrosarcoma cell lines that possess or lack endogenous mutant N-ras alleles as experimental model for Ras signaling pathways

We present here a human cell model for examination of mutant N-ras function. The HT1080 human fibrosarcoma cell line is pseudodiploid and contains a single endogenous mutant N-ras allele. MCH603c8 cells are a variant of HT1080 cells, in which the mutant allele has been deleted. The two cell lines differ dramatically in the constitutive levels of activation of downstream members of the Ras signaling pathways, and in biological features of transformation and tumorigenicity. Downregulation or activation of individual Ras-dependent pathways can be accomplished via transfection of dominant negatives or activated mutant cDNAs into HT1080 and MCH603c8 cells, respectively. The biochemical and biological consequences of expression of these mutant cDNAs can be assessed. There are dramatic effects on both the transformed and tumorigenic phenotype, depending on the cell line and mutant cDNA that is transfected.

Proliferation, cell cycle and apoptosis in cancer

Beneath the complexity and idiopathy of every cancer lies a limited number of 'mission critical' events that have propelled the tumour cell and its progeny into uncontrolled expansion and invasion. One of these is deregulated cell proliferation, which, together with the obligate compensatory suppression of apoptosis needed to support it, provides a minimal 'platform' necessary to support further neoplastic progression. Adroit targeting of these critical events should have potent and specific therapeutic consequences.

Functional characterization of cultured cells derived from an intraepidermal carcinoma of the skin (IEC-1)

We have successfully isolated a cell line (IEC-1) from an intraepidermal carcinoma of the skin of a patient and compared its behavior, in vitro, to normal human epidermal keratinocytes (HEK) and squamous cell carcinoma cell lines (SCCs). HEK differentiation comprises an initial growth arrest followed by an induction of squamous differentiation-specific genes such as transglutaminase type 1 (TG-1). Using thymidine uptake and TG-1 induction as markers of proliferation and differentiation, respectively, we were able to show that HEKs and the IEC-1 cells undergo growth arrest and induce TG-1 mRNA expression in response to various differentiation-inducing stimuli, while neoplastic SCC cell lines did not. However, differentiation in HEKs was an irreversible process whereas differentiation of the IEC-1 cells was reversible. Furthermore, growth of IEC-1 cells in organotypic raft cultures revealed differences in their ability to complete a squamous differentiation program compared with that of normal HEKs. The IEC-1 cells also exhibited a transitional phenotype with respect to replicative lifespan; HEKs had a lifespan of 4-6 passages, IEC-1 cells of 15-17 passages, and SCC cells were immortal. These alterations in IEC-1 cell behavior were not associated with functional inactivation or mutations of the p53 gene. These data indicate that the IEC-1 cells, derived from a preneoplastic skin tumor, exhibit differences in their ability to undergo terminal differentiation and have an extended replicative lifespan.

Lineage selection and the evolution of multistage carcinogenesis

A wide array of proto-oncogenes and tumour suppressor genes are involved in the prevention of cancer. Each form of cancer requires mutations in a characteristic group of genes, but no single group controls all cancers. This lack of generality shows that the control of cancer is not an ancient, fixed property of cells. By contrast, it supports a dynamic evolutionary model, whereby genetic controls over unregulated cell growth are recruited independently through evolutionary time in different tissues within different taxa. The complexity of this genetic control can be predicted from a population genetic model of lineage selection driven by the detrimental fitness effects of cancer. Cancer occurs because the genetic control of cell growth is vulnerable to somatic mutations (or 'hits'), particularly in large, continuously dividing tissues. Thus, compared to small rodents, humans must have evolved more complex genetic controls over cell growth in at least some of their tissues because of their greater size and longevity; an expectation relevant to the application of mouse data to humans. Similarly, the 'two-hit' model so successfully applied to retinoblastoma, which originates in a small embryonic tissue, is unlikely to be generally applicable to other human cancers; instead, more complex scenarios are expected to dominate, with complexity depending upon a tissue's size and its pattern of proliferation.