Transgenic overexpression of transforming growth factor alpha bypasses the need for c-Ha-ras mutations in mouse skin tumorigenesis

Injury prevents Ras mutant cell expansion in mosaic skin

Healthy skin is a mosaic of wild-type and mutant clones1,2. Although injury can cooperate with mutated Ras family proteins to promote tumorigenesis3-12, the consequences in genetically mosaic skin are unknown. Here we show that after injury, wild-type cells suppress aberrant growth induced by oncogenic Ras. HrasG12V/+ and KrasG12D/+ cells outcompete wild-type cells in uninjured, mosaic tissue but their expansion is prevented after injury owing to an increase in the fraction of proliferating wild-type cells. Mechanistically, we show that, unlike HrasG12V/+ cells, wild-type cells respond to autocrine and paracrine secretion of EGFR ligands, and this differential activation of the EGFR pathway explains the competitive switch during injury repair. Inhibition of EGFR signalling via drug or genetic approaches diminishes the proportion of dividing wild-type cells after injury, leading to the expansion of HrasG12V/+ cells. Increased proliferation of wild-type cells via constitutive loss of the cell cycle inhibitor p21 counteracts the expansion of HrasG12V/+ cells even in the absence of injury. Thus, injury has a role in switching the competitive balance between oncogenic and wild-type cells in genetically mosaic skin.

Mouse models of nonmelanoma skin cancer

The skin is the largest organ of the mammalian body, made up of multiple layers, which include the epidermis, dermis, and subcutis (Alam and Ratner, N Engl J Med 344(13):975-983, 2001). The human interfollicular epidermis can be subdivided into five different layers: (1) stratum basale, (2) stratum spinosum, (3) stratum granulosum, (4) stratum lucidum, and (5) stratum corneum, all originating from basal keratinocytes by differentiation (Hameetman et al., BMC cancer 13:58, 2013; Ramirez et al., Differentiation 58(1):53-64, 1994). The epidermis is also able to generate different appendages: hair follicles (HF) and their associated sebaceous glands (Sibilia et al., Cell 102(2):211-220, 2000) as well as sweat glands (Luetteke et al., Genes Dev 8(4):399-413, 1994). The skin has important functions in several biological processes like environmental barrier, tissue regeneration, hair cycling, and wound repair. During these processes, stem cells from the interfollicular epidermis and from the hair follicle bulge are activated to renew the epidermis or hair. The epidermis and hair undergo continuous homeostatic regeneration and mutations, upon mutations which disturb the balance of homeostatic regeneration of epidermis and hair and lead to enhanced proliferation of keratinocytes, development of skin cancer is developed. Tumors that arise in the skin are mainly of three types: malignant melanoma, arising from melanocytes, basal cell carcinoma (BCC), and squamous cell carcinoma (SCC), the latter two both arising from keratinocytes or hair follicle cells. In this chapter, we will describe some genetically engineered mouse models (GEMM) that aim at modeling human BCC and SCC and their respective precancerous lesions. We will describe the experimental approaches used in our laboratory to analyze tumor-bearing mice focusing on methods necessary for the induction of tumor growth as well as for the molecular and histological analysis of tumor tissue.

Molecular mechanisms of mouse skin tumor promotion

Multiple molecular mechanisms are involved in the promotion of skin carcinogenesis. Induction of sustained proliferation and epidermal hyperplasia by direct activation of mitotic signaling pathways or indirectly in response to chronic wounding and/or inflammation, or due to a block in terminal differentiation or resistance to apoptosis is necessary to allow clonal expansion of initiated cells with DNA mutations to form skin tumors. The mitotic pathways include activation of epidermal growth factor receptor and Ras/Raf/mitogen-activated protein kinase signaling. Chronic inflammation results in inflammatory cell secretion of growth factors and cytokines such as tumor necrosis factor-α and interleukins, as well as production of reactive oxygen species, all of which can stimulate proliferation. Persistent activation of these pathways leads to tumor promotion.

Ectopic expression of Zmiz1 induces cutaneous squamous cell malignancies in a mouse model of cancer

Cutaneous squamous cell carcinoma (SCC) is the second most common form of cancer in the human population, yet the underlying genetic mechanisms contributing to the disease are not well understood. We recently identified Zmiz1 as a candidate oncogene in non-melanoma skin cancer through a transposon mutagenesis screen. Here we show that transposon-induced mutations in Zmiz1 drive expression of a truncated transcript that is similar to an alternative endogenous ZMIZ1 transcript found to be overexpressed in human SCCs relative to normal skin. We also describe an original mouse model of invasive keratoacanthoma driven by skin-specific expression of the truncated Zmiz1 transcript. Unlike most mouse models, Zmiz1-induced skin tumors develop rapidly and in the absence of promoting agents such as phorbol esters. Additionally, we found that the alternative Zmiz1 isoform has greater protein stability than its full-length counterpart. Finally, we provide evidence that ZMIZ1 is overexpressed in a significant percentage of human breast, ovarian, and colon cancers in addition to human SCCs, suggesting ZMIZ1 may play a broader role in epithelial cancers.

Epithelial stem cells, wound healing and cancer

It is well established that tissue repair depends on stem cells and that chronic wounds predispose to tumour formation. However, the association between stem cells, wound healing and cancer is poorly understood. Lineage tracing has now shown how stem cells are mobilized to repair skin wounds and how they contribute to skin tumour development. The signalling pathways, including WNT and Hedgehog, that control stem cell behaviour during wound healing are also implicated in tumour formation. Furthermore, tumorigenesis and wound repair both depend on communication between epithelial cells, mesenchymal cells and bone marrow-derived cells. These studies suggest ways to harness stem cells for wound repair while minimizing cancer risk.

Inhibition of ERK oscillations by ionizing radiation and reactive oxygen species

The shuttling of activated protein kinases between the cytoplasm and nucleus is an essential feature of normal growth factor signaling cascades. Here we demonstrate that transforming growth factor alpha (TGFα) induces oscillations in extracellular signal regulated kinase (ERK) cytoplasmic-nuclear translocations in human keratinocytes. TGFα-dependent ERK oscillations mediated through the epidermal growth factor receptor (EGFR) are inhibited by low dose X-irradiation (10 cGy) and low concentrations of hydrogen peroxide (0.32-3.26 µM H(2)O(2)) used as a model reactive oxygen species (ROS). A fluorescent indicator dye (H2-DCFDA) was used to measure cellular ROS levels following X-irradiation, 12-O-tetradecanoyl phorbol-13-acetate (TPA) and H(2)O(2). X-irradiation did not generate significant ROS production while 0.32 µM H(2)O(2) and TPA induced significant increases in ROS levels with H(2)O(2)  > TPA. TPA alone induced transactivation of the EGFR but did not induce ERK oscillations. TPA as a cotreatment did not inhibit TGFα-stimulated ERK oscillations but qualitatively altered TGFα-dependent ERK oscillation characteristics (amplitude, time-period). Collectively, these observations demonstrate that TGFα-induced ERK oscillations are inhibited by ionizing radiation/ROS and perturbed by epigenetic carcinogen in human keratinocytes.

Growth factor signaling pathways as targets for prevention of epithelial carcinogenesis

Growth factor receptor (GFR) signaling controls epithelial cell growth by responding to various endogenous or exogenous stimuli and subsequently activating downstream signaling pathways including Stat3, PI3K/Akt/mTOR, MAPK, and c-Src. Environmental chemical toxicants and UVB irradiation cause enhanced and prolonged activation of GFR signaling and downstream pathways that contributes to epithelial cancer development including skin cancer. Recent studies, especially those with tissue-specific transgenic mouse models, have demonstrated that GFRs and their downstream signaling pathways contribute to all three stages of epithelial carcinogenesis by regulating a wide variety of biological functions including proliferation, apoptosis, angiogenesis, cell adhesion, and migration. Inhibiting these signaling pathways early in the carcinogenic process results in reduced cell proliferation and survival, leading to decreased tumor formation. Collectively, these studies suggest that GFR signaling and subsequent downstream signaling pathways are potential targets for the prevention of epithelial cancers including skin cancer.

Basic fibroblast growth factor regulates persistent ERK oscillations in premalignant but not malignant JB6 cells

The regulation of extracellular signal-regulated kinase (ERK) oscillations in the context of wound healing and carcinogenesis have been investigated in premalignant and malignant JB6 mouse epidermal cells stimulated with basic fibroblast growth factor (bFGF) and 12-O-tetradecanoyl phorbol-13-acetate (TPA). In premalignant JB6 cells, bFGF stimulation (1) increases cellular phospho-ERK and phospho-c-Jun levels, (2) increases serum-dependent cell proliferation, (3) induces an apparent epithelial-to-mesenchymal transition, and (4) induces the persistent nuclear-cytosolic oscillation of an ERK1-green fluorescent protein (ERK1-GFP) chimera. In contrast, TPA induces persistent activation of ERK in the absence of oscillations and does not induce efficient migration. Treatment of malignant or transformed JB6 cells with bFGF is associated with a transient nuclear translocation of ERK1-GFP but not oscillations or efficient cell migration. Our data suggest that bFGF regulates ERK oscillations in premalignant but not malignant JB6 cells.

Cellular dichotomy between anchorage-independent growth responses to bFGF and TPA reflects molecular switch in commitment to carcinogenesis

We have investigated gene expression patterns underlying reversible and irreversible anchorage-independent growth (AIG) phenotypes to identify more sensitive markers of cell transformation for studies directed at interrogating carcinogenesis responses. In JB6 mouse epidermal cells, basic fibroblast growth factor (bFGF) induces an unusually efficient and reversible AIG response, relative to 12-O-tetradecanoyl phorbol-13-acetate (TPA)-induced AIG which is irreversible. The reversible and irreversible AIG phenotypes are characterized by largely nonoverlapping global gene expression profiles. However, a subset of differentially expressed genes were identified as common to reversible and irreversible AIG phenotypes, including genes regulated in a reciprocal fashion. Hepatic leukemia factor (HLF) and D-site albumin promoter-binding protein (DBP) were increased in both bFGF and TPA soft agar colonies and selected for functional validation. Ectopic expression of human HLF and DBP in JB6 cells resulted in a marked increase in TPA- and bFGF-regulated AIG responses. HLF and DBP expression were increased in soft agar colonies arising from JB6 cells exposed to gamma radiation and in a human basal cell carcinoma tumor tissue, relative to paired nontumor tissue. Subsequent biological network analysis suggests that many of the differentially expressed genes that are common to bFGF- and TPA-dependent AIG are regulated by c-Myc, SP-1, and HNF-4 transcription factors. Collectively, we have identified a potential molecular switch that mediates the transition from reversible to irreversible AIG.

RasGRP1 transgenic mice develop cutaneous squamous cell carcinomas in response to skin wounding: potential role of granulocyte colony-stimulating factor

Models of epidermal carcinogenesis have demonstrated that Ras is a critical molecule involved in tumor initiation and progression. Previously, we have shown that RasGRP1 increases the susceptibility of mice to skin tumorigenesis when overexpressed in the epidermis by a transgenic approach, related to its ability to activate Ras. Moreover, RasGRP1 transgenic mice develop spontaneous papillomas and cutaneous squamous cell carcinomas, some of which appear to originate in sites of injury, suggesting that RasGRP1 may be responding to signals generated during the wound-healing process. In this study, we examined the response of the RasGRP1 transgenic animals to full-thickness incision wounding of the skin, and demonstrated that they respond by developing tumors along the wounded site. The tumors did not present mutations in the H-ras gene, but Rasgrp1 transgene dosage correlated with tumor susceptibility and size. Analysis of serum cytokines showed increased levels of granulocyte colony-stimulating factor in transgenic animals after wounding. Furthermore, in vitro experiments with primary keratinocytes showed that granulocyte colony-stimulating factor stimulated Ras activation, although RasGRP1 was dispensable for this effect. Since granulocyte colony-stimulating factor has been recently associated with proliferation of skin cancer cells, our results may help in the elucidation of pathways that activate Ras in the epidermis during tumorigenesis in the absence of oncogenic ras mutations.

Beyond wavy hairs: the epidermal growth factor receptor and its ligands in skin biology and pathology

The epidermal growth factor receptor (EGFR) network, including its seven ligands and four related receptors, represents one of the most complex signaling systems in biology. In many tissues, including the skin and its appendages (notoriously the hair follicles), its correct function is necessary for proper development and tissue homeostasis, and its deregulation rapidly results in defects in cellular proliferation and differentiation. The consequences are impaired wound healing, development of psoriasis-like lesions, structural and functional defects of the hair follicles, and tumorigenesis. In addition to in vitro experiments and data from clinical studies, several genetically modified mouse models displaying alterations in the interfollicular skin and hair follicles attributable to mutations in components of the EGFR system have been reported. These animals, in many cases representing bona fide models of known human diseases, have been seminal in the study of the role of EGFR and its ligands in the skin and its appendages. In this review, we take the multiple phenotypes of these animal models as a basis to summarize and discuss the effects elicited by members of the EGFR system in diverse aspects of skin biology and pathology, including cellular proliferation and differentiation, wound healing, hair follicle morphogenesis, and tumorigenesis.

Betacellulin regulates hair follicle development and hair cycle induction and enhances angiogenesis in wounded skin

Betacellulin (BTC) belongs to the EGF family, whose members play important roles in skin morphogenesis, homeostasis, and repair. However, the role of BTC in skin biology is still unknown. We employed transgenic mice overexpressing BTC ubiquitously to study its role in skin physiology. Immunohistochemistry revealed increased levels of BTC especially in the hair follicles and in the epidermis of transgenic animals. Expression of key markers of epithelial differentiation was unaltered, but keratinocyte proliferation was significantly increased. At post-natal day 1 (P1), transgenic mice displayed a significant retardation of hair follicle morphogenesis. At P17, when most follicles in control mice had initiated hair follicle cycling and had already entered into their first late catagen or telogen phase, all follicles of transgenic mice were still at the mid- to late catagen phases, indicating retarded initiation of hair follicle cycling. Healing of full-thickness excisional wounds and bursting strength of incisional wounds were similar in control and transgenic mice. However, an increase in the area covered by blood vessels at the wound site was detected in transgenic animals. These results provide evidence for a role of BTC in the regulation of epidermal homeostasis, hair follicle morphogenesis and cycling, and wound angiogenesis.

Amphiregulin-EGFR signaling regulates PTHrP gene expression in breast cancer cells

Parathyroid hormone-related protein (PTHrP) is an autocrine/paracrine factor produced by breast cancer cells that is speculated to play a major role in permitting breast cancer cells to grow into the bone microenvironment by stimulating the bone resorption axis. It has been previously shown that EGFR signaling induces the production of PTHrP in several primary and transformed epithelial cell types. Therefore, we investigated the relationship between EGFR and PTHrP gene expression in human breast cancer cells. Of a panel of 7 breast epithelial and cancer cell lines, the osteolytic, EGFR- positive lines (MDA-MB-231 and NS2T2A1) exhibited higher levels of PTHrP transcript expression. Amphiregulin mRNA levels in all lines were approximately 2 orders of magnitude higher than those of TGFalpha or HB-EGF. In the EGFR bearing lines, the receptor was phosphorylated at tyrosine 992 under basal conditions, and the addition of 100 nM amphiregulin did not lead to the phosphorylation of other tyrosine residues typically phosphorylated by the prototypical ligand EGF. Treatment of the EGFR positive lines with the EGFR inhibitor PD153035 and amphiregulin-neutralizing antibodies reduced PTHrP mRNA levels by 50-70%. Stable EGFR expression in the MCF7 line failed to increase basal PTHrP mRNA levels; however, treatment of this cell line with exogenous EGF or amphiregulin increased PTHrP transcription 3-fold. Transient transfection analysis suggests that the MAPK pathway and ETS transcription factors mediate EGFR coupling to PTHrP gene expression. Taken together, it appears that autocrine stimulation of EGFR signaling by amphiregulin is coupled to PTHrP gene expression via EGFR Tyr992 and MAPK, and that this pathway may contribute to PTHrP expression by breast tumor cells.

Expression of cell-cycle regulator CDK2-associating protein 1 (p12CDK2AP1) in transgenic mice induces testicular and ovarian atrophy in vivo

The novel cell-cycle regulator p12(CDK2AP1) (p12) gene encodes a cyclin-dependent kinase 2 (CDK2) partner that participates in cell-cycle regulation, apoptosis, and proliferation. CDK2 has been implicated in maintenance of gonadal homeostasis, as knockout mice display reproductive abnormalities. To investigate the role of p12 in homeostasis of gonadal tissues in vivo, we generated a transgenic mouse model driven by the human keratin 14 promoter, reported to target transgene expression to gonadal tissues and also stratified epithelia. Overexpression of the transgene was associated with a gonadal atrophy phenotype in mice of both sexes, yet fertility was not impaired. Histological evaluation of testes showed seminiferous tubule degeneration and decreased tubule diameter. Female transgenic mice had small ovaries, with a higher number of atretic follicles/mm(2) as compared to control nontransgenic mice. Also observed was increased germ cell apoptosis in both sexes (TUNEL). These results suggest that overexpression of p12 leads to testicular and ovarian abnormalities, a phenotype closely related to that of cdk2-/- mice. In combination, these observations suggest that the p12/CDK2 signaling pathways are carefully orchestrated to maintain proper gonadal tissue homeostasis. We suggest that the mechanisms of this regulation may be through p12-mediated altered expression of gonadal-specific genes and apoptotic pathways.

Mig6 is a negative regulator of EGF receptor-mediated skin morphogenesis and tumor formation

The growing number of recently identified negative feedback regulators of receptor tyrosine kinases (RTKs) highlights the importance of signal attenuation and modulation for correct signaling outcome. Mitogen-inducible gene 6 (Mig6 also known as RALT or Gene 33) is a multiadaptor protein thought to be involved in the regulation of RTK and stress signaling. Here, we show that deletion of the mouse gene encoding Mig6 (designated Errfi1, which stands for ERBB receptor feedback inhibitor 1) causes hyperactivation of endogenous epidermal growth factor receptor (EGFR) and sustained signaling through the mitogen-activated protein kinase (MAPK) pathway, resulting in overproliferation and impaired differentiation of epidermal keratinocytes. Furthermore, Errfi1-/- mice develop spontaneous tumors in various organs and are highly susceptible to chemically induced formation of skin tumors. A tumor-suppressive role for Mig6 is supported by our finding that MIG6 is downregulated in various human cancers. Inhibition of endogenous Egfr signaling with the Egfr inhibitor gefitinib (Iressa) or replacement of wild-type Egfr with the kinase-deficient protein encoded by the hypomorphic Egfr(wa2) allele completely rescued skin defects in Erffi1-/- mice. Carcinogen-induced tumors displayed by Errfi1-/- mice were highly sensitive to gefitinib. These results indicate that Mig6 is a specific negative regulator of Egfr signaling in skin morphogenesis and is a novel tumor suppressor of Egfr-dependent carcinogenesis.

Independent inheritance of genes regulating two subpopulations of mouse clonogenic keratinocyte stem cells

Mouse keratinocyte stem cells originate from the bulge of hair follicle, and, according to definition, possess a clonogenic activity in vitro. We have investigated seven inbred (C57BL/6, C3H, DBA/2, BALB/c, FVB) and outbred (SENCAR, CD-1) mouse strains and found that three genetically distinct subsets of mouse strains differ significantly in the frequency of clonogenic activity in vitro. The analysis of keratinocyte colonies in two reciprocal backcross [C57BL/6 x (BALB/c x C57BL/6); BALB/c x (BALB/c x C57BL/6)] and intercross [(BALB/c x C57BL/ 6)F2] of BALB/c and C57BL/6 mice allowed us to identify two subpopulations of clonogenic keratinocytes able to produce small (less than 2 mm2) and large (more than 2 mm2) colonies. We conducted linkage analysis and found that small colonies associated with mouse chromosomes 1, 6, 7, 8, and 9; but large colonies--with the chromosome 4. We defined locus on the chromosome 9 that associated with small colonies as keratinocyte stem cell locus 1 (Ksc1), and locus on the mouse chromosome 4 associated with large colonies-keratinocyte stem cell locus 2 (Ksc2). Ksc1 and loci on chromosomes 6 and 7 are close if not equal to loci associated with sensitivity to skin carcinogenesis. We conclude that two subpopulations of stem cells able to produce small and large colonies regulated by different genes and genes regulating small colonies might be responsible for sensitivity to skin carcinogenesis.

Genetic regulation of mouse stem cells: identification of two keratinocyte stem cell regulatory loci

It is well documented that the bulge of hair follicle is a 'niche' for a significant population of mouse keratinocyte stem cells, and 95% of rodent clonogenic keratinocytes originate from the bulge region. The ability to form colonies in vitro is a well recognized test for keratinocyte stem cells. We analyzed the epidermis of seven mouse strains and their segregating crosses [(BALB/c x C57BL/6)F1; (BALB/c x CB6F1); (C57BL/ 6 x CB6F1); (CBF1 x CBF1)F2] for their clonogenic activity in vitro. We found that keratinocyte colony (KC) number is a new quantitative multigenic trait. The analysis of KC size in two parental strains (C57BL/6 and BALB/c), the F1 generation and the segregating crosses demonstrated that the size of KC is a quantitative complex trait also. We determined that mouse epidermis has at least two subpopulations of keratinocytes that gave small (< 2 mm2) and large (> 2 mm2) colonies. The differences in the number of small and large colonies between parental strains (C57BL/6, BALB/c) were significant (P < 0.01). A genome-wide scan of the intercross and the two backcrosses maps the number of small KC to the central region of mouse Chromosome 9 (genomewide P value = 0.01). We define this locus as Ksc1. The proximal region of chromosome 4 is associated with the high number of large KC. We defined this locus as Ksc2. We found that Ksc1 and minor loci on chromosomes 6 and 7 map close, if not equal to, loci associated with mouse skin carcinogenesis. We conclude that mouse epidermis has at least two subpopulations of clonogenic keratinocyte stem cells that are regulated by different genes. We suggest that keratinocyte stem cells responsible for small colonies may play a major role in the regulation of resistance or sensitivity to skin carcinogenesis. Investigation of the genes regulating the stem cell number should provide new insight into the mechanisms of skin carcinogenesis, and should help to develop new approaches for therapies not only against active proliferating tumor cells but also quiescent tumor stem cells.

Contribution of stem cells and differentiated cells to epidermal tumours

The outer covering of the skin--the epidermis--is subject to sustained environmental assaults. As a result, many cells acquire potentially oncogenic mutations. Most cells are lost through differentiation, and only long-term epidermal residents, such as stem cells, accumulate the number of genetic hits that are necessary for tumour development. So, what genetic and environmental factors determine whether a mutant stem cell forms a tumour and what type of tumour will develop?

Physiological functions of Pten in mouse tissues

PTEN is a tumor suppressor gene mutated in many human sporadic cancers and in hereditary cancer syndromes such as Cowden disease, Bannayan-Zonana syndrome and Lhermitte-Duclos disease. The major substrate of PTEN is PIP3, a second messenger molecule produced following PI3K activation induced by variety of stimuli. PIP3 activates the serine-threonine kinase PKB/Akt which is involved in anti-apoptosis, proliferation and oncogenesis. In mice, heterozygosity for a null mutation of Pten (Pten(+/-) mice) frequently leads to the development of a variety of cancers and autoimmune disease. Homozygosity for the null mutation (Pten (-/-) mice) results in early embryonic lethality, precluding the functional analysis of Pten in various organs. To investigate the physiological functions of Pten in viable mice, various tissue-specific Pten mutations have been generated using the Cre-loxP system. This review will summarize the phenotypes of conditional mutant mice lacking Pten function in specific tissues, and discuss how these phenotypes relate to the physiological roles of Pten in various organ systems.

Progress in cutaneous cancer research

Cutaneous cancers represent a major public health concern due to the very high incidence, associated medical costs, substantial mortality, and cosmetic deformities associated with treatment. Considerable progress in basic research has provided new insights into the underlying genetic basis of the major human cutaneous cancers, malignant melanoma, basal cell carcinoma, and squamous cell carcinoma. In turn, these genetic insights have illuminated biochemical pathways that promise to provide new approaches to the prevention and treatment of cutaneous neoplasms. This review will detail the evolving genetic information and indicate how this information is being used to refine experimental models that serve to both define the biochemistry of cancer pathogenesis and test novel approaches to cancer therapy. Combined with preventive measures to reduce exposure to sunlight, these advances are likely to reduce this major public health burden in the coming decade.

A spontaneous CD8 T cell-dependent autoimmune disease to an antigen expressed under the human keratin 14 promoter

Using a previously described human keratin 14 (K14) promoter, we created mice expressing a peptide Ag (OVAp) in epithelial cells of the skin, tongue, esophagus, and thymus. Double transgenic mice that also express a TCR specific for this Ag (OT-I) showed evidence for Ag-driven receptor editing in the thymus. Surprisingly, such mice exhibited a severe autoimmune disease. In this work we describe the features of this disease and demonstrate that it is dependent on CD8 T cells. Consistent with the Ag expression pattern dictated by the human K14 promoter, an inflammatory infiltrate was observed in skin and esophagus and around bile ducts of the liver. We also observed a high level of TNF-alpha in the serum. Given that Ag expression in the thymus induced development of T cells with dual TCR reactivity, and that dual-reactive cells have been suggested to have autoimmune potential, we tested whether they were a causal factor in the disease observed here. We found that OT-I/K14-OVAp animals on a recombinase-activating gene-deficient background still suffered from disease. In addition, OT-I animals expressing OVA broadly in all tissues under a different promoter did not experience disease, despite having a similar number of dual-specific T cells. Thus, in this model it would appear that dual-reactive T cells do not underlie autoimmune pathology. Finally, we extended these observations to a second transgenic system involving 2C TCR-transgenic animals expressing the SIY peptide Ag with the hK14 promoter. We discuss the potential relationship between autoimmunity and self-Ags that are expressed in stratified epithelium.

Analysis of c-Ha-ras gene mutations in skin tumors induced in carcinogenesis-susceptible and carcinogenesis-resistant mice by different two-stage protocols or tumor promoter alone

In the present study we describe the molecular analysis of c-Ha-ras gene mutations in 47 papillomas and 17 carcinomas developed in two lines of mice, carcinogenesis-susceptible (Car-S) and carcinogenesis-resistant (Car-R), selectively bred for extreme susceptibility or resistance to chemical skin carcinogenesis initiated and promoted with different doses of 7,12-dimethylbenz[a]anthracene (DMBA) and 12-O-tetradecanoylphorbol-13-acetate (TPA). This study also presents the analysis of c-Ha-ras gene mutations in 22 papillomas and 22 carcinomas in Car-S mice initiated with DMBA and promoted with benzoyl peroxide (BzPo) and in seven papillomas and one carcinoma from a group of uniniated Car-S mice that received only BzPo treatment. The data showed that a A(182)-->T transversion in the c-Ha-ras gene was present in 100% and 81% of the skin tumors developed in Car-S and Car-R mice, respectively, after DMBA initiation and TPA promotion, suggesting that differences in genetic susceptibility can influence the frequency of c-Ha-ras mutations in the skin tumors produced. The same A(182)-->T mutation with an incidence of 68% was found in papillomas from DMBA-initiated and BzPo-promoted Car-S mice. The difference in the mutation frequency between DMBA/BzPo and DMBA/TPA papillomas suggested that the promotion step contributes to the final mutation pattern. The tumor induction experiment with BzPo alone showed that this compound can induce tumor development in 26% of Car-S mice, and the molecular analysis of the tumors showed a broad mutation spectrum, including mutations in codons 12, 13, and 61 of the c-Ha-ras gene. Mol. Carcinog. 30:111-118, 2001.

A causal relationship between unscheduled eicosanoid signaling and tumor development: cancer chemoprevention by inhibitors of arachidonic acid metabolism

Cancer results from disturbances of cellular signal transduction and data processing at the genetic and epigenetic level. In the early phase of the disease these disturbances are mainly caused by environmental toxic agents, i.e. genotoxic and non-genotoxic carcinogens, whereas endogenous agents derived from dys-regulated metabolic reactions may take over this role at later stages, thereby leading to a state of 'genetic instability' and 'growth autonomy'. Among these metabolic reactions becoming dys-regulated in the course of tumorigenesis, eicosanoid biosynthesis from arachidonic acid seems to play a particular role. A steadily increasing body of evidence indicates a causal relationship between cancer development and an abnormal overexpression of eicosanoid-forming enzymes, i.e. cyclooxygenases and lipoxygenases, in a wide variety of human and animal tumors. This overexpression seems to result from disturbances of cellular signaling cascades such as the Ras-Raf-MAPkinase cascade due to oncogenic gene mutations. Presently, research is focussed on the proinflammatory enzyme cyclooxygenase-2 (COX-2) the pathological overexpression of which has been found to be related to key events of tumor promotion such as cellular hyperproliferation, inhibition of programmed cell death, and tumor angiogenesis. In the mouse skin model of multistage carcinogenesis COX-2-derived prostaglandin F(2alpha) has been indentified as an endogenous tumor promoter. Moreover, genotoxic byproducts of both cylooxygenase and lipoxygenase-catalyzed arachidonic acid metabolism (such as active oxygen species, free radicals etc.) are suspected to contribute to 'genetic instability' and thus to malignant progression of tumor cells. The enzymes of eicosanoid biosynthesis rank therefore among the most attractive targets for cancer chernoprevention. In fact, both nonsteroidal antiinflammatory drugs, i.e. non-specific COX inhibitors, and isozyme-specific COX-2 inhibitors have been shown to inhibit experimental and human cancer development, in the latter case in particular in the large bowel. Beside their role as indicators of neoplastic development eicosanoids may be also used as reporters of skin irritation. Based to this concept an in vitro test system for skin toxicity has been developed in which the release of arachidonic acid and interleukin-1alpha, i.e. two key mediators of acute inflammation, from a human keratinocyte cell line is measured. The excellent correlation found between this mediator release and the effects of various chemical irritants on human skin in vivo indicates that, in the near future, this and related methods may help to do without animal experiments in toxicological testing.

MDM2 induces hyperplasia and premalignant lesions when expressed in the basal layer of the epidermis

The MDM2 oncogene is overexpressed in 5-10% of human tumours. Its major physiological role is to inhibit the tumour suppressor p53. However, MDM2 has p53-independent effects on differentiation and does not predispose to tumorigenesis when it is expressed in the granular layer of the epidermis. These unexpected properties of MDM2 could be tissue specific or could depend on the differentiation state of the cells. Strikingly, we found that MDM2 has p53-dependent effects on differentiation, proliferation and apoptosis when it is expressed in the less differentiated basal layer cells. MDM2 inhibits UV induction of p53, the cell cycle inhibitor p21(WAF1/CIP1) and apoptosis ('sunburn cells'). Importantly, MDM2 increases papilloma formation induced by chemical carcinogenesis and predisposes to the appearance of premalignant lesions and squamous cell carcinomas. p53 has a natural role in the protection against UV damage in the basal layer of the epidermis. Our results show that MDM2 predisposes to tumorigenesis when expressed at an early stage of differentiation, and provide a mouse model of MDM2 tumorigenesis relevant to p53's tumour suppressor functions.

Epstein-Barr virus latent membrane protein 2A has no growth-altering effects when expressed in differentiating epithelia

Previous studies using transgenic mice with B-cell expression of LMP2A demonstrated that LMP2A drives B-cell development and survival signal in the absence of normal B-cell receptor (BCR) signal transduction. To determine if LMP2A may have similar effects in epithelial differentiation, six transgenic murine lines were constructed and analysed with LMP2A expression directed to the epidermis by a keratin 14 (K14) promoter cassette. LMP2A protein expression was verified by immunofluorescence and immunoprecipitation of skin samples using LMP2A-specific antibodies. To evaluate the effects of LMP2A expression on epidermal differentiation, immunofluorescence and histochemistry were performed on tongue and tail samples of transgenic mice and their wild-type littermate controls using differentially expressed keratins. The analysis indicated that LMP2A does not alter the normal epithelial differentiation program in the epithelia of K14-LMP2A transgenic mice.

Constitutive expression of erbB2 in epidermis of transgenic mice results in epidermal hyperproliferation and spontaneous skin tumor development

The erbB family of receptor tyrosine kinases, which consists of the epidermal growth factor receptor (EGFr/erbB1), erbB2 (neu), erbB3 and erbB4, has been shown to be important for both normal development as well as neoplasia. The expression of rat erbB2 was targeted to the basal layer of mouse epidermis with the bovine keratin 5 promoter. Overexpression of wild type rat erbB2 in the basal layer of epidermis led to alopecia, follicular hyperplasia and sebaceous gland enlargement as well as hyperplasia of the interfollicular epidermis. Spontaneous papillomas, some of which converted to squamous cell carcinomas, arose in homozygous erbB2 transgenic mice as early as 6 weeks of age with >90% incidence by 6 months. Analysis of several proliferation/differentiation markers indicated that erbB2 overexpression led to epidermal hyperproliferation and a possible delay in epidermal differentiation. Transgenic mice were also hypersensitive to the proliferative effects of the skin tumor promoter, 12-0-tetradecanoylphorbol-13-acetate (TPA) and were more sensitive to two-stage carcinogenesis. Elevations in EGFr and erbB2 protein as well as erbB2:EGFr and erbB2:erbB3 heterodimers were observed in skin of the erbB2 transgenic mice. Phosphotyrosine levels of the EGFr, erbB2 and erbB3 proteins were also elevated. These results indicate an important role for erbB2 signaling in epidermal growth, development and neoplasia. Oncogene (2000) 19, 4243 - 4254

Cancer chemoprevention through interruption of multistage carcinogenesis. The lessons learnt by comparing mouse skin carcinogenesis and human large bowel cancer

Whilst in the early stages, neoplastic development is predominantly triggered by environmental genotoxic and non-genotoxic carcinogens, tumour progression becomes more and more autonomous at later stages. In this context a dysregulation of arachidonic acid metabolism seems to play a disastrous role. Conversely, non-steroidal anti-inflammatory drugs (NSAIDs) rank among the most potent and most promising agents for cancer chemoprevention probably because of their ability to inhibit prostaglandin biosynthesis, in particular, at the level of the 'pro-inflammatory' enzyme cyclooxygenase-2 (COX-2). A pathological overexpression of COX-2 resulting in excessive prostaglandin production has been found already in early stages of carcinogenesis and seems to be a consistent feature of neoplastic development in a wide variety of tissues. COX-2 overexpression is thought to occur along signalling pathways of inflammation and tissue repair which become activated in the course of tumour promotion and, due to autocrine and auto-stimulatory mechanisms, finally lead to some autonomy of tumour development (self-promotion). Prostaglandins formed along a dysregulated COX pathway have been shown to mediate tumour promotion in animal experiments and may play a role, in addition, in other processes involved in tumour growth such as angiogenesis, metastasis and immunosuppression. Moreover, genotoxic byproducts such as organic free radicals, reactive oxygen species and malondialdehyde produced in the course of prostanoid biosynthesis may contribute to genetic instability (mutator phenotype) of neoplastic cells thereby promoting malignant progression. Such mixtures of physiologically highly active mediators and genotoxic byproducts are, in addition, formed along the various lipoxygenase-catalysed pathways of arachidonic acid metabolism some of which also become dysregulated during tumour development and, therefore, provide novel targets of future chemopreventive approaches.

Pharmacological inhibition of Ras-transformed epithelial cell growth is linked to down-regulation of epidermal growth factor-related peptides

BACKGROUND & AIMS

Posttranslational farnesylation is required for Ras activation. Farnesyl transferase inhibitors (FTIs) selectively block protein farnesylation and reduce the growth of many Ras-transformed cells in vitro and in vivo. Activated Ras transforms rat intestinal epithelial (RIE-1) cells by a mechanism distinct from NIH 3T3 fibroblasts in that an epidermal growth factor receptor (EGFR) autocrine loop contributes significantly to the Ras-transformed RIE-1 phenotype.

METHODS

The ability of FTIs to block growth of Ras-transformed RIE-1 cells was evaluated, and these results were correlated with decreased EGFR ligand production.

RESULTS

FTI L744,832 caused a selective, dose-dependent, reversible blockade in proliferation of H-Ras-transformed RIE-1 cells, whereas control cell lines, K-Ras-transformed cells, and activated raf-transfected RIE cells were unaffected. The growth-inhibitory effects of L744,832 correlated with loss of farnesylated H-Ras protein and a marked reduction in transforming growth factor (TGF)-alpha and amphiregulin expression. Inhibition of proliferation of H-Ras RIE-1 cells by L744,832 was overcome by exogenous TGF-alpha, and enhanced growth inhibition was achieved by EGFR blockade in combination with L744,832. +

CONCLUSIONS

These data suggest that one mechanism by which FTIs inhibit growth of H-Ras-transformed epithelial cells is by reducing Ras-induced EGFR ligand production.

Metabolic targets of cancer chemoprevention: interruption of tumor development by inhibitors of arachidonic acid metabolism

Tumor promotion is understood as a process that favors the clonal outgrowth of single mutated (initiated) cells to premalignant lesions through co-mitogenic and anti-apoptotic effects. This process can be evoked by repeated induction of a regenerative tissue response as achieved either by irritation and wounding or by agents (tumor promoters) that interact with the corresponding pathways of cellular signaling. Metabolic processes regulated by such pathways and essential for tumor development are potential targets of cancer chemoprevention. Examples are provided by the expression of ornithine decarboxylase and the activation of eicosanoid formation from arachidonic acid. Arachidonic acid metabolism is a particularly attractive and important target of chemopreventive measures. Its induction is a characteristic response to tissue damage and irritation and an apparently critical event in epithelial tumor promotion. Inhibitors of eicosanoid formation, such as nonsteroidal anti-inflammatory drugs, rank among the most powerful chemopreventive agents in animal models and have been shown to halve the incidence of colorectal cancer in man. Recently, the role of cyclooxygenase-2 (COX-2)-catalyzed prostaglandin synthesis has been the subject of much attention. COX-2 is a typical 'emergency enzyme', since in most tissues it is transiently induced only in the course of repair and defense reactions. In epithelial neoplasia, i.e. in skin and colorectal tumors, the enzyme is constitutively overexpressed along different molecular pathways, and it seems to be critically involved in tumor promotion. Consequently, specific COX-2 inhibitors have been shown to exhibit considerable cancer chemopreventive potential. The putative role of other pathways of arachidonic acid metabolism in tumor promotion and malignant progression is presently under investigation.

Low antibody responsiveness is found to be associated with resistance to chemical skin tumorigenesis in several lines of Biozzi mice

High and low antibody responder lines of mice from Selections I, III and G were assayed for two-step skin tumorigenesis using a protocol consisting in initiation with 7,12-dimethylbenz[a]anthracene (DMBA) and promotion with 12-O-tetradecanoylphorbol-13-acetate (TPA). Concordant results were obtained in the three selections: low antibody responder mice were shown to be significantly more resistant to tumor induction than the high responder counterparts. The difference was observed for all parameters: kinetics and percentages of tumor incidence and tumor multiplicity. The three bidirectional selective breeding experiments differed in several respects namely, the origin of the foundation populations, the antigens and immunization protocols used during the selection, as well as the breeding unit environments. Therefore, the consistent results relative to tumorigenesis strongly suggest that some of the alleles relevant to multispecific 'low' antibody production could contribute to the resistance to cutaneous chemical tumorigenesis.

Contributions of the epidermal growth factor receptor to keratinocyte motility

The epidermal growth factor (EGF) receptor plays a central role in numerous aspects of keratinocyte biology. In normal epidermis, the EGF receptor is important for autocrine growth of this renewing tissue, suppression of terminal differentiation, promotion of cell survival, and regulation of cell migration during epidermal morphogenesis and wound healing. In wounded skin, the EGF receptor is transiently up-regulated and is an important contributor to the proliferative and migratory aspects of wound reepithelialization. In keratinocytic carcinomas, aberrant expression or activation of the EGF receptor is common and has been proposed to play a role in tumor progression. Many cellular processes such as altered cell adhesion, expression of matrix degrading proteinases, and cell migration are common to keratinocytes during wound healing and in metastatic tumors. The EGF receptor is able to regulate each of these cellular functions and we propose that transient and dynamic elevation of EGF receptor during wound healing, or constitutive overexpression in tumors, provides an important contribution to the migratory and invasive potential of keratinocytes.

Short- and intermediate-term carcinogenicity testing--a review. Part 1: the prototypes mouse skin tumour assay and rat liver focus assay

Carcinogenicity testing is by far the most expensive and time-consuming study type of toxicology. For many years, the lifetime exposure with the maximum tolerated dose in two rodent species has been the gold standard of carcinogenicity testing of pharmaceuticals. Major change was introduced by the Fourth International Conference on Harmonization in July 1997; a chronic rodent bioassay in one species and a short-term carcinogenicity assay are regarded as sufficient for registration. Such requirements provide the opportunity to redirect the vast resources previously spent on the lifetime study in the second species. Numerous experimental protocols for short- and intermediate-term carcinogenicity testing in many target tissues have been available for years. The first part of this review describes the basic principles of short- and intermediate-term carcinogenicity testing using the examples of the widely used mouse skin tumour assay and the rat liver foci assay. In the context of these experimental models, the discrimination and quantification of initiating and promoting activity and the use of preneoplastic lesions as endpoints in carcinogenicity testing are described. The review includes the limitations of the models with regard to the extrapolation from effects observed in animal experiments to a potential exposure of humans.

Enhanced sensitivity to tumor growth and development in multistage skin carcinogenesis by transforming growth factor-alpha-induced epidermal growth factor receptor activation but not p53 inactivation

Transforming growth factor-alpha (TGF alpha) can stimulate keratinocyte proliferation and function as an autocrine tumor promoter in 7,12-dimethylbenz[a]anthracene (DMBA)-initiated TGF alpha-transgenic mouse skin. In this study, we examined the effect of ectopic TGF alpha transgene expression on skin tumor growth and progression after DMBA initiation in the presence of 12-O-tetradecanoylphorbol-13-acetate (TPA). Both the multiplicity and size of skin tumors arising in TGF alpha-transgenic mice were significantly higher than those of the nontransgenic parental CD-1 strain. There were more dysplastic papillomas and squamous cell carcinomas (SCCs) in the transgenic animals as well. ProTGF alpha protein was expressed in transgenic papillomas, but mature TGF alpha was not detected. The epidermal growth factor receptor (EGFR) appeared to be downregulated and was associated with enhanced tyrosine phosphorylation of several substrates in TGF alpha-transgenic mouse tumors. Characteristic codon 61 mutations in the Ha-ras gene were found in most of the papillomas and SCCs induced by DMBA and TPA in transgenic as well as nontransgenic mice. However, no p53 gene mutations were found in any skin tumors from either transgenic or control animals. Analysis of cellular proliferation in both DMBA-TPA-induced papillomas and in skin 48 h after TPA treatment alone revealed significantly more DNA synthesis in TGF alpha-transgenic mice relative to controls. These results demonstrate that TGF alpha, through EGFR overstimulation, can act synergistically with TPA to induce the formation, growth, and development of DMBA-initiated skin tumors containing classic Ha-ras gene mutations but not p53 gene inactivation.

Genetic predisposition and parameters of malignant progression in K14-HPV16 transgenic mice

Reproducible multi-stage progression to invasive squamous carcinoma of the epidermis has been achieved in transgenic mice expressing the HPV16 early-region genes, including the E6/E7 oncogenes, under the control of the human keratin-14 promoter/enhancer. Although 100% of K14-HPV16 transgenic animals develop hyperplastic and/or dysplastic lesions in several inbred backgrounds, including C57BL/6, BALB/c, and SSIN/SENCAR, only mice backcrossed into the FVB/n background progress to malignant squamous cell carcinomas of two pathological grades, well differentiated and moderate/poorly differentiated (WDSC or MPDSC, respectively), each displaying characteristic patterns of malignant behavior. WDSCs typically arise within the epidermis of the ear and invade deeply into the underlying dermis but fail to metastasize, whereas MPDSCs develop on the chest and truncal skin and invariably metastasize to regional lymph nodes. The transition to the malignant state, in 21% of FVB/n transgenic mice, is characterized by alteration of the repertoire of keratin intermediate filament proteins expressed within neoplastic epidermis, such that WDSCs maintain expression of keratins common to terminally differentiating stratified keratinocytes (K10), whereas MPDSCs are distinguished from WDSCs by activation of embryonic and mucosal keratins (K13, K8, and K19). Precursor hyperplastic and dysplastic lesions are characterized by a progressively increased proliferative index, striking morphological alterations in keratinocyte cell-cell and cell-matrix interactions, and extensive remodeling of the underlying dermal stroma. Remarkably, this extensive stromal remodeling, which may facilitate both angiogenesis and eventual tumor cell invasion, develops early at the dysplastic stage in all animals well before malignant conversion.

Potentiation of diethylstilbestrol-induced alterations in the female mouse reproductive tract by transforming growth factor-alpha transgene expression

Neonatal estrogen exposure causes numerous abnormalities in the female reproductive tract, including carcinogenesis. One mechanism by which neonatal estrogen elicits teratogenic and carcinogenic effects is epigenetic and involves the modulation of a number of estrogen-regulated genes including epidermal growth factor (EGF). Because of the evidence that there is an integral relationship between the EGF family, estrogen action, and the regulation of the growth and differentiation of the reproductive tract, we used transforming growth factor-alpha (TGF alpha) transgenic mice to investigate the interaction of constitutive TGF alpha expression with the potent estrogen diethylstilbestrol (DES) in the induction of reproductive-tract alterations. Our study was designed to determine whether TGF alpha expression could modulate DES-induced carcinogenesis of the female mouse reproductive tract. The animals were homozygous TGF alpha transgenic female mice from the MT42 line and the parental CD-1 outbred mice. The presence of the TGF alpha transgene significantly increased the incidence of DES-induced vaginal adenosis, uterine endometrial hyperplasia, uterine polyps, hypospadia, benign ovarian cysts, and pituitary adenomas. However, constitutive TGF alpha expression did not promote reproductive-tract neoplasia. This study demonstrates that TGF alpha participates in the regulation of developmental and morphogenic events in the Müllerian duct and urogenital sinus, suggesting a role for TGF alpha in the pathogenesis of reproductive-tract diseases. Furthermore, we showed that although constitutive expression of the TGF alpha transgene did have an effect on the reproductive tract, TGF alpha overexpression alone could not substitute for DES as a reproductive-tract carcinogen or as a promoter of uterine neoplasia, indicating that DES-induced carcinogenesis requires events in addition to the overexpression of this single peptide growth factor.

Activation of the epidermal growth factor receptor signal transduction pathway stimulates tyrosine phosphorylation of protein kinase C delta

The expression of an oncogenic rasHa gene in epidermal keratinocytes stimulates the tyrosine phosphorylation of protein kinase C delta and inhibits its enzymatic activity (Denning, M. F., Dlugosz, A. A., Howett, M. K., and Yuspa, S. H. (1993) J. Biol. Chem. 268, 26079-26081). Keratinocytes expressing an activated rasHa gene secrete transforming growth factor alpha (TGFalpha) and have an altered response to differentiation signals involving protein kinase C (PKC). Because the neoplastic phenotype of v-rasHa expressing keratinocytes can be partially mimicked in vitro by chronic treatment with TGF alpha and the G protein activator aluminum fluoride (AlF4-), we determined if TGF alpha or AlF4- could induce tyrosine phosphorylation of PKCdelta. Treatment of primary keratinocyte cultures for 4 days with TGFalpha induced tyrosine phosphorylation of PKCdelta, whereas AlF4- only slightly stimulated PKCdelta tyrosine phosphorylation. The PKCdelta that was tyrosine-phosphorylated in response to TGFalpha had reduced activity compared with the nontyrosine-phosphorylated PKCdelta. Treatment of keratinocytes expressing a normal epidermal growth factor receptor (EGFR) with TGFalpha or epidermal growth factor for 5 min induced PKCdelta tyrosine phosphorylation. This acute epidermal growth factor treatment did not induce tyrosine phosphorylation of PKCdelta in keratinocytes isolated from waved-2 mice that have a defective epidermal growth factor receptor. In addition, the level of PKCdelta tyrosine phosphorylation in v-rasHa-transduced keratinocytes from EGFR null mice was substantially lower than in v-rasHa transduced wild type cells, suggesting that activation of the EGFR is important for PKC delta tyrosine phosphorylation in ras transformation. However, purified EGFR did not phosphorylate recombinant PKC delta in vitro, whereas members of the Src family (c-Src, c-Fyn) and membrane preparations from keratinocytes did. Furthermore, clearing c-Src or c-Fyn from keratinocyte membrane lysates decreased PKCdelta tyrosine phosphorylation, and c-Src and c-Fyn isolated from keratinocytes treated with TGFalpha had increased kinase activity. Acute or chronic treatment with TGFalpha did not induce significant PKCdelta translocation in contrast to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate, which induced both translocation and tyrosine phosphorylation of PKCdelta. This suggests that TGFalpha-induced tyrosine phosphorylation of PKC delta results from the activation of a tyrosine kinase rather than physical association of PKCdelta with a membrane-anchored tyrosine kinase. Taken together, these results indicate that PKCdelta activity is inhibited by tyrosine phosphorylation in response to EGFR-mediated signaling and activation of a member of the Src kinase family may be the proximal tyrosine kinase acting on PKCdelta in keratinocytes.

Transforming growth factor-alpha

Major advances in understanding growth factor biology, especially in epithelial cells, have resulted from work with TGF-alpha over the past decade. It is clear that TGF-alpha is a potent epithelial oncoprotein, but equally important biological activities in normal epithelial homeostasis have been described. A number of major challenges lie ahead. Foremost is the formidable task of dissecting out the individual contributions of each EGF-related peptide in the biological response to stimulation of the EGFR. Appreciation of the complexity of heterodimerization of receptors within the EGFR family will be equally important in the final analysis. These considerations assure the continued vitality and productivity of investigation of the EGF-related peptide/EGFR axis.

Liver regeneration and hepatocarcinogenesis in transforming growth factor-alpha-targeted mice

Transforming growth factor-alpha (TGF alpha), a member of the epidermal growth factor receptor ligand family, has been implicated in the regeneration and transformation of liver. Our recent development of mice that are homozygous for a disrupted TGF alpha gene allowed us to assess the requirement for this growth factor in these complex processes. We report here that although a 70% hepatectomy produced a significant increase in hepatic TGF alpha protein levels in wild-type mice, liver regeneration nevertheless proceeded normally in the absence of the growth factor. The hepatocyte labeling indices determined for homozygous targeted and wild-type mice at 36 and 48 h after hepatectomy were comparable, and the total liver DNA to body weight ratios 8 d after hepatectomy were essentially identical for the two genotypes. These results indicate that TGF alpha, is not necessary for liver regeneration. To test its requirement in liver carcinogenesis, young mice were administered single doses of diethylnitrosamine (DEN) with or without subsequent chronic treatment with the promoting agent phenobarbital (PB). Both wild-type and homozygous mutant male mice treated with DEN or DEN plus PB developed multiple preneoplastic foci or tumors by 9 mo of age with relatively high incidence. However, while five of 88 tumors in wild-type mice attained a diameter greater than 5 mm and were classified as hepatocellular carcinomas, none of 132 tumors in livers of targeted mice reached this size. Furthermore, three of these large wild-type tumors expressed significantly elevated levels of TGF alpha protein compared with normal liver. These results indicate that TGF alpha is not required for early events in chemically induced hepatocarcinogenesis but suggest that it could be important in the progression from small preneoplastic foci to large tumors.

Distinct genetic loci control development of benign and malignant skin tumours in mice

Genetic susceptibility to chemically induced skin cancer in mice is controlled by multiple unlinked genetic loci. Mus spretus mice have dominant resistance genes which confer resistance to interspecific F1 hybrids with susceptible Mus musculus strains. We have mapped three major resistance loci using a combination of Mapmaker/QTL analysis and multiple regression analysis to mouse chromosomes 5 and 7. At least two independent loci on chromosome 7 exert their effects primarily during benign tumour development and have very little influence on tumour progression. On the other hand, probably a single locus on chromosome 5 affects both early and late stages of malignancy. The results indicate that benign and malignant tumours are largely under independent genetic control.

Transgenic mice and squamous multistage skin carcinogenesis

The use of animals models of human cancers has proved useful in the elucidation of molecular events which occur during tumour development. Mouse skin has been used as a model for human squamous cancer for a number of decades, and analysis of this model has identified a number of changes important for the evolution of malignancy. Transgenic mice offer a further avenue of advancement, allowing refinement of the model, and the ability to examine the consequences of individual events in vivo in greater detail. This article reviews the impact of transgenic approaches to our understanding of multistage squamous carcinogenesis in mouse skin.

Activated Ha-Ras but not TPA induces transcription through binding sites for activating transcription factor 3/Jun and a novel nuclear factor

We report the identification of a 20-base pair sequence mediating induced transcription in response to an activated Ha-ras gene and epidermal growth factor (EGF) but not 12-O-tetradecanoylphorbol-13-acetate stimulation. This signal-specific nuclear target is present in the long terminal repeat of a mouse VL30 retrotransposon expressed in epidermis. Functional studies and in vitro binding analyses using cultured keratinocytes (Balb/MK) reveal that the response element is composed of two cooperating sequence motifs in juxtaposed position, both of which are targets for induced binding activity 1-2 h after EGF stimulation. Of many different activating transcription factor/cAMP-responsive element binding protein/activating protein 1 factors tested, one part of the sequence selectively binds endogenous proteins immunologically related to activating transcription factor 3 (ATF3) and Jun isotypes. The other sequence is a target for a nuclear factor showing binding specificity unrelated to factors known to mediate EGF- or ras-induced transcription as determined by its sequence specificity and by antibody experiments. This component has been characterized and partially purified by gel filtration chromatography and velocity centrifugation revealing a Stokes radius of 43.6 A and a sedimentation coefficient of 9.7 S in solution. Based on these parameters, a molecular mass of 178,000 Da was calculated. The results indicate that the specific binding of ATF3/Jun and a previously uncharacterized factor account for signal-specific transcription in response to EGF or an activated Ha-ras gene in a cell type in which the cooperative action of an activated Ha-ras gene and 12-O-tetradecanoylphorbol-13-acetate cause tumor growth.

Chemical skin carcinogenesis is prevented in mice by the induced expression of a TGF-beta related transgene

Skin papillomas and squamous cell carcinomas (SCCs) are induced in mice by tumor initiation with a carcinogen followed by tumor promotion with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). These usually arise from preneoplastic lesions characterized by epidermal proliferation and hyperplasia, dermal edema, and inflammation. To evaluate the role of polypeptide growth factors in chemically induced skin carcinogenesis, we used transgenic mice carrying the cDNA for a TGF-beta related molecule, bone morphogenetic protein-4 (BMP-4), under the control of the regulatory elements of the cytokeratin IV* gene in a skin carcinogenesis protocol. Control non-transgenic littermates and BMP-4 transgenic mice were treated with a single dose of a carcinogen, N-methyl-N'-nitrosoguanidine (MNNG), and biweekly with the tumor promoter TPA for 9 months. In control littermates TPA induced epidermal hyperproliferation, atypia with "dark" cells, and dermal inflammation, resulting in papillomas and SCCs in 13 of 26 animals tested. In BMP-4 transgenic mice, TPA treatment induced the expression of the BMP-4 transgene in interfollicular epidermis but only minimal epidermal thickening, hyperproliferation, and inflammation were noted after the initial dose of TPA. Furthermore, the mitotic indices in transgenic epidermis after 9 months of TPA treatment were significantly lower than the corresponding indices from untreated transgenic epidermis. Consequently, none of the 22 transgenic animals tested developed papillomas or SCCs. In conclusion, we have shown that the TPA induced expression of the BMP-4 transgene blocks proliferation and inflammation in skin, steps that are critical to the subsequent formation of papillomas and SCCs and we characterized an inducible promotersystem which expresses polypeptides in interfollicular epidermis after exogenous stimulation.

The epidermis: rising to the surface

At the skin surface, the epidermis serves an important protective function which it manifests by building an extensive cytoskeletal architecture of keratin filaments, spanning from the nuclear envelope to hemidesmosomes and desmosomes. Recent studies on epidermal proteins and their interactions have provided insights into human skin diseases, including genetic disorders of keratins, laminins, and collagen. Explorations into the regulatory mechanisms underlying epidermal genes have underscored the importance of transcription factors AP-1 and AP-2, retinoic acid receptors, and POU proteins. Transgenic and gene ablation experiments on TGF-alpha and TGF-beta genes have yielded clues as to how the epidermis maintains a balance of growing and differentiating cells.

Progressive squamous epithelial neoplasia in K14-human papillomavirus type 16 transgenic mice

To model human papillomavirus-induced neoplastic progression, expression of the early region of human papillomavirus type 16 (HPV16) was targeted to the basal cells of the squamous epithelium in transgenic mice, using a human keratin 14 (K14) enhancer/promoter. Twenty-one transgenic founder mice were produced, and eight lines carrying either wild-type or mutant HPV16 early regions that did not express the E1 or E2 genes were established. As is characteristic of human cancers, the E6 and E7 genes remained intact in these mutants. The absence of E1 or E2 function did not influence the severity of the phenotype that eventually developed in the transgenic mice. Hyperplasia, papillomatosis, and dysplasia appeared at multiple epidermal and squamous mucosal sites, including ear and truncal skin, face, snout and eyelids, and anus. The ears were the most consistently affected site, with pathology being present in all lines with 100% penetrance. This phenotype also progressed through discernible stages. An initial mild hyperplasia was followed by hyperplasia, which further progressed to dysplasia and papillomatosis. During histopathological progression, there was an incremental increase in cellular DNA synthesis, determined by 5-bromo-2'-deoxyuridine incorporation, and a profound perturbation in keratinocyte terminal differentiation, as revealed by immunohistochemistry to K5, K14, and K10 and filaggrin. These K14-HPV16 transgenic mice present an opportunity to study the role of the HPV16 oncogenes in the neoplastic progression of squamous epithelium and provide a model with which to identify genetic and epigenetic factors necessary for carcinogenesis.