Enhanced malignant tumorigenesis in Cdk4 transgenic mice

In a previous study, we reported that overexpression of cyclin-dependent kinase-4 (CDK4) in mouse epidermis results in epidermal hyperplasia, hypertrophy and severe dermal fibrosis. In this study, we have investigated the susceptibility to skin tumor formation by forced expression of CDK4. Skin tumors from transgenic mice showed a dramatic increase in the rate of malignant progression to squamous cell carcinomas (SCC) in an initiation-promotion protocol. Histopathological analysis of papillomas from transgenic mice showed an elevated number of premalignant lesions characterized by dysplasia and marked atypia. Interestingly, transgenic mice also developed tumors in initiated but not promoted skin, demonstrating that CDK4 replaced the action of tumor promoters. These results suggest that expression of cyclin D1 upon ras activation synergizes with CDK4 overexpression. However, cyclin D1 transgenic mice and double transgenic mice for cyclin D1 and CDK4 did not show increased malignant progression in comparison to CDK4 transgenic mice. Biochemical analysis of tumors showed that CDK4 sequesters the CDK2 inhibitors p27Kip1 and p21Cip1, suggesting that indirect activation of CDK2 plays an important role in tumor development. These results indicate that, contrary to the general assumption, the catalytic subunit, CDK4, has higher oncogenic activity than cyclin D1, revealing a potential use of CDK4 as therapeutic target.

Advances in molecular carcinogenesis: current and future use of mouse models to screen and validate molecularly targeted anticancer drugs

Survival of patients with advanced solid tumors has not significantly improved over the past 30 years. Although molecularly targeted anticancer drugs offer promise, few drugs make it through the end of the Food and Drug Administration approval process. Animal models that more closely resemble human carcinogenesis may bridge the gap between preclinical success and benefits for patients. We discuss pros and cons of several mouse models, including genetically engineered mice that each represent different aspects of human cancer, and the screening of targeted drugs in these models.

Tumor formation in mice with conditional inactivation of Brca1 in epithelial tissues

The BRCA1 tumor-suppressor protein has been implicated in the regulation of transcription, DNA repair, proliferation, and apoptosis. BRCA1 is expressed in many proliferative tissues and this is at least in part due to E2F-dependent transcriptional control. In this study, inactivation of a conditional murine Brca1 allele was achieved in a variety of epithelial tissues via expression of the Cre recombinase under the control of a keratin 5 (K5) promoter. The K5 Cre:Brca1 conditional knockout mice exhibited modest epidermal hyperproliferation, increased apoptosis, and were predisposed to developing tumors in the skin, the inner ear canal, and the oral epithelium after 1 year of age. Overexpression of the E2F1 transcription factor in K5 Cre:Brca1 conditional knockout mice dramatically accelerated tumor development. In addition, Brca1 heterozygous female mice that had elevated E2F1 expression developed tumors of the reproductive tract at high incidence. These findings demonstrate that in mice Brca1 functions as a tumor suppressor in other epithelial tissues in addition to the mammary gland. Moreover, inactivation of Brca1 is shown to cooperate with deregulation of the Rb-E2F1 pathway to promote tumorigenesis.

Cyclin D1 splice variants. Differential effects on localization, RB phosphorylation, and cellular transformation

Cyclin D1 is a proto-oncogene that functions by inactivation of the retinoblastoma tumor suppressor protein, RB. A common polymorphism in the cyclin D1 gene is associated with the production of an alternate transcript of cyclin D1, termed cyclin D1b. Both the polymorphism and the variant transcript are associated with increased risk for multiple cancers and the severity of a given cancer; however, the underlying activities of cyclin D1b have not been elucidated relative to the canonical cyclin D1a. Because cyclin D1b does not possess the threonine 286 phosphorylation site required for nuclear export and regulated degradation, it has been hypothesized to encode a stable nuclear protein that would constitutively inactivate the RB pathway. Surprisingly, we find that cyclin D1b protein does not inappropriately accumulate in cells and exhibits stability comparable to cyclin D1a. As expected, the cyclin D1b protein was constitutively localized in the nucleus, whereas cyclin D1a was exported to the cytoplasm in S-phase. Despite enhanced nuclear localization, we find that cyclin D1b is a poor catalyst of RB phosphorylation/inactivation. However, cyclin D1b potently induced cellular transformation in contrast to cyclin D1a. In summary, we demonstrate that cyclin D1b specifically disrupts contact inhibition in a manner distinct from cyclin D1a. These data reveal novel roles for d-type cyclins in tumorigenesis.

Modulation of the angiogenesis response through Ha-ras control, placenta growth factor, and angiopoietin expression in mouse skin carcinogenesis

Tumor angiogenesis is governed by a complex balance of positive and negative angiogenic factors. Development of chemically-induced mouse skin tumors appears to be highly dependent on an early burst of neovascularization. We have previously shown that Ha-ras-driven vascular endothelial growth factor (VEGF) expression plays a pivotal role in this process. However, the status of other critical positive and negative angiogenic factors throughout skin tumorigenesis has not been studied to the same extent. In the present study, we show that another VEGF family member, placenta growth factor (PlGF), was highly upregulated at all tumor stages in a ras-dependent manner. The study of angiopoietin-1 (Ang-1) and angiopoietin-2 (Ang-2), ligands of receptor tyrosine kinase 2 (Tie-2), showed that while stroma-derived Ang-2 was increased, epidermal Ang-1 expression was completely abolished at early papilloma formation. Studies using epidermal tumor cell lines suggest that the disappearance of Ang-1 also depends on ras activation, extending the plethora of events controlled by this oncogene in mouse skin carcinogenesis. Our results indicated that tumor development occurred in a strong angiogenesis-prone scenario in which PlGF and Ang-2 acted cooperatively with VEGF, whereas the negative or stabilizing effect of Ang-1 was abrogated. A time-course sequence of expression of angiogenic factors expressed throughout tumor growth, as well as the identification of key signaling molecules triggering the angiogenic response, may contribute to the development and testing of antiangiogenic therapeutic strategies with this in vivo tumor model.

Multidrug resistance modulators PSC 833 and CsA show differential capacity to induce apoptosis in lymphoid leukemia cell lines independently of their MDR phenotype

Among the mechanisms that induce multidrug resistance (MDR), one of those most frequent is over-expression of a phosphoglycoprotein (Pgp) encoded in the mouse by the mdr-1 and mdr-3 genes. We have demonstrated that cyclosporin-A (CsA) as well as its analogue PSC 833 were able to revert the MDR phenotype in murine cell lines resistant to vincristine (LBR-V160) or doxorubicin (LBR-D160). The aim of this work was to evaluate the ability of PSC 833 and CsA to modulate mdr-1, mdr-3 and mrp-1 genes as well as to induce apoptosis analyzing the mechanism involved in the above tumor cell lines. By semi-quantitative RT-PCR, we demonstrated that mdr-3 was over-expressed in both resistant lines while mdr-1 was over-expressed only in LBR-V160; in contrast, mrp-1 expression was not evidenced in any of the cell lines. After treatment with 0.1 microg ml(-1) of either PSC 833 or CsA, LBR-V160 showed no changes in mdr-1 but decreased mdr-3 expression, while LBR-D160 failed to display any modification in the expression of these genes. Apoptosis was evidenced by fluorescence microscopy, S minuscule accumulation and agarose gel electrophoresis. Our results demonstrated that CsA (1 microg ml(-1)) was able to induce apoptosis in all cell lines: 18.31% (+/-4.46) for LBR-, 25.96% (+/-5.24) for LBR-V160 and 27.36% (+/-4.12) for LBR-D160, while PSC 833 (1 microg ml(-1)) only induced apoptosis 21.51% (+/-5.73) in LBR-V160 cell line. The expression of Bcl-2 family proteins (Bcl-2, Bax and Bcl-x(L)) was analyzed by flow cytometry showing high expression of the three proteins which was not significantly modified after treatment with either PSC 833 or CsA on the sensitive as well as on the resistant cell lines. Single stranded conformation polymorphisms analysis of p53 (Trp53) gene in the cell lines showed no mutation in exons 5-8 of the tumor suppressor gene. We conclude that depending on the concentration used, PSC 833 and CsA may act either by modulating the mdr-3 gene (0.1 microg ml(-1)) or by direct impact on the cells through induction of apoptosis (1 microg ml(-1)), in the latter case through a mechanism that might act independent of the Bcl-2 family proteins.

Celecoxib and difluoromethylornithine in combination have strong therapeutic activity against UV-induced skin tumors in mice

The cyclooxygenase-2 (COX-2) inhibitor celecoxib and the ornithine decarboxylase (ODC) inhibitor difluoromethylornithine (DFMO) were each previously shown to prevent skin tumor development when administered throughout the course of UV irradiation. This raised the question of whether maintenance or continued growth of existing tumors required prostaglandins, the product of COX, or polyamines, the product of ODC. To address this question, SKH hairless mice were irradiated 3 times/week with 90 mJ/cm(2); this dose was increased 10% weekly to a maximum of 175 mJ/cm(2). UV was stopped at 27 weeks, at which time there were an average of 5 papillomas/mouse. The mice were then placed in one of four treatment groups: group 1, no treatment; group 2, 0.4% DFMO in the drinking water; group 3, 500 p.p.m. celecoxib in the diet (AIN76); group 4, both DFMO and celecoxib. The control group continued to produce new tumors in a nearly linear manner such that by week 31 the tumor number had nearly doubled, i.e. approximately 10 tumors/mouse. The group receiving DFMO showed significant tumor regression, losing an average of 1 tumor/mouse/week, such that 50% of the tumors remained at week 31. The celecoxib group showed a 25% reduction in tumor number. The group receiving the combination of celecoxib and DFMO showed the greatest regression, with an 89% reduction in tumor number compared with the control group. There was also a corresponding reduction in the size of the tumors. To determine whether tumor regression was permanent or required continued treatment, all treatments were stopped at 31 weeks. Over the next 4 weeks, tumors reappeared at the same rate in all treatment groups. It is concluded that the combination of celecoxib and DFMO are potent therapeutic agents for skin cancer, although the benefits are lost with the cessation of treatment.

Vascular endothelial growth factor: regulation in the mouse skin carcinogenesis model and use in antiangiogenesis cancer therapy

Of the various mechanisms responsible for tumor neovascularization, the angiogenesis process, in particular vascular endothelial growth factor (VEGF), is described here as a target for cancer therapy. While hypoxia is a trigger of tumor angiogenesis, various alterations in oncogenes and tumor suppressor genes also have been reported to induce VEGF expression in tumors. The regulation of VEGF has been investigated in chemically induced mouse squamous cell carcinoma of the skin. In this cancer model, VEGF expression appears to be dependent on ras oncogene activation as well as the epidermal growth factor receptor. Thus, in addition to VEGF, oncogene signaling pathways may be relevant targets in antiangiogenesis cancer therapies. The central role of VEGF in angiogenesis has led to the development of several drugs targeting the pathway of this growth factor. The present paper provides an overview of these drugs and their stage of development. In the near future, clinical trials using anti-VEGF drugs and other antiangiogenic agents, such as endostatin and angiostatin, will yield valuable information about their potential for cancer therapy.

Loss of heterozygosity analysis of mouse pulmonary adenomas induced by coal tar

Manufactured gas plant (MGP) residues, commonly known as coal tars, were generated several decades ago as a byproduct of residential and industrial gas production from the distillation of coal. Previous short-term exposure studies have shown MGP residues to be tumorigenic in mouse liver and lung. In order to gain further insight into carcinogenesis by complex mixtures of environmental chemicals containing known carcinogenic polycyclic aromatic hydrocarbons, we investigated mouse pulmonary tumors for loss of heterozygosity (LOH) as a result of multiple exposure to MGP residues. Twenty mouse lung adenomas produced in (C57BL/6 x C3H)F1 hybrid mice and manually microdissected were selected to examine genome-wide allelic losses at 58 microsatellite loci. Regions of chromosomes 1, 4, 5, 8, and 11 were affected in 30-40% of tumors. The elevated rates of allelic imbalance in these chromosomes may indicate the location of unknown tumor suppressor genes significant to neoplastic transformation in mouse lung tissues. Laser capture microdissection-based LOH analysis of pulmonary adenomas showed that contamination of nonneoplastic tissues was not masking the allelic losses in the manually microdissected tumor analysis. The low frequency of chromosome instability in these tumors, measured by means of inter-simple sequence repeat PCR, suggests the presence of discrete regions of LOH instead of extensive structural rearrangements.

Application of inter-simple sequence repeat PCR to mouse models: assessment of genetic alterations in carcinogenesis

Genomic instability is believed to play a significant role in cancer development by facilitating tumor progression and tumor heterogeneity. Inter-simple sequence repeat (inter-SSR) PCR has been proved to be a fast and reproducible technique for quantitation of genomic instability (amplifications, deletions, translocations, and insertions) in human sporadic tumors. However, the use of inter-SSR PCR in animal models of cancer has never been described. This new technique has been adapted in our laboratory for the analysis of spontaneous and induced mouse tumors. We established the best PCR conditions for each microsatellite-anchored primer and critically evaluated the reproducibility of the band patterns. We also studied the variation of the fingerprints between and within various inbred mouse strains, including wild-derived lines. Tumor-specific alterations were detected as gains, losses, or intensity changes in bands when compared with matched normal DNA. We quantitated the extent of alterations by dividing the number of altered bands in the tumor by the total number of bands in normal DNA (instability index). By means of inter-SSR PCR, we successfully analyzed genomic alterations in various mouse tumors, including spontaneous thymic lymphomas developed in Msh2 knockout mice as well as chemically induced squamous cell carcinomas and thymic lymphomas. Instability index values ranged between 0 and 9%, the highest levels observed in N-methyl-N-nitrosourea-induced thymic lymphomas generated in Trp53 (p53) nullizygote (-/-) mice. We report here, for the first time, the use of inter-SSR PCR to detect somatic mutations in mouse tumoral DNA, including laser-capture microdissected, methanol-fixed tissues. These PCR-based fingerprints provide a novel approach to assessing the number and onset of mutational events in mouse tumors and will help to understand better the mechanisms of carcinogenesis in mouse models.

Genetic analyses of mouse skin tumor progression susceptibility using SENCAR inbred derived strains

Susceptibility to tumor development varies among individuals in the human population. This variability can also be found among different strains of mice, particularly in the mouse skin chemical carcinogenesis model. The genetic mechanisms underlying mouse skin tumor susceptibility are not fully understood. The SENCAR stock has been found to be the most sensitive mice for skin carcinogenesis studies; however, little is known about the genes underlying tumor susceptibility, particularly, those involved in tumor progression. Experiments with the SSIN/Sprd mice, an inbred strain derived from the outbred SENCAR stock, suggested that papilloma development, tumor promotion, and their conversion into squamous cell carcinomas (SCCs), progression, are regulated by different genes. In the highly sensitive SSIN/Sprd mice, papillomas rarely progress to SCC. Using crosses between the outbred SENCAR and the SSIN/Sprd mice, we previously determined that papilloma progression in the SENCAR stock could be controlled by at least one autosomal dominant gene. However, the outbred nature of the SENCAR stock precluded us from extending those findings. More recently, another inbred strain was developed from the outbred SENCAR stock, the SENCARB/Pt. These mice have similar tumor promotion sensitivity to the SSIN/Sprd but in contrast, have high papilloma progression susceptibility, similar to the outbred original stock. In the present study, we generated F(1), F(2), and backcross hybrids between the SSIN/Sprd and SENCARB/Pt mice to determine a possible model for tumor progression susceptibility and to map the putative tumor susceptibility genes. Our tumor data suggests that papilloma progression susceptibility in the SENCAR mouse skin model could be genetically determined by one susceptibility gene. Our preliminary linkage analysis failed to identify one strong susceptibility locus to confirm this but provided some evidence for at least one possible susceptibility locus in mouse chromosome 14.

Impaired hair follicle morphogenesis and cycling with abnormal epidermal differentiation in nackt mice, a cathepsin L-deficient mutation

We previously described an autosomal-recessive mutation named nackt (nkt) exhibiting partial alopecia associated with CD4(+) T-cell deficiency. Also, we recently reported that nkt (now Ctsl(nkt)) comprises a deletion in the cathepsin L (Ctsl) gene. Another recent study reported that Ctsl knockout mice have CD4(+) T-cell deficiency and periodic shedding of hair, which recapitulate the nkt mutation and the old furless (fs) mutation. The current study focuses on the dermatological aspects of the nkt mutation. Careful histological analysis of skin development of homozygous nkt mice revealed a delayed hair follicle morphogenesis and late onset of the first catagen stage. The skin of Ctsl(nkt)/Ctsl(nkt) mice showed mild epidermal hyperplasia and hyperkeratosis, severe hyperplasia of the sebaceous glands, and structural alterations of hair follicles. Epidermal differentiation seems to be affected in nkt skin, with overexpression of involucrin and profilaggrin/filaggrin along with focal areas of keratin 6 expression in the interfollicular epidermis. Severe epidermal hyperplasia, acanthosis, orthokeratosis, and hyperkeratosis were only observed in mice maintained in nonpathogen-free environments. The analysis of Rag2-/- Ctsl(nkt)/Ctsl(nkt) double-mutant mice indicates that the skin defect remains under the absence of T and B cells. This animal model provides in vivo evidence that cysteine protease cathepsin L plays a critical role in hair follicle morphogenesis and cycling, as well as epidermal differentiation.

Cdk4 deficiency inhibits skin tumor development but does not affect normal keratinocyte proliferation

Most human tumors have mutations that result in deregulation of the cdk4/cyclin-Ink4-Rb pathway. Overexpression of D-type cyclins or cdk4 and inactivation of Ink4 inhibitors are common in human tumors. Conversely, lack of cyclin D1 expression results in significant reduction in mouse skin and mammary tumor development. However, complete elimination of tumor development was not observed in these models, suggesting that other cyclin/cdk complexes play an important role in tumorigenesis. Here we described the effects of cdk4 deficiency on mouse skin proliferation and tumor development. Cdk4 deficiency resulted in a 98% reduction in the number of tumors generated through the two-stage carcinogenesis model. The absence of cdk4 did not affect normal keratinocyte proliferation and both wild-type and cdk4 knockout epidermis are equally affected after topical treatment with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), resulting in epidermal hyperplasia. In similar fashion, cdk4 knockout keratinocytes proliferated well in an in vivo model of wound-induced proliferation. Biochemical studies in mouse epidermis showed that cdk6 activity increased twofold in cdk4-deficient mice compared to wild-type siblings. These results suggest that therapeutic approaches to inhibit cdk4 activity could provide a target to inhibit tumor development with minimal or no effect in normal tissue.

Overexpression of cyclin D2 is associated with increased in vivo invasiveness of human squamous carcinoma cells

Overexpression of cyclin D2 was studied in 10 human squamous cell carcinoma lines, to establish whether this gene plays a role in tumor progression. We found that those cell lines that overexpressed cyclin D2 (CCND2) had the most invasive in vivo behavior. The invasive ability of the cell lines was determined by evaluating the penetration of carcinoma cells into the tracheal wall in an in vivo assay with de-epithelialized tracheas transplanted into the subcutaneous tissue of nude mice. From five cell lines that exhibited low invasive ability, we selected two that had very little CCND2 expression (SCC9 and SCC15), to evaluate whether CCND2 gene transfer would increase the invasive behavior. After confirming the successful transfer of CCND2 by Northern, Western, and kinase-activity assays, we assessed the in vivo invasive behavior of the CCND2-transfected cells and their respective vector alone-transfected controls. The cell lines containing the transferred CCND2 gene had a significantly higher invasive ability than respective controls. This was accompanied by a moderate increase in gelatinase activity. In addition, the in vitro proliferative abilities, under normal culture conditions, of the parental CCND2-transfected and vector alone-transfected cells were found to be similar, as was the in vivo labeling index of Ki-67 in the tracheal transplants. These results indicated that the overexpression of CCND2 in squamous cell carcinoma lines modulates cell proliferation after induced quiescence and also has a powerful enhancing effect on in vivo aggressive growth behavior.

A critical role for ras-mediated, epidermal growth factor receptor-dependent angiogenesis in mouse skin carcinogenesis

Epidermal growth factor receptor (EGFR) plays a critical role in epidermal biology. Abnormal EGFR function has been described in epithelial tumors including those induced by two-stage chemical carcinogenesis in mouse skin. A large body of evidence indicates that in this model, activation of Ha-ras is the critical event in papilloma formation, a process that involves epidermal proliferation and stroma remodeling, which includes angiogenesis. This study reports that activated Ha-ras results in a dramatic induction of EGFR in epidermal tumor cells and provides experimental evidence that EGFR signaling is responsible for Ha-ras-dependent vascular endothelial growth factor (VEGF) induction, as well as for the repression of other angiogenic factors such as angiopoietin 1. The pivotal role of functional EGFR in throwing the angiogenic switch necessary for tumor growth was confirmed by s.c. injection of immunodeficient mice with epidermal tumor cells carrying a dominant negative (dn) EGFR and by in vivo chemical skin carcinogenesis assays in transgenic mice expressing the same dn EGFR form in the epidermis. Immunohistochemical analysis of the tumors obtained by both ex vivo and in vivo approaches showed that dn EGFR expression abolished the changes in blood vessels that occurred during tumor progression. A strong reduction of VEGF expression in dn EGFR tumors appears to be the key event responsible for angiogenesis and tumor growth suppression. The apoptotic rate was increased, and Akt activity was decreased, suggesting that impaired nutrient and oxygen supply might contribute to diminished cell survival in dn EGFR tumors. Support for this mechanism is provided by the fact that the ectopic expression of VEGF in dn EGFR-expressing tumor cell lines restored tumor growth capacity. Although ras activation might suffice for epidermal transformation and the stroma-remodeling events of tumor induction, such effects may not be operative without a functional upstream EGFR. It is tempting to speculate that EGFR family members may function as angiogenic regulators in other epithelial tumors such as those of the colon, breast, and prostate, reinforcing their value as targets for therapeutic intervention.

Inactivation of E2f1 enhances tumorigenesis in a Myc transgenic model

Previous studies have demonstrated both oncogenic and tumor suppressive properties for the E2F1 transcription factor. In this study, E2f1-null mice were crossed with transgenic mice expressing Myc under the control of an epithelial-specific keratin 5 promoter to determine whether the absence of E2F1 would modulate the oncogenic activity of Myc. Inactivation of E2f1 was found to significantly accelerate tumor development in keratin 5 Myc transgenic mice. Acceleration of tumorigenesis occurred despite the fact that apoptosis levels were increased in transgenic tissue and tumors null for E2f1, whereas Myc-induced proliferation was unaffected by the status of E2f1. These findings provide new insight into the tumor suppressive activity of E2F1 and identify for the first time a specific oncogenic alteration that cooperates with the loss of E2F1 in tumorigenesis.

p21, p27 and p53 in estrogen and antiprogestin-induced tumor regression of experimental mouse mammary ductal carcinomas

Metastatic mammary carcinoma tumor lines 59-2-HI and C7-2-HI originated in female BALB/c mice treated with medroxyprogesterone acetate and are maintained by syngeneic transplantation. Both lines express estrogen (ER) and progesterone receptors (PR) and regress completely after estradiol (E(2)) or antiprogestin treatment. The BET tumor line, of similar origin and biological features, regresses only after E(2) treatment. To investigate possible differences between E(2)- and antiprogestin-mediated effects we evaluated the morphological features, mitosis and apoptosis, and the differential expression of cell-cycle inhibitors associated with tumor regression. Treatments started when tumors reached 50-100 mm(2). After 24-96 h, tumors were excised and processed for morphological and immunohistochemical studies. Regression was associated with a significant and early decrease in the number of mitosis and with higher percentages of apoptotic cells. These phenomena were accompanied by an increase in p21 and p27 expression in the E(2) and antiprogestin-responsive lines treated with E(2), RU 38.486 or ZK 98.299 (P < 0.05). In BET tumors treated with E(2), p21 expression remained within basal levels and only p27 increased (P < 0.05). p53 was low in control 59-2-HI and C7-2-HI tumors and increased after treatment (P < 0.05) whereas BET untreated tumors already expressed high levels of p53 and MDM2. Although the immunohistochemical findings were compatible with alterations of p53, SSCP evaluation failed to disclose the presence of mutations, suggesting that the defective expression of p21 is related to an impaired p53 pathway. UV irradiation failed to increase p21 expression in BET, but was able to induce p53 and p21 in 59-2-HI and C7-2-HI tumors. The absence of an increased expression of p21 in E(2)-regressing BET lesions suggests that this protein is not necessary for estrogen-induced regression, but may be essential for antiprogestin action. Our results also suggest that p53/MDM2 alterations may be one of the mechanisms responsible for selected hormone resistance in breast carcinomas.

A variant of pulmonary alveolar microlithiasis in nackt mice

Four adult mutant nackt mice, which demonstrate alopecia and CD4+ T-cell deficiency, and two outbred SENCAR mice (sentinels) were presented for routine health surveillance. Lesions were not observed at necropsy. Microscopically, all four nackt mice demonstrated multiple concretions that were 30-100 microm in diameter, irregularly rounded to oval, nonbirefringent, and amphophilic to basophilic. Many of the concretions appeared attached to or within the alveolar walls of all lung lobes. Approximately half of the concretions had irregular fracture lines. All concretions were periodic acid-Schiff positive, and Von Kassa staining revealed diffuse calcification. None of the concretions were associated with inflammatory cell infiltrates, and metaplastic ossification was not evident. A diagnosis of pulmonary alveolar microlithiasis, a rare disease in both humans and animals, was made based on the size and location of the concretions and the lack of an inflammatory response. This is the first report of a laboratory mouse demonstrating pulmonary alveolar microlithiasis.

Simultaneous PAGE, immunoblotting, and immunohistochemical analysis of differentiation associated keratins in lesions of the oral mucosa

The expression of differentiation associated high PM Keratin polypeptides of the oral mucosa lesions were studied by immunohistochemical and immunoblotting techniques applied to adjacent sections of each biopsy specimen. The material studied included specimens of leukoplakia, verrucous carcinoma, squamous cell carcinoma, adenocarcinoma and keratoacanthoma. Little or no expression of 65-67 Kd keratins was evident in squamous cell carcinoma and adenocarcinoma. Hyperkeratotic (both benign and dysplastic) lesions such as verrucous carcinoma, leukoplakia, and keratoacanthoma, showed great variations in the intensity of 65-67 bands and a very irregular immunohistochemical staining pattern. Increased amounts of horny substance was usually accompanied by absence of, or decreased expression of 65-67 Kd keratins, thus indicating a change in the polypeptide composition of the horny layer in pathological conditions of the oral epithelium.

ARF differentially modulates apoptosis induced by E2F1 and Myc

The ARF tumor suppressor participates in a p53-dependent apoptotic pathway that is stimulated in response to some oncogenic stimuli. The E2F1 transcription factor is a critical downstream target of the Rb tumor suppressor and, when active, can promote proliferation as well as apoptosis. The finding that E2F1 transcriptionally regulates the ARF gene has led to the suggestion that ARF contributes to E2F1-induced apoptosis. Counter to this hypothesis, this study demonstrates not only that ARF is unnecessary for E2F1 to induce apoptosis but also that inactivation of ARF actually enhances the ability of E2F1 to promote apoptosis. Inactivation of ARF also cooperates with E2F1 activity to promote entry into the S phase of the cell cycle. This relationship between ARF and E2F1 is demonstrated in transgenic epidermis in vivo and in mouse embryo fibroblast cultures in vitro. In contrast, the ability of Myc to induce apoptosis is diminished in the absence of ARF. E2F1 induces the accumulation of p53 in the absence of ARF, and this is associated with the phosphorylation of p53 on several residues. These findings demonstrate that ARF is a negative regulator of E2F1 activity and is not required for E2F1-induced apoptosis.

Regulation of the differentiation-related gene Drg-1 during mouse skin carcinogenesis

Differentiation-related gene-1 (Drg-1) has been identified as a gene whose expression is increased in several processes related to differentiation, but its function is currently unknown. In this report, we show that Drg-1 was expressed in keratinocytes, this expression being rapidly increased as a result of induction by 12-O-tetradecanoylphorbol-13-acetate (TPA) or the presence of an activating form of Ha-ras. Induction by TPA occurred both in cultured cell lines and primary keratinocytes as well as in mouse skin after a single TPA application. Overexpression of Drg-1 was also observed in TPA-induced hyperplastic skin. In agreement, mouse skin papillomas and carcinomas also overexpressed Drg-1. In addition, Drg-1 was induced when keratinocytes were forced to differentiate by calcium switch or serum starvation. Analysis of the expression of Drg-1 during the keratinocyte cell cycle demonstrated relatively high levels of Drg-1 mRNA in G(0), which increased in early G(1) and decreased afterwards in late G(1)/S. In situ analysis showed an accumulation of Drg-1 in the suprabasal layers of the skin, as well as in the more differentiated areas of mouse skin papillomas. These results suggest that, in addition to being upregulated during keratinocyte differentiation, the Drg-1 gene might have a complex function in skin tumorigenesis.

Myc lacks E2F1's ability to suppress skin carcinogenesis

Myc and E2F1 can each stimulate proliferation, induce apoptosis, and contribute to oncogenic transformation. However, only E2F1 has been shown to have a tumor suppressive activity under some conditions. To examine the potential of Myc to suppress tumorigenesis under one of the conditions in which E2F1 functions to suppress tumorigenesis, transgenic mice expressing Myc under the control of a keratin 5 (K5) promoter were generated. Like K5 E2F1 transgenic mice, K5 Myc transgenic mice have hyperplastic and hyperproliferative epidermis and develop spontaneous tumors in the skin and oral epithelium. In addition, K5 Myc and K5 E2F1 transgenic mice both display aberrant, p53-dependent apoptosis in the epidermis. It has been demonstrated that deregulated expression of E2F1 in the epidermis of transgenic mice inhibits tumorigenesis in a two-stage skin carcinogenesis assay. In sharp contrast to those results, deregulated expression of Myc in the epidermis of transgenic mice resulted in an enhanced response to two-stage skin carcinogenesis. We conclude that while Myc and E2F1 have similar proliferative, apoptotic and oncogenic properties in mouse epidermis, Myc lacks E2F1's tumor suppressive property. This suggests that E2F1's unique ability to inhibit skin carcinogenesis is not simply a consequence of promoting p53-dependent apoptosis.

Transgenic expression of cyclin-dependent kinase 4 results in epidermal hyperplasia, hypertrophy, and severe dermal fibrosis

In a previous report we have described the effects of expression of D-type cyclins in epithelial tissues of transgenic mice. To study the involvement of the D-type cyclin partner cyclin-dependent kinase 4 (CDK4) in epithelial growth and differentiation, transgenic mice were generated carrying the CDK4 gene under the control of a keratin 5 promoter. As expected, transgenic mice showed expression of CDK4 in the epidermal basal-cell layer. Epidermal proliferation increased dramatically and basal cell hyperplasia and hypertrophy were observed. The hyperproliferative phenotype of these transgenic mice was independent of D-type cyclin expression because no overexpression of these proteins was detected. CDK4 and CDK2 kinase activities increased in transgenic animals and were associated with elevated binding of p27(Kip1) to CDK4. Expression of CDK4 in the epidermis results in an increased spinous layer compared with normal epidermis, and a mild hyperkeratosis in the cornified layer. In addition to epidermal changes, severe dermal fibrosis was observed and part of the subcutaneous adipose tissue was replaced by connective tissue. Also, abnormal expression of keratin 6 associated with the hyperproliferative phenotype was observed in transgenic epidermis. This model provides in vivo evidence for the role of CDK4 as a mediator of proliferation in epithelial cells independent of D-type cyclin expression.

PCR-based microsatellite analysis for differentiation and genetic monitoring of nine inbred SENCAR mouse strains

Sixteen DNA microsatellites or simple sequence length polymorphisms (SSLPs), generated by polymerase chain reaction (PCR) were selected for use in the genetic quality control of the nine inbred SENCAR strains currently available. The SENCAR strains constitute a powerful tool for mechanistic studies of multi-stage skin carcinogenesis, as well as for studies to understand the underlying genetic basis of resistance to tumour promotion and progression. SSLP analysis is a fast and economical way for detecting genetic contamination (unexpected outcrosses) among these closely-related albino strains, where standard immunological and biochemical markers have been shown to be insufficient.