Interdependence of cortical thymic epithelial cell differentiation and T-lineage commitment

Thymocyte and thymic epithelial cell (TEC) development are interdependent processes. Although lineage relationships among progressively maturing thymocyte subsets have been characterized, the developmental relationships among TEC subsets are obscure. Because epithelial cells express distinct keratin (K) species as a function of differentiation stage and proliferative status, we used K expression patterns to identify mouse TEC subsets and determine their lineage relationships. As expected, cortical and medullary TEC subsets express distinct K expression patterns in the normal thymus. However, we detected two distinct cortical TEC subsets, a major K8(+)K5(-) subset and a minor K8(+)K5(+) subset, which is highly represented at the cortico-medullary junction. Both cortical TEC subsets are also present in recombination activating gene 1 (RAG-1(-/-)) and TCRbetaxdelta-/- thymi in which T-cell development is blocked at the CD4(-)CD8(-)CD25(+)CD44(-) pre-T cell stage. In contrast, K8(+)K5(+) TECs predominate in the thymi of human CD3epsilon transgenic mice in which thymocyte development is blocked at an earlier CD4(-)CD8(-)CD25(-)CD44(+) stage. Transplantation of newborn human CD3epsilon transgenic thymi under the kidney capsule of RAG-1(-/-) mice results in the emergence of K8(+)K5(-) TECs concomitant with the appearance of CD25(+) thymocytes. Together, the data suggest that cortical TEC development proceeds from a K8(+)K5(+) precursor subset to a K8(+)K5(-) stage in a differentiation process concomitant with T-cell lineage commitment.

Reduced skin tumor development in cyclin D1-deficient mice highlights the oncogenic ras pathway in vivo

Cyclin D1 is part of a cell cycle control node consistently deregulated in most human cancers. However, studies with cyclin D1-null mice indicate that it is dispensable for normal mouse development as well as cell growth in culture. Here, we provide evidence that ras-mediated tumorigenesis depends on signaling pathways that act preferentially through cyclin D1. Cyclin D1 expression and the activity of its associated kinase are up-regulated in keratinocytes in response to oncogenic ras. Furthermore, cyclin D1 deficiency results in up to an 80% decrease in the development of squamous tumors generated through either grafting of retroviral ras-transduced keratinocytes, phorbol ester treatment of ras transgenic mice, or two-stage carcinogenesis.

VEGF/VPF overexpression in skin of transgenic mice induces angiogenesis, vascular hyperpermeability and accelerated tumor development

Upregulation of keratinocyte-derived VEGF-A expression has recently been established in non-neoplastic processes of skin such as wound healing, blistering diseases and psoriasis, as well as in skin neoplasia. To further characterize the effects of VEGF-A in skin in vivo, we have developed transgenic mice expressing the mouse VEGF120 under the control of a 2.4 kb 5' fragment of keratin K6 gene regulatory sequences that confers transgene inducibility upon hyperproliferative stimuli. As expected from the inducible nature of the transgene, two of the three founder mice obtained (V27 and V208), showed no apparent phenotype. However, one founder (V2), mosaic for transgene integration, developed scattered red spots throughout the skin at birth. The transgenic offspring derived from this founder developed a striking phenotype characterized by swelling and erythema, resulting in early postnatal lethality. Histological examination of the skin of these transgenics demonstrated highly increased vascularization and edema leading to disruption of skin architecture. Expression of the transgene was silent in adult animals of lines derived from founders V27 and V208. Phorbol ester-induced hyperplasia resulted in transgene induction and increased cutaneous vascularization in adult transgenic mice of these lines. Skin carcinogenesis experiments performed on hemizygous crosses of V208 mice with activated H-ras-carrying transgenic mice (TG.AC) resulted in accelerated papilloma development and increased tumor burden. Previous results from our laboratory showed that VEGF upregulation is a major angiogenic stimulus in mouse epidermal carcinogenesis. By overexpressing VEGF in the skin of transgenic mice we now move a step further toward showing that VEGF-mediated angiogenesis is a rate-limiting step in the genesis of premalignant lesions, such as mouse skin papilloma. Our transgenic mice constitute an interesting model system for in vivo study of the cutaneous angiogenic process and its relevance in tumorigenesis and other skin diseases.

Increased E2F1 activity induces skin tumors in mice heterozygous and nullizygous for p53

The p16(INK4a)-cyclin D-retinoblastoma tumor suppressor pathway is disrupted in most human cancers, and it has been suggested that the subsequent release of E2F transcription factors from inhibitory complexes may be a key event in tumor development. We described recently the generation of transgenic mice with E2F1 gene expression targeted to squamous epithelial tissues by a keratin 5 (K5) promoter. In the present study, K5 E2F1 transgenic mice were crossed with p53 null mice to examine functional interactions between E2F1 and p53 in vivo. We find that E2F1-induced apoptosis of epidermal keratinocytes is reduced in K5 E2F1 transgenic mice lacking p53, whereas E2F1-induced hyperproliferation is unaffected by p53 status. We also find that K5 E2F1 transgenic mice heterozygous or nullizygous for p53 develop spontaneous skin carcinomas, which normally are rare in p53-deficient mice. The timing of tumor development correlates with the level of E2F1 transgene expression and the status of p53. In primary transgenic keratinocytes, the major change in E2F1 DNA-binding activity is the generation of a complex also containing the retinoblastoma tumor suppressor protein. Nevertheless, the expression and associated kinase activity of cyclin E, a known target for E2F transcriptional activity, is elevated significantly in K5 E2F1 transgenic keratinocytes. These findings firmly establish that increased E2F1 expression can contribute to tumor development and suggest that p53 plays an important role in eliminating cells with deregulated E2F1 activity.

Alterations of p53, cyclin D1, Rb, and H-ras in human oral carcinomas related to tobacco use

BACKGROUND

Epidemiologic studies have indicated that environmental and personal habits, particularly tobacco use and alcohol abuse, are major etiologic factors in the induction and progression of head and neck squamous cell carcinomas (HNSCC). Molecular studies have focused on HNSCC related to smoking but not those associated with smokeless tobacco.

METHODS

The authors studied immunohistochemical evidence of alterations of p53, cyclin D1, and Rb in 34 human oral carcinomas related to tobacco use. They also examined p53 and H-ras using single strand conformation polymorphism (SSCP) and sequencing analysis.

RESULTS

Overexpression of cyclin D1 was found in 41% of cases, and accumulation of p53 was found in 59%. Only 9% of the samples did not show Rb staining. In SSCP and sequencing analysis, 17 cases showed mutations in the conserved region of the p53 gene. No mutations were detected in codons 12, 13, or 61 of the H-ras gene.

CONCLUSIONS

Overexpression of cyclin D1 and p53 mutations are common alterations in HNSCC. In contrast, the loss of Rb function seems to occur infrequently, and mutations in the H-ras gene apparently do not play a role in this cancer. HNSCC associated with smokeless tobacco contained the same alterations as those related to smoking.

Papillomas at high risk for malignant progression arising both early and late during two-stage carcinogenesis in SENCAR mice

The current study was designed to further establish that most papillomas produced in SENCAR mice during two-stage skin carcinogenesis are, in fact, premalignant lesions and to specifically determine the malignant conversion potential of papillomas that arise at different times during the carcinogenesis process. A method was established to physically map and monitor the lifespan of all papillomas produced in SENCAR mice during the course of an initiation-promotion experiment using DMBA as the initiator and 12-O-tetradecanoylphorbol-13-acetate (TPA) as the promoter. The results from these experiments showed that in groups of mice initiated with either 0.5 or 2.0 microg DMBA, long-term (60 weeks) treatment with TPA yielded a significantly higher number of SCCs compared to short-term treatment (7 weeks). Papillomas that emerged after 11 weeks and thereafter in all treatment groups had the ability to progress to SCCs. The median conversion time for all papillomas in all groups was 26 weeks. When corrected for median conversion time, papillomas that emerged in week 11 and thereafter in all treatment groups had similar or greater conversion ratios compared to those that emerged within the first 10 weeks. Interestingly, the median conversion time was significantly shorter (18 versus 27 weeks, respectively; P<0.0002) for papillomas that emerged in week 11 and thereafter compared to those that emerged at or prior to 10 weeks for all groups. The data in this study demonstrate that papillomas arising throughout a two-stage carcinogenesis protocol in SENCAR mice progress to SCCs. Many papillomas that arise later in two-stage carcinogenesis protocols do not have sufficient time to allow for conversion and should be excluded from the analyses. Furthermore, another novel finding of the current study was the observation that papillomas arising later in the two-stage protocol (>11 weeks) progressed to SCCs at a faster rate than those that arose earliest in the protocol.

Increased cell growth and tumorigenicity in human prostate LNCaP cells by overexpression to cyclin D1

Deregulated expression of cyclin D1 has been found in several types of human tumors. In order to investigate factors involved in human prostate cancer progression, we studied the effects of cyclin D1 overexpression on human prostate cancer cell proliferation and tumorigenicity by transfecting LNCaP cells with a retroviral vector containing human cyclin D1 cDNA. When compared to the parental and control-vector transfected LNCaP cells, these cyclin D1-transfected cells had more cells in S-phase and lower growth factor requirements. Furthermore, these cells grew more in androgen-free medium. We also detected higher levels of Rb phosphorylation and E2F-1 protein levels in LNCaP/cyclin D1 cells than that in the parental and vector control cells in medium with or without androgen. Cyclin D1 transfected clones formed tumors more rapidly than control and parental cells. These tumors were refractory to the androgen-ablation treatment by castration, whereas tumors from parental and vector-control LNCaP cells regressed within 4 weeks after castration. These results suggest that overexpression of cyclin D1 changes the growth properties, increases tumorigenicity and decreases the requirement for androgen stimulation in LNCaP cells both in vitro and in vivo.

Deregulated expression of E2F1 induces hyperplasia and cooperates with ras in skin tumor development

In cell culture studies, overexpression of the E2F1 transcription factor has been shown to stimulate proliferation, induce apoptosis, and cooperate with an activated ras gene to oncogenically transform primary rodent cells. To study the effect of increased E2F1 activity on epithelial growth and tumorigenesis in vivo, transgenic mice expressing E2F1 under the control of a keratin 5 (K5) promoter were generated. Expression of E2F1 in the epidermis results in hyperplasia but does not inhibit terminal differentiation. In a transgenic line expressing high levels of E2F1, mice have decreased hair growth likely as a result of aberrant apoptosis in developing hair follicles. Coexpression of a cyclin D1 transgene with E2F1 augments epidermal hyperplasia and further disrupts hair follicle development suggesting that hypophosphorylated Rb antagonizes the proliferative and apoptotic-promoting activities of E2F1. Finally, the E2F1 transgene is found to cooperate with a v-Ha-ras transgene to induce skin tumors in double transgenic animals. These findings confirm that many of the activities ascribed to E2F1 through in vitro studies can be reproduced in vivo and demonstrate for the first time that deregulated E2F activity can contribute to tumor development.

Beta2 integrin/ICAM-1 adhesion molecule interactions in cutaneous inflammation and tumor promotion

The beta2 integrin (CD 18/CD 11 a, b, c) family of proteins mediate adherence of leukocytes to vascular endothelium and the associated ligand, intercellular adhesion molecule-1 (ICAM-1; CD 54), interacts with beta2 integrin proteins to allow transendothelial migration of leukocytes into sites of inflammation. The present study examines the function of these proteins in a murine model of acute cutaneous inflammation induced following topical application of 12-O-tetradecanoylphorbol-13-acetate (TPA) to the dorsal epidermis of SENCAR mice and in a model of skin multistage carcinogenesis. At 24 h following topical application of TPA to the dorsal epidermis of mice, dermal leukocytes expressed higher levels of beta2 integrin protein compared with the lower levels of beta2 integrin protein expression by peripheral blood leukocytes. ICAM-1 protein was localized to epidermal keratinocytes and vascular endothelium in TPA-treated skin and to proliferating papilloma cells. Intravenous (i.v.) injection of either 50 microg anti-beta2 integrin antibody alone or in combination with anti-ICAM-1 antibody significantly inhibited both TPA-stimulated neutrophil infiltration into the dermis (P < 0.001) and myeloperoxidase (MPO) activity (P < 0.03 anti-beta2 integrin antibody; P < 0.01 anti-beta2 integrin + ICAM-1 adhesion molecule antibodies), but had no effect on TPA-induced epidermal hyperplasia. In addition, injection of either anti-ICAM-1 adhesion molecule antibody alone (P < 0.004) or in combination with anti-beta2 integrin antibody (P < 0.001) significantly inhibited TPA-induced production of 7,8-dihydroxy-2'-deoxyguanosine (8-OHdG) immunoreactive proteins by epidermal keratinocytes. Beta2 integrin/ICAM-1 adhesion molecules work in concert to regulate migration, retention and functional activation of leukocytes within the dermis during TPA-induced skin inflammation and within stromal tissue of papillomas that form during multi-stage carcinogenesis. Agents that inhibit these receptor/ligand interactions may be useful in defining the roles of specific cell populations in cutaneous inflammation and multistage carcinogenesis and may also have potential as anti-promoting and anti-progression agents.

Synchronized proliferation induced by 12-O-tetradecanoylphorbol-13-acetate treatment of mouse skin: an in vivo model for cell cycle regulation

Much of what is known about the mammalian cell cycle comes from studies using established cell lines in culture. In this study, cell cycle-regulatory events were analyzed in vivo after treatment of mouse epidermis with 12-O-tetradecanoylphorbol-13-acetate. A synchronized wave of basal keratinocyte proliferation occurred; over 80% of the cells were in S phase 15 h after treatment. c-myc protein expression was induced, and p57Kip2 protein levels dropped early after stimulation. Before S phase, cyclin D1 and cyclin-dependent kinase (CDK) 6 levels increased, and expression of cyclins E and A was induced. Rb was phosphorylated in late G1, and this correlates with the formation of cyclin D1/CDK4 and cyclin D1/CDK6 complexes. At the end of S phase, the p57Kip2 and p21Cip1 protein levels increased. These findings demonstrate that stimulation of basal epidermal cells by 12-O-tetradecanoylphorbol-13-acetate results in several classic cell cycle events and suggests that p57Kip2 plays a key role in regulating proliferation in the epidermis.

Analysis of two inbred strains of mice derived from the SENCAR stock with different susceptibility to skin tumor progression

The SENCAR stock of mice has proved to be a useful model in dissecting out the multistage nature as well as the critical mechanisms involved in skin tumorigenesis. This outbred stock was selectively bred to be susceptible to initiation with 7,12-dimethylbenz[a]anthracene (DMBA) and promotion with 12-O-tetradecanoylphorbol-13-acetate (TPA). In order to obtain mice more suitable for genetic analyses of tumor susceptibility and tissue transplantation studies, several inbred lines of mice were derived from the SENCAR stock. One of these lines, the SSIN mice, has a higher susceptibility to tumor promotion compared to the SENCAR stock but is very resistant to tumor progression. On the other hand, the SENCAR B/Pt mice, derived also from the outbred stock, not only have a tumor promotion susceptibility almost identical to the SSIN mice, but they also have a high susceptibility to tumor progression. In order to understand the nature of the phenotypic differences between these two inbred lines we have characterized them using several parameters and markers that are associated with the progression of papillomas to squamous cell carcinoma (SCC). In this sense we analysed the tumor multiplicity and SCC incidence, and the expression of markers of progression and cell cycle related proteins in papillomas derived from both strains. Our results showed that while both strains have a similar papilloma multiplicity and incidence the SENCAR B/Pt mice have 67% incidence of SCC, compared to 0% in the SSIN. SENCAR B/Pt papillomas at 30 weeks of promotion have a higher and aberrant expression of K13, and loss of connexin 26. TGF-beta1 was found to be over-expressed in the suprabasal and superficial cells in the SENCAR B/Pt papillomas, while it was only expressed in the superficial cell layer in those derived from SSIN. The SENCAR B/Pt papillomas also showed an enlarged proliferative compartment with overexpression of cyclin D1 and PCNA as seen by immunohistochemistry and Western blot.

Evidence that initiated keratinocytes clonally expand into multiple existing hair follicles during papilloma histogenesis in SENCAR mouse skin

We have previously shown that the precursors of cutaneous papillomas in SENCAR mice initiated with 7,12-dimethylbenz[a]anthracene and promoted with 12-O-tetradecanoylphorbol-13-acetate are focal hyperplastic lesions that we refer to as squamous cell hyperplastic foci (SCHF). Ha-ras gene codon 61 mutations were frequently found in SCHF, providing evidence that these lesions represent clones of initiated cells. We report here the pathogenesis of multiple hair follicle involvement in more advanced SCHF and describe the role of the hair follicle in papilloma histogenesis. Detailed histological evaluation of 83 SCHF and 25 early papillomas revealed a morphological continuum from the least developed SCHF, involving only one hair follicle, to advanced SCHF and early papillomas, which involved more than 10 hair follicles. These results provide evidence of the recruitment of additional hair follicles as SCHF progress. In advanced SCHF and early papillomas the bulk of the epithelial component in all cases consisted of several markedly hyperplastic adjacent hair follicles, whereas the involved interfollicular epidermis (IFE) was generally less hyperplastic. All of the hair follicles involved in SCHF appeared to have been preexisting, based on their pattern of spacing, that they were consistently normal appearing below the level of the sebaceous glands, and that they were in the same phase of the hair cycle as surrounding, uninvolved hair follicles. Also, no evidence of follicular neogenesis was observed in serially sectioned SCHF, and coalescence of smaller lesions was rare. To investigate whether the involvement of multiple hair follicles in SCHF was due to expansion of initiated cells into existing hair follicles or, possibly, to a paracrine mechanism, we analyzed different levels of three serially sectioned SCHF and one early papilloma for Ha-ras mutations. These analyses revealed cells with Ha-ras gene codon 61 mutations at multiple levels that involved different hair follicles. Overall, our results provide evidence that as initiated cells clonally expand, they spread across the IFE and populate the upper permanent portions of existing hair follicles. The abnormal proliferation of the infundibula of the hair follicles involved in SCHF appears to give rise to most of the epithelial component of papillomas.

Transplantation analyses of the immunogenicity of epidermal tumors generated in murine skin two-stage carcinogenesis protocols

SSIN mice are very sensitive to tumor promoters in two-stage skin carcinogenesis protocols. It was recently reported that SSIN mice have fewer CD8+ T-cells than other strains of mice and develop a weaker cytotoxic T-cell response upon challenge with an allogeneic tumor transplant. The significance of this muted immune response to processes involved in two-stage carcinogenesis depends on the immunogenicity of the tumors generated in such protocols. Although they have low CD8+ T-cell contents, SSIN rejected a variety of subcutaneously transplanted allogeneic murine tumors. Analyses of the growth of primary papillomas derived from 7,12-dimethylbenz[a]anthracene-initiated/12-O-tetradecanoylphorbol-13-ac etate-promoted SSIN mice and then subcutaneously transplanted into triple-deficient (bg-nu-xid), athymic nude and immune-competent and immunosuppressed SSIN mice revealed that few tumors took and tumor takes were not markedly influenced by the immumological status of the transplant recipient. Two tumor cell lines (RS1 and RS2) were derived from the transplantation studies and could be passaged in normal SSIN mice (H-2q haplotype). Both tumors were squamous cell carcinomas (SCCs) by the second in vivo passage and were rejected in allogeneic mice (BALB/c) but grew in FVB/N mice, a strain having the H-2q haplotype. Transplantation studies revealed that prior exposure to RS1 and RS2 did not prime SSIN mice to reject a subsequent tumor challenge. Three primary SCC tumors derived from SSIN mice in a two-stage carcinogenesis protocol also grew when subcutaneously transplanted in SSIN mice and could be serially passaged. Consequently, the epidermal SCCs that develop in two-stage carcinogenesis protocols appear to be nonimmunogenic.

Increased apoptosis during papilloma development in mice susceptible to tumor progression

Programmed cell death (apoptosis) is known to occur not only during normal development and tissue remodeling but also during neoplasia. Despite the suggested role of apoptosis in preventing the proliferation of malignant cells, a positive correlation between tumor progression and the presence of apoptotic cells has been found in different types of cancer, including epithelial tumors. In normal mouse skin, the role of apoptosis is not completely understood, and it has been suggested that terminal differentiation may be a special case of apoptosis. In the work reported here, we counted apoptotic cells in mouse skin tumors generated with a two-stage chemical carcinogenesis protocol. We analyzed papillomas from outbred SENCAR mice at different times during promotion, and to better determine the correlation between apoptosis and tumor progression, we compared papillomas generated from two inbred strains derived from the SENCAR stock that differ in their susceptibility to tumor progression. Our results showed that in mouse skin chemical carcinogenesis, the number of apoptotic cells was greater in papillomas that may have been in the process of progressing to squamous cell carcinomas. This conclusion is also supported by the fact that papillomas from SENCAR P/Bt. mice, a tumor progression-susceptible strain derived from outbred SENCAR mice, had more apoptotic cells than papillomas from progression-resistant SSIN mice.

Changes in protein expression during multistage mouse skin carcinogenesis

To directly compare the expression patterns of different proteins known to be altered during mouse skin carcinogenesis, serial sections of normal and hyperplastic skin and tumors from various stages of 7,12-dimethylbenz[a]anthracene-initiated, 12-O-tetradecanoylphorbol-13-acetate-promoted female SENCAR mice were examined by immunohistochemistry. In untreated, normal mouse skin, keratin 1 (K1) and transforming growth factor-beta1 (TGFbeta1) were strongly expressed in the suprabasal layers, whereas integrin alpha6beta4 was expressed only in basal cells and only moderate staining for transforming growth factor-alpha (TGFalpha) was seen. In hyperplastic skin, TGFalpha expression became stronger, whereas expression of another epidermal growth factor (EGF) receptor ligand, heparin-binding EGF-like growth factor (HB-EGF), was strongly induced in all epidermal layers from no expression in normal skin. Likewise, the gap-junctional protein connexin 26 (Cx26) became highly expressed in the differentiated granular layers of hyperplastic skin relative to undetectable expression in normal skin. Expression of cyclin D1 in the proliferative cell compartment was seen in all benign and malignant tumors but not in hyperplastic skin. Beginning with very early papillomas (after 10 wk of promotion), expression of alpha6beta4 in suprabasal cells and small, focal staining for keratin 13 (K13) were seen in some tumors. Later (after 20-30 wk), focal areas of gamma-glutamyl transpeptidase (GGT) activity appeared in a few papillomas, whereas TGFbeta1 expression began to decrease. Cx26 and TGFalpha staining became patchier in some late-stage papillomas (30-40 wk), whereas suprabasal alpha6beta4, K13, and GGT expression progressively increased and K1 expression decreased. Finally, in squamous cell carcinomas (SCCs), there was an almost complete loss of K1 and a further decline in TGFalpha, HB-EGF, TGFbeta1, and Cx26 expression. On the other hand, almost all SCCs showed suprabasal staining for alpha6beta4 and widespread cyclin D1 and K13 expression, whereas only about half showed positive focal staining for GGT activity.

During development, 17alpha-estradiol is a potent estrogen and carcinogen

Neonatal administration of estradiol-17beta (E2-17beta) increases the nuclear DNA content in the mouse reproductive tract. Similar responses have been demonstrated for synthetic estrogens such as diethylstilbestrol. One of the questions raised regarding environmental estrogens such as organochlorines is whether they are potent enough to result in abnormal changes such as those demonstrated by both natural and synthetic estrogens. To test this hypothesis, female BALB/c mice were treated neonatally (days 1-5) with either E2-17beta or estradiol-17alpha (E2-17alpha), an inactive stereoisomer in adult reproductive tissues. We also proposed whether neonatal administration of (E2-17alpha) was tumorigenic and whether the effects were age dependent. To answer these questions, one set each of 10 day-old treated and control mice received short-term secondary administration of E2-17beta, E2-17alpha, or cholesterol. Cervicovaginal tracts from intact BALB/c mice were examined histologically and by flow cytometry at 70 days of age and by histology alone at 18 to 22 months of age. The results include several important findings: a) like E2-17beta, neonatal E2-17alpha treatment induced persistent vaginal cornification, hypospadias, vaginal concretions, and hyperproliferation in nearly 100% of the animals regardless of the secondary treatment for most groups; b) neonatal E2-17alpha treatment increased the nuclear DNA content of cervicovaginal epithelium at one-half both the level (mean DNA index of 1.02 vs 1.04) and incidence (22 vs 46% of the animals) of E2-17beta; c) short-term secondary treatment with E2-17alpha, unlike E2-17beta, did not significantly augment the increase in DNA content (13% for E2-17alpha vs 37 and 56% for control and E2-17beta, respectively); and d) neonatal administration with E2-17alpha induced adenosquamous tumors in the reproductive tract in 25% of the animals. Therefore, the biological effects (estrogenic potency) of E2-17alpha may be age dependent.

Induction of mammary cancer and lymphoma by multiple, low oral doses of 7,12-dimethylbenz[a]anthracene in SENCAR mice

Existing models of mouse mammary carcinogenesis induced by the model polycyclic aromatic hydrocarbon 7,12-dimethylbenz[a]anthracene (DMBA) typically use a small number of bolus doses applied intragastrically. In contrast to this, typical human exposures to carcinogens are thought to be at lower doses and to occur with chronic or sporadic timing. When the classical dosage (1 mg DMBA given once a week for 6 weeks) was split into five daily doses of 200 microg given intragastrically to female SENCAR mice each week for 6 weeks, toxicity was high and the major tumor type seen was lymphoma. Lowering the dose to 60 microg/day gave less toxicity, a 75% incidence of lymphoma and a 30% incidence of mammary carcinoma. However, 20 microg DMBA given five times per week for 6 weeks resulted in a 65-70% incidence of mammary carcinoma within approximately 50 weeks. This represents a 50-fold lower daily dosage of DMBA than that used in the classical model. DNA was prepared from 10 mammary adenocarcinomas and 10 lymphomas and exons 1 and 2 of the H-ras1, K-ras and N-ras genes were sequenced using PCR techniques. Mutations altering codons 12 or 61 of one of the ras family genes were found in 4/10 mammary carcinomas and 5/10 lymphomas. Three mammary tumors exhibited codon 61 mutations, one in each of the genes studied, and a fourth tumor contained a codon 12 mutation in the K-ras gene. Among the lymphomas, two mutations in codon 12 of K-ras, one mutation in codon 61 of K-ras and two mutations in codon 61 of N-ras were also found. Each of the mutations could be interpreted as a G-->T or A-->T transversion. It is suggested that the high incidence of lymphoma at the higher, repetitive doses may be related to immunotoxicity. These low dose models of lymphomagenesis and mammary carcinogenesis should prove useful for tests of chemopreventive agents that target the initiation phase of carcinogenesis.

Loss of heterozygosity on murine chromosome 6 in two-stage carcinogenesis: evidence for a conserved tumor suppressor gene

We have recently demonstrated that loss of heterozygosity (LOH) at 7q31 is frequent in many kinds of human primary tumors and that introducing a single human chromosome (hchr) 7 into a murine squamous cell carcinoma (SCC)-derived cell line suppresses the malignant phenotype. To investigate whether the putative tumor suppressor gene on hchr 7 is conserved in mice, we studied LOH on mouse chromosome (mchr) 6 in chemically induced SCCs in (C57BL x DBA2) F1 (B6D2F1) females. LOH analysis was performed by polymerase chain reaction amplification of 17 (CA)n microsatellite repeats in mchr 6 A1-C3. As expected, all the B6D2F1-derived tumors were informative for all the locus assayed. The highest percentage of LOH in the hchr 7q-homologous segment was found at D6Mit50 (60.0%) and the other markers in this segment had LOH incidences normally distributed around the peak. The high incidence of LOH in the tumors studied suggests that a tumor suppressor gene relevant to the development of epithelial cancers is present on mchr 6 A2. As this segment is homologous with hchr 7q31, these data suggest that the putative tumor suppressor gene is conserved in the two species and explains the suppression of tumorigenicity when a single hchr 7 is introduced to a murine SCC cell line.

Effect of spray-dried yogurt and lactic acid bacteria on the initiation and promotion stages of chemically induced skin carcinogenesis in mice

The effects of dietary spray-dried yogurt powder product (YPP) and two strains of lactic acid bacteria on the initiation and promotion stages of carcinogenesis were investigated using the 7,12-dimethylbenz[a]anthracene (DMBA)-12-O-tetradecanoylphorbol-13-acetate (TPA) mouse skin carcinogenesis model. In two independent studies, Sencar mice were fed a diet containing 86%, 43%, or 0% YPP or the 0% YPP diet supplemented with viable cultures of Lactobacillus acidophilus or bifidobacteria. Animals were initiated with a single topical application of DMBA (10 nmol). Promotion began three weeks later with twice weekly treatment of TPA (1 microgram/200 microliters acetone). During the initiation study (Study 1) the experimental diets were fed for four weeks before and one week after DMBA treatment. All mice were then switched to the AIN-76 diet. For the promotion study (Study 2) the experimental diets were begun one week after initiation and fed during the remainder of the study. Gross appearance of tumors was assessed weekly. No statistically significant differences in body weight or food disappearance were observed among the diet groups during the studies. For Studies 1 and 2, final histologically verified papilloma incidence and multiplicity and carcinoma incidence were not statistically different. These data suggest that different levels of YPP or lactic acid bacteria fed during the initiation or promotion stage of carcinogenesis do not significantly affect chemically induced skin tumor development.

Up-regulation of vascular endothelial growth factor/vascular permeability factor in mouse skin carcinogenesis correlates with malignant progression state and activated H-ras expression levels

Angiogenesis is a crucial process for tumor growth and metastasis regulated by the balance of positive and negative factors. Vascular endothelial growth factor (VEGF/VPF) is a specific mitogen for endothelial cells that has been shown to be overexpressed in a variety of tumors and other inflammatory diseases. To analyze the implication of VEGF/VPF during tumorigenesis, we have studied its expression at different stages of tumor development using the mouse skin carcinogenesis model. VEGF/VPF mRNA was induced in skin in vivo after 12-O-tetradecanoylphorbol-13-acetate treatment. Constitutive up-regulation of VEGF/VPF at the mRNA and protein levels was also observed in premalignant papillomas and, at a higher level, in squamous carcinomas, suggesting a correlation between VEGF/VPF expression and tumor progression. A direct positive correlation between VEGF/VPF mRNA expression and the level of activated H-ras gene was found in a series of cell lines representing different stages of epidermal tumor development. Consequently, a clone of one of these cell lines, HaCa4, which has lost most of its v-ras expression, down-regulated VEGF mRNA expression concomitantly with its metastatic potential. Direct evidence of H-ras involvement in VEGF induction was obtained when an immortalized mouse keratinocyte cell line transduced with a retrovirus carrying v-H-ras showed highly increased VEGF/VPF mRNA levels. These data show that in mouse skin carcinogenesis, the VEGF/VPF angiogenic stimulus occurs early during premalignant papilloma development and further increases at later stages. Moreover, we demonstrate that increasing the activated H-ras dose, a phenomenon that takes place sequentially throughout mouse skin tumor development, may play an additional role by facilitating malignant in vivo progression through the modulation of VEGF/VPF-mediated angiogenesis.

Characterization of mutant HA-ras gene expression in transformed murine keratinocyte lines grown under in vitro and in vivo conditions

We investigated the Ha-ras and Ki-ras gene status, tumorigenicity, pathology, line derivation, and intercellular communication of 7,12-dimethylbenz[a]anthracene-initiated papilloma-, carcinoma-, and hyperplastic skin-producing cell lines to further characterize them. Six of nine tumor cell lines grown in vitro expressed both mutant and normal Ha-ras proteins, and three lines expressed only normal Ha-ras. However, when grown subcutaneously in nude mice, seven of the nine lines expressed both mutant and normal Ha-ras, one line expressed normal Ha-ras, and one line did not grow subcutaneously. One papilloma line, P2/15, appeared to have an inducible mutant Ha-ras gene, as it was expressed only in vivo. These findings suggest that mutant Ha-ras genes may be lost in only a minor population of tumor lines during growth in culture. Finally, we found that mutant Ha-ras gene expression was strongly correlated with tumorigenicity in nude mice and that intercellular communication was strongly correlated with the derivation of the lines.