ras gene activation and aberrant expression of keratin K13 in ultraviolet B radiation-induced epidermal neoplasias of mouse skin

Development and homeostasis of the skin epidermis

The skin epidermis is a stratified epithelium that forms a barrier that protects animals from dehydration, mechanical stress, and infections. The epidermis encompasses different appendages, such as the hair follicle (HF), the sebaceous gland (SG), the sweat gland, and the touch dome, that are essential for thermoregulation, sensing the environment, and influencing social behavior. The epidermis undergoes a constant turnover and distinct stem cells (SCs) are responsible for the homeostasis of the different epidermal compartments. Deregulation of the signaling pathways controlling the balance between renewal and differentiation often leads to cancer formation.

Defining the origins of Ras/p53-mediated squamous cell carcinoma

The precise identity of cancer cells of origin and the molecular events of tumor initiation in epidermal squamous cell carcinoma (SCC) are unknown. Here we show that malignancy potential is related to the developmental capacity of the initiating cancer cell in a genetically defined, intact, and inducible in vivo model. Specifically, these data demonstrate that SCCs can originate from inside the hair follicle stem cell (SC) niche or from immediate progenitors, whereas more developmentally restricted progeny, the transit amplifying (TA) cells, are unable to generate even benign tumors in the same genetic context. Using a temporal model of tumorigenesis in situ, we highlight the phenotypes of cancer progression from the hair follicle SC niche, including hyperplasia, epithelial to mesenchymal transition, and SCC formation. Furthermore, we provide insights into the inability of hair follicle TA cells to respond to tumorigenic stimuli.

Identifying the cellular origin of squamous skin tumors

Squamous cell carcinoma (SCC) is the second most frequent skin cancer. The cellular origin of SCC remains controversial. Here, we used mouse genetics to determine the epidermal cell lineages at the origin of SCC. Using mice conditionally expressing a constitutively active KRas mutant (G12D) and an inducible CRE recombinase in different epidermal lineages, we activated Ras signaling in different cellular compartments of the skin epidermis and determined from which epidermal compartments Ras activation induces squamous tumor formation. Expression of mutant KRas in hair follicle bulge stem cells (SCs) and their immediate progeny (hair germ and outer root sheath), but not in their transient amplifying matrix cells, led to benign squamous skin tumor (papilloma). Expression of KRas(G12D) in interfollicular epidermis also led to papilloma formation, demonstrating that squamous tumor initiation is not restricted to the hair follicle lineages. Whereas no malignant tumor was observed after KRas(G12D) expression alone, expression of KRas(G12D) combined with the loss of p53 induced invasive SCC. Our studies demonstrate that different epidermal lineages including bulge SC are competent to initiate papilloma formation and that multiple genetic hits in the context of oncogenic KRas are required for the development of invasive SCC.

Combined subcarcinogenic benzo[a]pyrene and UVA synergistically caused high tumor incidence and mutations in H-ras gene, but notp53, in SKH-1 hairless mouse skin

Combined subcarcinogenic doses of benzo[a]pyrene (BaP) and UVA induced H-ras, but not p53, gene mutations 8 weeks before tumor emergence in SKH-1 mice. Neither UVA (40 kJ/m2) nor BaP (8 nmol) induced any tumors after mice were topically treated 3 times/week for 25 weeks. However, combined BaP-UVA treatment synergistically increased tumor incidence and multiplicity. All tumors induced by BaP-UVA were malignant. The epidermis was collected from mice treated for 2, 6 and 10 weeks. DNA from UVB- (0.3 kJ/m2) or BaP-UVA-(8 nmol and 40 kJ/m2-induced tumors was isolated and screened for H-ras and p53 mutations. Four types of point mutation, GGC-->GAC, GCC, GTC and CGC, occurred in UVB-induced tumors at H-ras codon 13; and one type of point mutation, GGA-->GAA, at codon 12. Treatment with either BaP alone or BaP-UVA for 10 weeks caused GGA-->GAA mutation at codon 12 or GGC-->GAC mutation at codon 13 in nontumor skin, respectively, as well as in tumors induced by BaP-UVA. All of the 10-week samples treated with either BaP or BaP-UVA showed detectable mutations at codons 12 and 13, but the genetic load was significantly higher in BaP-UVA-treated mice than in those exposed only to BaP. UVA alone induced mutations at codon 12 in only one-third of samples. G-->A mutations induced by BaP or BaP-UVA at position 38 of codon 13 have not been reported previously. C-->T transitions were detected in p53 hot spots of exon 8 in 2 of 19 BaP-UVA-induced tumors but were not found in nontumor skin.

Inhibition of Rel/Nuclear Factor-kappaB signaling in skin results in defective DNA damage-induced cell cycle arrest and Ha-ras- and p53-independent tumor development

In recent years a growth inhibitory role in skin for the Rel/NF-kappaB transcription factors has been established, and the block of Rel/NF-kappaB signaling results in rapid development of spontaneous skin cancer. The molecular mechanism underlying tumor development is however unknown. In the present study, we show that inhibition of NF-kappaB signaling in mouse skin by targeted expression of degradation resistant IkappaB-alpha generates transgenic keratinocytes unable to arrest the cell cycle in response to DNA damage induced by gamma-radiation. The results indicate that transgenic keratinocytes have a defect at the G1-S checkpoint whereas the G2-M checkpoint response was found to be intact. However, transgenic keratinocytes still respond by induction of the cyclin dependent kinase inhibitor p21(Cip1/Waf) after exposure to gamma-radiation. In the spontaneous skin tumors that develop in transgenic mice no mutations were found in the Ha-ras or p53 gene, suggesting that inhibition of NF-kappaB signaling in skin can induce cancer development independently of initiating mutations in the Ha-ras gene or additional mutations in the p53 gene. These findings demonstrate an involvement of NF-kappaB signaling in the DNA damage response and cell cycle checkpoint control in the skin.

Skin tumor development and keratin expression in different experimental models. Relation to inducing agent and target tissue structure

The applicability of the experimental skin carcinogenesis model for studies of tumor development was examined by exposing the skin of various mouse strains to different chemical carcinogens and UV radiation regimens, in order to analyze the development and progression of the neoplastic process and the role of differentiation markers such as keratins. In tumor-sensitive hairy NMRI mouse skin, the chemical carcinogen, 7,12-dimethylbenz(a)-anthracene (DMBA) induced an abnormal epidermal cell differentiation and structural irregularities associated with an altered keratin expression, as well as numerous papillomas and squamous cell carcinomas. A suboptimal dose of UVB irradiation increased the number of DMBA-induced benign squamous neoplasms. Low doses of benzo(a)pyrene resulted in mild epidermal alterations, but only in one tumor. High doses of UVB induced a large number of undifferentiated spindle cell tumors with few keratinpositive cells in NMRI mice, similar though fewer tumors in hairy, heavily pigmented C57BL/6 mice, numerous papillomas and squamous cell carcinomas in hairless hr/hr mice but only two papillomas in hairy, moderately pigmented DBA/2 mice while UVA exposure produced only two papillomas in hairless SKH-1 mice. In conclusion, the extent and type of skin tumor development depended upon the induction regimen: physical, chemical, dose and duration, as well as on the skin structure: pigmentation and adnexal development, all of which have to be taken into account when relating experimental results to human conditions.

Chronic ultraviolet exposure-induced p53 gene alterations in Sencar mouse skin carcinogenesis model

Alterations of the tumor suppresser gene p53 have been found in ultraviolet radiation (UVR) related human skin cancers and in UVR-induced murine skin tumors. However, links between p53 gene alterations and the stages of carcinogenesis induced by UVR have not been clearly defined. We established a chronic UVR exposure-induced Sencar mouse skin carcinogenesis model to determine the frequency of p53 gene alterations in different stages of carcinogenesis, including UV-exposed skin, papillomas, squamous-cell carcinomas (SCCs), and malignant spindle-cell tumors (SCTs). A high incidence of SCCs and SCTs were found in this model. Positive p53 nuclear staining was found in 10/37 (27%) of SCCs and 12/24 (50%) of SCTs, but was not detected in normal skin or papillomas. DNA was isolated from 40 paraffin-embedded normal skin, UV-exposed skin, and tumor sections. The p53 gene (exons 5 and 6) was amplified from the sections by using nested polymerase chain reaction (PCR). Subsequent single-strand conformation polymorphism (SSCP) assay and sequencing analysis revealed one point mutation in exon 6 (coden 193, C-->A transition) from a UV-exposed skin sample, and seven point mutations in exon 5 (codens 146, 158, 150, 165, and 161, three C-->T, two C-->A, one C-->G, and one A-->T transition, respectively) from four SCTs, two SCCs and one UV-exposed skin sample. These experimental results demonstrate that alterations in the p53 gene are frequent events in chronic UV exposure-induced SCCs and later stage SCTs in Sencar mouse skin.

Molecular aspects of chemical carcinogenesis: the roles of oncogenes and tumour suppressor genes

The observation that oncogenes are frequently activated in human tumours raises the question of whether these genes are involved in chemical carcinogenesis. H-ras activation is probably an initiating event in mouse skin and rat mammary gland systems. The H-ras oncogene is also important in mouse liver tumours; in mouse lung the K-ras gene is commonly activated. In both, the mutations observed are usually those predicted from the adduct-forming properties of the carcinogen. Among non-ras oncogenes, only raf and neu have been detected in experimental tumours. Tumour suppressor genes are frequently inactivated in human tumours. Searches for such phenomena in animal tumours have generally had disappointing results. p53 and Rb gene alterations are rarely observed in chemically-induced tumours. The reason may be that unknown tumour suppressor genes are involved in animal tumour development. Several novel genes have been identified using animal tumour susceptibility models. Thus, ras genes are important in chemical carcinogenesis, but as the methodology for studying other genes improves, their roles will be seen in perspective.