Development and initial characterization of several new inbred strains of SENCAR mice for studies of multistage skin carcinogenesis

Protein kinase C family: on the crossroads of cell signaling in skin and tumor epithelium

The protein kinase C (PKC) family represents a large group of phospholipid dependent enzymes catalyzing the covalent transfer of phosphate from ATP to serine and threonine residues of proteins. Phosphorylation of the substrate proteins induces a conformational change resulting in modification of their functional properties. The PKC family consists of at least ten members, divided into three subgroups: classical PKCs (alpha, betaI, betaII, gamma), novel PKCs (delta, epsilon, eta, theta), and atypical PKCs (zeta, iota/lambda). The specific cofactor requirements, tissue distribution, and cellular compartmentalization suggest differential functions and fine tuning of specific signaling cascades for each isoform. Thus, specific stimuli can lead to differential responses via isoform specific PKC signaling regulated by their expression, localization, and phosphorylation status in particular biological settings. PKC isoforms are activated by a variety of extracellular signals and, in turn, modify the activities of cellular proteins including receptors, enzymes, cytoskeletal proteins, and transcription factors. Accordingly, the PKC family plays a central role in cellular signal processing. Accumulating data suggest that various PKC isoforms participate in the regulation of cell proliferation, differentiation, survival and death. These findings have enabled identification of abnormalities in PKC isoform function, as they occur in several cancers. Specifically, the initiation of squamous cell carcinoma formation and progression to the malignant phenotype was found to be associated with distinct changes in PKC expression, activation, distribution, and phosphorylation. These studies were recently further extended to transgenic and knockout animals, which allowed a more direct analysis of individual PKC functions. Accordingly, this review is focused on the involvement of PKC in physiology and pathology of the skin.

Altered methylation in gene-specific and GC-rich regions of DNA is progressive and nonrandom during promotion of skin tumorigenesis

Altered DNA methylation, an epigenetic mechanism, likely contributes to tumorigenesis, with an inverse relationship existing between methylation in a promoter region and transcription. Using the SENCAR two-stage mouse skin tumorigenesis model, altered methylation was characterized in precancerous tissue and in tumor tissue. Mouse skin was initiated with 7,12-dimethylbenz[a]anthracene and promoted three times a week with 3, 9, 18, or 27 mg cigarette smoke condensate (CSC) for 4, 8, or 29 weeks; tumors were collected at 29 weeks. In addition, reversibility of changes in methylation was assessed following cessation of the promoting stimulus. DNA was isolated, and GC-rich methylation was assessed quantitatively via methylation-sensitive restriction digestion, arbitrarily primed PCR, and electrophoretic separation of PCR products. Analysis focused on regions of altered methylation (RAMs), which persisted from 4 to 8 weeks and from 8 weeks to tumor tissue. Persistent RAMs (i.e., seen in precancerous tissue and carried forward to tumors) are likely to play a key role in tumorigenesis. Twenty-two CpG sites in the upstream region of the Ha-ras promoter were unmethylated in control skin, 27 mg CSC, and tumor tissue. At two CpG sites closer to the transcriptional start site the incidence of hypomethylation increased with the dose of CSC. Hypomethylation was detected in all tumor samples. Expression of Ha-ras increased with 18 and 27 mg CSC promotion and more so in tumor tissue. These data support our hypothesis that tumor promotion involves instability of the epigenome, providing an environment where changes in the methylation status of specific regions of the genome accumulate progressively and contribute to the clonal expansion of initiated cells that leads to tumor formation.

Differential susceptibility to chemically induced thymic lymphomas in SENCARB and SSIN inbred mice

In the past 20 yr, several inbred strains have been derived from SENCAR outbred mice. These strains display different susceptibility to the induction of papillomas and progression to squamous cell carcinomas (SCC) in the skin after chemical carcinogenesis. In the present study, we showed that one of these strains SENCARB/Pt was highly susceptible to the development of N-methyl-N-nitrosourea (MNU)- and 7,12-dimethylbenz[a]anthracene (DMBA)-induced lymphomas. In contrast, the SSIN/Sprd inbred strain is completely resistant to T-cell lymphomagenesis by both carcinogens. Within 175 d after a single injection of 75 mg/kilogram body weight (kbw) of MNU, SENCARB/Pt mice exhibited a 91.6% incidence of lymphoma. In addition, during an independent tumorigenesis study with repeated doses of intragastric DMBA, SENCARB/Pt mice showed an incidence of 75% lymphoma development 300 d after the last treatment. Histopathological and flow cytometric parameters indicated that the lymphomas were of the T-cell lineage. In order to study the genetics of MNU-induced tumorigenesis, we generated F1 hybrid mice between SSIN/Sprd and SENCARB/Pt mice. Tumor incidence in MNU-injected F1 mice suggested that the high tumor incidence is a dominant trait. Loss of heterozygosity (LOH) analysis in these tumor samples revealed allelic imbalances on chromosomes 15 and 19. Given that these inbred strains are closely related, it is likely that a relatively small number of loci are responsible for the observed differences in susceptibility. Therefore, these SENCAR inbred strains constitute important new tools to study the genetic basis of resistance and susceptibility to chemically induced thymic lymphoma formation.

Persistent activation of the Akt pathway in head and neck squamous cell carcinoma: a potential target for UCN-01

Squamous carcinomas of the head and neck (HNSCC) represent the sixth most common cancer among men worldwide and a major cause of morbidity and mortality due to its relatively poor prognosis. As part of ongoing studies addressing the molecular events underlying tumor progression in HNSCC, we have explored the nature of the proliferative pathways in which dysregulation may promote aberrant cell growth in this tumor type. The serine/threonine protein kinase Akt is a downstream target of phosphatidylinositol 3-kinase and a key regulator of normal and cancerous growth and cell fate decisions. Therefore, in this study, we have examined the status of activation of Akt in different stages of squamous cell carcinoma development in mice and in clinical samples from HNSCC patients. By immunohistochemical analysis, using a recently developed phosphorylation state-specific antibody, we demonstrated that Akt activation correlates closely with the progression of mouse skin squamous cell carcinoma. We also observed that activation of Akt is a frequent event in human HNSCC because active Akt can be detected in these tumors with a pattern of expression and localization correlating with the progression of the lesions. In line with these observations, Akt was constitutively activated in a large fraction of HNSCC-derived cell lines. We also provide evidence that the Akt signaling pathway may represent a biologically relevant target for a novel antineoplastic agent, UCN-01, which recently has been shown to be active in cellular and xenograft models for HNSCC at concentrations safely achievable in clinically relevant situations.

Confirmation of the mapping of a 12-O-tetradecanoylphorbol-13-acetate promotion susceptibility locus, Psl1, to distal mouse chromosome 9

Susceptibility to two-stage skin carcinogenesis in the mouse is affected by several genes. In addition, studies suggest that genes that modify the response of mice to skin tumor promotion by 12-O-tetradecanoylphorbol-13-acetate (TPA) also may influence histologic changes in the skin as the result of TPA treatment. One TPA susceptibility locus, Psl1, previously was mapped to distal chromosome 9. The mapping of this locus was confirmed by marker-based genotypic selection. Furthermore, Psl1 or a gene closely linked to Psl1 influenced epidermal hyperplasia and epidermal labeling index of mice treated with TPA.

Role of 'platelet-type' 12-lipoxygenase in skin carcinogenesis

Murine and human skin express an abundance of lipoxygenase isoforms whose functions are not understood. Substantial data have implicated a role for the 'platelet-type' 12-lipoxygenase (P-12LO) metabolite, 12(S)-hydroxy-eicosatetraenoic acid (12-HETE), in a variety of tumor functions. Using P-12LO deficient mice, we sought to examine the role of the P-12LO pathway in tumor initiation and progression. Two distinct genetic strains of P-12LO deficient and wild-type mice, B6/129 Sv and SENCAR, were evaluated in two-stage carcinogenesis experiments. Carcinoma incidence was significantly reduced in the P-12LO deficient mice of the B6/129 Sv background but not the SENCAR-backcrossed mice. In contrast, papilloma incidence was reduced on the SENCAR background but not in the B6/129 Sv strain mice. A separate experiment employing a complete carcinogenesis protocol failed to find any difference in papilloma or carcinoma incidence. Overall, these data suggest that the P-12LO pathway may contribute to tumor incidence and progression in two-stage, but not complete, carcinogenesis, depending on the genetic background.

Microsatellite DNA variants between the inbred SENCAR mouse strains

The two-stage model, initiation with 7,12-dimethylbenz[a]anthracene and promotion with 12-O-tetradecanoylphorbol-13-acetate, of mouse skin carcinogenesis has been the protocol of choice to study the genetic susceptibility to carcinogens, the outbred SENCAR mouse being the most widely used skin tumor-sensitive animal model. Squamous cell carcinomas (SCCs) develop from many of the papillomas, making these mice a useful model for epithelial tumorigenesis and for the progression to malignant tumors. Nine different inbred strains derived from outbred SENCAR mice have been recently reported. Interestingly, these strains display different sensitivities to two-stage carcinogenesis, and, in particular, some of them show a dissociation between susceptibility to papilloma development and the malignant conversion of these into SCC. However, the utility of these SENCAR strains for genetic mapping is limited by the lack of information regarding DNA variant alleles among them. Therefore, we analyzed the nine inbred strains with microsatellite markers distributed along the 20 chromosomes and in this article report the variant alleles found. The information presented is likely to be helpful for linkage analysis and marker-assisted development of congenic strains between SENCAR-derived inbred strains.