7,12-Dimethylbenz[a]anthracene-induced mouse keratinocyte malignant transformation independent of Harvey ras activation

Immunoprevention of chemical carcinogenesis through early recognition of oncogene mutations

Prevention of tumors induced by environmental carcinogens has not been achieved. Skin tumors produced by polyaromatic hydrocarbons, such as 7,12-dimethylbenz(a)anthracene (DMBA), often harbor an H-ras point mutation, suggesting that it is a poor target for early immunosurveillance. The application of pyrosequencing and allele-specific PCR techniques established that mutations in the genome and expression of the Mut H-ras gene could be detected as early as 1 d after DMBA application. Further, DMBA sensitization raised Mut H-ras epitope-specific CTLs capable of eliminating Mut H-ras(+) preneoplastic skin cells, demonstrating that immunosurveillance is normally induced but may be ineffective owing to insufficient effector pool size and/or immunosuppression. To test whether selective pre-expansion of CD8 T cells with specificity for the single Mut H-ras epitope was sufficient for tumor prevention, MHC class I epitope-focused lentivector-infected dendritic cell- and DNA-based vaccines were designed to bias toward CTL rather than regulatory T cell induction. Mut H-ras, but not wild-type H-ras, epitope-focused vaccination generated specific CTLs and inhibited DMBA-induced tumor initiation, growth, and progression in preventative and therapeutic settings. Transferred Mut H-ras-specific effectors induced rapid tumor regression, overcoming established tumor suppression in tumor-bearing mice. These studies support further evaluation of oncogenic mutations for their potential to act as early tumor-specific, immunogenic epitopes in expanding relevant immunosurveillance effectors to block tumor formation, rather than treating established tumors.

NF-κB repression by PIAS3 mediated RelA SUMOylation

Negative regulation of the NF-κB transcription factor is essential for tissue homeostasis in response to stress and inflammation. NF-κB activity is regulated by a variety of biochemical mechanisms including phosphorylation, acetylation, and ubiquitination. In this study, we provide the first experimental evidence that NF-κB is regulated by SUMOylation, where the RelA subunit of NF-κB is SUMOylated by PIAS3, a member of the PIAS (protein inhibitor of activated STAT) protein family with E3 SUMO ligase activity. PIAS3-mediated NF-κB repression was compromised by either RelA mutant resistant to SUMOylation or PIAS3 mutant defective in SUMOylation. PIAS3-mediated SUMOylation of endogenous RelA was induced by NF-κB activation thus forming a negative regulatory loop. The SUMOylation of endogenous RelA was enhanced in IκBα null as compared with wild type fibroblasts. The RelA SUMOylation was induced by TNFα but not leptomycin B mediated RelA nuclear translocation. Furthermore, RelA mutants defective in DNA binding were not SUMOylated by PIAS3, suggesting that RelA DNA binding is a signal for PIAS3-mediated SUMOylation. These results support a novel negative feedback mechanism for NF-κB regulation by PIAS3-mediated RelA SUMOylation.

Antioxidative effects of the spice cardamom against non-melanoma skin cancer by modulating nuclear factor erythroid-2-related factor 2 and NF-κB signalling pathways

The role of dietary factors in inhibiting or delaying the development of non-melanoma skin cancer (NMSC) has been investigated for many years. Cardamom, which is a dietary phytoproduct, has been commonly used in cuisines for flavour and has numerous health benefits, such as improving digestion and stimulating metabolism and having antitumorigenic effects. We have investigated the efficacy of dietary cardamom against 7,12-dimethylbenz[a]anthracene (DMBA)-induced skin papillomatogenesis in Swiss albino mice that closely resembles human NMSC. Mice were grouped into normal wild type (untreated), vehicle-treated (acetone), carcinogen-treated (DMBA), and DMBA and cardamom-treated (DMBA+CARD) to delineate the role of cardamom against DMBA-induced papillomatogenesis. Oral administration of cardamom to DMBA-treated mice up-regulated the phase II detoxification enzymes, such as glutathione-S-transferase and glutathione peroxidase, probably via activation of nuclear factor erythroid-2-related factor 2 transcription factor in 'DMBA+CARD' mice. Furthermore, reduced glutathione, glutathione reductase, superoxide dismutase and catalase were also up-regulated by cardamom in the same 'DMBA+CARD' group of mice compared with DMBA-treated mice. Cardamom ingestion in DMBA-treated mice blocked NF-κB activation and down-regulated cyclo-oxygenase-2 expression. As a consequence, both the size and the number of skin papillomas generated on the skin due to the DMBA treatment were reduced in the 'DMBA+CARD' group. Thus, the results from the present study suggest that cardamom has a potential to become a pivotal chemopreventive agent to prevent papillomagenesis on the skin.

Novel initiation genes in squamous cell carcinomagenesis: a role for substrate-specific ubiquitylation in the control of cell survival

The study of experimental epidermal carcinogenesis offers several advantages over other epithelial carcinogenesis models, including easy accessibility and a database of research findings spanning over a century. Our studies make use of a clonal in vitro/in vivo keratinocyte carcinogenesis model with low frequency of ras mutation and derivative clonal-initiated lineages with distinct tumor fate. Analysis of this model has yielded candidate genes involved in the stages of initiation and tumorigenic progression, and has revealed novel roles for ubiquitylation in transcriptional control of survival and apoptotic pathways during the early stages of carcinogenesis. The expression of a recently described E3-ubiquitin ligase, Trim32, is elevated during initiation, and ectopic expression of Trim32 confers extended survival in response to terminal differentiation and ultraviolet light (UV) B/TNF-alpha death signals. Trim32 binds and ubiquitylates Piasy, controlling its stability and accumulation. Piasy is a SUMOylation factor involved in the control of apoptosis, senescence, and NF-kappaB activation. NF-kappaB is a survival factor for keratinocytes in response to UV irradiation, the main carcinogenic stimulus for the epidermis. Piasy inhibits NF-kappaB activity, and promotes keratinocyte apoptosis in response to UV and TNF-alpha. In human skin squamous cell carcinoma (SCC) samples, we found an inverse correlation between Trim32 and Piasy expression supporting a role for Trim32-Piasy interaction in human epidermal carcinogenesis. Our hypothesis is that increased expression of Trim32 may enhance epidermal carcinogenesis, by increasing the threshold of NF-kappaB activity through Piasy downmodulation.

Melanocyte and keratinocyte carcinogenesis: p53 family protein activities and intersecting mRNA expression profiles

Melanocytes and keratinocytes were analyzed for potential roles of p53, p73, and p63 tumor suppressor family proteins and of malignancy-specific gene expression changes in the etiology of multi-step cancer. Melanocytes expressed deltaNp73alpha, two p63 isoforms and p53. Although p21 and Noxa mRNA levels increased following DNA damage, p53 family member binding to p21 and Noxa DNA probes was undetectable, suggesting p53 family-independent responses. In contrast, keratinocytes expressed multiple isoforms each of p73 and p63 that were induced to bind p21 and Noxa DNA probes after ionizing (IR) or after ultraviolet B (UVB) irradiation, correlating with p21 and Noxa mRNA induction and with apoptosis. Interestingly, IR-resistant malignant melanocytes and keratinocytes both exhibited Noxa mRNA induction after UVB treatment, correlating with DNA binding of p53 family proteins to the Noxa probe only in keratinocytes. To uncover other malignancy-specific events, we queried mouse initiated keratinocyte clones for early changes that were exacerbated in malignant derivatives and also differentially expressed in human advanced melanoma versus normal melanocytes. Using a new method for ranking and normalization of microarray data for 5000 probe sets, 27 upregulated and 13 downregulated genes satisfied our query. Of these, the majority was associated with late-stage human cancers and six were novel genes. Thus, clonal lineage mouse models representing early through late cancer progression stages may inform the focus on early, potentially causal events from microarray studies of human cancers, facilitating prognosis and molecular therapy.

SPAF, a new AAA-protein specific to early spermatogenesis and malignant conversion

A novel spermatogenesis associated factor (SPAF) was found to be aberrantly expressed at the malignant conversion stage in a clonal epidermal model of chemical carcinogenesis. Sequence analysis revealed two ATPase modules, classifying this gene as a new member of the AAA-protein family (ATPase associated with diverse activities). Immunohistochemical staining of mouse testis sections with SPAF antibody localized expression to spermatogonia and early spermatocytes in the basal compartment of the seminiferous tubules. Northern and Western analysis of SPAF expression in testes of mice at different developmental stages confirmed its expression at early stages of spermatogenesis. In view of a mitochondrial-localization-like signal, sequence similarities to membrane-associated proteins, ATP binding properties, and intracellular expression patterns in testis, we speculate that SPAF protein may be involved in morphological and functional mitochondrial transformations during spermatogenesis. Ectopic expression of the SPAF gene in malignant epidermal cells may signify adoption of an early germ cell-like phenotype advantageous in malignant conversion.

Expression of proopiomelanocortin, corticotropin-releasing hormone (CRH), and CRH receptor in melanoma cells, nevus cells, and normal human melanocytes

Proopiomelanocortin (POMC) is a 31 kDa prohormone that is processed to various bioactive peptides, including adrenocorticotropin (ACTH), melanotropins (alpha, beta, gamma-MSH), lipotropins, and endorphins. POMC is expressed not only in the pituitary gland but also in a variety of nonpituitary organs and tumors, including melanomas. We previously showed that normal human melanocytes produce and secrete alpha-MSH and ACTH, and furthermore, that advanced melanoma cells generally produce higher amounts of POMC peptides that correlate with tumor progression. To elucidate the mechanism of this upregulation, the expression of genes encoding corticotropin-releasing hormone (CRH) and its receptor, CRH-R, as well as POMC and the MSH receptor (MC1-R), was evaluated by reverse transcriptase-polymerase chain reaction using cultured human melanoma cells, nevus cells, and normal melanocytes. Our results show that all melanocytic cells express CRH, CRH-R, POMC, and MC1-R, with highest intensities in melanoma cells. Furthermore, immunohistochemistry shows that CRH as well as POMC is strongly expressed in advanced melanomas, such as vertically growing lesions of acral lentiginous, nodular and metastatic melanomas, in contrast to negative expression in nevus cells. These results indicate that tumor progression accentuates CRH, CRH-R, and POMC expression by melanoma cells.

Alterations in focal adhesion kinase activity and associated proteins during malignant conversion of mouse keratinocytes

Focal adhesion kinase (pp125FAK) has well-established functions in the attachment and growth of cells in culture and has been implicated as a marker of malignant progression in human tumors. To evaluate its role in the metastatic conversion of mouse skin tumors, pp125FAK activity and protein expression were examined in normal and transformed keratinocyte cell lines. Malignant mouse keratinocyte lines exhibited a reproducible increase in the specific activity of pp125FAK compared with that of nontransformed control cells. An increase in pp125FAK activity was not observed in papilloma-derived keratinocytes, indicating that this response correlated with malignant progression of cells and not cell transformation per se. Immune complex kinase assays and metabolic labeling with [32P]orthophosphate also revealed the specific loss of pp125FAK-associated proteins in the metastatic keratinocytes. Furthermore, immunocytochemical examination revealed an altered distribution of pp125FAK in the cells with malignant potential compared with normal and papilloma-inducing keratinocytes. The cells with malignant potential also exhibited reduced levels of paxillin and integrin beta1 as well as altered distribution of paxillin, reinforcing the notion that specific changes in the composition of focal adhesions contribute to the malignant conversion of mouse keratinocytes.

Activities and response to DNA damage of latent and active sequence-specific DNA binding forms of mouse p53

The mouse p53 protein generated by alternative splicing (p53as) has amino acid substitutions at its C terminus that result in constitutively active sequence-specific DNA binding (active form), whereas p53 protein itself binds inefficiently (latent form) unless activated by C-terminal modification. Exogenous p53as expression activated transcription of reporter plasmids containing p53 binding sequences and inhibited growth of mouse and human cells lacking functional endogenous p53. Inducible p53as in stably transfected p53 null fibroblasts increased p21(WAF1/Cip-1/Sdi) and decreased bcl-2 protein steady-state levels. Endogenous p53as and p53 proteins differed in response to cellular DNA damage. p53 protein was induced transiently in normal keratinocytes and fibroblasts whereas p53as protein accumulation was sustained in parallel with induction of p21(WAF1/Cip-1/Sdi) protein and mRNA, in support of p53as transcriptional activity. Endogenous p53 and p53as proteins in epidermal tumor cells responded to DNA damage with different kinetics of nuclear accumulation and efficiencies of binding to a p53 consensus DNA sequence. A model is proposed in which C-terminally distinct p53 protein forms specialize in functions, with latent p53 forms primarily for rapid non-sequence-specific binding to sites of DNA damage and active p53 forms for sustained regulation of transcription and growth.

Role of the Ha-ras gene in the malignant transformation of rat liver oval cells

We have shown that the oval cell line OC/CDE 22 can be transformed by the highly carcinogenic fjord-region diol epoxides of benzo[c]phenanthrene. Mutational activation of the ras proto-oncogene family has been proposed to be a critical event in the formation of tumors induced by polycyclic aromatic hydrocarbons. Therefore, we investigated whether in the earlier transformed OC/CDE 22 cells any point mutations were detected in the ras proto-oncogene. The results indicate that the malignant transformation of OC/CDE 22 cells by the 4 stereoisomeric benzo[c]phenanthrene diol epoxides in vitro is independent of activation of the Ha-ras proto-oncogene. In addition, Northern and Western blot analyses revealed no overexpression of the Ha-ras protooncogene in the transformed OC/CDE 22 cell lines. However, transfection of the OC/CDE 22 cells with an activated Ha-ras oncogene malignantly transformed the OC/CDE 22 cells, and the transfected cells served as precursor cells of tumors with a cholangiocellular morphology and phenotype. Our latter finding reinforces the view that OC/CDE 22 cells are committed to the bile duct epithelial cell lineage.

Induced terminal differentiation and tumorigenic suppression in murine keratinocyte somatic-cell hybrids

The development of malignancy has been associated with both the activation of oncogenes and the inactivation of tumor suppressor genes. Whereas recent data implicate tumor suppressor genes as cell-cycle check-points, the nature and timing of tumor suppressor gene inactivation during multistage carcinogenesis is still largely uncharacterized. To address this issue, we used a syngeneic mouse epidermal model system. By creating somatic-cell hybrids between nontumorigenic x benign (291 x 291.09RAT), nontumorigenic x malignant (291 x 291.05RAT and 291 x 291.03RAT), benign x malignant (291.09RAT x 291.03RAT) and malignant x malignant (291.03RAT x 291.05RAT) clones, multiple tumor suppressor activities were detected. Most importantly, we demonstrated the first example of the complete suppression of benign papillomas in vivo, thus implicating tumor suppressor gene activity loss an early event in skin carcinogenesis. In addition, the carcinoma phenotype was suppressed in vivo by nontumorigenic, benign, and heterologous malignant keratinocytes. The somatic-cell hybrids expressed the differentiation-specific keratins, K1 and K10, in response to high extracellular calcium concentrations (1.4 mM) in vitro. All of the hybrids had fewer local metastases than did the parental lines, and when tumor formation was not suppressed, the resulting tumors were highly differentiated. Polymerase chain reaction analysis of the neomycin-resistance gene at nontumorigenic injection sites indicated an absence of injected hybrids, and subsequent analyses failed to detect nontumorigenic 291 cells 1 wk after transplantation. These data demonstrate that distinct tumor suppressor gene activities are lost at discrete stages during multistage carcinogenesis and are consistent with the hypothesis that tumor suppression can occur through induction of terminal differentiation.

Endogenous p53 protein generated from wild-type alternatively spliced p53 RNA in mouse epidermal cells

We previously demonstrated that a wild-type alternatively spliced p53 (p53as) RNA exists in mouse cultured cells and normal mouse tissues at approximately 25 to 33% of the level of the major p53 RNA form. The alternative RNA transcript is 96 nucleotides longer than the major transcript as a result of alternative splicing of intron 10 sequences. The protein expected to be generated from the p53as transcript is 9 amino acids shorter than the major p53 protein and has 17 different amino acids at the carboxyl terminus. We report here that p53as protein exists in nontransformed and malignant epidermal cells and is localized to the nucleus. In addition, p53as protein is preferentially expressed during the G2 phase of the cell cycle and in cells with greater than G2 DNA content compared with the major p53 protein, which is preferentially expressed in G1. The p53as immunoreactivity is elevated and shifted to the G1 phase of the cell cycle following actinomycin D treatment of nontransformed cells but not malignant cells. In view of the dimerization and tetramerization of p53 protein which may be necessary for its DNA binding and transcriptional activation activities, the presence of p53as protein in cells has important implications for understanding the physiological function(s) of the p53 gene.

Endogenous p53 protein generated from wild-type alternatively spliced p53 RNA in mouse epidermal cells

We previously demonstrated that a wild-type alternatively spliced p53 (p53as) RNA exists in mouse cultured cells and normal mouse tissues at approximately 25 to 33% of the level of the major p53 RNA form. The alternative RNA transcript is 96 nucleotides longer than the major transcript as a result of alternative splicing of intron 10 sequences. The protein expected to be generated from the p53as transcript is 9 amino acids shorter than the major p53 protein and has 17 different amino acids at the carboxyl terminus. We report here that p53as protein exists in nontransformed and malignant epidermal cells and is localized to the nucleus. In addition, p53as protein is preferentially expressed during the G2 phase of the cell cycle and in cells with greater than G2 DNA content compared with the major p53 protein, which is preferentially expressed in G1. The p53as immunoreactivity is elevated and shifted to the G1 phase of the cell cycle following actinomycin D treatment of nontransformed cells but not malignant cells. In view of the dimerization and tetramerization of p53 protein which may be necessary for its DNA binding and transcriptional activation activities, the presence of p53as protein in cells has important implications for understanding the physiological function(s) of the p53 gene.