JB6 Mouse Model
The mouse Balb/C JB6 model (1) is the only well characterized model of genetic variants for a neoplastic transformation response to tumor promoters. These cells are not differentially sensitive to tumor promoter induced mitogenesis or differentiation (2), thus increasing the likelihood that molecular response differences will be transformation-relevant. These cells are also not differential at the level of tumor promoter receptors such as protein kinase C or epidermal growth factor receptor, thus indicating that these are post-receptor signaling variants (3). Moreover the JB6 response variants are stably free of spontaneous transformation (a property found to be mouse strain dependent) and stably responsive (P+) or nonresponsive (P-) to tumor promoter induced transformation. Thus molecular events that occur only in the P+ cells are candidates for mediating tumor promotion while those that occur only in the P- cells are candidate inhibitors. The JB6 model is not suitable for studying epidermal differentiation or keratin synthesis. The JB6 model is uniquely suited for studying tumor promotion and promotion-relevant molecular events. Research using the Balb/C mouse JB6 model of transformation response variants has yielded milestones that propelled research in several areas.
The JB6 model was among the first (along with those reported by M. Oren and his colleagues (4) in which expression of a mutant of the tumor suppressor p53 was shown to produce a gain of oncogenic response (5). The first cloning of mouse TIMP-3 was done as part of a differential display analysis of JB6 cells (6). The demonstration that TIMP-3 functions as a tumor suppressor in human colon carcinoma cells (7) was supported by others who isolated TIMP-3 by gene trapping (8). The first observation of a requirement for active oxygen generation in tumor promotion was made with JB6 cells (9) and has been followed up extensively (10-15). The ubiquitous secreted cell adhesion glycophosphoprotein osteopontin was first cloned in 1987 from JB6 cells by subtractive hybridization (16). Osteopontin expression plays important roles in tumor promotion and tumor progression, inflammation, bone remodeling and other processes (17-19). The 1989 discovery (20) and subsequent reports (21, 22) that the JB6 transformation variants are also AP-1 response variants stimulated the demonstration that AP-1 activation is causal for progression/invasion (23, 24), maintenance (25), and induction (26) of tumor phenotype in mouse cell culture models. These findings stimulated in vivo investigations that implicated cFos in papilloma-to- carcinoma progression but not in tumor promotion (papilloma induction) (27). That tumor promotion in vivo requires AP-1 activation was first established using dominant negative jun(TAM67) expressing transgenic mice (28). Transformation relevant pathways to AP-1 activation have been elucidated in JB6 cells, implicating MAP kinase ERK activation (29, 30). The ERK requirement for AP-1 activation has been extended to AP-1 activation by UVB (31) and by arsenite (32). The observation that UVB activates AP-1 has been extended from JB6 cells to human keratinocytes (33, 34) and to mouse skin in vivo (35, 36). NF B-dependent gene expression is also required for transformation in the JB6 model (14). P- cells are nonresponsive for NF B activation (14). Inducible nitric oxide synthase (iNOS) is induced in P+ but not in P- JB6 cells cocultured with an interferon-stimulated macrophage cell line (37). INOS induction is NF B-dependent. iNOS is one of a small set of genes that appears to be targeted in mice expressing AP-1/NF B inhibitor (TAM67) (Young et al., unpublished).
In addition to propelling mechanistic insights, studies with the JB6 model have impacted research in cancer prevention. A number of tumor promotion inhibitors studied in the JB6 model have been discovered to owe their transformation inhibiting activity at least in part to their AP-1 repressing activity. Among these inhibitors are turmeric-derived curcumin (38), aspirin (39), the tea polyphenol EGCG (40-42), potato proteinase inhibitors (43, 44), gingerol (45), citrifolin A (46), Omega 3 fatty acids (47), and novel glycosides (48). Some of these inhibitors have been shown to target AP-1 also in mouse skin (35). Recently Birt and colleagues have discovered that the basis for caloric restriction inhibiting tumor promotion lies at least in part, in its inhibition of ERK and AP-1 activation (49, 50). Other molecular targets of cancer prevention agents have been identified using the JB6 model. The grape skin inhibitor resveratrol activates p53 (51).
Cancer Risk Assessment
Finally, the JB6 model has also been valuable for cancer risk assessment. The sensitivity of the JB6 model has made it possible to convincingly conclude that exposure to electric and magnetic fields around power lines poses little or no human risk (52, 53). Environmental metals such as arsenic and vanadium have been shown to act as tumor promoters in the JB6 model (32, 54).
In summary the use of the mouse JB6 model has spawned a number of advances not only in basic cell biology but also in understanding the molecular mechanism of tumor promotion. Moreover these studies have contributed to progress in cancer prevention and risk assessment research. Many of these advances have been extended from the JB6 model to human epithelial cell (solid tumor) progression models or to in vivo mouse multistage carcinogenesis models. This progress reflects a response of many laboratories to the unique opportunities for discovery presented by the mouse JB6 model.
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