Previous studies indicated that genetic deletion of c-fos prevents malignant conversion of benign skin tumors. Conversely, overexpression of c-fos accelerates conversion, and AP-1 transcriptional activity increases as skin tumors progress from benign to malignant. To assess the contribution of AP-1 activity on tumor development, a dominant-negative AP-1 construct has been targeted to the epidermis of transgenic mice using a tetracycline regulated promoter system. Expression of the dominant-negative transgene enhances epidermal hyperplasia in response to phorbol esters, but also increases the apoptotic response. Furthermore, the incidence of benign tumors increases in the transgenic mice during initiation-promotion protocols, but all of the tumors are sebaceous adenomas as contrasted to squamous papillomas in control groups. Squamous tumors produced when the transgene is suppressed either regress or undergo sebaceous metaplasia upon activation of the transgene. Under conditions of transgene expression, malignant conversion is rare while control groups have a high conversion frequency. Current analysis of phorbol ester treated skin and skin tumors by gene profiling is revealing a trove of AP-1 regulated genes associated with epidermal differentiation, inflammation and retinoid metabolism. Expression of the dominant-negative AP-1 transgene for prolonged periods in older mice produces a cutaneous Vitamin A deficiency phenotype. Unexpectedly, the keratin 5 targeting vector was expressed in the retinal pigment epithelium, the retinal compartment responsible for Vitamin A cycling in production of the visual pigment. Such mice are blind and lack the outer segment of rods and cones similar to pathological changes seen in systemic Vitamin A deficiency syndromes and in opsin null mice. The p53 regulated pro-apoptotic chloride channel protein, mtCLIC, is localized to human chromosome 1p36, a site of abnormalities in many human tumors. MtCLIC RNA expression is reduced in many human tumors, most commonly in breast, renal and ovarian sites. MtCLIC protein is also reduced in mouse skin carcinomas. MtCLIC translocates from the cytoplasm to the nucleus after DNA damage, induction of differentiation or senescence, and treatment with TNFa. Deletion mutant studies indicate that the lysine rich nuclear localization sequence (NLS) is required for translocation, and the NH2 terminus of mtCLIC masks the NLS in resting cells. mtCLIC interacts with Ran, Importin a and NTF2 when induced to translocate to the nucleus. Targeting mtCLIC to the nucleus with adenovirus encoding full-length mtCLIC modified with an enhanced NLS accelerates apoptosis. The mtCLIC promoter contains 3 functional p53 consensus binding sequences. Furthermore, the promoter contains a TNFa responsive element and an NFkB suppressing element. Stable keratinocyte tumor cell lines and human Saos osteosarcoma cells expressing tetracycline regulated mtCLIC or mtCLIC antisense have been produced and are being tested for tumor formation in vivo in mice with or without supplementation with doxycycline in the diet.
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