Constitutive ap‐1 dna binding and transactivating ability of malignant but not benign mouse epidermal cells

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Abstract
The mouse epidermal cell line 308 contains an activated Ha‐ras gene and forms benign papillomas when transplanted to the skin of athymic nude mice. A radiation‐associated malignant variant of this cell line, 308‐10Gy5, has been isolated and shown to form squamous cell carcinomas in nude mice. To further examine the molecular events involved in malignant conversion of 308‐10Gy5, we assessed the activator protein‐1 (AP‐1) binding and transactivating ability of 308 and 308‐10Gy5. In nuclear protein extracts of 308, AP‐1 sequence‐specific binding to an oligonucleotide containing a single high‐affinity AP‐1 binding site was induced by the tumor promoter 12‐O‐tetradecanoylphorbol‐13‐acetate, as determined by gel shift analysis. Nuclear extracts of 308‐10Gy5 bound to the AP‐1 oligonucleotide without treatment with tumor promoters. Not only was sequence‐specific AP‐1 DNA binding constitutively active in malignant versus benign tumor cells, but so was transactivation of a unique AP‐1‐responsive chloramphenicol acetyltransferase reporter construct, pTiCTaK. Constitutive transactivation of this AP‐1–responsive reporter construct was observed in the malignant but not the benign tumor cells. Furthermore, steady‐state transcript levels of the tumor‐associated AP‐1–responsive genes stromelysin, urokinase‐type plasminogen activator, c‐jun, and c‐fos were higher in malignant 308‐10Gy5 cells than in benign 308 cells. These results suggest that acquisition of constitutive AP‐1 DNA binding and transactivation can result in sustained deregulation of gene expression. While malignant progression in keratinocytes is probably not due solely to the acquisition of constitutive cellular AP‐1 activity, the effect of deregulated expression of AP‐1–regulated genes, especially basement membrane–degrading enzymes, may be functionally related to malignant conversion.

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