Overexpression of plasminogen activator inhibitor type 2 in basal keratinocytes enhances papilloma formation in transgenic mice

The serpin plasminogen activator inhibitor (PAI) type 2 is expressed in dif ferentiated epidermal keratinocytes. To explore its role in this tissue, we studied the impact of PAI-2. overexpression on epidermal differentiation a nd skin carcinogenesis. ii mouse PAI-2-encoding transgene was targeted to b asal epidermis and hair follicles under the control of the bovine keratin t ype 5 gene promoter. Two mouse lines were established, one of which strongl y expressed the transgene and produced elevated levels of PAI-2 in the epid ermis, Although it had no manifest impact on cellularity or differentiation of skin or hair follicles, PAI-2 overexpression rendered the mice highly s usceptible to skin carcinogenesis induced by a single application of 7,12-d imethylbenz(n)anthracene (initiation) followed by twice weekly applications of 12-O-tetradecanoylphorbol-13-acetate [TPA (promotion)]. In transgenic m ice, papillomas could be observed after 3 weeks of promotion; after 8 weeks , 94% (31 of 33) of transgenic mice had developed readily visible papilloma s, whereas only 35% (7 of 20) of control mice (transgene-negative littermat es) had barely detectable lesions. After 11 weeks, all but 1 (32 of 33) of the transgenic mice had papillomas as compared with only 65% (13 of 20) of control mice. After 11 weeks of promotion, application of TPA was terminate d. In control mice, papillomas regressed and eventually disappeared; in tra nsgenic mice, there was continued growth of papillomas, some of which furth er progressed to carcinomas, In contrast to massive apoptosis in regressing papillomas of control mice, only a few apoptotic cells were detected in tr ansgenic papillomas after the cessation of TPA application, The effect of P AI-2 on papilloma formation did not appear to involve inhibition of the sec reted protease urokinase-type plasminogen activator (uPA): PAI-2 accumulate d predominantly in cells, and PAI-2 overexpression failed to alleviate a ph enotype induced by uPA secretion, as demonstrated by a double transgenic st rategy. In addition, in situ hybridization revealed that uPA mRNA is not ex pressed concomitantly with PAI-2 in developing papillomas. We conclude that overexpression of PAI-2 promotes the development and progression of epider mal papillomas in a manner that does not involve inhibition of its extracel lular target protease, uPA, but appears to be related to an inhibition of a poptosis.