PTEN Loss Promotes rasHa-Mediated Papillomatogenesis via Dual Up-Regulation of AKT Activity and Cell Cycle Deregulation but Malignant Conversion Proceeds via PTEN-Associated Pathways

Abstract PTEN tumor suppressor gene failure in rasHa-activated skin carcinogenesis was investigated by mating exon 5 floxed-PTEN (Δ5PTEN) mice to HK1.ras mice that expressed a RU486-inducible cre recombinase (K14.creP). PTEN inactivation in K14.cre/PTENflx/flx keratinocytes resulted in epidermal hyperplasia/hyperkeratosis and novel 12-O-tetradecanoylphorbol-13-acetate (TPA)–promoted papillomas, whereas HK1.ras/K14.cre/PTENflx/flx cohorts displayed a rapid onset of papillomatogenesis due to a synergism of increased AKT activity and extracellular signal-regulated kinase (ERK) elevation. High 5-bromo-4-deoxyuridine labeling in Δ5PTEN papillomas showed that a second promotion mechanism centered on failures in cell cycle control. Elevated cyclin D1 was associated with both HK1.ras/ERK– and Δ5PTEN-mediated AKT signaling, whereas cyclin E2 overexpression seemed dependent on PTEN loss. Spontaneous HK1.ras/Δ5PTEN malignant conversion was rare, whereas TPA promotion resulted in conversion with high frequency. On comparison with all previous HK1.ras carcinomas, such TPA-induced carcinomas expressed atypical retention of keratin K1 and lack of K13, a unique marker profile exhibited by TPA-induced K14.cre/PTENflx/flx papillomas that also lacked endogenous c-rasHa activation. Moreover, in all PTEN-null tumors, levels of rasHa-associated total ERK protein became reduced, whereas phosphorylated ERK and cyclin D1 were lowered in late-stage papillomas returning to elevated levels, alongside increased cyclin E2 expression, in TPA-derived carcinomas. Thus, during early papillomatogenesis, PTEN loss promotes rasHa initiation via elevation of AKT activity and synergistic failures in cyclin regulation. However, in progression, reduced rasHa-associated ERK protein and activity, increased Δ5PTEN-associated cyclin E2 expression, and unique K1/K13 profiles following TPA treatment suggest that PTEN loss, rather than rasHa activation, gives rise to a population of cells with greater malignant potential. (Cancer Res 2006; 66(3): 1302-12)