PTEN loss promotes ras<SUP>Ha</SUP>-mediated papillomatogenesis via dual up-regulation of AKT activity and cell cycle deregulation but malignant conversion proceeds via rasHa-independent pathways.

PTEN tumor suppressor gene failure in ras&lt;SUP&gt;Ha&lt;/SUP&gt;-activated skin carcinogenesis was investigated by mating exon 5 floxed-PTEN [5PTEN] mice to HK1.ras mice that expressed a RU486-inducible cre recombinase [K14.creP]. PTEN inactivation in K14.cre/Pten&lt;SUP&gt;flx/flx&lt;/SUP&gt; keratinocytes resulted in epidermal hyperplasia/hyperkeratosis and novel TPA-promoted papillomas, whilst HK1.ras/K14.cre/Pten&lt;SUP&gt;flx/fl&lt;/SUP&gt;x cohorts displayed a rapid onset of papillomatogenesis due to a synergism of increased AKT activity and ERK elevation. High BrdU labeling in D5PTEN papillomas demonstrated that a second promotion mechanism centered on failures in cell cycle control. Elevated cyclin D1 was associated with both HK1.ras/ERK and D5PTEN-mediated AKT signaling, while cyclin E2 overexpression appeared dependent on PTEN loss. Spontaneous HK1.ras/DPTEN malignant conversion was rare, whereas TPA promotion resulted in conversion with high frequency. On comparison to 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/Pten&lt;SUP&gt;flx/flx&lt;/SUP&gt; papillomas that also lacked endogenous c-rasHa activation. Moreover, in all PTEN-null tumors levels of rasHa-associated total-ERK protein became reduced, whilst phospho-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 ras-associated ERK protein and activity, increased D5PTEN-associated cyclin E2 expression and unique K1/K13 profiles following TPA treatment, suggests that PTEN loss, rather than rasHa activation, gives rise to a population of cells with greater malignant potential