Generación de un modelo murino de tumores orales a través del estudio del papel de la quinasa Akt en la homeostasis epitelial y en los procesos tumorigénicos

Abstract

PKB/Akt is a serine-threonine kinase frequently overactivated in multiple types of human cancer, including head and neck squamous cell carcinomas (HNSCC). Its role in mouse skin tumorigénesis, particularly in epidermal tumors, has been widely proved in the last years. However, its function in normal epidermal homeostasis is not that well established. To investigate both aspects of Akt functions in epithelial cells, we have generated transgenic mouse models for either AktWT or a permanently activated form of Akt through myristoilation (myrAkt) in the basal layer of stratified epithelial tissues (transgene expression regulated by the bovine keratin 5 promoter; BK5AktWT and BK5myrAkt mice respectively). We have studied the epidermal alterations and tumor susceptibility in the three transgenic mouse lines (one expressing AktWT and the other two expressing myrAkt) and several founders obtained and we have found that increased Akt leads, in a level-dependent fashion, to epidermal hyperplasia, altered ectodermal-derived organ development, expansion of stem cell-like population, increased susceptibility to chemically induced carcinogenesis protocols and spontaneous development of tumors. Specifically, transgenic mice with high Akt activity levels develop oral tumors with 100% incidence that resemble human oral dysplasia and in situ carcinomas, including alterations in Akt targets and KLF4 overexpression. Interestingly, these tumors seldom progress into aggressive carcinomas. We have studied these tumors in depth, and found that this lack of malignization is due to induction of p53-dependent premature senescence. We have then generated three transgenic mouse lines in which active Akt, by means of myrAkt expression or Pten ablation, is combined with the somatic deletion of the Trp53 gene in stratified epithelia (BK5myrAkt;p53f/f;K14Cre and p53f/f;Ptenf/f;K14Cre mice). Whereas oral tumors developed by BK5myrAkt;pRbf/f;K14Cre are essentially identical to BK5myrAkt tumors, the other two transgenic mice lines develop oral tumors with a more aggressive behavior than those found in BK5myrAkt mice: higher frequency of squamous cell carcinomas and invasive SCCs that also arise earlier in the life of the animal, even compromising survival in the case of p53f/f;Ptenf/f;K14Cre mice. These tumors display histopathological characteristics that resemble human HNSCC. We have studied the possible mechanisms that might help explain malignization of oral tumors arising in BK5myrAkt;p53f/f;K14Cre with respect to BK5myrAkt, and found absence of the premature senescence response together with increased proliferation and expansion of the stem cell-like population. We have thus generated mice models that on one hand might help define roles of Akt in normal epithelial homeostasis and epidermal stem cell regulation, and on the other hand shed light on its role in oral tumorigenesis as well as provide a model with preclinical utility in which to test possible antitumoral drugs that specifically target the PI3K/Akt pathway.