Exploring the Gain of Function Contribution of AKT to Mammary Tumorigenesis in Mouse Models

Abstract

Elevated expression of AKT has been noted in a significant percentage of primary human breast cancers, mainly as a consequence of the PTEN/PI3K pathway deregulation. To investigate the mechanistic basis of the AKT gain of function-dependent mechanisms of breast tumorigenesis, we explored the phenotype induced by activated AKT transgenes in a quantitative manner. We generated several transgenic mice lines expressing different levels of constitutively active AKT in the mammary gland. We thoroughly analyzed the preneoplastic and neoplastic mammary lesions of these mice and correlated the process of tumorigenesis to AKT levels. Finally, we analyzed the impact that a possible senescent checkpoint might have in the tumor promotion inhibition observed, crossing these lines to mammary specific p53(R172H) mutant expression, and to p27 knock-out mice. We analyzed the benign, premalignant and malignant lesions extensively by pathology and at molecular level analysing the expression of proteins involved in the PI3K/AKT pathway and in cellular senescence. Our findings revealed an increased preneoplastic phenotype depending upon AKT signaling which was not altered by p27 or p53 loss. However, p53 inactivation by R172H point mutation combined with myrAKT transgenic expression significantly increased the percentage and size of mammary carcinoma observed, but was not sufficient to promote full penetrance of the tumorigenic phenotype. Molecular analysis suggest that tumors from double myrAKT;p53(R172H) mice result from acceleration of initiated p53(R172H) tumors and not from bypass of AKT-induced oncogenic senescence. Our work suggests that tumors are not the consequence of the bypass of senescence in MIN. We also show that AKT-induced oncogenic senescence is dependent of pRb but not of p53. Finally, our work also suggests that the cooperation observed between mutant p53 and activated AKT is due to AKT-induced acceleration of mutant p53-induced tumors. Finally, our work shows that levels of activated AKT are not essential in the induction of benign or premalignant tumors, or in the cooperation of AKT with other tumorigenic signal such as mutant p53, once AKT pathway is activated, the relative level of activity seems not to determine the phenotype.

Bypassing cellular senescence by genetic screening tools

“…Our studies in mouse models with activated AKT or PI3Ka indicated that the activation of these oncogenes do not fully recapitulates the tumorigenic phenotype of PTEN loss [85][86][87]. We have observed an increase in benign lesions, as expected, if senescence is induced upon PI3K and AKT activation [44,88], perhaps by AKT or PI3K activation in a progenitor cell unable to progress to full tumorigenesis. On the other hand, Majumder and colleagues [89] recently published their fi nding that a p27kip1-dependent checkpoint also induces senescence upon AKT activation and that ablation of this checkpoint allows progression from PIN to carcinoma.…”

“…It is possible that the induction of the senescent checkpoint is not a direct effect of PI3K pathway deregulation but a general checkpoint imposed on the progression from benign to malignant tumours. However, in mouse models, activation of AKT1 in the mammary gland triggers senescence that seems to be dependent upon p16 activation but not of p27 or p53 absences [44,86].…”

“…Comparable with p53, which functions as a failsafe mediator, the cyclin-dependent kinase (CDK) inhibitor p16 INK4a has been implicated in both response to DNA damage and control of stress-induced senescence [25,43,44]. Although the molecular mechanism used by p16 INK4a to control not only a temporary but a permanent G1 arrest is largely unclear, p16 INK4a responds to DNA damage in a delayed manner and appears to be indispensable for maintaining cellular senescence [45,46].…”

Senescence in Oncogenesis: From Molecular Mechanisms to Therapeutic Opportunities

“…Studies in murine mouse models have shown that p53 is the preferred mutation upon PTEN loss. In constitutively active AKT or PI3K transgenic models, an increase in benign lesions are observed if senescence is induced upon AKT activation (Blanco-Aparicio et al 2010 ;Renner et al 2008 ).…”

The novel miR-9500 regulates the proliferation and migration of human lung cancer cells by targeting Akt1

“…29 Other studies have shown that aberrant AKT activation has a critical role in tumorigenesis. 30 In this study, we identified small RNAs in lung cancer cells. To analyze a novel miRNA signature, we examined the structure and sequence of the small RNAs, analyzed the expression patterns of the novel miRNAs in lung cancer tissues and assessed the miRNA target genes.…”