PTEN loss and activation of K‐RAS and β‐catenin cooperate to accelerate prostate tumourigenesis

Aberrant phosphoinositide 3‐kinase (PI3K), mitogen‐activated protein kinase (MAPK) and WNT signalling are emerging as key events in the multistep nature of prostate tumourigenesis and progression. Here, we report a compound prostate cancer murine model in which these signalling pathways cooperate to produce a more aggressive prostate cancer phenotype. Using Cre‐LoxP technology and the probasin promoter, we combined the loss of Pten (Pten^fl/fl), to activate the PI3K signalling pathway, with either dominant stabilized β‐catenin [Catnb^+/lox(ex3)] or activated K‐RAS (K‐Ras^+/V12) to aberrantly activate WNT and MAPK signalling, respectively. Synchronous activation of all three pathways (triple mutants) significantly reduced survival (median 96 days) as compared with double mutants [median: 140 days for Catnb^+/lox(ex3)Pten^fl/fl; 182 days for Catnb^+/lox(ex3)K‐Ras^+/V12; 238 days for Pten^fl/flK‐Ras^+/V12], and single mutants [median: 383 days for Catnb^+/lox(ex3); 407 days for Pten^fl/fl], reflecting the accelerated tumourigenesis. Tumours followed a stepwise progression from mouse prostate intraepithelial neoplasia to invasive adenocarcinoma, similar to that seen in human disease. There was significantly elevated cellular proliferation, tumour growth and percentage of invasive adenocarcinoma in triple mutants as compared with double mutants and single mutants. Triple mutants showed not only activated AKT, extracellular‐signal regulated kinase 1/2, and nuclear β‐catenin, but also significantly elevated signalling through mechanistic target of rapamycin complex 1 (mTORC1). In summary, we show that combined deregulation of the PI3K, MAPK and WNT signalling pathways drives rapid progression of prostate tumourigenesis, and that deregulation of all three pathways results in tumours showing aberrant mTORC1 signalling. As mTORC1 signalling is emerging as a key driver of androgen deprivation therapy resistance, our findings are important for understanding the biology of therapy‐resistant prostate cancer and identifying potential approaches to overcome this. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.