Inhibition of the PI3K/AKT/mTOR pathway activates autophagy and compensatory Ras/Raf/MEK/ERK signalling in prostate cancer

// Dominika E. Butler 1 , Christopher Marlein 2 , Hannah F. Walker 1 , Fiona M. Frame 1 , Vincent M. Mann 3 , Matthew S. Simms 4, 5 , Barry R. Davies 6 , Anne T. Collins 1 and Norman J. Maitland 1 1 The Cancer Research Unit, Department of Biology, University of York, York YO10 5DD, UK 2 Norwich Medical School, University of East Anglia, Norwich NR4 7TJ, UK 3 Gastroenterology Research Department, Hull Royal Infirmary, Hull HU3 2JZ, UK 4 Department of Urology, Castle Hill Hospital (Hull & East Yorkshire Hospital NHS Trust), Cottingham HU16 5JQ, UK 5 Hull York Medical School, University of York, York YO10 5DD, UK 6 AstraZeneca, Cancer Research UK Cambridge Institute, Cambridge CB2 0RE, UK Correspondence to: Norman J. Maitland, email: njm9@york.ac.uk Keywords: prostate cancer, AKT, mTOR, RAS, MAPK signalling, autophagy Received: May 27, 2016     Accepted: April 24, 2017     Published: May 23, 2017 ABSTRACT The PI3K/AKT/mTOR pathway is frequently activated in advanced prostate cancer, due to loss of the tumour suppressor PTEN, and is an important axis for drug development. We have assessed the molecular and functional consequences of pathway blockade by inhibiting AKT and mTOR kinases either in combination or as individual drug treatments. In established prostate cancer cell lines, a decrease in cell viability and in phospho-biomarker expression was observed. Although apoptosis was not induced, a G1 growth arrest was observed in PTEN null LNCaP cells, but not in BPH1 or PC3 cells. In contrast, when the AKT inhibitor AZD7328 was applied to patient-derived prostate cultures that retained expression of PTEN, activation of a compensatory Ras/MEK/ERK pathway was observed. Moreover, whilst autophagy was induced following treatment with AZD7328, cell viability was less affected in the patient-derived cultures than in cell lines. Surprisingly, treatment with a combination of both AZD7328 and two separate MEK1/2 inhibitors further enhanced phosphorylation of ERK1/2 in primary prostate cultures. However, it also induced irreversible growth arrest and senescence. Ex vivo treatment of a patient-derived xenograft (PDX) of prostate cancer with a combination of AZD7328 and the mTOR inhibitor KU-0063794, significantly reduced tumour frequency upon re-engraftment of tumour cells. The results demonstrate that single agent targeting of the PI3K/AKT/mTOR pathway triggers activation of the Ras/MEK/ERK compensatory pathway in near-patient samples. Therefore, blockade of one pathway is insufficient to treat prostate cancer in man.