E6AP promotes prostate cancer by reducing p27 expression

// Dinesh Raghu 1, 2 , Piotr Jan Paul 1, 2 , Twishi Gulati 1, 2 , Siddhartha Deb 3 , Christine Khoo 4 , Andrea Russo 5 , Enzo Gallo 5 , Giovanni Blandino 6 , Ai-Leen Chan 2, 11 , Elena Takano 4 , Shahneen K. Sandhu 7 , Stephen B. Fox 4 , Scott Williams 8 , Sue Haupt 1, 2 , Cristina Gamell 1, 2, * and Ygal Haupt 1, 2, 9, 10, * 1 The Sir Peter MacCallum Department of Oncology, The University of Melbourne, Melbourne, Victoria, Australia 2 Tumor Suppression Laboratory, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia 3 Anatpath Services Pty Ltd, Gardenvale, Victoria, Australia 4 Department of Pathology, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia 5 Department of Surgical Pathology, Regina Elena Cancer Institute, Rome, Italy 6 Oncogenomic and Epigenetic Unit, Italian National Cancer Institute, Rome, Italy 7 Division of Cancer Medicine, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia 8 Division of Radiation Oncology and Cancer Imaging, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia 9 Department of Biochemistry and Molecular Biology, Monash University, Melbourne, Victoria, Australia 10 Department of Pathology, The University of Melbourne, Melbourne, Victoria, Australia 11 Current address: Australian Regenerative Medicine Institute, Monash University, Melbourne, Victoria, Australia * Both authors have shared co-senior authorship Correspondence to: Ygal Haupt, email: ygal.haupt@petermac.org Keywords: prostate cancer, E6AP, p27, E2F1, tumor suppression Received: February 07, 2017      Accepted: March 16, 2017      Published: April 19, 2017 ABSTRACT Prostate cancer (PC) is the most common cancer in men. Elevated levels of E3 ligase, E6-Associated Protein (E6AP) were previously linked to PC, consistent with increased protein expression in a subset of PC patients. In cancers, irregular E3 ligase activity drives proteasomal degradation of tumor suppressor proteins. Accordingly, E3 ligase inhibitors define a rational therapy to restore tumor suppression. The relevant tumor suppressors targeted by E6AP in PC are yet to be fully identified. In this study we show that p27, a key cell cycle regulator, is a target of E6AP in PC. Down regulation of E6AP increases p27 expression and enhances its nuclear accumulation in PC. We demonstrate that E6AP regulates p27 expression by inhibiting its transcription in an E2F1-dependent manner. Concomitant knockdown of E6AP and p27 partially restores PC cell growth, supporting the contribution of p27 to the overall effect of E6AP on prostate tumorigenesis. Overall, we unravelled the E6AP-p27 axis as a new promoter of PC, exposing an attractive target for therapy through the restoration of tumor suppression.