1. ¹Urology Research Group, Northern Institute for Cancer Research, University of Newcastle, Tyne and Wear, UK
2. ²Cancer Research UK Laboratories, Beatson Institute for Cancer Research, Glasgow, UK
3. ³Section of Surgical and Translational Research, Division of Cancer Sciences and Molecular Pathology, University of Glasgow, Glasgow, UK
4. ⁴Department of Cellular Pathology, Royal Victoria Infirmary, Tyne and Wear, UK
5. ⁵Medical Research Council Protein Phosphorylation Unit, School of Life Sciences, University of Dundee, Dundee, UK

Correspondence: Professor HY Leung, University Section of Surgery, University Tower Level 2, Glasgow Royal Infirmary, 10 Alexandra Parade, Glasgow G31 2ER, UK. E-mail: h.leung@beatson.gla.ac.uk

Received 12 June 2007; Revised 9 October 2007; Accepted 10 October 2007; Published online 10 December 2007.

Abstract

Abnormal intracellular signaling contributes to carcinogenesis and may represent novel therapeutic targets. mitogen/extracellular signal-regulated kinase kinase-5 (MEK5) overexpression is associated with aggressive prostate cancer. In this study, we examined the role of extracellular signal-regulated kinase (ERK5, an MAPK and specific substrate for MEK5) in prostate cancer. ERK5 immunoreactivity was significantly upregulated in high-grade prostate cancer when compared to benign prostatic hyperplasia (P<0.0001). Increased ERK5 cytoplasmic signals correlated closely with Gleason sum score (P<0.0001), bony metastases (P=0.0044) and locally advanced disease at diagnosis (P=0.0023), with a weak association with shorter disease-specific survival (P=0.036). A subgroup of patients showed strong nuclear ERK5 localization, which correlated with poor disease-specific survival and, on multivariant analysis, was an independent prognostic factor (P<0.0001). Analysis of ERK5 expression in matched tumor pairs (before and after hormone relapse, n=26) revealed ERK5 nuclear expression was significantly associated with hormone-insensitive disease (P=0.0078). Similarly, ERK5 protein expression was increased in an androgen-independent LNCaP subline. We obtained the following in vitro and in vivo evidence to support the above expression data: (1) cotransfection of ERK5wt and MEK5D constructs in PC3 cells results in predominant ERK5 nuclear localization, similar to that observed in aggressive clinical disease; (2) ERK5-overexpressing PC3 cells have enhanced proliferative, migrative and invasive capabilities in vitro (P<0.0001), and were dramatically more efficient in forming tumors, with a shorter mean time for tumors to reach a critical volume of 1000 mm³, in vivo (P<0.0001); (3) the MEK1 inhibitor, PD184352, blocking ERK1/2 activation at low dose, did not suppress proliferation but did significantly decrease proliferation at a higher dose required to inhibit ERK5 activation. Taken together, our results establish the potential importance of ERK5 in aggressive prostate cancer.