The mangosteen (Garcinia mangostana) fruit has been a popular food in Southeast Asia for centuries and is increasing in popularity in Western countries. We identified alpha-Mangostin as a primary phytochemical modulating ER stress proteins in prostate cancer cells and propose that alpha-Mangostin is responsible for exerting a biological effect in prostate cancer cells. Two human prostate cancer cell lines, 22Ry1 and LNCaP, and prostate epithelial cells procured from two patients undergoing radical prostatectomy were treated with alpha-Mangostin and evaluated by RT-PCR, Western blot, fluorescent microscopy and siRNA transfection to evaluate ER stress. Next, we evaluated alpha-Mangostin for microsomal stability, pharmacokinetic parameters, and anti-cancer activity in nude mice. alpha-Mangostin significantly upregulated ER stress markers in prostate cancer cells. Interestingly, alpha-Mangostin did not promote ER stress in prostate epithelial cells (PrECs) from prostate cancer patients. CHOP knockdown enhanced alpha-Mangostin-induced apoptosis in prostate cancer cells. alpha-Mangostin significantly suppressed tumor growth in a xenograft tumor model without obvious toxicity. Our study suggests that alpha-Mangostin is not the only active constituent from the mangosteen fruit requiring further work to understand the complex chemical composition of the mangosteen. (C) 2014 Elsevier Inc. All rights reserved.