BACKGROUNDRecent evidence suggests a particular role for obesity in prostate cancer (PCa) progression. Adiponectin (ADN) is a hormone secreted by adipose tissue and has a variety of functions including the inhibition of PCa cell proliferation. Although serum ADN levels have been identified to be related with carcinogenesis in a tissue‐specific context, the exact role of endogenous ADN in PCa cells remains largely unknown.METHODSTwo tissue microarrays were constructed and immunohistochemistry (IHC) was utilized to detect ADN's expression in a cohort of 96 Chinese PCa patients with radical prostatectomy as well as 15 cases with Benign Prostatic Hyperplasia (BPH). MTS and transwell assays were applied to validate the effects of ADN on proliferation and invasive capacity of PCa cells. Real‐time PCR and Western blot were performed to evaluate the expression at transcript and protein levels. Epigenetic modifications of ADN's promoter after TGF‐β1 treatment in 22RV1 cells was monitored by chromatin immunoprecipitation (ChIP). Methylation‐Specific PCR (MSP) was performed to determine the methylation status of ADN's promoter.RESULTSIHC showed decreased levels of ADN in 1 of 15 (6.7%) BPH cases, 6 of 27 (22.2%) PCa cases with low Gleason score (<7), 18 of 26 (69.2%) cases with Gleason score 7, but 32 of 43 (74.4%) cases with high Gleason score (>7). Silencing endogenous ADN could promote proliferation and invasion of 22RV1 cells via orchestrating Epithelial‐to‐mesenchymal Transition (EMT) process. TGF‐β1, a potent EMT inducer, could decrease levels of chromatin markers associated with active genes (H3K4me3, H4acetylK16), and increase levels of repressive marker (H3K27me3) at ADN promoter in 22RV1 cells. Additionally, 5‐aza and TSA treatment restored ADN expression in LNCaP cells in which the ADN expression was almost absent. MSP analysis revealed that methylation in the promoter might be involved in decreased expression of ADN in PCa tissues.CONCLUSIONOur findings indicated that endogenous ADN may function as a tumor suppressor gene through inhibiting EMT of PCa cells but is down‐regulated in PCa via promoter hypermethylation. Prostate 75: 1197–1205, 2015. © 2015 Wiley Periodicals, Inc.