AbstractRecent studies have introduced prosaposin (PSAP) as a pleiotrophic growth factor for prostate cancer (PCa). We have previously reported that PSAP or one of its known active molecular derivatives, saposin C functions as an androgen‐agonist and androgen‐regulated gene (ARG) for androgen‐sensitive (AS) PCa cell lines. Due to the potential significance of androgen receptor (AR)‐expressing stroma in PCa, we evaluated a possible bi‐directional paracrine regulatory interactions between DHT and PSAP in AR‐positive prostate stromal (PrSt) cells. We report that saposin C in a ligand‐independent manner increased AR expression, its nuclear content, and tyrosine phosphorylation. DHT treatment of PrSt cells increased PSAP expression. We also demonstrated both serum‐ and androgen‐inducibility of a previously characterized hormone‐responsive element (HRE) located in the proximal region of PSAP promoter. In addition, conditioned‐media derived from PrSt cells and bone fibroblasts (i.e., MSF) differentially increased PSAP‐promoter activity in androgen‐independent (AI) PC‐3 and AS LNCaP cells. Our data for the first time demonstrate that not only saposin C or PSAP regulates AR expression/activity, but also function as an ARG in PrSt. Ligand‐independent activation of AR by PSAP or saposin C in PCa and stromal cells may contribute not only to prostate carcinogenesis at an early stage, but also in AI progression of the disease in an androgen‐deprived tumor microenvironment. J. Cell. Biochem. 104: 2272–2285, 2008. © 2008 Wiley‐Liss, Inc.