AbstractBACKGROUNDMRP4/ABCC4 is an ATP‐binding cassette transporter expressed in normal prostate. This study aimed to define the pattern of MRP4/ABCC4 expression in normal and malignant prostate tissue and the relationship of MRP4/ABCC4 expression and function in response to androgen signaling.METHODSEighty‐four radical prostatectomy specimens from patients with localized prostate cancer (PC) (22 neoadjuvant androgen ablation, AA, 62 no AA), 42 non‐cancer and 16 advanced PCs were assessed for MRP4/ABCC4 mRNA/protein expression. The effect of DHT and bicalutamide on LNCaP cells was assessed by immunoblotting. HEK293 cells (+/−MRP4/ABCC4) were assessed for the ability to efflux androgens and anti‐androgens.RESULTSMRP4/ABCC4 mRNA/protein levels were higher in localized PC compared to non‐cancer (P = 0.006). MRP4/ABCC4 levels were significantly decreased in PCs treated with AA compared to cancers exposed to normal testosterone levels (P < 0.0001). MRP4/ABCC4 expression in normal human tissues was limited to the prostate and the renal tubules. MRP4/ABCC4 protein levels increased in LNCaP cells after DHT which was partially blocked by bicalutamide. However, DHT did not alter the activation of the MRP4/ABCC4 promotor in luciferase reporter assays and testosterone, DHT, flutamide and hydroxy‐flutamide were not substrates for MRP4/ABCC4.DISCUSSIONElevated MRP4/ABCC4 expression is found in malignant compared to benign prostate tissue while lower MRP4/ABCC4 expression is seen after AA. Furthermore, MRP4/ABCC4 is upregulated by androgen and downregulated by anti‐androgen treatment in vitro potentially through an indirect mode of action. These data strongly suggest that MRP4/ABCC4 is an androgen‐regulated gene important in the progression to PC and may be a potential drug target. Prostate 68: 1421–1429, 2008. © 2008 Wiley‐Liss, Inc.