Prostate cancer is an indolent tumor, and the mechanisms responsible are not understood. The normal prostate produces arachidonic acid (AA) metabolites through the cyclooxygenase (COX) pathway that are added to seminal fluids and aid in reproduction. Our research shows that tumor cell growth is suppressed by AA pathway metabolites that are products of lipocalcin type-prostaglandin D synthase (PGDS) in normal prostate cells and may act through high levels of nuclear receptor peroxisome proliferator activated receptor-gamma (PPARγ) expressed by prostate cancer cells. All normal prostate cells studied, prostate epithelial (PrE), stromal (PrSt), and smooth muscle (PrSM) cells, expressed PGDS mRNA by RT-PCR and protein by Western analysis. In contrast, prostate cancer cells, PC-3, LNCaP, and DU145 cells, lacked any detectable PGDS mRNA by RT-PCR at 25, 35, or 45 amplification cycles and any protein expression by Western analysis. These high levels of PGDS expression in normal PrSt, PrSM, and PrE cells corresponded to significant levels of PGD2 (PrSt [2.75 + 0.001 ng/106 cells], PrSM [1.49 + 0.28 ng/106 cells], and PrE [1.24 + 0.01 ng/106 cells]) as measured by LC/MS/MS analysis and were uniformly restricted to normal cells. We then tested the biologic effects of PGDS and its metabolic products on prostate tumor cells. Conditioned medium from PrSt cells fed 10μM AA for 30 minutes inhibited prostate cancer cell growth by 20-40% at 24 hours as measured by calcein-AM vital dye. Neutralizing antibody against PGDS abrogated the inhibitory effect of PrSt-conditioned medium. This suggests that PrSt cells produce inhibitory factors that are enzymatic metabolites of PGDS. In coculture, PrSt cells induced suppression of PC-3, LNCaP, and DU145 cell growth. Additionally, PGDS subcloned into a Neo, HA tag, and Myc tag vector was expressed in COS-1 cells, which were previously determined to express both PPARγ and COX-1. Significant time-dependent changes in morphology resembling differentiation were observed in COS-1 cells expressing these vectors but not in cells transfected with empty vectors. These results suggest that prostate stromal PGDS-derived eicosanoids inhibit prostate cancer cell growth, that this effect may be mediated by activation of PPARγ, and that this prostate stromal cell-cancer cell interaction may be the mechanism that explains prostate cancer’s indolent growth. Supported by a grant from the American Cancer Society, TPRN-99-240-01-CNE-1, to DGM.
[Proc Amer Assoc Cancer Res, Volume 45, 2004]