Retinoic acid via RARa inhibits the expression of 24-hydroxylase in human prostate stromal cells Yan-Ru Lou a, * , Susanna Miettinen b , Hiroyuki Kagechika c , Hinrich Gronemeyer d , Pentti Tuohimaa a,e a Department of Anatomy Medical School, FIN-33014 University of Tampere, Finland b Cell Biology, Medical School, FIN-33014 University of Tampere, Finland c School of Biomedical Science, Tokyo Medical and Dental University, 2-3-10 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-0062, Japan d Department of Cell Biology and Signal Transduction, IGBMC-B.P. 10142, F-67404 Illkirch Cedex, C.U. de Strasbourg, France e Department of Clinical Chemistry, Tampere University Hospital, Tampere, Finland Received 28 October 2005 Available online 7 November 2005 Abstract 25-Hydroxyvitamin D 3 -24-hydroxylase (24-hydroxylase) is an important inactivating enzyme and its expression is induced by 25- hydroxyvitamin D 3 (25OHD 3 ) and 1a,25-dihydroxyvitamin D 3 (1a,25-(OH) 2 D 3 ) through action of heterodimers of vitamin D receptor (VDR) and retinoid X receptor (RXR). RXRs also act as heterodimer partners for retinoic acid receptors (RARs), mediating the action of all-trans-retinoic acid (ATRA). Prostate stroma plays a crucial role in prostate cancer development and benign prostatic hyperplasia. We demonstrate here that ATRA markedly reduced the expression of 24-hydroxylase mRNA induced by 25OHD 3 and 1a,25-(OH) 2 D 3 in human prostatic stromal cells P29SN and P32S but not in epithelial cells PrEC or cancer cells LNCaP. By using transfection and RAR- selective ligands, we found that the inhibitory effect of ATRA on 24-hydroxylase expression in stromal cells was mediated by RARa but not by RARb. Moreover, the ATRA-induced expression of RARb was also mediated by RARa. The combined treatment of 1a,25- (OH) 2 D 3 and RARa agonist Am80 at 10 nM exhibited strong growth-inhibitory effect whereas either alone had no effect. Our data sug- gest that ATRA suppresses 24-hydroxylase expression through RARa-dependent signaling pathway and can enhance vitamin D action in suppression of cell growth. Ó 2005 Elsevier Inc. All rights reserved. Keywords: All-trans-retinoic acid; 24-Hydroxylase; Retinoic acid receptor; Stroma; RAR-selective ligands; Prostate cancer; Benign prostatic hyperplasia; Calcidiol; Calcitriol 25-Hydroxyvitamin D 3 (25OHD 3 ) and 1a,25- dihydroxyvitamin D 3 (1a,25-(OH) 2 D 3 ) are active hor- mones, regulating the proliferation and differentiation in a variety of cell types [1–5]. The actions of 25OHD 3 and 1a,25-(OH) 2 D 3 are mediated by vitamin D receptor (VDR), which acts as a heterodimer with retinoid X recep- tor (RXR) and binds to the vitamin D response element (VDRE) within the promoter region of the target genes. 25OHD 3 -24-hydroxylase (24-hydroxylase, CYP24) con- trols the first steps of the inactivation of both active hor- mones and its expression is highly induced by its substrates, which thereby regulates their own metabolism. The high expression of 24-hydroxylase has been shown to cause vitamin D 3 resistance and the suppression by enzyme inhibitors is beneficial to the growth-inhibitory effect of 1a,25-(OH) 2 D 3 [6]. In addition, 24-hydroxylase has been suggested to be an oncogene in breast cancer [7]. The chro- mosomal region where 24-hydroxylase gene is located has been found to be amplified in human breast carcinoma [8,9], prostate cancer [10], and ovarian cancer [11] as well as mouse islet carcinomas [12]. Retinoic acid (RA), the most potent form of vitamin A, also plays an important role in the growth and differentia- tion of many cells [13]. RA has been shown to function via retinoic acid receptors (RARs) and RXRs [14,15]. Six 0006-291X/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.bbrc.2005.10.178 * Corresponding author. Fax: +358 3 35516170. E-mail address: loyalo@uta.fi (Y.-R. Lou). www.elsevier.com/locate/ybbrc Biochemical and Biophysical Research Communications 338 (2005) 1973–1981 BBRC