Mechanistic studies of the effects of the retinoid N-(4-hydroxyphenyl)retinamide on prostate cancer cell growth and apoptosis

To explore the mechanisms underlying the chemopreventive effects of the syn thetic retinoid N-(4-hydroxyphenyl)retinamide (4-HPR) in prostate cancer, w e evaluated the anti-proliferative and apoptosis-inducing effects of 4-HPR in the androgen-sensitive human prostate cancer cell line LNCaP. 4-HPR decr eased the number of viable LNCaP cells las measured by the 3-(4,5-dimethylt hiazol-2-yl)-2,5-diphenyltetrazolium bromide assay) in a dose-dependent man ner. Although 4-HPR exerted a modest G(1) cell-cycle block (as determined b y flow cytometry), its effect on reduced cell number appeared to result pri marily from induction of apoptosis las measured by an enzyme-linked immunos orbent assay and flow-cytometric assays). The mitogenic effects of R1881, a non-metabolizable androgen that potently induces LNCaP cell proliferation, was completely blocked by greater than 0.5 mu M 4-HPR. Furthermore, increa sing the R1881 concentration in the presence of 2.0 mu M 4-HPR increased ap optotic cell death. 4-HPR decreased prostate-specific antigen (PSA) protein levels in conditioned medium and decreased PSA mRNA expression. 4-HPR also decreased the ratio of bcl-2 to bar mRNA expression in LNCaP cells by appr oximately 45%, indicating that the apoptotic effects of 4-HPR may be mediat ed, at least in part, by alterations in the bcl-2/bax-regulated apoptotic p athway. N-acetylcysteine (4 mM) completely blocked the anti-proliferative a nd apoptotic-inducing effects of 4-HPR suggesting that an oxidative mechani sm may be involved. We concluded that (i) 4-HPR exerts growth-suppressive a nd apoptotic effects on LNCaP cells, (ii) 4-HPR can interact with androgen to suppress proliferation and induce apoptosis, (iii) the apoptotic effects of 4-HPR may be mediated in part by the bcl-2/bax pathway, and (iv) a pro- oxidant mechanism may contribute to the anti-proliferative a nd a apoptotic -inducing effects of 4-HPR. Mol. Carcinog. 24:160-168, 1999. (C) 1999 Wiley -Liss, Inc.