N-(4-HYDROXYPHENYL)RETINAMIDE (4-HPR)-MEDIATED BIOLOGICAL ACTIONS INVOLVE RETINOID RECEPTOR-INDEPENDENT PATHWAYS IN HUMAN BREAST-CARCINOMA

Retinoid response pathways involve retinoic acid receptors (RARs) and retinoid X receptors, N-(4-hydroxyphenyl) retinamide (4-HPR), a deriva tive of all-trans-retinoic acid (RA) is currently in clinical trials a s a chemopreventive agent for breast cancer. The issue whether 4-HPR m ediates its biological actions via classical retinoid receptor pathway s remains to be investigated. In this study, we provide several lines of evidence that 4-HPR mediates its biological actions via a novel pat hway(s) that does not involve the classical retinoid receptor pathways . For example, 4-HPR was more potent than RA as an antiproliferative a gent and inhibited growth of otherwise RA-resistant human breast carci noma cells. Exposure to 4-HPR resulted in the generation of DNA fragme ntation with subsequent cell death in both RA-positive estrogen recept or (ER)-positive as well as RA-refractory ER-negative breast carcinoma cell lines. N-(4-Methoxyphenyl)retinamide (4-MPR), which is the major 4-HPR metabolite in circulation, was biologically inert in this syste m, 4-HPR and 4-MPR bound poorly to the RAR alpha, beta and gamma in vi tro and only minimally activated the retinoic acid receptor element (R ARE) and retinoid X receptor response elements (RXREs) in human breast carcinoma cells. Neither 4-HPR nor 4-MPR are metabolized to any of th e known conventional retinoids. In addition, 4-HPR or 4-MPR transactiv ation of RAREs or RXREs transfected into MCF-7 and MDA-MB-231 cells wa s not noted at 48 h. Nevertheless 4-HPR-mediated cell death was observ ed at 48 h, further suggesting that neither 4-HPR nor 4-MPR are metabo lized to retinoids which activate the RAREs or RXREs in breast carcino ma cells. Furthermore, unlike RA, which exhibited anti-AP1 activity, 4 -HPR inhibition of growth did not involve anti-AP1 activity. These res ults suggest that 4-HPR acts by a unique pathway that is not mediated by retinoid receptors.