Identification of a Novel Class of Retinoic Acid Receptor b-Selective Retinoid Antagonists and Their Inhibitory Effects on AP-1 Activity and Retinoic Acid-induced Apoptosis in Human Breast Cancer Cells*

Four candidate retinoid antagonists (LE135, LE511, LE540, and LE550) were designed on the basis of the ligand superfamily concept and synthesized. Analysis of these related retinoids by transient transfection assay demonstrated that LE135, LE540, and LE550 are effective retinoic acid receptor (RAR) antagonists, whereas LE511 selectively induced RARb transcriptional activity. Both LE135 and LE540 inhibited retinoic acid (RA)induced transcriptional activation of RARb, but not RARa, RARg or retinoid X receptor a (RXRa), on a variety of RA response elements. The retinoid antagonists also inhibited all-trans-RA-induced transcriptional activation of RARb/RXRa heterodimers, although they did not show any effect on transactivation activity of RXR/ RXR homodimers. In ZR-75-1 human breast cancer cells, cotreatment of LE135 and LE540 with all-trans-RA inhibited all-trans-RA-induced apoptosis of the cells, further demonstrating that RARb plays a role in RA-induced apoptosis of breast cancer cells. We also evaluated the effect of these retinoids on AP-1 activity. Our data showed that LE135 and LE540 strongly repressed 12-O-tetradecanoylphorbol-13-acetate-induced AP-1 activity in the presence of RARb and RXRa. Interestingly, LE550 induced AP-1 activity when RARb and RXRa were expressed in HeLa cells but not in breast cancer cells. These results demonstrate that LE135 and LE540 were a novel class of RARb-selective antagonists and anti-AP-1 retinoids and should be useful tools for studying the role of retinoids and their receptors.