N-(4-hydroxyphenyl)retinamide activation of transforming growth factor-β and induction of apoptosis in human breast cancer cells

N-(4 hydroxyphenyl)retinamide (4-HPR), a synthetic derivative of all-trans-retinoic acid, induces DNA synthesis arrest and apoptosis in human breast cancer cells in a dose- and time-dependent manner. MDA-MB-435 cells treated with 3 mu M 4-HPR exhibited 58% and 75% DNA synthesis arrest after 1 and 2 days of treatment and 31%, 39%, 48%, and 56% apoptosis after 3, 4, 5, and 6 days of treatment, respectively. Conditioned media from 4-HPR-treated MDA-MB-435 cells contained 63 and 57 pg of active transforming growth factor-beta (TGF-beta) per 10(6) cells after 1 and 2 days of treatment, whereas conditioned media from control cells contained only 9 pg/10(6) cells. TGF-beta involvement in 4-HPR- induced apoptosis, but not DNA synthesis arrest, in MDA-MB-435 cells was demonstrated by 1) blockage of 4-HPR-induced apoptosis by 66-75% after treatment of cells with neutralizing antibodies to TGF-beta s, 2) blockage of 4-HPR-induced apoptosis by 64-67% after transient transfection of cells with antisense oligomers to TGF-beta(1) or TGF-beta type II receptor, 3) blockage of 4-HPR-induced apoptosis by approximately 50% after inhibition of latent TGF-beta activation, and 4) demonstration that human breast cancer cells (T47D) defective in TGF-beta signaling were refractive to 4-HPR-induced apoptosis. These data indicate that 4-HPR is a patent activator of TGF-beta and that TGF-beta participates in 4-HPR-induced apoptosis of human breast cancer cells.