Growth inhibition of human in vitro and mouse in vitro and in vivo mammarytumor models by retinoids in comparison with tamoxifen and the RU-486 anti-progestagen

Retinoids constitute a very promising class of agents for the chemopreventi on or treatment of breast cancer. These retinoids exert their biological ac tivity through two distinct classes of retinoic acid (KA) receptors (R), th e RAR isotypes (alpha, beta, and gamma) and the three RXR isotypes (alpha, beta, and gamma) and their numerous isoforms which bind as RX R/RAR heterod imers to the polymorphic cis-acting response elements of RA target genes. W ith respect. to these numerous receptor sub-types, the retinoid-induced eff ects at the biological level include marked modifications with respect to b oth cell proliferation and cell death (apoptosis), and also in the inductio n of differentiation processes. The present study aims to characterize the effect which fc,ur retinoids (TTNPB,9-cis-RA, LGD 1069, 9-HPR) with distinc t RAR/RXR binding properties induced on various in vitro and in vivo mouse and human breast cancer models. The experiments with the retinoids were car ried out in comparison with the anti-estrogen tamoxifen and the anti-proges tagen RU-486 compounds. The results show that the 6 compounds under study w ere markedly more efficient in terms of growth inhibition in the human T-47 D cell line when maintained under anchorage-independent culture conditions than when maintain ed under anchorage-dependent ones. While RU-486 exhibite d a weak statistically significant (p < 0.05) influence on the growth of th e T-47D stem cells, tamoxifen had a marked inhibitory influence on the grow th of these cells. Of the four retinoids, I-HPR was the least effective sin ce the lowest doses tested (1 and 0.1 nM) exhibited Ilo statistically (p >0 .05) significant influence on the growth of the stem cells. The: most effic ient retinoid was TTNPB. It was only at the highest dose (10 mu M) that tam oxifen and RU-486 showed a weak inhibitory influence on the growth of the T -47D non-stem cells while all 1 retinoids exerted a significant inhibitory influence on the growth of these non-stem cells, with 4-HPR being the most efficient (P < 0.001) at the highest dose, but ineffective (P > 0.05) at th e: lowest. Tamoxifen and TTNPB were tested in vivo on hormone-senstive (HS) and hormone-insensitive (HI) strains of the MXT murine mammary carcinoma. While TTNPB appeared to be equally efficient in terms of growth inhibition in both MXT-HS and MXT-HI models, tamoxifen had only a marginal inhibitory influence on the growth of the MXT-HI strain but did inhibit growth in the cast: of the MXT-HS one, TTNPB was markedly more efficient than tamoxifen i n terms of both inhibiting the cell proliferation level (measured by means Of computer-assisted microscopy applied to Feulgen-stained nuclei, a method which enables the percentage of cells in the ii phase of the cell cycle to be determined) and triggering cell death (measured by means of the determi nation of the transglutaminase activity) in both the MXT-HI and MXT-HS mode ls. The very significant TTNPB-induced inhibition of the macroscopic MXT-HS growth rate relates to the triggering of cell death (apoptosis) rather tha n to an inhibition of cell proliferation. All these results clearly indicat e that retinoids are very efficient agents against breast cancer, at least as efficient as tamoxifen.