Pathways through which a regimen of melatonin and retinoic acid induces apoptosis in MCF-7 human breast cancer cells

It has been established that melatonin (Mlt) and retinoic acid, individuall y, inhibit the proliferation of the estrogen receptor-alpha (ER alpha)-posi tive MCF-7 breast cancer cell line. Our laboratory has previously demonstra ted that Mlt and all-trans-retinoic acid (atRA) not only inhibit the prolif eration, but also induce apoptosis of MCF-7 cells when used in a sequential regimen of Mlt followed 24 h later by atRA. Using this same MCF-7 breast c ancer cell line, we investigated the potential pathways through which apopt osis is being induced. We found that treatment of MCF-7 cells with Mlt for 24 h before the addition of atRA decreased the protein levels of the death suppressor, Bcl-2, and increased, although with different time courses, the levels of the death promoters, Bax and Bak; however, there was no change i n the levels of the tumor suppressor gene, p53. MCF-7 cells treated sequent ially with Mlt and atRA also demonstrated an enhanced sensitivity to the ap optotic effects of atRA, which did not appear to be due to increased expres sion of the retinoic acid receptors, RAR alpha or RXR alpha, but rather to enhanced transcriptional activity of the RAR alpha. These data suggest that the sequential treatment regimen of Mlt and atRA may induce apoptosis by m odulation of members of the Bcl-2 family of proteins. Thus, this combinator ial regimen, which reduces the concentration of atRA needed for clinical ef ficacy while enhancing its anti-tumorigenic activity, could be of great the rapeutic benefit, and may, in fact, specifically induce the regression of e stablished breast tumors due to its apoptosis-promoting effects.