A SEQUENTIAL TREATMENT REGIMEN WITH MELATONIN AND ALL-TRANS-RETINOIC ACID INDUCES APOPTOSIS IN MCF-7 TUMOR-CELLS

Neoplastic events are marked by uncontrolled cell proliferation. One m ajor focus of cancer research has been to identify treatments that red uce or inhibit cell growth. Over the years, various compounds, both na turally occurring and chemically synthesized, have been used to inhibi t neoplastic cell proliferation. Two such oncostatic agents, melatonin and retinoic acid, have been shown to suppress the growth of hormone- responsive breast cancer. Currently, separate clinical protocols exist for the administration of retinoids and melatonin as adjuvant therapi es for cancer. Using the oestrogen receptor (ER)-positive MCF-7 human breast tumour cell line, our laboratory has studied the effects of a s equential treatment regimen of melatonin followed by all-trans retinoi c acid (atRA) on breast tumour cell proliferation in vitro. Incubation of hormonally responsive MCF-7 and T47D cells with melatonin (10(-9) M) followed 24 h later by atRA (10(-9) M) resulted in the complete ces sation of cell growth as well as a reduction in the number of cells to below the initial plating density. This cytocidal effect is in contra st to the growth-suppressive effects seen with either hormone alone. T his regimen of melatonin followed by atRA induced cytocidal effects on MCF-7 cells by activating pathways leading to apoptosis (programmed c ell death) as evidenced by decreased ER and Bcl-2 and increased Bar an d transforming growth factor beta 1 (TGF-beta 1) expression. Apoptosis was reflected morphologically by an increase in the number of lysosom al bodies and perinuclear chromatin condensation, cytoplasmic blebbing and the presence of apoptotic bodies. The apoptotic effect of this se quential treatment with melatonin and atRA appears to be both cell and regimen specific as (a) ER-negative MDA-MB-231 and BT-20 breast tumou r cells were unaffected, and (b) the simultaneous administration of me latonin and atRA was not associated with apoptosis in any of the breas t cancer cell lines studied. Taken together, the results suggest that use of an appropriate regimen of melatonin and atRA should be consider ed for preclinical and clinical evaluation against ER-positive human b reast cancer.