EFFECTS OF ALL-TRANS-RETINOIC ACID AND ANTIRECEPTOR ANTIBODIES ON GROWTH AND PROGRAMMED CELL-DEATH OF HUMAN MYELOMA CELLS

In contrast to cytotoxic agents inducing rapid cell death, biological agents such as hormones, vitamins (e.g., retinoids), cytokines, and an tireceptor antibodies act slowly and may alter ratios between cell gro wth and programmed cell death (apoptosis), We showed previously that a nti-interleukin 6 (IL-6) and antitransferrin (Tf) receptor antibodies inhibited in vitro growth and induced death of myeloma cells, Retinoid s also inhibit lit vitro growth of human cancer cells and decrease IL- 6 receptor display and autosecretion by some myeloma cells, Retinoids may also antagonize in vitro growth-promoting effects of iron and tran sferrin, To develop a novel strategy for treating myeloma, we examined antiproliferative and cytotoxic effects of retinoids in combination w ith anti-Tf or anti-IL-6 receptor antibodies, Myeloma cell lines were cultured with retinoids with or without anti-growth factor receptor mo noclonal antibodies, Both all-trans retinoic acid (ATRA) and 13-cis-re tinoic acid showed variable, dose-dependent inhibition of myeloma cell line growth, ATRA also induced significant down-regulation of myeloma IL-6 receptors and inhibited IL-6 autosecretion by myeloma cells, Ant iproliferative effects of ATRA were increased by coculture with anti-T f but not anti-IL-6 receptor antibodies. Colony-forming assays showed that antiproliferative effects of anti-Tf receptor antibodies were lar gely reversible, but 1 mu M ATRA was cytotoxic to myeloma cells, To as sess apoptosis, a flow cytometry assay detecting DNA damage was used, Using previously studied cell line models, flow cytometry detected pro grammed cell death induced by transforming growth factor pi in leukemi a cells and by anti-growth factor receptor antibody treatment of IL-6- dependent myeloma cells, treatments which caused only modest increases in the percentage of cells undergoing morphological apoptosis and inc reased internucleosomal DNA degradation, Flow cytometry analysis of AT RA and anti-Tf antibody-treated myeloma cells also showed evidence for apoptosis induced by ATRA, but not with anti-Tf receptor antibodies, These changes were apparent several days before detection of internucl eosomal DNA degradation on agarose gels in 8226 cells but were not det ected at any time in U266 cells, which underwent cell death but showed no DNA damage using flow cytometry or degradation on agarose gels, Re tinoids merit further study as possible maintenance or chemoprevention therapies for clonal plasma cell disorders and for treating paraneopl astic disorders such as Castleman's disease, Flow cytometry rapidly de tects apoptosis induced by biological agents and may be useful for in vitro screening of novel biological therapies.