Induction of apoptosis by all-trans retinoic acid in the human myeloma cell line RPMI 8226 and negative regulation of some of its typical morphological features by dexamethasone

We investigated the effects of all-trans retinoic acid (RA) and dexamethaso ne (Dex) on the in vitro growth of the human myeloma cell line RPMI 8226, R A inhibited RPMI 8226 cell growth by both antiproliferative effect and indu ction of apoptosis, Typical morphological and biochemical characteristics o f apoptosis including chromatin condensation, apoptotic bodies formation an d internucleosomal DNA cleavage were detected after 4 days of treatment wit h 1 mu m RA, In situ TUNEL assay demonstrated that DNA cleavage preceded ch romatin condensation. The expression of tissue transglutaminase (tTG), an e nzyme proposed to play a role in apoptosis was induced with RA, as shown by both enzymatic assay and in situ immunofluorescence detection, Dex, when u sed alone, had no effect on cell growth and apoptosis, When combined to RA, Dex did not interfere with the RA-dependent inhibition of cell proliferati on, but unexpectedly inhibited both quantitatively and qualitatively severa l morphological and biochemical features of the apoptosis induced by RA, De x did not affect RA-induced DNA breaks formation but impeded the progressio n of chromatin condensation and the formation of apoptotic bodies. Interest ingly, Dex also inhibited the RA-dependent induction of tTG. RU486, a gluco corticoid antagonist, counteracted all Dex effects, Taken together these da ta demonstrate that key cytoplasmic and nuclear events occurring during apo ptosis are differentially regulated by RA and Dex in myeloma cell line RPMI 8226.