Downregulation of Bcl-xL and activation of caspases during retinoic acid-induced apoptosis in an adult T-cell leukemia cell line.

INTRODUCTION: Adult T-cell leukemia (ATL) is a peripheral T-cell neoplasm caused by human T-cell leukemia virus type I (HTLV-I). We previously reported that retinoic acid (RA) isomers, all-trans-RA (ATRA), 9-cis-RA and 13-cis-RA, induce growth arrest and apoptosis in ATL cell lines. In this study we investigate the mechanisms of apoptosis induced by RAs.
MATERIALS AND METHODS: An ATL cell line, KK1, established from an ATL patient was cultured with or without RAs. DNA fragmentation was analyzed using propidium iodide for staining DNA on a FACS. The apoptosis-related proteins and their transcriptional levels were thereafter analyzed by Western blotting and reverse transcription polymerase chain reaction (RT-PCR). Caspase activity was determined by using specific fluorogenic substrates.
RESULTS: Bcl-2 and Bcl-xL are constitutively expressed in KK1. Treatment with RAs caused the level of Bcl-x protein decreased, while the levels of Bcl-2 and Bcl-xS remained unchanged. The level of Bax tended to decrease. There was a reduction of mitochondrial membrane potential and activation of caspase-3 and -6 without any activation of caspase-1. Broad range caspase inhibitors, Z-Asp and Z-VAD, prevented DNA fragmentation.
CONCLUSION: These results suggest that the RA-induced apoptotic signals were transduced via downregulation of Bcl-xL and the decrease in the mitochondrial membrane function leading to caspase-3 activation.