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


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.


CONCLUSIONS
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.
D000328:Adult; D017209:Apoptosis / Q000187:drug effects*; D053148:Caspase 3; D053178:Caspase 6; D020169:Caspases / Q000378:metabolism*; D045744:Cell Line, Tumor; D053938:DNA Fragmentation / Q000187:drug effects; D015536:Down-Regulation; D004789:Enzyme Activation / Q000187:drug effects; D006801:Humans; D015458:Leukemia, T-Cell / Q000473:pathology*; D008564:Membrane Potentials; D008928:Mitochondria / Q000502:physiology; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000096:biosynthesis*; D012333:RNA, Messenger / Q000032:analysis; D014212:Tretinoin / Q000494:pharmacology*; D051020:bcl-X Protein
C494811:BCL2L1 protein, human; D019253:Proto-Oncogene Proteins c-bcl-2; D012333:RNA, Messenger; D051020:bcl-X Protein; D014212:Tretinoin; C505407:CASP3 protein, human; C505514:CASP6 protein, human; D053148:Caspase 3; D053178:Caspase 6; D020169:Caspases