GENOTOXICITY AND MITOCHONDRIAL DAMAGE IN HUMAN LYMPHOCYTIC CELLS CHRONICALLY EXPOSED TO 3'-AZIDO-2',3'-DIDEOXYTHYMIDINE

AZT (3'-azido-2',3'-dideoxythymidine), the first nucleoside analog app roved for the treatment of AIDS (acquired immunodeficiency syndrome), induces significant toxic effects in humans exposed to therapeutic dos es. As an inhibitor of the HIV-1 (human immunodeficiency virus 1) reve rse transcriptase, AZT blocks the incorporation of nucleotides into th e host's newly synthesized DNA. Incorporation of AZT into mammalian DN A as well as specific localization of the drug into telomeric DNA, has been previously documented by immunohistochemistry. As with other nuc leoside analogs, AZT has affinity for polymerase-gamma, the enzyme res ponsible for the replication of mitochondrial DNA. In order to examine the mechanisms of toxic events induced by long-term AZT exposure, hum an T-lymphocytic H-9 cells were cultured with 25 mu M AZT for 7 months . In the resulting H9-AZT cells, incorporation of AZT into DNA was dem onstrated by radioimmunoassay and immunohistochemistry, chromosomal ab errations and micronuclei were scored and intracellular lipid distribu tion was determined. Two pmol of AZT per microgram of DNA were detecte d by radioimmunoassay in H9-AZT cells. Control cells showed negative v alues in the radioimmunoassay. Cytogenetic observations on H9-AZT cell s showed an increase in chromosomal aberrations and nuclear fragmentat ion when compared with unexposed H-9 cells. Electron microscopy reveal ed mitochondrial damage and an elevated accumulation of neutral intrac ellular lipid deposits probably as a consequence of a distortion in th e beta-oxidation of fatty acids normally carried out by this organelle . The toxicities explored here suggest that the mechanisms of AZT indu ced cytotoxicity in bone marrow of the patients chronically exposed to the drug in vivo may involve both chromosomal and mitochondrial DNA d amage.