Long-term AZT Exposure Alters the Metabolic Capacity of Cultured Human Lymphoblastoid Cells

The antiretroviral efficacy of 3′-azido-3′-deoxythymidine (AZT) is dependent upon intracellular mono-, di-, and triphosphorylation and incorporation into DNA in place of thymidine. Thymidine kinase 1 (TK-1) catalyzes the first step of this pathway. MOLT-3, human lymphoblastoid cells, were exposed to AZT continuously for 14 passages (P(1)–P(14)) and cultured for an additional 14 passages (P(15)–P(28)) without AZT. Progressive and irreversible depletion of the enzymatically active form of the TK-1 24-kDa monomer with loss of active protein was demonstrated during P(1)–P(5) of AZT exposure. From P(15) to P(28), both the 24- and the 48-kDa forms of TK-1 were undetectable and a tetrameric 96-kDa form was present. AZT-DNA incorporation was observed with values of 150, 133, and 108 molecules of AZT/10(6) nucleotides at the 10μM plasma-equivalent AZT dose at P(1), P(5), and P(14), respectively. An exposure-related increase in the frequency of micronuclei (MN) was observed in cells exposed to either 10 or 800μM AZT during P(1)–P(14). Analysis of the cell cycle profile revealed an accumulation of S-phase cells and a decrease in G(1)-phase cells during exposure to 800μM AZT for 14 passages. When MOLT-3 cells were grown in AZT-free media (P(15)–P(29)), there was a reduction in AZT-DNA incorporation and MN formation; however, TK-1 depletion and the persistence of S-phase delay were unchanged. These data suggest that in addition to known mutagenic mechanisms, cells may become resistant to AZT partially through inactivation of TK-1 and through modulation of cell cycle components.