Cellular factors may contribute to the decreased efficacy of chemotherapy in HIV infection. Indeed, prolonged treatment with nucleoside analogues, such as azidothymidine (AZT), 2′,3′-deoxycytidine or 9-(2-phosphonylmethoxyethyl)adenine, induces cellular resistance. We have developed a human T lymphoblastoid cell line (CEM3TC) that is selectively resistant to the antiproliferative effect of 2′,3′-dideoxy-3′-thiacytidine (3TC) because the CEM3TC cells were equally sensitive to AZT, as well as the antimitotic agent, vinblastine. The anti-retroviral activity of 3TC against HIV-1 was also severely impaired in the CEM3TC cells. Despite similar deoxycytidine kinase activity and unchanged uptake of nucleosides such as AZT and 2′-deoxycytidine, CEM3TC had profoundly impaired 3TC accumulation. Further studies indicated that CEM3TC retained much less 3TC. However, despite a small overexpression of multidrug resistance protein (MRP) 4, additional studies with cells specifically engineered to overexpress MRP4 demonstrated there was no impact on either 3TC accumulation or efflux. Finally, an increased expression of the MRP5 homologue, ATP-binding cassette C11 (ABCC11) was observed in the CEM3TC cells. We speculate that the decreased 3TC accumulation in the CEM3TC might be due to the upregulation of ABCC11.
Abbreviations used: ABC, ATP-binding cassette; AZT, 3′-azido-3′-deoxythymidine; CNT, concentrative nucleoside carrier; dCK, deoxcytidine kinase; dCyd, deoxycytidine; ddC, 2′-3′-dideoxycytidine; ENT, equilibrative nucleoside carrier; GFP, green fluorescence protein; HAART, highly active anti-retroviral therapy; [3H]Cyd, [5-3H]cytidine; ID50, 50% inhibitory dose; MDR, multidrug resistance; [3H]AZT, [Me-3H]AZT; MRP, multidrug resistant protein; MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide; NRTI, nucleoside reverse transcriptase inhibitor; Pgp, P-glycoprotein; PMEA, 9-(2-phosphonylmethoxyethyl)adenine; RT, reverse transcriptiase; 3TC, 2′-3′-dideoxy-3′-thiacytidine (also called lamivudine); [3H]3TC, [Me-3H]3TC.