Alteration of thymidine kinase activity in cells treated with an antiviral agent

A lymphoblastoid cell line, CEM, was rendered resistant to zidovudine (AZT) in vitro by exposure to low but gradually increasing concentrations of the drug. This type of cellular resistance seems to be due to a defect of thymidine kinase (TK) activity that is acquired by cells grown in the presence of AZT: In fact, enzymatic studies with extracts from AZT-resistant cells (CEMazt), have shown that the value of the maximum velocity (Vmax) of TK activity measured with AZT and for deoxythymidine (dThd) is decreased as compared to sensitive CEM cells. Further-move, the enzyme affinity for AZT and dThd is reduced in CEMazt. Further experiments have shown that such cells do not show resistance to other nucleoside analogs, such as ddI, ddC: AraT and D4T, suggesting that the phosphorylation pathways different fr om those involving TK are unaltered. Ex vivo experiments performed by using peripheral blood mononuclear cells (PBMC) from HN infected individuals revealed that a prolonged treatment with AZT may modify the affinity of TK for dThd thus suggesting that the aforementioned phenomenon may occur also in vivo

Impaired 2’-3’-dideoxy-3’-thiacytidine accumulation in CEM lymphoblastoid cells as a mechanism of acquired resistance independent of MRP4 with a possible role for ABCC11.

We have developed a human T lymphoblastoid cell line (CEM3TC) that is selectively resistant to the anti-proliferative 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

Impaired 2',3'-dideoxy-3'-thiacytidine accumulation in T-lymphoblastoid cells as a mechanism of acquired resistance independent of multidrug resistant protein 4 with a possible role for ATP-binding cassette C11.

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 (CEM 3TC) that is selectively resistant to the antiproliferative effect of 2',3'-dideoxy-3'-thiacytidine (3TC) because the CEM 3TC 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 CEM 3TC cells. Despite similar deoxycytidine kinase activity and unchanged uptake of nucleosides such as AZT and 2'-deoxycytidine, CEM 3TC had profoundly impaired 3TC accumulation. Further studies indicated that CEM 3TC 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 CEM 3TC cells. We speculate that the decreased 3TC accumulation in the CEM 3TC might be due to the upregulation of ABCC11