FIG 7.

USP18 enhances HIV-1 replication by upregulating intracellular dNTP concentrations. (A) The intracellular levels of dATP, dCTP, dGTP, and dTTP were quantified by a single nucleotide primer extension assay in differentiated THP-1.Control, THP-1.USP18, SAMHD1KO_CTL, and SAMHD1KO_USP18 cells. Mean differences of three replicates for each dNTP level in a single experiment were analyzed and compared between THP-1.Control and THP-1.USP18 (A) cells, as well as between SAMHD1KO_CTL and SAMHD1KO_USP18 (B) cells, by a Student t test, and are expressed as means ± the SD. A P value of <0.05 was considered statistically significant (*). The higher the number of asterisks, the lower the P value. Each panel is representative of at least three independent experiments. (C) PMA-differentiated THP-1 cells were treated with 2.5 mM dNs and transduced with HIV-1 luciferase reporter virus (HIV-1luc) from the three-plasmid system (see Materials and Methods) and from the NL-LucE⁻R⁻ construct. At 48 h postinfection, the intracellular luciferase activity was measured. The values (i.e., for the luciferase activity) obtained in cells untreated with dNs were set as 100%, and the viral gene expression from cells treated with dNs was calculated relative to the untreated cells. The data are representative of two independent experiments.