THE EFFECTS OF MULTIPLE UV EXPOSURES ON HIV‐LTR EXPRESSION

Abstract— Previous studies have shown that cellular stress agents such as UV radiation induce transcription from the long terminal repeat (LTR) of the human immunodeficiency virus (HIV). Using HeLa cells stably transfected with the HIV‐LTR sequence, which transcriptionally drives the chloramphenicol acetyl transferase (CAT) reporter gene, we examined the effects of multiple exposures to UVC (254 nm) on HIV‐LTR‐CAT expression. Low doses (± 5 J m^‐2) had no effect on CAT expression, but up to 29‐fold induction was observed with 10 J m^‐2 when cells were harvested 48 h after completion of the exposure. Little difference was noted in induction levels when cells were exposed to one 25 J m^‐2 dose, viable cells were harvested at 24 h, 48 h or 72 h, and cell lysates were assayed for CAT expression. Two sequential 12.5 J m^‐2 exposures, given 24 h apart, resulted in an additive effect on CAT expression; these two exposures produced CAT activity equivalent to that induced following a single 25 J m^‐2 dose. This additive effect was not evident at the lower doses (≤5 J m^‐2) or at the higher doses. Maximal induction was observed using doses from 25 to 37.5 J m^‐2. Multiple exposures with either the low (≤5 J m ^‐2) or high doses (>25 J m ^‐2) did not result in an additive effect. Our data suggest that HIV‐LTR requires a specific threshold UV dose in order to elicit induction; a maximal induction dose is also evident; exposures higher than this maximal dose contribute no more to HIV‐LTR induction in viable cells.