THE EFFECTS OF MULTIPLE UV EXPLOSURES ON HIV-LTR EXPRESSION

Previous studies have shown that cellular stress agents such as UV rad iation induce transcription from the long terminal repeat (LTR) of the human immunodeficiency virus (HIV). Using HeLa cells stably transfect ed with the HIV-LTR sequence, which transcriptionally drives the chlor amphenicol acetyl transferase (CAT) reporter gene, we examined the eff ects of multiple exposures to UVC (254 nm) on HIV-LTR-CAT expression. Low doses (less than or equal to 5 J m(-2)) had no effect on CAT expre ssion, but up to 29-fold induction was observed with 10 J m(-2) when c ells were harvested 48 h after completion of the exposure. Little diff erence 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 t hat induced following a single 25 J m(-2) dose. This additive effect w as not evident at the lower doses (less than or equal to 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 (less than or equal to 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 do se is also evident; exposures higher than this maximal dose contribute no more to HIV-LTR induction in viable cells.