PUVA inhibits DNA replication, but not gene transcription at nonlethal dosages

The combination of psoralens and UVA radiation (PUVA photochemotherapy) is an established treatment for many skin disorders. UVA-induced psoralen-DNA interactions are assumed to contribute to the cutaneous anti-inflammatory and anti-proliferative effects of PUVA. PUVA-induced DNA modifications might interfere not only with DNA replication, but also with gene transcription of proinflammatory genes. We therefore studied the effect of PUVA on cell proliferation and on the transcription of the c-jun and intercellular adhesion molecule-1 genes in a promyelocytic (HL60) and a keratinocyte (HaCaT) cell line. PUVA inhibited cell proliferation increasingly with increasing 8-methoxypsoralen concentrations or UVA doses. The inhibition was observed at conditions not affecting cell viability up to 48 h after PUVA. In contrast, PUVA did not inhibit gene transcription at anti-proliferative, yet nonlethal conditions. Baseline and phorbol-ester induced c-jun mRNA expression was not inhibited, nor was baseline and IFN-gamma or phorbol-ester induced intercellular adhesion molecule-1 mRNA expression. In order to assess possible transcriptional effects of PUVA-generated reactive oxygen intermediates, the reactive oxygen intermediates-sensitive transcription factor nuclear factor kappaB was assayed in mobility shift experiments. Nuclear factor kappaB-specific binding activity was not induced 1-24 h after PUVA in extracts from PUVA-treated cells when compared with controls, whereas the pro-oxidant cytokine TNF-alpha caused a marked increase in nuclear factor kappaB binding. The presented data suggest that PUVA inhibits cell proliferation, but not transcription, at nonlethal PUVA conditions. Furthermore, the data do not support a major role for PUVA-generated reactive oxygen intermediates in the regulation of gene transcription.