U937 promonocytic cells, either treated or untreated with phorbol-esters, were used for transient expression assays. We analyzed a series of visna LTR plasmids containing either the AP-1 or the AP-4 or both target responsive sequences for visna Tat transactivation. A 5′ deletion mutant of the LTR containing a truncated AP-4 target sequence lost the Tat-mediated transactivation, while phorbol ester-mediated transactivation was not affected. Furthermore, the absence of this AP-4 sequence dramatically decreased the additive effect observed when U937 cells were both treated by phorbol ester and expressed the tat gene product, suggesting a high interdependence of the AP-1 and AP-4 sequences for the regulation of the transcription driven by the visna LTR. The c-Jun/AP-1 factor was a prerequisite for the modulation of the activity of the LTR since no Tat-mediated transactivation was found when transfection experiments were carried out in F9 teratocarcinoma cells which are deficient for AP-1 activity. Because the Tat product enhanced the transcription of the visna LTR via the AP-1 site, we asked whether this viral factor could regulate the expression of cellular factors involved in one of the cellular activation pathways. Northern analysis of U937 cells clearly indicated that visna Tat promoted the c-jun mRNA expression, in contrast to the c-fos mRNA expression. Next, we examined nuclear extracts prepared at various times after infection of permissive ovine cells with visna virus, and showed an increased level in the c-Jun DNA binding activity. These data indicated that viral infection can induce a cellular activation pathway in permissive cells.
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