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.