Paracrine activation of the HIV-1 LTR promoter by the viral Tat protein is mechanistically similar to trans-activation within a cell

The HIV-1 Tat protein activates transcription of the viral LTR promoter through interaction with the nuclear transcription machinery of the host cell. Tat can also activate the LTR promoter in a paracrine or inter-cellular manner by a yet unknown mechanism. One possibility is that Tat protein itself is secreted by cells and taken up by other cells. According to this mechanism, inter-cellular transcriptional activation by Tat should be very similar to intra-cellular trans-activation in Tat-producing cells. A large number of cytokine genes was recently reported to be Tat-responsive, raising the possibility that such cytokines and the corresponding cellular transduction pathways are involved in inter-cellular Tat action. The transcriptional events in such an indirect route are likely to differ from intra-cellular Tat action. To discriminate between a direct or indirect mechanism of inter-cellular Tat action, we compared the activity of a set of Tat mutants and different promoter constructs in inter-cellular and intra-cellular transcriptional activation. Identical results were obtained in both assays, suggesting that Tat protein itself is exported by one and transported into the nucleus of another cell. The demonstration that Tat antibodies specifically inhibit the inter-cellular route is also consistent with cell-to-cell transport of the Tat protein. Furthermore, we found that the second Tat coding exon, including the RGD motif that has been proposed to interact with an integrin receptor, is not required for cellular uptake of the Tat protein