Stable activation of phosphatidylinositol 3-kinase in the T cell immunological synapse stimulates Akt signaling to FoxO1 nuclear exclusion and cell growth control.

We have previously reported at the single cell level that PI3K is activated after conjugate formation between T lymphocytes and APCs. However, in contrast to cells exposed to an asymmetrical signal that usually increase 3'-phosphoinositides (3'-PI) transiently in the region of the activated receptors, T cells contacting APC accumulate 3'-PI across their whole plasma membrane far beyond the region of the immunological synapse (IS). Importantly, this effect is maintained over time, for hours, and although PI3K-dependent pathways translate in various cell types extracellular stimuli into a wide range of biological events, in primary T cells this stability is mostly required for cell division induced by Ag. Using imaging methodologies, the present article elucidates the molecular mechanisms responsible for this particular functioning of the PI3K pathway in primary human T lymphocytes interacting with APCs, especially with dendritic cells. The results reveal that the IS unremittingly recruits PI3K to maintain high 3'-PI levels in T cells through phosphotyrosine-dependent mechanisms, suggesting a major participation of class Ia PI3K. This persistent activation of PI3K results in the Akt-dependent sequestration of the FoxO transcription factor, FoxO1, outside the nucleus of T cells interacting with APCs. Using an active form of FoxO1, we demonstrate that this compartmentalization process can affect T cell growth after Ag recognition. We conclude that the need for sustained PI3K signaling within the consolidated IS is probably an undemanding tactic used by primary T cells critical for initiating cell cycle progression through the prolonged inactivation of FoxO1, one important factor that can control cell quiescence.

HIV-1 Vpu sequesters beta-transducin repeat-containing protein (betaTrCP) in the cytoplasm and provokes the accumulation of beta-catenin and other SCFbetaTrCP substrates.

The human immunodeficiency virus type 1 Vpu protein acts as an adaptor for the proteasomal degradation of CD4 by recruiting CD4 and beta-transducin repeat-containing protein (betaTrCP), the receptor component of the multisubunit SCF-betaTrCP E3 ubiquitin ligase complex. We showed that the expression of a Vpu-green fluorescent fusion protein prevented the proteosomal degradation of betaTrCP substrates such as beta-catenin, IkappaBalpha, and ATF4, which are normally directly targeted to the proteasome for degradation. Beta-catenin was translocated into the nucleus, whereas the tumor necrosis factor-induced nuclear translocation of NFkappaB was impaired. Beta-catenin was also up-regulated in cells producing Vpu+ human immunodeficiency virus type 1 but not in cells producing Vpu-deficient viruses. The overexpression of ATF4 also provoked accumulation of beta-catenin, but to a lower level than that resulting from the expression of Vpu. Finally, the expression of Vpu induces the exclusion of betaTrCP from the nucleus. These data suggest that Vpu is a strong competitive inhibitor of betaTrCP that impairs the degradation of SCFbetaTrCP substrates as long as Vpu has an intact phosphorylation motif and can bind to betaTrCP.