CD4+ cells of most individuals infected with HIV-1 harbor a C-terminally truncated and constitutively activated form of signal transducer and activator of transcription-5 (STAM). We report that the chronically HIV-Infected U1 cell line expresses STAT5,1 but not full-length STAT5. Granulocyte-macrophage colonystimulating factor (GM-CSF) stimulation of U1 cells promoted early activation of STAT5A and of extracellular signal regulated kinases (ERKs), followed by later activation of activator protein 1 (AP-1) and HIV expression. Inhibition of ERK/ AP-1 by PD98,059 abolished, whereas either tyrphostin AG490 or a STAT5 small interfering RNA (siRNA) enhanced, virion production in GM-CSF-stimulated U1 cells. Chromatin immunoprecipitation demonstrated the induction of STAT5A binding to STAT consensus sequences in the HIV-1 promoter together with a decreased recruitment of RNA polymerase 11 after 1 hour of GMI-CSF stimulation of U1 cells. Down-regulation of STAT5A by siRNA resulted in the up-regulation of both HIV-1 gag-pol RNA and p24 Gag antigen expression in CD8-depleted leukocytes of several HIV-positive individuals cultivated ex vivo in the presence of interleukin-2 but not of interleukin-7. Thus, the constitutively activated STAT5A present in the leukocytes of most HIV-positive individuals acts as a negative regulator of HIV expression.
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