Selective Inhibition of Receptor Pathways for Macrophage-Specific Cytokines by HIV-1 Nef Protein.

HIV-1 Nef protein is a well-known major determinant of the pathogenicity of the virus and much attention has been given to Nef to explain its contribution to HIV-1 pathogenesis. One of the important features of Nef is the binding to a variety of host intracellular proteins. Hck, a member of Src family tyrosine kinases, is one of the binding partners of Nef. Meanwhile, Src kinases including Hck are key regulators for signaling pathways of several cytokine receptor systems. We therefore investigated whether Nef would affect cytokine receptor signaling pathways. Both GM-CSF and M-CSF are primary cytokines for monocytes/macrophages, and regulates their development and effector functions. We introduced a conditionally active Nef (Nef-estrogen receptor hormone binding-domain fusion protein) into the human leukemia cell line of which proliferation was dependent on those cytokines, and analyzed their ligand-dependent proliferation and receptor signaling events. Nef activation did not apparently affect the proliferation mediated by GM-CSF but markedly caused the inhibition of proliferation mediated by M-CSF, indicating that Nef selectively inhibited the M-CSF receptor pathways. In fact, Nef inhibited the tyrosine-phosphorylation/activation of M-CSF receptor and the molecular association with the activated receptor, which was the earliest signaling events induced by M-CSF. We found that Nef-activated Hck constitutively associated with M-CSF receptor complex. Since the formation of the molecular complex should occur under physiological conditions, i.e., upon M-CSF stimulation, the constitutive association occurred in Nef-active cells was an aberrant interaction. These results suggested that HIV-1 Nef interfered with M-CSF receptor signaling through Hck activation and thereby inhibited M-CSF bioactivity. The experiments in which Nef, Hck and M-CSF receptor were transiently expressed within 293 T cells in various combinations supported the notion (Suzu S et al., Blood 2005). The selective inhibition of Nef on cytokine signaling might result in the modification of macrophage functions such as cytokine/chemokine production, creating an immunologic environment favorable for HIV-1 virus production.