Human immunodeficiency virus-1 (HIV-1) impairs
tumor necrosis factor-alpha (
TNF-alpha)-mediated macrophage apoptosis induced by Mycobacterium tuberculosis (Mtb).
HIV Nef protein plays an important role in the pathogenesis of
AIDS. We have tested the hypothesis that exogenous Nef is
a factor that inhibits
TNF-alpha production/apoptosis in macrophages infected with Mtb. We demonstrate that Mtb and Nef individually trigger
TNF-alpha production in macrophages. However,
TNF-alpha production is dampened when the two are present simultaneously, probably through cross-regulation of the individual signaling pathways leading to activation of the
TNF-alpha promoter. Mtb-induced
TNF-alpha production is abrogated upon mutation of the Ets, Egr, Sp1, CRE, or AP1 binding sites on the
TNF-alpha promoter, whereas Nef-mediated promoter activation depends only on the CRE and AP1 binding sites, pointing to differences in the mechanisms of activation of the promoter. Mtb-dependent promoter activation depends on the
mitogen-activated
kinase (MAPK)
kinase kinase ASK1 and on
MEK/ERK signaling. Nef inhibits ASK1/
p38 MAPK-dependent Mtb-induced
TNF-alpha production probably by inhibiting binding of ATF2 to the
TNF-alpha promoter. It also inhibits
MEK/ERK-dependent Mtb-induced binding of FosB to the promoter. Nef-driven
TNF-alpha production occurs in an ASK1-independent, Rac1/PAK1/
p38 MAPK-dependent, and
MEK/ERK-independent manner. The signaling pathways used by Mtb and Nef to trigger
TNF-alpha production are therefore distinctly different. In addition to attenuating Mtb-dependent
TNF-alpha promoter activation, Nef also reduces Mtb-dependent
TNF-alpha mRNA stability probably through its ability to inhibit ASK1/
p38 MAPK signaling. These results provide new insight into how HIV Nef probably exacerbates
tuberculosis infection by virtue of its ability to dampen Mtb-induced
TNF-alpha production.