The Human Immunodeficiency Virus Type 1 Accessory Protein Vpu Induces Apoptosis by Suppressing the Nuclear Factor κB–dependent Expression of Antiapoptotic Factors

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Abstract
Human immunodeficiency virus (HIV) type 1 Vpu is an integral membrane protein with a unique affinity for βTrCP (TrCP), a key member of the SkpI-Cullin-F-box E3 ubiquitin ligase complex that is involved in the regulated degradation of cellular proteins, including IκB. Remarkably, Vpu is resistant to TrCP-mediated degradation and competitively inhibits TrCP-dependent degradation of IκB, resulting in the suppression of nuclear factor (NF)-κB activity in Vpu-expressing cells. We now report that Vpu, through its interaction with TrCP, potently contributes to the induction of apoptosis in HIV-infected T cells. Vpu-induced apoptosis is specific and independent of other viral proteins. Mutation of a TrCP-binding motif in Vpu abolishes its apoptogenic property, demonstrating a close correlation between this property of Vpu and its ability to inhibit NF-κB activity. The involvement of NF-κB in Vpu-induced apoptosis is further supported by the finding that the levels of antiapoptotic factors Bcl-xL, A1/Bfl-1, and TNF receptor-associated factor (TRAF)1, all of which are expressed in an NF-κB–dependent manner, are reduced and, at the same time, levels of active caspase-3 are elevated. Thus, Vpu induces apoptosis through activation of the caspase pathway by way of inhibiting the NF-κB–dependent expression of antiapoptotic genes.