Limits...
The human immunodeficiency virus type 1 accessory protein Vpu induces apoptosis by suppressing the nuclear factor kappaB-dependent expression of antiapoptotic factors.

Akari H, Bour S, Kao S, Adachi A, Strebel K - J. Exp. Med. (2001)

Bottom Line: 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-kappaB activity.The involvement of NF-kappaB 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-kappaB-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-kappaB-dependent expression of antiapoptotic genes.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Molecular Microbiology, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892, USA.

ABSTRACT
Human immunodeficiency virus (HIV) type 1 Vpu is an integral membrane protein with a unique affinity for betaTrCP (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 IkappaB. Remarkably, Vpu is resistant to TrCP-mediated degradation and competitively inhibits TrCP-dependent degradation of IkappaB, resulting in the suppression of nuclear factor (NF)-kappaB 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-kappaB activity. The involvement of NF-kappaB 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-kappaB-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-kappaB-dependent expression of antiapoptotic genes.

Show MeSH

Related in: MedlinePlus

Model for Vpu-induced apoptosis through activation of the caspase pathway. Details of the model are explained in the Discussion. Broken arrows symbolize inhibitory effects. Steps inhibited by Vpu are marked in red.
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC2195969&req=5

fig7: Model for Vpu-induced apoptosis through activation of the caspase pathway. Details of the model are explained in the Discussion. Broken arrows symbolize inhibitory effects. Steps inhibited by Vpu are marked in red.

Mentions: Both Vpr- and Vpu-induced apoptosis involve the activation of the caspase pathway (references 48 and 49, and this study). Although the precise mechanism for Vpr-induced apoptosis is still unclear, recent observations suggest that it might be caused by a Vpr-induced permeabilization of mitochondrial membranes resulting in the release of apoptogenic proteins such as cytochrome c or apoptosis inducing factor and the subsequent activation of caspase (50). While it was suggested that Vpu itself might have poreforming properties (51, 52) making a mechanism for induction of apoptosis similar to that of Vpr conceivable, our data suggest that Vpu instead functions by inhibiting the NF-κB–dependent expression of antiapoptotic genes. This is supported by the observation that mutation of the TrCP-binding motif (Ser52, 56Asn), which in fact stabilized the pore-forming property of Vpu (52), abolished its apoptogenic potential (Table I). Based on the available experimental evidence, we therefore propose the following model for Vpu-induced apoptosis (Fig. 7) : in unstimulated cells, NF-κB resides in the cytoplasm in an inactive complex with its inhibitor IκB (15). Upon stimulation of cells by cytokines such as TNF-α (Fig. 7 no. 1), IκB is rapidly phosphorylated by an IκB-specific kinase (Fig. 7 no. 2), which results in the rapid degradation of IκB via a TrCP-dependent pathway (Fig. 7 no. 3). Infection of cells by HIV-1 results in the gradual intracellular accumulation of Vpu. Because of its constitutively active TrCP-binding motif and the fact that it is not sensitive to TrCP-mediated proteolysis, Vpu functions as a competitive inhibitor of TrCP. This results in the gradual accumulation of IκB and the progressive impairment of the cell's ability to activate NF-κB (Fig. 7 no. 4). The inhibition of NF-κB blocks the synthesis of antiapoptotic proteins such as the Bcl-2 family proteins (e.g., Bcl-xL and A1/Bfl-1) or TNFR complex proteins (e.g., TRAF1; Fig. 7 no. 5). TRAF1 is induced by TNF-α treatment and normally inhibits activation of caspase-8 (Fig. 7 no. 6). In Vpu-expressing cells, the levels of TRAF1, in response to TNF stimulation, are reduced and no longer sufficient to inhibit the cytokine-induced activation of caspase-8 (Fig. 7 no. 6). Activated caspase-8 in turn induces the release of cytochrome c from the mitochondria (Fig. 7 no. 7). Release of cytochrome c is normally inhibited by the Bcl-2 family of proteins. However, in Vpu-expressing cells the levels of Bcl-2 proteins are limiting and no longer sufficient to block cytochrome c release (Fig. 7 no. 8). After its release from the mitochondria, cytochrome c forms ternary complexes with Apaf-1 and caspase-9 (Fig. 7 no. 9), resulting in the activation of caspase-3 (Fig. 7 no. 10). Active caspase-3 finally triggers a reaction that results in the cleavage of a number of target proteins including Bcl-2 family proteins (Fig. 7 no. 11) and leads to cell death (Fig. 7 no. 12).


The human immunodeficiency virus type 1 accessory protein Vpu induces apoptosis by suppressing the nuclear factor kappaB-dependent expression of antiapoptotic factors.

Akari H, Bour S, Kao S, Adachi A, Strebel K - J. Exp. Med. (2001)

Model for Vpu-induced apoptosis through activation of the caspase pathway. Details of the model are explained in the Discussion. Broken arrows symbolize inhibitory effects. Steps inhibited by Vpu are marked in red.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2195969&req=5

fig7: Model for Vpu-induced apoptosis through activation of the caspase pathway. Details of the model are explained in the Discussion. Broken arrows symbolize inhibitory effects. Steps inhibited by Vpu are marked in red.
Mentions: Both Vpr- and Vpu-induced apoptosis involve the activation of the caspase pathway (references 48 and 49, and this study). Although the precise mechanism for Vpr-induced apoptosis is still unclear, recent observations suggest that it might be caused by a Vpr-induced permeabilization of mitochondrial membranes resulting in the release of apoptogenic proteins such as cytochrome c or apoptosis inducing factor and the subsequent activation of caspase (50). While it was suggested that Vpu itself might have poreforming properties (51, 52) making a mechanism for induction of apoptosis similar to that of Vpr conceivable, our data suggest that Vpu instead functions by inhibiting the NF-κB–dependent expression of antiapoptotic genes. This is supported by the observation that mutation of the TrCP-binding motif (Ser52, 56Asn), which in fact stabilized the pore-forming property of Vpu (52), abolished its apoptogenic potential (Table I). Based on the available experimental evidence, we therefore propose the following model for Vpu-induced apoptosis (Fig. 7) : in unstimulated cells, NF-κB resides in the cytoplasm in an inactive complex with its inhibitor IκB (15). Upon stimulation of cells by cytokines such as TNF-α (Fig. 7 no. 1), IκB is rapidly phosphorylated by an IκB-specific kinase (Fig. 7 no. 2), which results in the rapid degradation of IκB via a TrCP-dependent pathway (Fig. 7 no. 3). Infection of cells by HIV-1 results in the gradual intracellular accumulation of Vpu. Because of its constitutively active TrCP-binding motif and the fact that it is not sensitive to TrCP-mediated proteolysis, Vpu functions as a competitive inhibitor of TrCP. This results in the gradual accumulation of IκB and the progressive impairment of the cell's ability to activate NF-κB (Fig. 7 no. 4). The inhibition of NF-κB blocks the synthesis of antiapoptotic proteins such as the Bcl-2 family proteins (e.g., Bcl-xL and A1/Bfl-1) or TNFR complex proteins (e.g., TRAF1; Fig. 7 no. 5). TRAF1 is induced by TNF-α treatment and normally inhibits activation of caspase-8 (Fig. 7 no. 6). In Vpu-expressing cells, the levels of TRAF1, in response to TNF stimulation, are reduced and no longer sufficient to inhibit the cytokine-induced activation of caspase-8 (Fig. 7 no. 6). Activated caspase-8 in turn induces the release of cytochrome c from the mitochondria (Fig. 7 no. 7). Release of cytochrome c is normally inhibited by the Bcl-2 family of proteins. However, in Vpu-expressing cells the levels of Bcl-2 proteins are limiting and no longer sufficient to block cytochrome c release (Fig. 7 no. 8). After its release from the mitochondria, cytochrome c forms ternary complexes with Apaf-1 and caspase-9 (Fig. 7 no. 9), resulting in the activation of caspase-3 (Fig. 7 no. 10). Active caspase-3 finally triggers a reaction that results in the cleavage of a number of target proteins including Bcl-2 family proteins (Fig. 7 no. 11) and leads to cell death (Fig. 7 no. 12).

Bottom Line: 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-kappaB activity.The involvement of NF-kappaB 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-kappaB-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-kappaB-dependent expression of antiapoptotic genes.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Molecular Microbiology, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892, USA.

ABSTRACT
Human immunodeficiency virus (HIV) type 1 Vpu is an integral membrane protein with a unique affinity for betaTrCP (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 IkappaB. Remarkably, Vpu is resistant to TrCP-mediated degradation and competitively inhibits TrCP-dependent degradation of IkappaB, resulting in the suppression of nuclear factor (NF)-kappaB 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-kappaB activity. The involvement of NF-kappaB 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-kappaB-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-kappaB-dependent expression of antiapoptotic genes.

Show MeSH
Related in: MedlinePlus