Gary D. Bren

Activation of IκB Kinase β by Protein Kinase C Isoforms

ABSTRACT The atypical protein kinase C (PKC) isotypes (λ/ιPKC and ζPKC) have been shown to be critically involved in important cell functions such as proliferation and survival. Previous studies have demonstrated that the atypical PKCs are stimulated by tumor necrosis factor alpha (TNF-α) and are required for the activation of NF-κB by this cytokine through a mechanism that most probably involves the phosphorylation of IκB. The inability of these PKC isotypes to directly phosphorylate IκB led to the hypothesis that ζPKC may use a putative IκB kinase to functionally inactivate IκB. Recently several groups have molecularly characterized and cloned two IκB kinases (IKKα and IKKβ) which phosphorylate the residues in the IκB molecule that serve to target it for ubiquitination and degradation. In this study we have addressed the possibility that different PKCs may control NF-κB through the activation of the IKKs. We report here that αPKC as well as the atypical PKCs bind to the IKKs in vitro and in vivo. In addition, overexpression of ζPKC positively modulates IKKβ activity but not that of IKKα, whereas the transfection of a ζPKC dominant negative mutant severely impairs the activation of IKKβ but not IKKα in TNF-α-stimulated cells. We also show that cell stimulation with phorbol 12-myristate 13-acetate activates IKKβ, which is entirely dependent on the activity of αPKC but not that of the atypical isoforms. In contrast, the inhibition of αPKC does not affect the activation of IKKβ by TNF-α. Interestingly, recombinant active ζPKC and αPKC are able to stimulate in vitro the activity of IKKβ but not that of IKKα. In addition, evidence is presented here that recombinant ζPKC directly phosphorylates IKKβ in vitro, involving Ser177 and Ser181. Collectively, these results demonstrate a critical role for the PKC isoforms in the NF-κB pathway at the level of IKKβ activation and IκB degradation.

Transcription of the RelB gene is regulated by NF-κB

RelA and RelB are two members of the NF-κB family that differ structurally and functionally. While RelA is regulated through its cytosolic localization by inhibitor proteins or IκB and not through transcriptional mechanisms, the regulation of RelB is poorly understood. In this study we demonstrate that stimuli (TNF or LPS) lead within minutes to the nuclear translocation of RelA, but require hours to result in the nuclear translocation of RelB. The delayed nuclear translocation of RelB correlates with increases in its protein synthesis which are secondary to increases in RelB gene transcription. RelA is alone sufficient to induce RelB gene transcription and to mediate the stimuli-driven increase in RelB transcription. Cloning and characterization of the RelB 5′ untranslated gene region indicates that RelB transcription is dependent on a TATA-less promoter containing two NF-κB binding sites. One of the NF-κB sites is primarily involved in the binding of p50 while the other one in the binding and transactivation by RelA and also RelB. Lastly, it is observed that p21, a protein involved in cell cycle control and oncogenesis known to be regulated by NF-κB, is upregulated at the transcriptional level by RelB. Thus, RelB is regulated at least at the level of transcription in a RelA and RelB dependent manner and may exert an important role in p21 regulation.

Human Immunodeficiency Virus–Induced Apoptosis of Human Hepatocytes via CXCR4

Human immunodeficiency virus (HIV) infection is commonly associated with liver dysfunction. The X4 HIV glycoprotein 120 envelope (env) induces apoptosis in T cells and neurons via the HIV coreceptor CXCR4. Therefore, we investigated whether hepatocyte death could result from the HIV env signaling through CXCR4 on the hepatocyte. We demonstrated that hepatocytes in humans express CXCR4 on the cell surface. Furthermore, we established that the X4 HIV env and the entire HIV virion signal hepatocyte apoptosis through CXCR4. The apoptotic process is dependent on G(ialpha) protein signaling, yet it is independent of caspase cascade activation. Thus, HIV can directly cause hepatocyte death in humans by signaling through CXCR4, without infecting the cell.

Inhibition of adenine nucleotide translocator pore function and protection against apoptosis in vivo by an HIV protease inhibitor

Inhibitors of HIV protease have been shown to have antiapoptotic effects in vitro, yet whether these effects are seen in vivo remains controversial. In this study, we have evaluated the impact of the HIV protease inhibitor (PI) nelfinavir, boosted with ritonavir, in models of nonviral disease associated with excessive apoptosis. In mice with Fas-induced fatal hepatitis, Staphylococcal enterotoxin B-induced shock, and middle cerebral artery occlusion-induced stroke, we demonstrate that PIs significantly reduce apoptosis and improve histology, function, and/or behavioral recovery in each of these models. Further, we demonstrate that both in vitro and in vivo, PIs block apoptosis through the preservation of mitochondrial integrity and that in vitro PIs act to prevent pore function of the adenine nucleotide translocator (ANT) subunit of the mitochondrial permeability transition pore complex.

IκB Kinase-Dependent Chronic Activation of NF-κB Is Necessary for p21WAF1/Cip1 Inhibition of Differentiation-Induced Apoptosis of Monocytes

ABSTRACT The molecular mechanisms regulating monocyte differentiation to macrophages remain unknown. Although the transcription factor NF-κB participates in multiple cell functions, its role in cell differentiation is ill defined. Since differentiated macrophages, in contrast to cycling monocytes, contain significant levels of NF-κB in the nuclei, we questioned whether this transcription factor is involved in macrophage differentiation. Phorbol 12-myristate 13-acetate (PMA)-induced differentiation of the promonocytic cell line U937 leads to persistent NF-κB nuclear translocation. We demonstrate here that an increased and persistent IKK activity correlates with monocyte differentiation leading to persistent NF-κB activation secondary to increased IκBα degradation via the IκB signal response domain (SRD). Promonocytic cells stably overexpressing an IκBα transgene containing SRD mutations fail to activate NF-κB and subsequently fail to survive the PMA-induced macrophage differentiation program. The differentiation-induced apoptosis was found to be dependent on tumor necrosis factor alpha. The protective effect of NF-κB is mediated through p21WAF1/Cip1, since this protein was found to be regulated in an NF-κB-dependent manner and to confer survival features during macrophage differentiation. Therefore, NF-κB plays a key role in cell differentiation by conferring cell survival that in the case of macrophages is mediated through p21WAF1/Cip1.

Elimination of senescent neutrophils by TNF-related apopotosis-inducing ligand

Neutrophils are phagocytic effectors which are produced in the bone marrow and released into the circulation. Thereafter, they are either recruited to sites of inflammation or rapidly become senescent, return to the bone marrow, and undergo apoptosis. Stromal cell-derived factor 1 (SDF-1) coordinates the return of senescent neutrophils to the bone marrow by interacting with CXCR4 that is preferentially expressed on senescent neutrophils. We demonstrate that CXCR4 ligation by SDF-1 or other CXCR4 agonists significantly increases the expression of both TNF-related apoptosis-inducing ligand (TRAIL) and of the death-inducing TRAIL receptors on neutrophils, which confers an acquired sensitivity to TRAIL-mediated death and results in TRAIL-dependent apoptosis. In vivo administration of TRAIL antagonists results in neutrophilic accumulation within the bone marrow and a reduction in neutrophil apoptosis; conversely recombinant TRAIL administration reduced neutrophil number within bone marrow. Thus, SDF-1 ligation of CXCR4 causes the parallel processes of chemotaxis and enhanced TRAIL and TRAIL death receptor expression, resulting in apoptosis of senescent neutrophils upon their return to the bone marrow.

Human Immunodeficiency Virus Type 1 Protease Cleaves Procaspase 8 In Vivo

ABSTRACT Human immunodeficiency virus type 1 (HIV-1) infection causes apoptosis of infected CD4 T cells as well as uninfected (bystander) CD4 and CD8 T cells. It remains unknown what signals cause infected cells to die. We demonstrate that HIV-1 protease specifically cleaves procaspase 8 to create a novel fragment termed casp8p41, which independently induces apoptosis. casp8p41 is specific to HIV-1 protease-induced death but not other caspase 8-dependent death stimuli. In HIV-1-infected patients, casp8p41 is detected only in CD4+ T cells, predominantly in the CD27+ memory subset, its presence increases with increasing viral load, and it colocalizes with both infected and apoptotic cells. These data indicate that casp8p41 independently induces apoptosis and is a specific product of HIV-1 protease which may contribute to death of HIV-1-infected cells.

HIV induces TRAIL sensitivity in hepatocytes.

Background HIV infected patients have an increased susceptibility to liver disease due to Hepatitis B Virus (HBV), Hepatitis C Virus (HCV), alcoholic, and non-alcoholic steatohepatitis. Clinically, this results in limited options for antiretroviral therapy and accelerated rates of liver disease, causing liver disease to be the second leading cause of death for HIV infected patients. The mechanisms causing this propensity for liver dysfunction during HIV remains unknown. Methodology/Principal Findings We demonstrate that HIV and/or the HIV glycoprotein gp120 ligation of CXCR4 on hepatocytes selectively up-regulates TRAIL R2 expression and confers an acquired sensitivity to TRAIL mediated apoptosis which is mediated by JNK II, but not p38 nor G-proteins. Conclusions/Significance These findings suggest that HIV infection renders hepatocytes more susceptible to liver injury during disease states associated with enhanced TRAIL production such as HBV, HCV, or steatohepatitis.

Human Immunodeficiency Virus Reactivation by Phorbol Esters or T-Cell Receptor Ligation Requires both PKCα and PKCθ

ABSTRACT Latently human immunodeficiency virus (HIV)-infected memory CD4+ T cells represent the major obstacle to eradicating HIV from infected patients. Antigens, T-cell receptor (TCR) ligation, and phorbol esters can reactivate HIV from latency in a protein kinase C (PKC)-dependent manner; however, it is unknown which specific PKC isoforms are required for this effect. We demonstrate that constitutively active (CA) forms of both PKCθ, PKCθA148E, and PKCα, PKCαA25E, induce HIV long terminal repeat (LTR)-dependent transcription in Jurkat and primary human CD4+ T cells and that both PKCθA148E and PKCαA25E cause HIV reactivation in J1.1 T cells. Suppression of both PKCα and PKCθ with short hairpinned (sh) RNA inhibited CD3/CD28-induced HIV LTR-dependent transcription and HIV reactivation in J1.1 T cells. Both prostratin and phorbol myristate 13-acetate induced HIV LTR-dependent transcription and HIV reactivation in J1.1 T cells that was blocked by shRNA against either PKCα or PKCθ. Since suppression of PKCα and PKCθ together has no greater inhibitory effect on HIV reactivation than inhibition of PKCα alone, our data confirm that PKCα and PKCθ act in sequence. The requirement for PKCα and PKCθ for prostratin-induced HIV reactivation and the ability of selective PKCα or PKCθ agonists to induce HIV transcription indicate that these PKC isoforms are important targets for therapeutic drug design.

Glycoprotein 120 Binding to CXCR4 Causes p38-Dependent Primary T Cell Death That Is Facilitated by, but Does Not Require Cell-Associated CD4

HIV-1 infection causes the depletion of host CD4 T cells through direct and indirect (bystander) mechanisms. Although HIV Env has been implicated in apoptosis of uninfected CD4 T cells via gp120 binding to either CD4 and/or the chemokine receptor 4 (CXCR4), conflicting data exist concerning the molecular mechanisms involved. Using primary human CD4 T cells, we demonstrate that gp120 binding to CD4 T cells activates proapoptotic p38, but does not activate antiapoptotic Akt. Because ligation of the CD4 receptor alone or the CXCR4 receptor alone causes p38 activation and apoptosis, we used the soluble inhibitors, soluble CD4 (sCD4) or AMD3100, to delineate the role of CD4 and CXCR4 receptors, respectively, in gp120-induced p38 activation and death. sCD4 alone augments gp120-induced death, suggesting that CXCR4 signaling is principally responsible. Supporting that model, AMD3100 reduces death caused by gp120 or by gp120/sCD4. Finally, prevention of gp120-CXCR4 interaction with 12G5 Abs blocks p38 activation and apoptosis, whereas inhibition of CD4-gp120 interaction with Leu-3a has no effect. Consequently, we conclude that gp120 interaction with CXCR4 is required for gp120 apoptotic effects in primary human T cells.