Extracellular human immunodeficiency virus type-1 Tat protein activates phosphatidylinositol 3-kinase in PC12 neuronal cells

Effect of HIV-1 Tat on the formation of the mitotic spindle by interaction with ribosomal protein S3

Human immunodeficiency virus type 1 (HIV-1) Tat, an important regulator of viral transcription, interacts with diverse cellular proteins and promotes or inhibits cell proliferation. Here, we show that ribosomal protein S3 (RPS3) plays an important role in mitosis through an interaction with α-tubulin and that Tat binds to and inhibits the localization of RPS3 in the mitotic spindle during mitosis. RPS3 colocalized with α-tubulin around chromosomes in the mitotic spindle. Depletion of RPS3 promoted α-tubulin assembly, while overexpression of RPS3 impaired α-tubulin assembly. Depletion of RPS3 resulted in aberrant mitotic spindle formation, segregation failure, and defective abscission. Moreover, ectopic expression of RPS3 rescued the cell proliferation defect in RPS3-knockdown cells. HIV-1 Tat interacted with RPS3 through its basic domain and increased the level of RPS3 in the nucleus. Expression of Tat caused defects in mitotic spindle formation and chromosome assembly in mitosis. Moreover, the localization of RPS3 in the mitotic spindle was disrupted when HIV-1 Tat was expressed in HeLa and Jurkat cells. These results suggest that Tat inhibits cell proliferation via an interaction with RPS3 and thereby disrupts mitotic spindle formation during HIV-1 infection. These results might provide insight into the mechanism underlying lymphocyte pathogenesis during HIV-1 infection.

The cross-talk of HIV-1 Tat and methamphetamine in HIV-associated neurocognitive disorders

Antiretroviral therapy has dramatically improved the lives of human immunodeficiency virus 1 (HIV-1) infected individuals. Nonetheless, HIV-associated neurocognitive disorders (HAND), which range from undetectable neurocognitive impairments to severe dementia, still affect approximately 50% of the infected population, hampering their quality of life. The persistence of HAND is promoted by several factors, including longer life expectancies, the residual levels of virus in the central nervous system (CNS) and the continued presence of HIV-1 regulatory proteins such as the transactivator of transcription (Tat) in the brain. Tat is a secreted viral protein that crosses the blood–brain barrier into the CNS, where it has the ability to directly act on neurons and non-neuronal cells alike. These actions result in the release of soluble factors involved in inflammation, oxidative stress and excitotoxicity, ultimately resulting in neuronal damage. The percentage of methamphetamine (MA) abusers is high among the HIV-1-positive population compared to the general population. On the other hand, MA abuse is correlated with increased viral replication, enhanced Tat-mediated neurotoxicity and neurocognitive impairments. Although several strategies have been investigated to reduce HAND and MA use, no clinically approved treatment is currently available. Here, we review the latest findings of the effects of Tat and MA in HAND and discuss a few promising potential therapeutic developments.

Upregulation of SOCS-1 by Nutlin-3 in acute myeloid leukemia cells but not in primary normal cells

OBJECTIVE

It has been shown that SOCS-1 plays an important role in the proper control of cytokine/growth factor responses and acts as a tumor suppressor in acute myeloid leukemias. Therefore, the objective of the present study was to evaluate the in vitro effect of treatment with Nutlin-3, a small molecule inhibitor of the MDM2/p53 interaction, on the expression of the suppressor of cytokine signaling 1 in primary acute myeloid leukemia cells and in myeloid cell lines with differential p53 status.

METHOD

The expression of the suppressor of cytokine signaling 1 was quantitatively analyzed by real-time PCR in myeloid p53wild-type (OCI and MOLM) and p53null HL-60, leukemic cell lines, in patient-derived acute myeloid leukemia blasts, and in primary normal cell types, such as macrophages, endothelial cells, and bone marrow mesenchymal stem cells. The p53-dependence of the suppressor of cytokine signaling 1 upregulation that is induced by Nutlin-3 was analyzed in experiments performed using siRNA for p53, while the functional upregulation of the suppressor of cytokine signaling 1 was analyzed by assessing the levels of phosphorylated STAT-3.

RESULTS

Nutlin-3 significantly upregulated the transcription of the suppressor of cytokine signaling 1 in p53wild-type OCI and MOLM but not in p53deleted p53null HL60, myeloid leukemic cell lines, as well as in primary acute myeloid leukemia blasts. Conversely, and somewhat unexpectedly, Nutlin-3 did not modulate the suppressor of cytokine signaling 1 expression in primary normal macrophages, endothelial cells, and bone marrow mesenchymal stem cells. The p53-dependent upregulation of the suppressor of cytokine signaling 1 by Nutlin-3 was associated with the downregulation of phosphorylated STAT-3, a major molecular target of the suppressor of cytokine signaling 1.

CONCLUSION

Overall, our data suggest a potential role for the suppressor of cytokine signaling 1 as a therapeutic target of Nutlin-3 in p53 wild-type acute myeloid leukemias.

Inhibitory effect of natural anti-inflammatory compounds on cytokines released by chronic venous disease patient-derived endothelial cells

Large vein endothelium plays important roles in clinical diseases such as chronic venous disease (CVD) and thrombosis; thus to characterize CVD vein endothelial cells (VEC) has a strategic role in identifying specific therapeutic targets. On these bases we evaluated the effect of the natural anti-inflammatory compounds α-Lipoic acid and Ginkgoselect phytosome on cytokines/chemokines released by CVD patient-derived VEC. For this purpose, we characterized the levels of a panel of cytokines/chemokines (n = 31) in CVD patients' plasma compared to healthy controls and their release by VEC purified from the same patients, in unstimulated and TNF-α stimulated conditions. Among the cytokines/chemokines released by VEC, which recapitulated the systemic profile (IL-8, TNF-α, GM-CSF, INF-α2, G-CSF, MIP-1β, VEGF, EGF, Eotaxin, MCP-1, CXCL10, PDGF, and RANTES), we identified those targeted by ex vivo treatment with α-Lipoic acid and/or Ginkgoselect phytosome (GM-CSF, G-CSF, CXCL10, PDGF, and RANTES). Finally, by investigating the intracellular pathways involved in promoting the VEC release of cytokines/chemokines, which are targeted by natural anti-inflammatory compounds, we documented that α-Lipoic acid significantly counteracted TNF-α-induced NF-κB and p38/MAPK activation while the effects of Ginkgo biloba appeared to be predominantly mediated by Akt. Our data provide new insights into the molecular mechanisms of CVD pathogenesis, highlighting new potential therapeutic targets.

HIV-1 Tat interacts with and regulates the localization and processing of amyloid precursor protein

HIV-1 Tat protein plays various roles in virus proliferation and in the regulation of numerous host cell functions. Accumulating evidence suggests that HIV-1 Tat also plays an important role in HIV-associated neurocognitive disorders (HAND) by disrupting intracellular communication. Amyloid beta (Aβ) is generated from amyloid precursor protein (APP) and accumulates in the senile plaques of Alzheimer's disease patients. This study demonstrates that Tat interacts with APP both in vitro and in vivo, and increases the level of Aβ42 by recruiting APP into lipid rafts. Co-localization of Tat with APP in the cytosol was observed in U-87 MG cells that expressed high levels of Tat, and redistribution of APP into lipid rafts, a site of increased β- and γ-secretase activity, was demonstrated by discontinuous sucrose density gradient ultracentrifugation in the presence of Tat. Furthermore, Tat enhanced the cleavage of APP by β-secretase in vitro, resulting in 5.5-fold higher levels of Aβ42. This was consistent with increased levels of β-C-terminal fragment (β-CTF) and reduced levels of α-CTF. Moreover, stereotaxic injection of a lentiviral Tat expression construct into the hippocampus of APP/presenilin-1 (PS1) transgenic mice resulted in increased Tat-mediated production and processing of Aβ in vivo. Increased levels of Aβ42, as well as an increase in the number and size of Aβ plaques, were observed in the hippocampus following injection of Tat virus compared with mock virus. These results suggest that HIV-1 Tat may contribute to HAND by interacting with and modifying APP processing, thereby increasing Aβ production.

Endothelial PDGF-BB produced ex vivo correlates with relevant hemodynamic parameters in patients affected by chronic venous disease

Surgical specimens of vein were obtained from the tertiary venous network and/or saphenous vein from patients (n=20) affected by chronic venous disease (CVD). Into the venous segments, which subsequently were surgically ablated, the following hemodynamic parameters were assessed by echo-color-doppler (ECD): peak systolic velocity, end diastolic velocity, whose combination allowed the calculation of the resistance index (RI) and the reflux time (RT). Highly purified venous endothelial cell (VEC) cultures derived from venous segments of these CVD patients were then characterized for the profile of cytokines and chemokines released in the culture supernatants. Among the 27 cytokines and chemokines examined, we found a positive and significant correlation (R=0.5; p=0.03) only between the spontaneous release of PDGF-BB by VEC cultures and the RT values of the patients from which the VEC were isolates. In addition, the release of PDGF-BB in the VEC culture supernatants was significantly (p<0.01) increased upon in vitro treatment with recombinant TNF-α. By using pharmacological inhibitors, specific for the main pathways, NF-kB, ERK1/2 and p38 MAPK, activated by exposure of endothelial cells to TNF-α, we found that only NF-kB appeared to be significantly involved in mediating the PDGF-BB induction by TNF-α. Of interest, the release of PDGF-BB in response to the in vitro inflammatory stimulation, maintained a positive and significant correlation with RT (R=0.6; p=0.01), while showing a negative correlation with RI (R=-0.5; p=0.03). The potential implications of our findings for the pathophysiology of CVD are discussed.

C-Reactive protein downregulates TRAIL expression in human peripheral monocytes via an Egr-1-dependent pathway

PURPOSE

To investigate the potential link between C-reactive protein (CRP), a known biomarker of acute and chronic inflammation, and TRAIL, a cytokine which plays a key role in the immune-surveillance against tumors.

EXPERIMENTAL DESIGN

Primary normal peripheral blood mononuclear cell (PBMC) and CD14(+) monocytes were exposed to recombinant CRP (1-10 μmol/L). TRAIL expression was analyzed by ELISA and/or by quantitative real-time PCR (qRT-PCR). In parallel, the potential role of the transcription factor Egr-1 was investigated by analyzing its modulation in response to CRP and by transfection experiments.

RESULTS

In vitro CRP exposure induced downregulation of TRAIL expression, both at the mRNA and protein level, in unfractionated PBMC and in purified CD14(+) monocytes. TRAIL downregulation was not due to a specific toxicity or to contaminating lipopolysaccharide (LPS), as shown by the lack of induction of monocyte apoptosis and by the inability of the inhibitor of LPS polymyxin B to interfere with CRP activity. Of note, CRP downregulated TRAIL expression/release in CD14(+) monocytes also in response to IFN-α, the most potent inducer of TRAIL. At the molecular level, the downmodulation of TRAIL by CRP was accompanied by a significant increase of Egr-1. Consistently, Egr-1 overexpression reduced the baseline levels of TRAIL mRNA, whereas knocking down Egr-1 counteracted the ability of CRP to downregulate TRAIL.

CONCLUSIONS

Our findings suggest that a chronic elevation of CRP, which occurs during systemic inflammation and often in patients with cancer, might contribute to promote cancer development and/or progression by downregulating TRAIL in immune cells.

Soluble TRAIL is elevated in recurrent miscarriage and inhibits the in vitro adhesion and migration of HTR8 trophoblastic cells

STUDY QUESTION

What is the potential physiopathological role of tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) in recurrent miscarriage (RM), characterized by at least three consecutive pregnancy losses.

SUMMARY ANSWER

The levels of serum TRAIL immediately after miscarriage in RM patients are significantly elevated with respect to that in first-trimester normal pregnant women, and recombinant TRAIL inhibits the adhesion and migration of HTR8 trophoblastic cells in vitro.

WHAT IS KNOWN ALREADY

Both TRAIL and its trans-membrane receptors (TRAIL-R1, TRAIL-R2, TRAIL-R3 and TRAIL-R4) have been documented in the placenta, but their physiopathological role is incompletely understood.

STUDY DESIGN, SIZE, DURATION

The study populations consisted of RM patients (n = 80) and first-trimester normal pregnant women (n = 80). Blood samples were obtained within 24 h after abortion (RM) or at gestational 12-week (normal pregnant women). As additional controls, third-trimester normal pregnant women (n = 28) were examined before (within 72 h) and after (within 24 h) partum.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The concentrations of TRAIL were analysed in serum samples by ELISA. In parallel, the effect of soluble recombinant TRAIL (0.1-1000 ng/ml) was analysed on the survival of primary extravillus trophoblasts (EVTs) and on the survival, proliferation, adhesion and migration of trophoblastic HTR8 cells.

MAIN RESULTS AND THE ROLE OF CHANCE

The circulating levels of TRAIL in RM women (median: 52.5 pg/ml; mean and SD: 55.5 ± 24.4 pg/ml) were significantly higher with respect to first-trimester normal pregnant women (median: 44.9 pg/ml; mean and SD: 47 ± 15.1 pg/ml) and third-trimester normal pregnant women, as assessed before (median: 45.1 pg/ml; mean and SD: 46 ± 12.4 pg/ml) and after partum (median: 35.4 pg/ml; mean and SD: 38 + 17.5 pg/ml). Both primary EVT and HTR8 cells expressed detectable levels of TRAIL death receptors, but exposure to soluble recombinant TRAIL did not induce cell death of trophoblastic cells. On the other hand, TRAIL dose-dependently inhibited the adhesion of HTR8 cells to decidual endothelial cells (DEC) as well as the migration of HTR8 in transwell assays using either fibronectin or DEC.

LIMITATIONS, REASONS FOR CAUTION

Although this study suggests that TRAIL might have a pathogenic role in RM by inhibiting both the adhesion and migration capabilities of first trimester trophoblastic cells, there is a possibility that the elevated serum levels of TRAIL in RM are not cause but rather the result of RM.

WIDER IMPLICATIONS OF THE FINDINGS

Our current findings together with data of other authors suggest that circulating TRAIL should be further analysed as a potential important biomarker in different physiopathological settings.

STUDY FUNDING/COMPETING INTEREST(S)

This study was funded by FIRB projects (RBAP11Z4Z9_002 to Giorgio Zauli and RBAP10447J_002 to Paola Secchiero). The authors have no competing interests to declare.

Ketone bodies protection against HIV-1 Tat-induced neurotoxicity

HIV-1-associated neurocognitive disorder (HAND) is a syndrome that ranges clinically from subtle neuropsychological impairments to profoundly disabling HIV-associated dementia. Not only is the pathogenesis of HAND unclear, but also effective treatments are unavailable. The HIV-1 transactivator of transcription protein (HIV-1 Tat) is strongly implicated in the pathogenesis of HAND, in part, because of its well-characterized ability to directly excite neurons and cause neurotoxicity. Consistent with previous findings from others, we demonstrate here that HIV-1 Tat induced neurotoxicity, increased intracellular calcium, and disrupted a variety of mitochondria functions, such as reducing mitochondrial membrane potential, increasing levels of reactive oxygen species, and decreasing bioenergetic efficiency. Of therapeutic importance, we show that treatment of cultured neurons with ketone bodies normalized HIV-1 Tat induced changes in levels of intracellular calcium, mitochondrial function, and neuronal cell death. Ketone bodies are normally produced in the body and serve as alternative energy substrates in tissues including brain and can cross the blood-brain barrier. Ketogenic strategies have been used clinically for treatment of neurological disorders and our current results suggest that similar strategies may also provide clinical benefits in the treatment of HAND.

Pegylated TRAIL retains anti-leukemic cytotoxicity and exhibits improved signal transduction activity with respect to TRAIL

To improve the pharmacokinetic profile of tumor necrosis factor-related apoptosis inducing ligand (TRAIL) an N-terminal specific pegylation was performed to generate pegylated TRAIL (PEG-TRAIL). In in vitro experiments, we found that although PEG-TRAIL was slightly less efficient than recombinant TRAIL in promoting leukemic cell apoptosis, it showed an improved ability to promote migration of bone-marrow mesenchymal stem cells and to elicit the ERK1/2 intracellular signal transduction pathway. Overall, these data suggest that TRAIL pegylation retains, or even enhances, the biological activities of TRAIL relevant for its therapeutic applications.

Dasatinib plus Nutlin-3 shows synergistic antileukemic activity in both p53 wild-type and p53 mutated B chronic lymphocytic leukemias by inhibiting the Akt pathway

PURPOSE

To analyze the effect of the combination of Dasatinib, a multikinase inhibitor, plus Nutlin-3, a nongenotoxic activator of the p53 pathway, in primary B chronic lymphocytic leukemia (B-CLL) patient samples and B leukemic cell line models.

EXPERIMENTAL DESIGN

The induction of cytotoxicity was evaluated in both primary B-CLL cell samples (n = 20) and in p53(wild-type) (EHEB, JVM-2) and p53(deleted/mutated) (MEC-2, BJAB) B leukemic cell lines. The role of Akt in modulating leukemic cell survival/apoptosis in response to Dasatinib or Dasatinib + Nutlin-3 was documented by functional experiments carried out using specific pharmacological inhibitors and by overexpression of membrane-targeted constitutively active form of Akt.

RESULTS

The combination of Dasatinib + Nutlin-3 exhibited a synergistic cytotoxicity in the majority (19 out of 20) of B-CLL samples, including patients carrying 17p- (n = 4), and in both p53(wild-type) and p53(deleted/mutated) B leukemic cell lines. At the molecular level, Dasatinib significantly counteracted the Nutlin-3-mediated induction of the p53 transcriptional targets MDM2 and p21 observed in p53(wild-type) leukemic cells. Conversely, Nutlin-3 did not interfere with the ability of Dasatinib to decrease the phosphorylation levels of ERK1/2, p38/MAPK, and Akt in both p53(wild-type) and p53(deleted/mutated) B leukemic cell lines. A critical role of Akt downregulation in mediating the antileukemic activity of Dasatinib and Dasatinib + Nutlin-3 was demonstrated in experiments carried out by specifically modulating the Akt pathway.

CONCLUSIONS

These findings suggest that Dasatinib + Nutlin-3 might represent an innovative therapeutic combination for both p53(wild-type) and p53(deleted/mutated) B-CLL.

Anti-leukemic activity of dasatinib in both p53(wild-type) and p53(mutated) B malignant cells

The multi-kinase inhibitor dasatinib induced a variable but significant decrease of viability in both p53(wild-type) (EHEB, JVM-2, JVM-3) and p53(mutated) (MEC-1, MEC-2, BJAB) prolymphocytic B leukemic cells, due to a combination of cell cycle block in G1 and apoptosis. Antibody phospho-kinase array analysis revealed that dasatinib inhibited the phosphorylation of various kinases, including ERK1/2 and p38/MAPK as well as of STAT3 transcription factors, in both p53(wild-type) and p53(mutated) cells. Therefore, dasatinib might offer a novel therapeutic strategy not only for p53(wild-type), but also for p53(mutated) B malignancies that have the worst prognosis and urgently need innovative therapeutic approaches.

The oncogene DEK promotes leukemic cell survival and is downregulated by both Nutlin-3 and chlorambucil in B-chronic lymphocytic leukemic cells

PURPOSE

To characterize the role of the oncogene DEK in modulating the response to either Nutlin-3, a small-molecule inhibitor of the MDM2/p53 interaction, or chlorambucil in primary B-chronic lymphocytic leukemia (B-CLL) cells.

EXPERIMENTAL DESIGN

DEK mRNA and protein levels were evaluated in primary B-CLL samples (n = 21), p53(wild-type) SKW6.4, p53(mutated) BJAB lymphoblastoid cell lines, and normal CD19(+) B lymphocytes-treated Nutlin-3 or chlorambucil (10 micromol/L, each). Knocking down experiments with either p53 or DEK small interfering RNA (siRNA) were done to investigate the potential role of p53 in controlling the expression of DEK and the role of DEK in leukemic cell survival/apoptosis.

RESULTS

Both Nutlin-3 and chlorambucil downregulated DEK in primary B-CLL samples (n = 21) and SKW6.4 but not in BJAB cells. Knocking down p53 attenuated the effect of Nutlin-3 on DEK expression, whereas knocking down DEK significantly increased both spontaneous and Nutlin-3-induced apoptosis. Conversely, counteracting DEK downmodulation by using p53 small interfering RNA reduced Nutlin-3-mediated apoptosis. On the other hand, Nutlin-3 potently induced p53 accumulation, but it did not affect DEK levels in normal CD19(+) B lymphocytes.

CONCLUSIONS

These data show that the downregulation of DEK in response to either Nutlin-3 or chlorambucil represents an important molecular determinant in the cytotoxic response of leukemic cells, and suggest that strategies aimed to downregulate DEK might improve the therapeutic potential of these drugs.

Extracellular HIV-Tat induces cyclooxygenase-2 in glial cells through activation of nuclear factor of activated T cells

Both the HIV-1 protein Tat and cyclooxygenase-2 (COX-2) have been involved in the neuropathogenesis associated with HIV-1 infection. However, the relationship among them has not been addressed. Here, we found that extracellular Tat was able to induce COX-2 mRNA and protein expression and PGE2 synthesis in astrocytoma cell lines and primary human astrocytes. Moreover, Tat induced COX-2 promoter transcription. Deletion of NF-kappaB sites of the promoter did not diminish Tat-dependent transcription. Interestingly, Tat did not induce NF-kappaB activity, suggesting that NF-kappaB was not necessary to control COX-2 transcription induced by Tat. In contrast, deletion or mutation of the NFAT and/or AP-1 site abrogated COX-2 induction by Tat. Moreover, Tat induced transcription of NFAT- and AP-1-dependent reporter genes. Transfection of a dominant negative c-Jun mutant protein, TAM-67, or of a dominant negative version of NFAT, efficiently blocked the induction of COX-2 promoter by Tat, confirming the requirement of both transcription factors. Moreover, Tat induced NFAT translocation to the nucleus and binding to the distal site of the COX-2 promoter. The importance of NFAT and AP-1 in COX-2 induction and PGE2 synthesis by Tat was corroborated by using pharmacological inhibitors of the NFAlphaTau, ERK, and JNK pathways. In summary, our results indicate that HIV-1 Tat was able to induce COX-2 and PGE2 synthesis in astrocytic cells through an NFAT/AP-1-dependent mechanism.

HIV accomplices and adversaries in macrophage infection

Cell surface and intracellular proteins in macrophages influence various steps in the life cycle of lentiviruses. Characterization of these restriction and/or cofactors is essential to understanding how macrophages become unwitting HIV hosts and in fact, can coexist with a heavy viral burden. Although many of the cellular pathways co-opted by HIV in macrophages mimic those seen in CD4+ T cells, emerging evidence reveals cellular constituents of the macrophage, which may be uniquely usurped by HIV. For example, in addition to CD4 and CCR5, membrane annexin II facilitates early steps in infection of macrophages, but not in T cells. Blockade of this pathway effectively diminishes macrophage infection. Viral binding engages a macrophage-centric signaling pathway and a transcriptional profile, including genes such as p21, which benefit the virus. Once inside the cell, multiple host cell molecules are engaged to facilitate virus replication and assembly. Although the macrophage is an enabler, it also possesses innate antiviral mechanisms, including apolipoprotein B mRNA-editing enzyme-catalytic polypeptide-like 3G (APOBEC3) family DNA-editing enzymes to inhibit replication of HIV. Differential expression of these enzymes, which are largely neutralized by HIV to protect its rebirth, is associated with resistance or susceptibility to the virus. Higher levels of the cytidine deaminases endow potential HIV targets with a viral shield, and IFN-alpha, a natural inducer of macrophage APOBEC expression, renders macrophages tougher combatants to HIV infection. These and other manipulatable pathways may give the macrophage a fighting chance in its battle against the virus.

HIV-1 Tat regulates endothelial cell cycle progression via activation of the Ras/ERK MAPK signaling pathway

Tat, the transactivator of HIV-1 gene expression, is released by acutely HIV-1-infected T-cells and promotes adhesion, migration, and growth of inflammatory cytokine-activated endothelial and Kaposi's sarcoma cells. It has been previously demonstrated that these effects of Tat are due to its ability to bind through its arginine-glycine-aspartic (RGD) region to the alpha5beta1 and alphavbeta3 integrins. However, the signaling pathways linking Tat to the regulation of cellular functions are incompletely understood. Here, we report that Tat ligation on human endothelial cells results in the activation of the small GTPases Ras and Rac and the mitogen-activated protein kinase ERK, specifically through its RGD region. In addition, we demonstrated that Tat activation of Ras, but not of Rac, induces ERK phosphorylation. We also found that the receptor proximal events accompanying Tat-induced Ras activation are mediated by tyrosine phosphorylation of Shc and recruitment of Grb2. Moreover, Tat enabled endothelial cells to progress through the G1 phase in response to bFGF, and the process is linked to ERK activation. Taken together, these data provide novel evidence about the ability of Tat to activate the Ras-ERK cascade which may be relevant for endothelial cell proliferation and for Kaposi's sarcoma progression.

HIV-dementia, Tat-induced oxidative stress, and antioxidant therapeutic considerations

Oxidative stress is thought to play a role in the onset of dementia. HIV-dementia has recently been demonstrated to be associated with oxidative stress as indexed by increased protein and lipid peroxidation in the brain and cerebrospinal fluid compared to HIV non-demented patients. The HIV protein Tat induces neurotoxicity, and, more recently, Tat was found to induce oxidative stress directly and indirectly. The role of Tat in HIV-dementia and possible therapeutic strategies involving endogenous and exogenous antioxidants are discussed.

Regulation of neural cell survival by HIV-1 infection

Infection by the lentivirus, human immunodeficiency virus type 1 (HIV-1), results in a variety of syndromes involving both the central (CNS) and the peripheral (PNS) nervous systems. Productive HIV-1 infection of the CNS is chiefly detectable in perivascular macrophages and microglia. HIV-1 encoded transcripts and proteins have also been detected in the PNS; however, productive viral replication appears to be sparse and restricted to the macrophage cell population. Despite the absence of productive infection of neurons, HIV-1 infection has been associated with neuronal loss in distinct regions of the brain. Neuronal cell loss may occur through both necrosis and apoptosis, although neuronal apoptosis appears to be a feature of AIDS, as only rare apoptotic neurons have been demonstrated in a few pre-AIDS cases. Although there is no clear consensus as to the underlying mechanism of HIV-induced neuropathogenesis, two complementary concepts predominate. Firstly, HIV-1 encoded proteins injure neurons directly without requiring the intermediary functions of nonneuronal cells. Alternatively, neuronal apoptosis may result indirectly from the secretion of neurotoxic host molecules by resident brain macrophages or microglia in response to HIV-1 infection, stimulation by viral proteins or immune activation. Herein, we review the neurological disorders and their underlying mechanisms associated with HIV infection, focusing on HIV-associated dementia (HAD) and HIV sensory neuropathy (HIV-SN). The evidence that neuronal loss in HIV-1-infected individuals may be due to neuronal apoptosis is then discussed. This review also summarizes the current data supporting both the direct and indirect mechanisms by which neuronal death may occur during infection with HIV-1 or the closely related lentiviruses SIV and FIV. Lastly, strategies are examined for treating or preventing HAD by targeting specific neurotoxic mechanisms.

Molecular targets of opiate drug abuse in neuroAIDS

Opiate drug abuse, through selective actions at mu-opioid receptors (MOR), exacerbates the pathogenesis of human immunodeficiency virus-1 (HIV-1) in the CNS by disrupting glial homeostasis, increasing inflammation, and decreasing the threshold for pro-apoptotic events in neurons. Neurons are affected directly and indirectly by opiate-HIV interactions. Although most opiates drugs have some affinity for kappa (KOR) and/or delta (DOR) opioid receptors, their neurotoxic effects are largely mediated through MOR. Besides direct actions on the neurons themselves, opiates directly affect MOR-expressing astrocytes and microglia. Because of their broad-reaching actions in glia, opiate abuse causes widespread metabolic derangement, inflammation, and the disruption of neuron-glial relationships, which likely contribute to neuronal dysfunction, death, and HIV encephalitis. In addition to direct actions on neural cells, opioids modulate inflammation and disrupt normal intercellular interactions among immunocytes (macrophages and lymphocytes), which on balance further promote neuronal dysfunction and death. The neural pathways involved in opiate enhancement of HIV-induced inflammation and cell death, appear to involve MOR activation with downstream effects through PI3-kinase/Akt and/or MAPK signaling, which suggests possible targets for therapeutic intervention in neuroAIDS.

Signaling mechanisms of HIV-1 Tat-induced alterations of claudin-5 expression in brain endothelial cells

Exposure of brain microvascular endothelial cells (BMEC) to human immunodeficiency virus-1 (HIV-1) Tat protein can decrease expression and change distribution of tight junction proteins, including claudin-5. Owing to the importance of claudin-5 in maintaining the blood-brain barrier (BBB) integrity, the present study focused on the regulatory mechanisms of Tat-induced alterations of claudin-5 mRNA and protein levels. Real-time reverse-transcription-polymerase chain reaction revealed that claudin-5 mRNA was markedly diminished in BMEC exposed to Tat. However, U0126 (an inhibitor of mitogen-activated protein kinase kinase1/2, MEK1/2) protected against this effect. In addition, inhibition of the vascular endothelial growth factor receptor type 2 (VEGFR-2) by SU1498, phosphatidylinositol-3 kinase (PI-3 K) by LY294002, nuclear factor-kappaB (NF-kappaB) by peptide SN50, and intracellular calcium by BAPTA/AM partially prevented Tat-mediated alterations in claudin-5 protein levels and immunoreactivity patterns. In contrast, inhibition of protein kinase C did not affect claudin-5 expression in Tat-treated cells. The present findings indicate that activation of VEGFR-2 and multiple redox-regulated signal transduction pathways are involved in Tat-induced alterations of claudin-5 expression. Because claudins constitute the major backbone of tight junctions, the present data are relevant to the disturbances of the BBB in the course of HIV-1 infection.

HIV-1 gp120-induced TNF-{alpha} production by primary human macrophages is mediated by phosphatidylinositol-3 (PI-3) kinase and mitogen-activated protein (MAP) kinase pathways

Human immunodeficiency virus type 1 (HIV-1) infection is initiated by binding of the viral envelope glycoprotein gp120 to CD4 followed by a chemokine receptor, but these interactions may also take place independently from infection. gp120 stimulation of primary human macrophages is known to trigger production of cytokines implicated in pathogenesis, particularly tumor necrosis factor alpha (TNF-alpha), but the mechanisms have not been determined. We sought to define the pathways responsible for TNF-alpha secretion by monocyte-derived macrophages (MDM) following HIV-1 gp120 stimulation. MDM exposure to recombinant macrophage-tropic (R5) gp120 led to dose- and donor-dependent release of TNF-alpha, which was cyclohexamide-sensitive and associated with up-regulated message. Pretreatment with specific inhibitors of the mitogen-activated protein kinases (MAPK) extracellular signal-regulated kinase 1/2 (ERK-1/2; PD98059, U0126) and p38 (SB202190, PD169316) inhibited the secretion of TNF-alpha. gp120-elicited TNF-alpha production was also blocked by phosphatidylinositol-3 kinase (PI-3K) inhibitors (wortmannin, LY294002). Moreover, PI-3K inhibition ablated gp120-induced phosphorylation of p38 and ERK-1/2. The response was inhibited by a CC chemokine receptor 5 (CCR5)-specific antagonist, indicating that CCR5 was in large part responsible. These results indicate that gp120-elicited TNF-alpha production by macrophages involves chemokine receptor-mediated PI-3K and MAPK activation, that PI-3K is an upstream regulator of MAPK in this pathway, and that p38 and ERK-1/2 independently regulate TNF-alpha production. These gp120-triggered signaling pathways may be responsible for inappropriate production of proinflammatory cytokines by macrophages, which are believed to play a role in immunopathogenesis and in neurological sequelae of AIDS.

HIV-Tat promotes cellular proliferation and inhibits NGF-induced differentiation through mechanisms involving Id1 regulation

Id1 is a helix-loop-helix transcriptional factor that controls growth and survival of neuronal cells. Downregulation of Id1 expression is required to initiate differentiation and cell-cycle withdrawal in primary neuronal culture as well as in PC12 cells. The HIV-1 transactivating factor, Tat, has been suspected of causing neuronal dysfunction that often leads to the development of HIV-associated dementia in AIDS patients. We found that the expression of Tat in PC12 cells promotes serum-independent growth, formation of large colonies in soft agar, and the acceleration of tumor growth in nude mice. In addition, Tat showed the ability to inhibit the nerve growth factor (NGF)-induced neuronal differentiation of PC12 cells. Our results show that the Tat-mediated signaling events, which lead to serum-independent growth and the inhibition of NGF-induced differentiation, have a common cellular target: the upregulation of Id1 expression. In the absence of NGF, expression of Id1 is required to promote serum-independent proliferation of PC12/Tat cells, as the inhibition of Id1 by antisense DNA restored the serum-dependent growth of PC12/Tat cells. In the presence of NGF, Tat utilizes an additional pathway that involves phosphorylation of Stat5a, to upregulate Id1 expression and block neuronal cell differentiation. Suppression of Stat5a by use of its dominant-negative mutant reversed the transient expression of Id1 and the blockage of NGF-mediated differentiation in PC12/Tat cells. Finally, the treatment of PC12 cells with recombinant Tat also enhanced the NGF-induced Id1 expression, further pointing to Id1 as a target for Tat. Taken together, these studies suggest additional targets for Tat action in neuronal cells and provide new insights into the mechanisms involved in the dysregulation of neuronal functions.

Effects of apolipoprotein E on the human immunodeficiency virus protein tat in neuronal cultures and synaptosomes

Human immunodeficiency virus type 1 (HIV-1)-associated dementia is observed in 20-30% of patients with acquired immunodeficiency syndrome (AIDS). The epsilon4 allele of the apolipoprotein E (APOE) gene currently is thought to play a role as a risk factor for the development of HIV dementia. The HIV protein Tat is neurotoxic and binds to the same receptor as apoE, the low-density lipoprotein receptor-related protein (LRP). In this study, we investigated the role apoE plays in Tat toxicity. Synaptosomes from wild-type mice treated with Tat had increased reactive oxygen species (ROS), increased lipid and protein oxidation, and decreased mitochondrial membrane potential. Synaptosomes from APOE-knockout mice also had increased ROS, increased protein oxidation, and decreased mitochondrial membrane potential, but to a significantly lesser degree. Treatment of synaptosomes with heparinase and Tat increased Tat-induced oxidative stress, consistent with the notion of Tat requiring interaction with neuronal membranes to induce oxidative damage. Human lipidated apoE3 greatly protected neurons from Tat-induced toxicity, whereas human lipidated apoE4 showed no protection. We demonstrated that human apoE3 has antioxidant properties against Tat-induced toxicity. Taken together, the data suggest that murine apoE and human apoE4 act similarly and do not protect the cell from Tat-induced toxicity. This would allow excess Tat to remain outside the cell and interact with synaptosomal membranes, leading to oxidative stress and neurotoxicity, which could contribute to dementia associated with HIV. We show that the antioxidant properties of apoE3 greatly outweigh the competition for clearance in deterring Tat-induced oxidative stress.

Transport of exogenous growth factors and cytokines to the cytosol and to the nucleus

In recent years a number of growth factors, cytokines, protein hormones, and other proteins have been found in the nucleus after having been added externally to cells. This review evaluates the evidence that translocation takes place and discusses possible mechanisms. As a demonstration of the principle that extracellular proteins can penetrate cellular membranes and reach the cytosol, a brief overview of the penetration mechanism of protein toxins with intracellular sites of action is given. Then problems and pitfalls in attempts to demonstrate the presence of proteins in the cytosol and in the nucleus as opposed to intracellular vesicular compartments are discussed, and some new approaches to study this are described. A detailed overview of the evidence for translocation of fibroblast growth factor, HIV-Tat, interferon-gamma, and other proteins where there is evidence for intracellular action is given, and translocation mechanisms are discussed. It is concluded that although there are many pitfalls, the bulk of the experiments indicate that certain proteins are indeed able to enter the cytosol and nucleus. Possible roles of the internalized proteins are discussed.

Calcium dysregulation and neuronal apoptosis by the HIV-1 proteins Tat and gp120

Patients with AIDS often develop cognitive and motor dysfunction that results from damage to synapses and death of neurons in brain regions such as the hippocampus and basal ganglia. This brain syndrome, called AIDS dementia or HIV encephalitis, is believed to result from the infection of one or more populations of mitotic brain cells with HIV-1, which then release viral proteins that are toxic to neurons. Two neurotoxic HIV-1 proteins have been identified, the viral coat protein gp120 and the transcription regulator Tat. Each of these proteins can induce apoptosis of cultured neurons and can render neurons vulnerable to excitotoxicity and oxidative stress. Gp120 and Tat also cause neuronal dysfunction and death in rodents in vivo. Both gp120 and Tat disrupt neuronal calcium homeostasis by perturbing calcium-regulating systems in the plasma membrane and endoplasmic reticulum. Accordingly, drugs that stabilize cellular calcium homeostasis can protect neurons against the toxic effects of gp120 and Tat. By altering voltage-dependent calcium channels, glutamate receptor channels, and membrane transporters, the HIV-1 proteins promote calcium overload, oxyradical production, and mitochondrial dysfunction. A better understanding of how gp120 and Tat disrupt neuronal calcium homeostasis may lead to the development of novel treatments for AIDS patients.

Interactions of human immunodeficiency virus-1 proteins with neurons: possible role in the development of human immunodeficiency virus-1-associated dementia

Human immunodeficiency virus-1 (HIV-1)-associated dementia is a severe neurological complication of HIV-1 infection that affects 15-20% of the patients in the late stages of acquired immunodeficiency syndrome. HIV-1-associated dementia is most probably a consequence of HIV-1 infection of the brain rather than of an opportunistic pathogen. The exact mechanism by which the virus causes this disorder, however, is not completely understood. A number of HIV-1 proteins have been shown to be released from HIV-1-infected cells and/or to be present in the extracellular milieu in the HIV-1-infected brain. Moreover, these proteins have been shown to possess neurotoxic and/or neuromodulatory features in vitro. This review describes the possible direct interactions of the HIV-1 proteins gp120, gp41, vpr, tat, rev, vpu and nef with neurons, which might play a role in the development of HIV-1-associated dementia in vivo.

Angiogenic effects of extracellular human immunodeficiency virus type 1 Tat protein and its role in the pathogenesis of AIDS-associated Kaposi's sarcoma

The Tat protein of human immunodeficiency virus (HIV) type 1 is a transactivator of viral gene expression that is required for virus replication and spread. Moreover, Tat is released by acutely HIV-infected cells via a leaderless secretory pathway and in a biologically active form that exerts effects on both HIV-infected and uninfected cells from different organs and systems. This review focuses on the activities of extracellular Tat protein on endothelial cells, on angiogenesis, and on the pathogenesis of AIDS-associated angioproliferative diseases such as Kaposi's sarcoma. In particular, we discuss results from different groups indicating that Tat mimics the proangiogenic activities of extracellular matrix molecules and that it enhances the effects of angiogenic factors.

Activation of endothelial cell mitogen activated protein kinase ERK(1/2) by extracellular HIV-1 Tat protein

Extracellular Tat protein, the transactivating factor of the human immunodeficiency virus type 1 (HIV-1), modulates gene expression, growth, and angiogenic activity in endothelial cells by interacting with the vascular endothelial growth factor (VEGF) receptor-2 (Flk-1/KDR). Recombinant Tat protein, produced as glutathione-S-transferase chimera (GST-Tat), activates mitogen-activated protein kinase (MAPK) ERK(1/2) in human, murine, and bovine endothelial cells whereas GST is ineffective. In bovine aortic endothelial cells, GST-Tat and the 165 amino acid VEGF isoform (VEGF165) induce transient ERK(1/2) phosphorylation with similar potency and kinetics. The synthetic peptide Tat(41-60), but not peptides Tat(1-21) and Tat(71-86), causes ERK(1/2) phosphorylation, thus implicating Tat/KDR interaction in the activation of this signalling pathway. Accordingly, GST-Tat induces ERK(1/2) phosphorylation in KDR-transfected porcine aortic endothelial cells but not in parental cells. MAPK kinase inhibitors PD098059 and U0126 prevent ERK(1/2) phosphorylation by Tat. However, they do not affect the angiogenic activity exerted by Tat in the murine Matrigel plug and chick embryo chorioallantoic membrane assays. Blocking of MAPK kinase activity impairs instead the angiogenic response to VEGF165 and to fibroblast growth factor-2 (FGF-2). Our data demonstrate that ERK(1/2) activation following the interaction of HIV-1 Tat protein with endothelial cell Flk-1/KDR receptor does not represent an absolute requirement for a full angiogenic response to this growth factor that appears to utilize mechanism(s) at least in part distinct from those triggered by other prototypic angiogenic growth factors.

Chemical modification of transducin with iodoacetic acid: transducin-alpha carboxymethylated at Cys(347) allows transducin binding to Light-activated rhodopsin but prevents its release in the presence of GTP

Modification of transducin (T) with iodoacetic acid (IAA) inhibited its light-dependent guanine nucleotide-binding activity. Approximately 1 mol of [(3)H]IAA was incorporated per mole of T. Cys(347), located on the alpha-subunit of T (T(alpha)), was identified as the major labeled residue in the [(3)H]IAA-modified holoenzyme. In contrast, Cys(135) and Cys(347) were modified with [(3)H]IAA in the isolated T(alpha). IAA-modified T was able to bind tightly to photoexcited rhodopsin (R*), but GTP did not promote the dissociation of the complex between alkylated T and R*. In addition, R* protected against the inhibition of T by IAA. A comparable inactivation of T and analogous interactions between T and R* were observed when 2-nitro 5-thiocyanobenzoic acid (NTCBA) was used as the modifying reagent (J. O. Ortiz and J. Bubis, 2001, Effects of differential sulfhydryl group-specific labeling on the rhodopsin and guanine nucleotide binding activities of transducin, Arch. Biochem. Biophys. 387, 233-242). However, while carboxymethylated T was capable of liberating GDP in the presence of R*, NTCBA-modified T was unable to release the guanine nucleotide diphosphate upon incubation with the photoactivated receptor. Thus, IAA-labeling stabilized a T:R* complex intermediate carrying the empty nucleotide pocket conformation of T. On the other hand, NTCBA-modified T seemed to be "locked" in the GDP-bound state of T, even in the presence of R*.

Phosphoinositide 3-kinase regulates crosstalk between Trk A tyrosine kinase and p75(NTR)-dependent sphingolipid signaling pathways

The mechanism of crosstalk between signaling pathways coupled to the Trk A and p75(NTR) neurotrophin receptors in PC12 cells was examined. In response to nerve growth factor (NGF), Trk A activation inhibited p75(NTR)-dependent sphingomyelin (SM) hydrolysis. The phosphoinositide 3-kinase (PI 3-kinase) inhibitor, LY294002, reversed this inhibition suggesting that Trk A activation of PI 3-kinase is necessary to inhibit sphingolipid signaling by p75(NTR). In contrast, SM hydrolysis induced by neurotrophin-3 (NT-3), which did not activate PI-3 kinase, was uneffected by LY294002. However, transient expression of a constituitively active PI 3-kinase inhibited p75(NTR)-dependent SM hydrolysis by both NGF and NT-3. Intriguingly, NGF induced an association of activated PI 3-kinase with acid sphingomyelinase (SMase). This interaction localized to caveolae-related domains and correlated with a 50% decrease in immunoprecipitated acid SMase activity. NGF-stimulated PI 3-kinase activity was necessary for inhibition of acid SMase but was not required for ligand-induced association of the p85 subunit of PI 3-kinase with the phospholipase. Finally, this interaction was specific for NGF since EGF did not induce an association of PI 3-kinase with acid SMase. In summary, our data suggest that PI 3-kinase regulates the inhibitory crosstalk between Trk A tyrosine kinase and p75(NTR)-dependent sphingolipid signaling pathways and that this interaction localizes to caveolae-related domains.

HIV-1 Tat protein down-regulates CREB transcription factor expression in PC12 neuronal cells through a phosphatidylinositol 3-kinase/AKT/cyclic nucleoside phosphodiesterase pathway

The addition of low concentrations (0.1-1 nM) of extracellular HIV-1 Tat protein to PC12 neuronal cells stimulated a rapid (peak at 5 min) elevation of the cAMP intracellular levels, which in turn induced the phosphorylation of CREB transcription factor (peak at 15 min) on serine-133 (Ser-133). On the contrary, at later time points (60-120 min) Tat induced a significant decline of intracellular cAMP with respect to the basal levels observed in control cells treated with bovine serum albumin. In blocking experiments performed with pharmacological inhibitors, Tat decreased the intracellular levels of cAMP and CREB Ser-133 phosphorylation through a signal transduction pathway involving the sequential activation of phosphatidylinositol 3-kinase, AKT, and cyclic nucleoside phosphodiesterases. Moreover, in transient transfection experiments, Tat inhibited transcription of CREB promoter in a manner strictly dependent on the presence of the cAMP-responsive elements (CRE) in the CREB promoter. Consistently, the expression of endogenous CREB protein was significantly reduced in PC12 cells by prolonged (24-48 h) treatment with Tat. This decline in the expression of CREB, which plays an essential role in the survival and function of neuronal cells, anticipated a progressive increase of apoptosis in Tat-treated cells. Although obtained in a neuronal cell line, our findings might help to explain some aspects of the pathogenesis of HIV-1-associated dementia.

Thrombospondin-1/HIV-1 tat protein interaction: modulation of the biological activity of extracellular Tat

Tat protein, a trans-activating factor of the human immunodeficiency virus type 1, acts also as an extracellular molecule modulating gene expression, cell survival, growth, transformation, and angiogenesis. Here we demonstrate that human thrombospondin-1 (TSP), a plasma glycoprotein and constituent of the extracellular matrix, binds to glutathione-S-transferase (GST)-Tat protein but not to GST. Scatchard plot analysis of the binding of free GST-Tat to immobilized TSP reveals a high-affinity interaction (Kd equal to 25 nM). Accordingly, TSP inhibits cell internalization and HIV-1 LTR trans-activating activity of extracellular Tat in HL3T1 cells with ID50 equal to 10-30 nM. Also, TSP inhibits cell interaction and mitogenic activity of extracellular Tat in T53 Tat-less cells. TSP is instead ineffective when administered after the interaction of Tat with cell surface heparan-sulfate proteoglycans has occurred, in keeping with its ability to prevent but not disrupt Tat/heparin interaction in vitro. Finally, TSP inhibits the autocrine loop of stimulation exerted by endogenous Tat in parental T53 cells. Accordingly, TSP overexpression inhibits cell proliferation, angiogenic activity, and tumorigenic capacity of stable T53 transfectants. Our data demonstrate the ability of TSP to bind to Tat protein and to affect its LTR trans-activating, mitogenic, angiogenic, and tumorigenic activity. These findings suggest that TSP may be implicated in the progression of AIDS and in AIDS-associated pathologies by modulating the bioavailability and biological activity of extracellular Tat.

Identification of specific molecular structures of human immunodeficiency virus type 1 Tat relevant for its biological effects on vascular endothelial cells

Human immunodeficiency virus type 1 (HIV-1) Tat transactivates viral genes and is released by infected cells, acting as a soluble mediator. In endothelial cells (EC), it activates a proangiogenic program by activating vascular endothelial growth factor receptor type 2 (VEGFR-2) and integrins. A structure-activity relationship study was performed by functional analysis of Tat substitution and deletion variants to define the Tat determinants necessary for EC activation. Variants were made (i) in the basic and (ii) in the cysteine-rich domains and (iii) in the C-terminal region containing the RGD sequence required for integrin recognition. Our results led to the following conclusions. (i) Besides a high-affinity binding site corresponding to VEGFR-2, EC express low-affinity binding sites. (ii) The basic and the cysteine-rich variants bind only to the low-affinity binding sites and do not promote tyrosine phosphorylation of VEGFR-2. Furthermore, they have a reduced ability to activate EC in vitro, and they lack angiogenic activity. (iii) Mutants with mutations in the C-terminal region are partially defective for in vitro biological activities and in vivo angiogenesis, but they activate VEGFR-2 as Tat wild type. In conclusion, regions encoded by the first exon of tat are necessary and sufficient for activation of VEGFR-2. However, the C-terminal region, most probably through RGD-mediated integrin engagement, is indispensable for full activation of an in vitro and in vivo angiogenic program.

Multiple interactions of HIV-I Tat protein with size-defined heparin oligosaccharides

Tat protein, a transactivating factor of the human immunodeficiency virus type I, acts also as an extracellular molecule. Heparin affects the bioavailability and biological activity of extracellular Tat (Rusnati, M., Coltrini, D., Oreste, P., Zoppetti, G., Albini, A., Noonan, D., D'Adda di Fagagna, F., Giacca, M., and Presta, M. (1997) J. Biol. Chem. 272, 11313-11320). Here, a series of homogeneously sized, (3)H-labeled heparin fragments were evaluated for their capacity to bind to free glutathione S-transferase (GST)-Tat protein and to immobilized GST-Tat. Hexasaccharides represent the minimum sized heparin fragments able to interact with GST-Tat at physiological ionic strength. Also, the affinity of binding increases with increasing the molecular size of the oligosaccharides, with large fragments (>/=18 saccharides) approaching the affinity of full-size heparin. 6-Mer heparin binds GST-Tat with a dissociation constant (K(d)) equal to 0.7 +/- 0.4 microM and a molar oligosaccharide:GST-Tat ratio of about 1:1. Interaction of GST-Tat with 22-mer or full-size heparin is consistent instead with two-component binding. At subsaturating concentrations, a single molecule of heparin interacts with 4-6 molecules of GST-Tat with high affinity (K(d) values in the nanomolar range of concentration); at saturating concentrations, heparin binds GST-Tat with lower affinity (K(d) values in the micromolar range of concentration) and a molar oligosaccharide:GST-Tat ratio of about 1:1. In agreement with the binding data, a positive correlation exists between the size of heparin oligosaccharides and their capacity to inhibit cell internalization, long terminal repeat-transactivating activity of extracellular Tat in HL3T1 cells, and its mitogenic activity in murine adenocarcinoma T53 Tat-less cells. The data demonstrate that the modality of heparin-Tat interaction is strongly affected by the size of the saccharide chain. The possibility of establishing multiple interactions increases the affinity of large heparin fragments for Tat protein and the capacity of the glycosaminoglycan to modulate the biological activity of extracellular Tat.

Involvement of inositol 1,4,5-trisphosphate-regulated stores of intracellular calcium in calcium dysregulation and neuron cell death caused by HIV-1 protein tat

HIV-1 infection commonly leads to neuronal cell death and a debilitating syndrome known as AIDS-related dementia complex. The HIV-1 protein Tat is neurotoxic, and because cell survival is affected by the intracellular calcium concentration ([Ca2+]i), we determined mechanisms by which Tat increased [Ca2+]i and the involvement of these mechanisms in Tat-induced neurotoxicity. Tat increased [Ca2+]i dose-dependently in cultured human fetal neurons and astrocytes. In neurons, but not astrocytes, we observed biphasic increases of [Ca2+]i. Initial transient increases were larger in astrocytes than in neurons and in both cell types were significantly attenuated by antagonists of inositol 1,4,5-trisphosphate (IP3)-mediated intracellular calcium release [8-(diethylamino)octyl-3,4,5-trimethoxybenzoate HCI (TMB-8) and xestospongin], an inhibitor of receptor-Gi protein coupling (pertussis toxin), and a phospholipase C inhibitor (neomycin). Tat significantly increased levels of IP3 threefold. Secondary increases of neuronal [Ca2+]i in neurons were delayed and progressive as a result of excessive calcium influx and were inhibited by the glutamate receptor antagonists ketamine, MK-801, (+/-)-2-amino-5-phosphonopentanoic acid, and 6,7-dinitroquinoxaline-2,3-dione. Secondary increases of [Ca2+]i did not occur when initial increases of [Ca2+]i were prevented with TMB-8, xestospongin, pertussis toxin, or neomycin, and these inhibitors as well as thapsigargin inhibited Tat-induced neurotoxicity. These results suggest that Tat, via pertussis toxin-sensitive phospholipase C activity, induces calcium release from IP3-sensitive intracellular stores, which leads to glutamate receptor-mediated calcium influx, dysregulation of [Ca2+]i, and Tat-induced neurotoxicity.

Extracellular HIV type 1 Tat protein induces CD69 expression through NF-kappaB activation: possible correlation with cell surface Tat-binding proteins

The HIV-1 Tat protein, essential for HIV-1 gene expression and viral replication, is known to be secreted by infected cells and has pleiotropic effects on various cell functions. It seems that extracellular Tat may exert its functions on cellular targets by at least two different mechanisms, namely, by adsorptive endocytosis, and by a possible interaction with cell surface receptor(s). Here we report that extracellular Tat activates AIM/CD69 gene transcription through an NF-kappaB-dependent pathway in the erythroleukemia cell line K562. Tat induces NF-kappaB binding to DNA as a result of IkappaBalpha phosphorylation and degradation, which depend on the intracellular redox state. We found that the second Tat-coding exon is required for CD69 gene trans-activation, but not for HIV LTR gene transcription. Fluorescein-labeled Tat proteins were used to study cell surface binding sites and cellular uptake of the proteins. Full-length Tat protein has specific binding sites on the surface of K562 cells, whereas truncated Tat1-48, which is efficiently internalized by the cells, does not bind to the cell surface. Our results suggest that extracellular Tat may activate a cell surface-mediated pathway that induces intracellular signal transduction in K562 cells, leading to the activation of NF-kappaB and the transcription of NF-kappaB-dependent genes, such as CD69.

HIV-1 Tat-mediated activation of glycogen synthase kinase-3beta contributes to Tat-mediated neurotoxicity

Human immunodeficiency virus type 1 (HIV-1) Tat induces neuronal apoptosis. To examine the mechanism(s) that contribute to this process, we studied Tat's effects on glycogen synthase kinase-3beta (GSK-3beta), an enzyme that has been implicated in the regulation of apoptosis. Addition of Tat to rat cerebellar granule neurons resulted in an increase in GSK-3beta activity, which was not associated with a change in protein expression and could be abolished by the addition of an inhibitor of GSK-3beta (lithium). Lithium also enhanced neuronal survival following exposure to Tat. Coprecipitation experiments revealed that Tat can associate with GSK-3beta, but direct addition of Tat to purified GSK-3beta had no effect on enzyme activity, suggesting that Tat's effects might be mediated indirectly. As the activation of platelet activating factor (PAF) receptors is critical for the induction of neuronal death by several candidate HIV-1 neurotoxins, we determined whether PAF can also activate GSK-3beta. Application of PAF to neuronal cultures activated GSK-3beta, and coincubation with lithium ameliorated PAF-induced neuronal apoptosis. These findings are consistent with the existence of one or more pathways that can lead to GSK-3beta activation in neurons, and they suggest that the dysregulation of this enzyme could contribute to HIV-induced neuronal apoptosis.

DNA Immunization with HIV-1 tat Mutated in the trans Activation Domain Induces Humoral and Cellular Immune Responses Against Wild-Type Tat

Intramuscular immunization of mice with plasmids encoding two transdominant negative mutants of the HIV-1 Tat protein (Tat22 and Tat22/37) elicited a humoral response to wild-type Tat that is comparable to that induced by inoculation of wild-type tat DNA or Tat protein. The percentage of the responders and the Ab titers continued to increase after three additional DNA boosts and pretreatment with bupivacaine at the site of inoculation, without a significant difference (p &gt; 0.05) among the three groups of mice immunized with mutant and wild-type tat genes. By utilizing synthetic peptides representing the amino acid sequence of Tat, one major B cell epitope was defined within the cysteine-rich domain of Tat. Anti-Tat IgG Abs directed against this epitope were found in mice immunized with all tat DNA constructs, whereas different Tat epitopes were detected in mice immunized with the Tat protein. Similarly, IgG2a was the predominant isotype in DNA-immunized mice, with both mutants and wild-type tat genes, as compared with protein immunization, which induced mostly IgG1 and IgG3. Sera from most immunized mice neutralized the effect of extracellular Tat in activating HIV-1 replication. A cellular response was also elicited as indicated by the proliferation of splenocytes when stimulated with wild-type Tat. These results indicate that the wild-type Tat Ag is recognized by Abs and T cells induced by DNA immunization with mutated tat genes, suggesting the possible use of these Tat transdominant mutants, lacking viral trans activation activity and capable of blocking wild-type Tat activity, in the development of an anti-HIV-1 vaccine.

Functional domains of Tat required for efficient human immunodeficiency virus type 1 reverse transcription

Tat expression is required for efficient human immunodeficiency virus type 1 (HIV-1) reverse transcription. In the present study, we generated a series of 293 cell lines that contained a provirus with a tat gene deletion (Deltatat). Cell lines that contained Deltatat and stably transfected vectors containing either wild-type tat or a number of tat mutants were obtained so that the abilities of these tat genes to stimulate HIV-1 gene expression and reverse transcription could be compared. tat genes with mutations in the amino terminus did not stimulate either viral gene expression or HIV-1 reverse transcription. In contrast, tat mutants in the activation, core, and basic domains of Tat did not stimulate HIV-1 gene expression but markedly stimulated HIV-1 reverse transcription. No differences in the levels of virion genomic RNA or tRNA3Lys were seen in the HIV-1 Deltatat viruses complemented with either mutant or wild-type tat. Finally, overexpression of the Tat-associated kinases CDK7 and CDK9, which are involved in Tat activation of HIV-1 transcription, was not able to complement the reverse transcription defects associated with the lack of a functional tat gene. These results indicate that the mechanism by which tat modulates HIV-1 reverse transcription is distinct from its ability to activate HIV-1 gene expression.

Granulocyte-macrophage colony-stimulating factor-activated signaling pathways in human neutrophils. Involvement of Jak2 in the stimulation of phosphatidylinositol 3-kinase

Granulocyte-macrophage colony-stimulating factor (GM-CSF) regulates many of the biological activities of human neutrophils. The signaling pathways via which these effects are mediated are not fully understood. We have shown previously that GM-CSF treatment of human neutrophils activates the Janus kinase/signal transducers and activators of transcription (Jak/STAT) pathway and, more specifically, Jak2, STAT3, and STAT5B in neutrophils. GM-CSF also stimulates the activity of the phosphatidylinositol 3-kinase (PI3-kinase) in a tyrosine kinase-dependent manner. Here we report that pretreating the cells with a Jak2 inhibitor (AG-490) abolishes tyrosine phosphorylation of the p85 subunit of PI3-kinase induced by GM-CSF. Furthermore, p85 was found to associate with Jak2, but not with Lyn, in stimulated cells in situ and with its autophosphorylated form in vitro; however, Jak2 did not bind to either of the two Src homology 2 (SH2) domains of the p85 subunit of PI3-kinase. Although STAT5B bound to the carboxyl-terminal SH2 domain of p85, it was absent from the complex containing PI3-kinase and Jak2. These results suggest that stimulation of the activity of PI3-kinase induced by GM-CSF is mediated by Jak2 and that the association between Jak2 and p85 depends on an adaptor protein yet to be identified.

Characterization of the effects of two polysulfonated distamycin A derivatives, PNU145156E and PNU153429, on HIV type 1 Tat protein

We examined whether two sulfonated distamycin A derivatives, PNU145156E and PNU153529, inhibit the trans-activating and angiogenic effects of HIV-1 Tat protein. The study was carried out by analyzing the activity of the two drugs on: (1) extracellular and intracellular Tat protein, introduced into HL3T1 cells containing an integrated HIV-1 LTR/CAT plasmid; (2) binding of Tat to 3H-labeled heparin and to 14C-labeled PNU145156E; and (3) the angiogenic response induced in vivo by culture medium conditioned by T53c14 cells, which release extracellular Tat. PNU145156E and PNU153429 interacted with extracellular Tat in the culture medium and physically bound the Tat protein, most likely sequestering it in the extracellular space. As a consequence, the two drugs inhibited trans-activation of the HIV-1 LTR on addition of the free Tat protein to HL3T1 cells. However, the two compounds inhibited the activity of intracellular Tat when they were introduced into the cells by lipofection. In vivo experiments showed that the two drugs blocked the neoangiogenesis induced by Tat released in the conditioned medium of T53c14 cells. Owing to the critical role of intracellular and extracellular Tat in HIV-1 replication, these drugs show promise as a means to control the progression of HIV-1 infection as well as the neoplastic and angiogenic effects induced by Tat in the course of AIDS.

HIV-1 Tat induces tyrosine phosphorylation of p125FAK and its association with phosphoinositide 3-kinase in PC12 cells

OBJECTIVE

To evaluate the signal transduction potential of HIV-1 Tat in a neuronal cell model.

METHODS

The tyrosine phosphorylation levels of the focal adhesion kinase p125FAK and its association with phosphoinositide 3-kinase (PI 3-K) were evaluated in serum-starved rat pheochromocytoma PC12 cells, either treated with low concentrations (0.1-1 nM) of extracellular HIV-1 Tat protein or stably transfected with Tat cDNA.

RESULTS

Extracellular Tat induced a rapid increase of p125FAK tyrosine phosphorylation and p125FAK-associated PI 3-K activity. By using recombinant mutated Tat proteins, it was found that deletion of amino acids 73-86 encoded by the second exon of the tat gene resulted in a significant decrease of the ability of Tat to induce p125FAK tyrosine phosphorylation. Paradoxically, mutations in the basic region encoded by the first exon of tat, which is essential for nuclear localization and HIV-1 LTR transactivation, increased the ability of Tat to stimulate p125FAK tyrosine phosphorylation. Moreover, in comparison with cells transfected with a control vector, PC12 cells stably transfected with tat cDNA showed greater amounts of p125FAK protein, an increase in p125FAK tyrosine phosphorylation and higher levels of p125FAK-associated PI 3-K activity. The addition of anti-Tat neutralizing antibody to tat-transfected PC12 cells in culture blocked both the p125FAK tyrosine phosphorylation and its association with PI 3-K but did not affect the total amount of p125FAK.

CONCLUSION

HIV-1 Tat protein enhanced both the expression and the functionality of p1 25FAK in PC12 neuronal cells. Whereas the first event required intracellular Tat, the increased p125FAK phosphorylation was strictly dependent upon extracellular Tat.

Human immunodeficiency virus tat modulates the Flk-1/KDR receptor, mitogen-activated protein kinases, and components of focal adhesion in Kaposi's sarcoma cells

Kaposi's sarcoma (KS) spindle cell growth and spread have been reported to be modulated by various cytokines as well as the human immunodeficiency virus (HIV) gene product Tat. Recently, HIV-1 Tat has been shown to act like a cytokine and bind to the Flk-1/KDR receptor for the vascular endothelial growth factor A (VEGF-A), which is expressed by KS cells. We have characterized signal transduction pathways stimulated by HIV-1 Tat upon its binding to surface receptors on KS cells. We observed that stimulation in KS 38 spindle cells resulted in tyrosine phosphorylation and activation of the Flk-1/KDR receptor. We also report that HIV-1 Tat treatment enhanced the phosphorylation and association of proteins found in focal adhesions, such as the related adhesion focal tyrosine kinase RAFTK, paxillin, and p130(cas). Further characterization revealed the activation of mitogen-activated protein kinase, c-Jun amino-terminal kinase (JNK), and Src kinase. HIV-1 Tat contains a basic domain which can interact with growth factor tyrosine kinase receptors and a classical RGD sequence which may bind to and activate the surface integrin receptors for fibronectin and vitronectin. We observed that stimulation of KS cells with basic as well as RGD sequence-containing Tat peptides resulted in enhanced phosphorylation of RAFTK and activation of MAP kinase. These studies reveal that Tat stimulation activates a number of signal transduction pathways that are associated with cell growth and migration.