Browsing by Author "Fan, Yan"

Now showing 1 - 4 of 4
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    Activation of Egr-1 expression in astrocytes by HIV-1 Tat: new insights into astrocyte-mediated Tat neurotoxicity
    (Springer Nature, 2011-03) Fan, Yan; Zou, Wei; Green, Linden A.; oh Kim, Byung; He, Johnny J.; Microbiology and Immunology, School of Medicine
    Human immunodeficiency virus type 1 (HIV-1) Tat plays an important role in HIV-associated neuropathogenesis; the underlying mechanisms are still evolving. We have recently shown that HIV-1 Tat induces expression of glial fibrillary acidic protein (GFAP), a characteristic of HIV-1 infection of the central nervous system (CNS). We have also shown that the Tat-induced GFAP expression in astrocytes is regulated by p300, and that deletion of the early growth response 1 (Egr-1) cis-transacting element within the p300 promoter abolishes Tat-induced GFAP expression. In this study, we further examined the relationship between Tat and Egr-1 in astrocytes. We found increased Egr-1 protein expression in Tat-expressing human astrocytoma cells and mouse primary astrocytes. Using the Egr-1 promoter-driven firefly luciferase reporter gene assay and the site-directed mutagenesis, we demonstrated that Tat increased Egr-1 expression by transactivating the Egr-1 promoter and involving specific serum response elements (SRE) within the promoter. Consistent with these data, we showed that Tat transactivation of the Egr-1 promoter was abrogated when astrocytes were cultured in serum-reduced media. Taken together, these results reveal that Tat directly transactivates Egr-1 expression and suggest that Tat interaction with Egr-1 is probably one of the very upstream molecular events that initiate Tat-induced astrocyte dysfunction and subsequent Tat neurotoxicity.
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    HIV Tat Impairs Neurogenesis through Functioning As a Notch Ligand and Activation of Notch Signaling Pathway
    (Society for Neuroscience., 2016-11-02) Fan, Yan; Gao, Xiang; Chen, Jinhui; Liu, Ying; He, Johnny J.; Neurological Surgery, School of Medicine
    Alterations in adult neurogenesis have been noted in the brain of HIV-infected individuals and are likely linked to HIV-associated neurocognitive deficits, including those in learning and memory. But the underlying molecular mechanisms are not fully understood. In the study, we took advantage of doxycycline-inducible and astrocyte-specific HIV-1 Tat transgenic mice (iTat) and determined the relationship between Tat expression and neurogenesis. Tat expression in astrocytes was associated with fewer neuron progenitor cells (NPCs), fewer immature neurons, and fewer mature neurons in the dentate gyrus of the hippocampus of the mouse brain. In vitro NPC-derived neurosphere assays showed that Tat-containing conditioned media from astrocytes or recombinant Tat protein inhibited NPC proliferation and migration and altered NPC differentiation, while immunodepletion of Tat from Tat-containing conditioned media or heat inactivation of recombinant Tat abrogated those effects. Notch signaling downstream gene Hes1 promoter-driven luciferase reporter gene assay and Western blotting showed that recombinant Tat or Tat-containing conditioned media activated Hes1 transcription and protein expression, which were abrogated by Tat heat inactivation, immunodepletion, and cysteine mutation at position 30. Last, Notch signaling inhibitor N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester (DAPT) significantly rescued Tat-impaired NPC differentiation in vitro and neurogenesis in vivo Together, these results show that Tat adversely affects NPCs and neurogenesis through Notch signaling and point to the potential of developing Notch signaling inhibitors as HIV/neuroAIDS therapeutics. SIGNIFICANCE STATEMENT: HIV infection of the CNS causes cognitive and memory deficits, which have become more prevalent in the era of combination antiretroviral therapy (cART). Neurogenesis is impaired in HIV-infected individuals. But the underlying molecular mechanisms remain largely unknown. In this study, we have discovered that HIV Tat impairs neurogenesis through the Notch signaling pathway. These findings are particularly important because Tat protein has recently been detected in the brain of HIV-infected individuals with HIV replication in the periphery being effectively controlled by cART. The current study not only further highlights the importance of HIV Tat protein in HIV/neuroAIDS, but also presents a new strategy to develop novel HIV/neuroAIDS therapeutics, particularly in the era of cART.
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    Involvement of p300 in constitutive and HIV-1 Tat-activated expression of glial fibrillary acidic protein in astrocytes
    (Wiley, 2010-10) Zou, Wei; Wang, Zhenyuan; Liu, Ying; Fan, Yan; Zhou, Betty Y.; Yang, X. Frank; He, Johnny J.; Microbiology and Immunology, School of Medicine
    HIV-1 Tat protein is an important pathogenic factor in HIV-1-associated neurological diseases. One hallmark of HIV-1 infection of the central nervous system (CNS) is astrocytosis, which is characterized by elevated GFAP expression in astrocytes. We have shown that Tat activates GFAP expression in astrocytes (Zhou, et al., Mol. Cell. Neurosci. 27:296, 2004) and that GFAP is an important regulator of Tat neurotoxicity (Zou, et. al., Am. J. Pathol. 171:1293, 2007). However, the underlying mechanisms for Tat-mediated GFAP up-regulation are not understood. In the current study, we reported concurrent up-regulation of adenovirus E1a-associated 300 kDa protein p300 and GFAP in Tat-expressing human astroytoma cells and primary astrocytes. We showed that p300 was indeed induced by Tat expression and HIV-1 infection and that the induction occurred at the transcriptional level through the cis-acting elements of early growth response 1 (Egr-1) within its promoter. Using siRNA, we further showed that p300 regulated both constitutive and Tat-mediated GFAP expression. Moreover, we showed that ectopic expression of p300 potentiated Tat transactivation activity and increased proliferation of HIV-1-infected astrocytes, but had little effect on HIV-1 replication in these cells. Taken together, these results demonstrate for the first time that Tat is a positive regulator of p300 expression, which in turn regulates GFAP expression, and suggest that the Tat-Egr-1-p300-GFAP axis likely contributes to Tat neurotoxicity and predisposes astrocytes to be an HIV-1 sanctuary in the CNS.
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    Tip110 Regulates the Cross Talk between p53 and Hypoxia-Inducible Factor 1α under Hypoxia and Promotes Survival of Cancer Cells
    (MCB, 2015-07) Timani, Khalid Amine; Liu, Ying; Fan, Yan; Mohammad, Khalid S.; He, Johnny J.; Department of Medicine, IU School of Medicine
    Hypoxia often occurs under various physiological and pathophysiological conditions, including solid tumors; it is linked to malignant transformation, metastatic progression, and treatment failure or resistance. Tip110 protein plays important roles in several known physiological and pathophysiological processes, including cancers. Thus, in the present study we investigated the regulation of Tip110 expression under hypoxia. Hypoxia led to Tip110 protein degradation through the ubiquitin-proteasome system. Under hypoxia, Tip110 stabilized p53, which in return destabilized Tip110. In addition, Tip110 regulated hypoxia-inducible factor 1α (HIF-1α), likely through enhancement of its protein stability. Furthermore, Tip110 upregulated p300, a known coactivator for both p53 and HIF-1α. Expression of a p53(22/23) mutant deficient in p300 binding accelerated Tip110 degradation under hypoxia. Tip110 knockdown resulted in the inhibition of cell proliferation and cell death in the presence of p53. Finally, significantly less Tip110, p53, and HIF-1α was detected in the hypoxic region of bone metastasis tumors in a mouse model of human melanoma cells. Taken together, these results suggest Tip110 is an important mediator in the cross talk between p53 and HIF-1α in response to hypoxic stress.