Opposing regulation of PROX1 by interleukin-3 receptor and NOTCH directs differential host cell fate reprogramming by Kaposi sarcoma herpes virus

The Kaposi's sarcoma progenitor enigma: KSHV-induced MEndT-EndMT axis

Endothelial-to-mesenchymal transition has been described in tumors as a source of mesenchymal stroma, while the reverse process has been proposed in tumor vasculogenesis and angiogenesis. A human oncogenic virus, Kaposi's sarcoma herpes virus (KSHV), can regulate both processes in order to transit through this transition 'boulevard' when infecting KS oncogenic progenitor cells. Endothelial or mesenchymal circulating progenitor cells can serve as KS oncogenic progenitors recruited by inflammatory cytokines because KSHV can reprogram one into the other through endothelial-to-mesenchymal and mesenchymal-to-endothelial transitions. Through these novel insights, the identity of the potential oncogenic progenitor of KS is revealed while gaining knowledge of the biology of the mesenchymal-endothelial differentiation axis and pointing to this axis as a therapeutic target in KS.

Meningeal lymphatic vessels mediate neurotropic viral drainage from the central nervous system

Recent studies have demonstrated that brain meningeal lymphatic vessels (MLVs) act as a drainage path directly into the cervical lymph nodes (CLNs) for macromolecules contained in the cerebrospinal fluid (CSF). However, the role of MLVs during CNS viral infection remains unexplored. Here, we found that infection with several neurotropic viruses in mice promotes MLV expansion but also causes impaired MLV-mediated drainage of macromolecules. Notably, MLVs could drain virus from the CNS to CLNs. Surgical ligation of the lymph vessels or photodynamic ablation of dorsal MLVs increased neurological damage and mortality of virus-infected mice. By contrast, pretreatment with vascular endothelial growth factor C promoted expansion of functional MLVs and alleviated the effects of viral infection. Together, these data indicate that functional MLVs facilitate virus clearance, and MLVs represent a critical path for virus spreading from the CNS to the CLNs. MLV-based therapeutic strategies may thus be useful for alleviating infection-induced neurological damage.

Kaposi's sarcoma-associated herpesvirus promotes mesenchymal-to-endothelial transition by resolving the bivalent chromatin of PROX1 gene

Increasing evidence suggests that Kaposi’s sarcoma (KS) arises from Kaposi’s sarcoma-associated herpesvirus (KSHV)-infected mesenchymal stem cells (MSCs) through mesenchymal-to-endothelial transition (MEndT). KSHV infection promotes MSC differentiation of endothelial lineage and acquisition of tumorigeneic phenotypes. To understand how KSHV induces MEndT and transforms MSCs to KS cells, we investigated the mechanism underlying KSHV-mediated MSC endothelial lineage differentiation. Like embryonic stem cells, MSC differentiation and fate determination are under epigenetic control. Prospero homeobox 1 (PROX1) is a master regulator that controls lymphatic vessel development and endothelial differentiation. We found that the PROX1 gene in MSCs harbors a distinctive bivalent epigenetic signature consisting of both active marker H3K4me3 and repressive marker H3K27me3, which poises expression of the genes, allowing timely activation upon differentiation signals or environmental stimuli. KSHV infection effectively resolves the bivalent chromatin by decreasing H3K27me3 and increasing H3K4me3 to activate the PROX1 gene. vIL-6 signaling leads to the recruitment of MLL2 and SET1 complexes to the PROX1 promoter to increase H3K4me3, and the vGPCR-VEGF-A axis is responsible for removing PRC2 from the promoter to reduce H3K27me3. Therefore, through a dual signaling process, KSHV activates PROX1 gene expression and initiates MEndT, which renders MSC tumorigenic features including angiogenesis, invasion and migration.

Numerous parallelisms between development and cancer led to the concept that cancer is a development problem over the past 50 years. As our knowledge of epigenetic regulation is advancing, the similarities between development and cancer are becoming more apparent, providing further support to the theory. KSHV infection of mesenchymal stem cells (MSCs) may result in Kaposi’s sarcoma (KS) through mesenchymal-to-endothelial transition (MEndT), a process resembling endothelial differentiation during development. KSHV initiates MEndT by activating the homeobox gene PROX1, a master regulator of the lymphatic endothelial cell differentiation, at the epigenetic level. Here we found that the PROX1 gene resides in bivalent domain chromatin in MSCs and KSHV infection resolves it through a dual signaling process to activates the PROX1 gene, which initiates MEndT and confers MSC KS-like phenotypes. The significance of this study is two-fold. First, the study elucidated the mechanism underlying KSHV-mediated MEndT and KS development at the transcription level. Second, KSHV uses two independent pathways to elevate activating histone modification and decrease repressive marker, respectively, to resolved bivalent chromatin, revealing a two-factor-authentication mechanism in the epigenetic regulation, which may grant a more efficient and accurate response to activate a gene in bivalent chromatin.

Protective Effects of Collagen Tripeptides in Human Aortic Endothelial Cells by Restoring ROS-Induced Transcriptional Repression

Collagen tripeptide (CTP) is defined as a functional food material derived from collagenase digests of type I collagen and contains a high concentration of tripeptides with a Gly-X-Y sequence. CTP has several biological effects, including the acceleration of fracture healing, ameliorating osteoarthritis, and improving dryness and photoaging of the skin. Recently, an antiatherosclerotic effect of CTP has been reported, although its molecular mechanism is yet to be determined. In this study, we examined the effects of CTP on primary cultured human aortic endothelial cells (HAECs) under oxidative stress, because oxidative endothelial dysfunction is a trigger of atherosclerosis. DNA microarray and RT-qPCR analyses showed that CTP treatment recovered the downregulated expression of several genes, including the interleukin-3 receptor subunit alpha (IL3RA), which were suppressed by reactive oxygen species (ROS) treatment in HAECs. Furthermore, IL3RA knockdown significantly decreased the viability of HAECs compared with control cells. RT-qPCR analysis also showed that solute carrier 15 family peptide transporters, which are involved in CTP absorption into cells, were expressed in HAECs at levels more than comparable to those of a CTP-responsive human osteoblastic cell line. These results indicated that CTP exerts a protective effect for HAECs, at least in part, by regulating the recovery of ROS-induced transcriptional repression.

The Dynamic Interface of Viruses with STATs

Viruses commonly antagonize the antiviral type I interferon response by targeting signal transducer and activator of transcription 1 (STAT1) and STAT2, key mediators of interferon signaling. Other STAT family members mediate signaling by diverse cytokines important to infection, but their relationship with viruses is more complex. Importantly, virus-STAT interaction can be antagonistic or stimulatory depending on diverse viral and cellular factors. While STAT antagonism can suppress immune pathways, many viruses promote activation of specific STATs to support viral gene expression and/or produce cellular conditions conducive to infection. It is also becoming increasingly clear that viruses can hijack noncanonical STAT functions to benefit infection. For a number of viruses, STAT function is dynamically modulated through infection as requirements for replication change. Given the critical role of STATs in infection by diverse viruses, the virus-STAT interface is an attractive target for the development of antivirals and live-attenuated viral vaccines. Here, we review current understanding of the complex and dynamic virus-STAT interface and discuss how this relationship might be harnessed for medical applications.

Oncogenic Herpesvirus Engages Endothelial Transcription Factors SOX18 and PROX1 to Increase Viral Genome Copies and Virus Production

Kaposi sarcoma is a tumor caused by Kaposi sarcoma herpesvirus (KSHV) infection and is thought to originate from lymphatic endothelial cells (LEC). While KSHV establishes latency in virtually all susceptible cell types, LECs support spontaneous expression of oncogenic lytic genes, high viral genome copies, and release of infectious virus. It remains unknown the contribution of spontaneous virus production to the expansion of KSHV-infected tumor cells and the cellular factors that render the lymphatic environment unique to KSHV life cycle. We show here that expansion of the infected cell population, observed in LECs, but not in blood endothelial cells, is dependent on the spontaneous virus production from infected LECs. The drivers of lymphatic endothelium development, SOX18 and PROX1, regulated different steps of the KSHV life cycle. SOX18 enhanced the number of intracellular viral genome copies and bound to the viral origins of replication. Genetic depletion or chemical inhibition of SOX18 caused a decrease of KSHV genome copy numbers. PROX1 interacted with ORF50, the viral initiator of lytic replication, and bound to the KSHV genome in the promoter region of ORF50, increasing its transactivation activity and KSHV spontaneous lytic gene expression and infectious virus release. In Kaposi sarcoma tumors, SOX18 and PROX1 expression correlated with latent and lytic KSHV protein expression. These results demonstrate the importance of two key transcriptional drivers of LEC fate in the regulation of the tumorigenic KSHV life cycle. Moreover, they introduce molecular targeting of SOX18 as a potential novel therapeutic avenue in Kaposi sarcoma. SIGNIFICANCE: SOX18 and PROX1, central regulators of lymphatic development, are key factors for KSHV genome maintenance and lytic cycle in lymphatic endothelial cells, supporting Kaposi sarcoma tumorigenesis and representing attractive therapeutic targets.

MMP14 in Sarcoma: A Regulator of Tumor Microenvironment Communication in Connective Tissues

Sarcomas are deadly malignant tumors of mesenchymal origin occurring at all ages. The expression and function of the membrane-type matrix metalloproteinase MMP14 is closely related to the mesenchymal cell phenotype, and it is highly expressed in most sarcomas. MMP14 regulates the activity of multiple extracellular and plasma membrane proteins, influencing cell–cell and cell–extracellular matrix (ECM) communication. This regulation mediates processes such as ECM degradation and remodeling, cell invasion, and cancer metastasis. Thus, a comprehensive understanding of the biology of MMP14 in sarcomas will shed light on the mechanisms controlling the key processes in these diseases. Here, we provide an overview of the function and regulation of MMP14 and we discuss their relationship with clinical and pre-clinical MMP14 data in both adult and childhood sarcomas.

An In Vitro Model for Studying Cellular Transformation by Kaposi Sarcoma Herpesvirus

Kaposi sarcoma (KS) is an unusual tumor composed of proliferating spindle cells that is initiated by infection of endothelial cells (EC) with KSHV, and develops most often in the setting of immunosuppression. Despite decades of research, optimal treatment of KS remains poorly defined and clinical outcomes are especially unfavorable in resource-limited settings. KS lesions are driven by pathological angiogenesis, chronic inflammation, and oncogenesis, and various in vitro cell culture models have been developed to study these processes. KS arises from KSHV-infected cells of endothelial origin, so EC-lineage cells provide the most appropriate in vitro surrogates of the spindle cell precursor. However, because EC have a limited in vitro lifespan, and as the oncogenic mechanisms employed by KSHV are less efficient than those of other tumorigenic viruses, it has been difficult to assess the processes of transformation in primary or telomerase-immortalized EC. Therefore, a novel EC-based culture model was developed that readily supports transformation following infection with KSHV. Ectopic expression of the E6 and E7 genes of human papillomavirus type 16 allows for extended culture of age- and passage-matched mock- and KSHV-infected EC and supports the development of a truly transformed (i.e., tumorigenic) phenotype in infected cell cultures. This tractable and highly reproducible model of KS has facilitated the discovery of several essential signaling pathways with high potential for translation into clinical settings.

Kaposi's Sarcoma-Associated Herpesvirus: Epidemiology and Molecular Biology

Kaposi's sarcoma-associated herpesvirus (KSHV), also known as Human herpesvirus 8 (HHV-8), is a member of the lymphotropic gammaherpesvirus subfamily and a human oncogenic virus. Since its discovery in AIDS-associated KS tissues by Drs. Yuan Chang and Patrick Moore, much progress has been made in the past two decades. There are four types of KS including classic KS, endemic KS, immunosuppressive therapy-related KS, and AIDS-associated KS. In addition to KS, KSHV is also involved in the development of primary effusion lymphoma (PEL) and certain types of multicentric Castleman's disease. KSHV manipulates numerous viral proteins to promote the progression of angiogenesis and tumorigenesis. In this chapter, we review the epidemiology and molecular biology of KSHV and the mechanisms underlying KSHV-induced diseases.

Fine-Tuning of the Kaposi's Sarcoma-Associated Herpesvirus Life Cycle in Neighboring Cells through the RTA-JAG1-Notch Pathway

Kaposi’s sarcoma (KS)-associated herpesvirus (KSHV) is an oncogenic pathogen that displays latent and lytic life cycles. In KS lesions, infiltrated immune cells, secreted viral and/or cellular cytokines, and hypoxia orchestrate a chronic pro-lytic microenvironment that can promote KSHV reactivation. However, only a small subset of viruses spontaneously undergoes lytic replication in this pro-lytic microenvironment while the majority remains in latency. Here, we show that the expression of the Notch ligand JAG1 is induced by KSHV-encoded replication and transcription activator (RTA) during reactivation. JAG1 up-regulation activates Notch signaling in neighboring cells and prevents viral lytic replication. The suppression of JAG1 and Notch1 with inhibitors or small interfering RNA promotes lytic replication in the presence of RTA induction or under conditions of hypoxia. The underlying mechanism involves the Notch downstream effector hairy and enhancer of split 1 (Hes1), which directly binds lytic gene promoters and attenuates viral lytic gene expression. RTA interacts with lymphoid enhancer-binding factor 1 (LEF1), disrupts LEF1/Groucho/TLE suppressive complexes and releases LEF1 to activate JAG1 expression. Taken together, our results suggest that cells with viral lytic replication can inhibit KSHV reactivation in neighboring cells through an RTA-JAG1-Notch pathway. These data provide insight into the mechanism by which the virus maintains the balance between lytic and latent infection in the pro-lytic tumor microenvironment.

KSHV infected cells display significant heterogeneity in viral lytic replication within the universal pro-lytic inflammatory milieu, suggesting that the balance between latency and reactivation is carefully regulated. This fine-tuned regulatory mechanism is essential for KSHV to persist in the host and drive cells to malignancy. In the present study, we show that KSHV can usurp the Notch signaling pathway to inhibit the viral lytic life cycle in neighboring cells. Notch signaling in surrounding cells can be activated through an RTA-JAG1-Notch pathway initiated by cells in which KSHV is reactivated. Activated Notch inhibits KSHV reactivation through its downstream effector Hes1. These findings suggest that the ability of Notch to determine the fate of adjacent cells is hijacked by KSHV to maintain its life cycle, providing a mechanistic explanation for the phenomenon by which only a small fraction of viruses enters lytic replication in the common pro-lytic microenvironment.

Nuclear receptor NR5A2 controls neural stem cell fate decisions during development

The enormous complexity of mammalian central nervous system (CNS) is generated by highly synchronized actions of diverse factors and signalling molecules in neural stem/progenitor cells (NSCs). However, the molecular mechanisms that integrate extrinsic and intrinsic signals to control proliferation versus differentiation decisions of NSCs are not well-understood. Here we identify nuclear receptor NR5A2 as a central node in these regulatory networks and key player in neural development. Overexpression and loss-of-function experiments in primary NSCs and mouse embryos suggest that NR5A2 synchronizes cell-cycle exit with induction of neurogenesis and inhibition of astrogliogenesis by direct regulatory effects on Ink4/Arf locus, Prox1, a downstream target of proneural genes, as well as Notch1 and JAK/STAT signalling pathways. Upstream of NR5a2, proneural genes, as well as Notch1 and JAK/STAT pathways control NR5a2 endogenous expression. Collectively, these observations render NR5A2 a critical regulator of neural development and target gene for NSC-based treatments of CNS-related diseases.

The molecular signals regulating the decision of neural stem cells (NSC) to proliferate versus differentiate are unclear. Here, the authors identify the nuclear receptor NR5A2 as coordinating cell-cycle exit with differentiation of NSCs via direct actions on Ink4, Prox1, Notch1 and JAK/STAT.

KSHV-Mediated Angiogenesis in Tumor Progression

Human herpesvirus 8 (HHV-8), also known as Kaposi's sarcoma-associated herpesvirus (KSHV), is a malignant human oncovirus belonging to the gamma herpesvirus family. HHV-8 is closely linked to the pathogenesis of Kaposi's sarcoma (KS) and two other B-cell lymphoproliferative diseases: primary effusion lymphoma (PEL) and a plasmablastic variant of multicentric Castleman's disease (MCD). KS is an invasive tumor of endothelial cells most commonly found in untreated HIV-AIDS or immuno-compromised individuals. KS tumors are highly vascularized and have abnormal, excessive neo-angiogenesis, inflammation, and proliferation of infected endothelial cells. KSHV directly induces angiogenesis in an autocrine and paracrine fashion through a complex interplay of various viral and cellular pro-angiogenic and inflammatory factors. KS is believed to originate due to a combination of KSHV's efficient strategies for evading host immune systems and several pro-angiogenic and pro-inflammatory stimuli. In addition, KSHV infection of endothelial cells produces a wide array of viral oncoproteins with transforming capabilities that regulate multiple host-signaling pathways involved in the activation of angiogenesis. It is likely that the cellular-signaling pathways of angiogenesis and lymph-angiogenesis modulate the rate of tumorigenesis induction by KSHV. This review summarizes the current knowledge on regulating KSHV-mediated angiogenesis by integrating the findings reported thus far on the roles of host and viral genes in oncogenesis, recent developments in cell-culture/animal-model systems, and various anti-angiogenic therapies for treating KSHV-related lymphoproliferative disorders.

Apolipoprotein A-I Limits the Negative Effect of Tumor Necrosis Factor on Lymphangiogenesis

OBJECTIVE

Lymphatic endothelial dysfunction underlies the pathogenesis of many chronic inflammatory disorders. The proinflammatory cytokine tumor necrosis factor (TNF) is known for its role in disrupting the function of the lymphatic vasculature. This study investigates the ability of apolipoprotein (apo) A-I, the principal apolipoprotein of high-density lipoproteins, to preserve the normal function of lymphatic endothelial cells treated with TNF.

APPROACH AND RESULTS

TNF decreased the ability of lymphatic endothelial cells to form tube-like structures. Preincubation of lymphatic endothelial cells with apoA-I attenuated the TNF-mediated inhibition of tube formation in a concentration-dependent manner. In addition, apoA-I reversed the TNF-mediated suppression of lymphatic endothelial cell migration and lymphatic outgrowth in thoracic duct rings. ApoA-I also abrogated the negative effect of TNF on lymphatic neovascularization in an ATP-binding cassette transporter A1-dependent manner. At the molecular level, this involved downregulation of TNF receptor-1 and the conservation of prospero-related homeobox gene-1 expression, a master regulator of lymphangiogenesis. ApoA-I also re-established the normal phenotype of the lymphatic network in the diaphragms of human TNF transgenic mice.

CONCLUSIONS

ApoA-I restores the neovascularization capacity of the lymphatic system during TNF-mediated inflammation. This study provides a proof-of-concept that high-density lipoprotein-based therapeutic strategies may attenuate chronic inflammation via its action on lymphatic vasculature.

Nuclear PROX1 is Associated with Hypoxia-Inducible Factor 1α Expression and Cancer Progression in Esophageal Squamous Cell Carcinoma

BACKGROUND

Transcription factor prospero homeobox 1 (PROX1) has been identified as a master regulator of lymphangiogenesis associated with metastasis. Although PROX1 expression has been investigated in several cancers, its clinical significance remains controversial and needs further validation. In this study, we investigated the clinical and functional significance of PROX1 and PROX1 regulator hypoxia-inducible factor 1α (HIF1α) in esophageal squamous cell carcinoma (ESCC).

METHODS

A total of 117 samples from ESCC patients were analyzed for PROX1, HIF1α, and E-cadherin expression by immunohistochemistry; correlation with clinicopathological characteristics was determined. PROX1 function was evaluated in PROX1 small interfering RNA (siRNA)-transfected human ESCC cells in vitro by assessing cell proliferation and migration.

RESULTS

PROX1 expression was higher in ESCC than in normal tissues. Patients with higher PROX1 expression (n = 26) had increased nuclear accumulation of HIF1α (p = 0.004) and more advanced metastasis, both lymph node (N factor; p = 0.09) and hematogenous (M factor; p = 0.04), than those with lower PROX1 expression (n = 91). In addition, high PROX1 and HIF1α expression correlated with low levels of E-cadherin, an epithelial cell marker. Analysis of overall and cancer-specific survival indicated that elevated PROX1 expression was significantly correlated with poor prognosis (p = 0.0064). PROX1 downregulation in ESCC cells inhibited cellular proliferation and migration (p < 0.05). Hypoxia restored PROX1 levels that were reduced by PROX1-specific siRNA.

CONCLUSION

Our data suggest that high expression of PROX1 in ESCC could be used as an indicator of poor prognosis, and that PROX1 is a promising candidate molecular target for ESCC treatment.

TH2 cells and their cytokines regulate formation and function of lymphatic vessels

Lymphatic vessels (LVs) are critical for immune surveillance and involved in the pathogenesis of diverse diseases. LV density is increased during inflammation; however, little is known about how the resolution of LVs is controlled in different inflammatory conditions. Here we show the negative effects of T helper type 2 (TH2) cells and their cytokines on LV formation. IL-4 and IL-13 downregulate essential transcription factors of lymphatic endothelial cells (LECs) and inhibit tube formation. Co-culture of LECs with TH2 cells also inhibits tube formation, but this effect is fully reversed by interleukin (IL)-4 and/or IL-13 neutralization. Furthermore, the in vivo blockade of IL-4 and/or IL-13 in an asthma model not only increases the density but also enhances the function of lung LVs. These results demonstrate an anti-lymphangiogenic function of TH2 cells and their cytokines, suggesting a potential usefulness of IL-4 and/or IL-13 antagonist as therapeutic agents for allergic asthma through expanding LV mediated-enhanced antigen clearance from the inflammatory sites.

Regulation of the life cycle of HPVs by differentiation and the DNA damage response

HPVs are the causative agents of cervical and other anogenital cancers. HPVs infect stratified epithelia and link their productive life cycles to cellular differentiation. Low levels of viral genomes are stably maintained in undifferentiated cells and productive replication or amplification is restricted to differentiated suprabasal cells. Amplification is dependent on the activation of the ATM DNA damage factors that are recruited to viral replication centers and inhibition of this pathway blocks productive replication. The STAT-5 protein appears to play a critical role in mediating activation of the ATM pathway in HPV-positive cells. While HPVs need to activate the DNA damage pathway for replication, cervical cancers contain many genomic alterations suggesting that this pathway is circumvented during progression to malignancy.

Manipulation of endothelial cells by KSHV: implications for angiogenesis and aberrant vascular differentiation

Kaposi sarcoma (KS), a viral cancer associated to Kaposi sarcoma herpesvirus (KSHV) infection, is currently the most common tumor in men in sub-Saharan Africa. KS is an angiogenic tumor and characterized by the presence of aberrant vascular structures in the lesion. Although our understanding of how KSHV causes the aberrant differentiation of endothelial cells and the typical vascular abnormalities in KS tumors is far from complete, the experimental evidence reviewed here provides a comprehensive description of the role of KSHV in the pathogenesis of this unusual tumor. In contrast to other tumor viruses, whose interference with cellular processes relating to cell cycle, apoptosis and DNA damage may be at the heart of their oncogenic properties, KSHV may cause KS primarily by its ability to engage with the differentiation and function of endothelial cells. Although the intracellular pathways engaged by KSHV in the endothelial cells are being explored as drug targets, a better understanding of the impact of KSHV on endothelial cell differentiation and vasculogenesis is needed before the encouraging findings can form the basis for new targeted therapeutic approaches to KS.

Hey bHLH transcription factors

Hey bHLH transcription factors are direct targets of canonical Notch signaling. The three mammalian Hey proteins are closely related to Hes proteins and they primarily repress target genes by either directly binding to core promoters or by inhibiting other transcriptional activators. Individual candidate gene approaches and systematic screens identified a number of Hey target genes, which often encode other transcription factors involved in various developmental processes. Here, we review data on interaction partners and target genes and conclude with a model for Hey target gene regulation. Furthermore, we discuss how expression of Hey proteins affects processes like cell fate decisions and differentiation, e.g., in cardiovascular, skeletal, and neural development or oncogenesis and how this relates to the observed developmental defects and phenotypes observed in various knockout mice.

Cellular origin of Kaposi's sarcoma and Kaposi's sarcoma-associated herpesvirus-induced cell reprogramming

Kaposi's sarcoma (KS) is the most common malignancy in untreated HIV patients. KS is characterised by abnormal neoangiogenesis, inflammation, and proliferation of tumour cells [KS spindle cells (SCs)]. Kaposi's sarcoma-associated herpesvirus (KSHV) is the aetiological agent of KS. KS SCs are the predominant KSHV-infected cells in KS lesions. In this review, we report advances in understanding of the cellular origin of the KS SC, a contentious topic in KSHV research. KS SCs are now known to be of endothelial cell (EC) origin, phenotypically most similar to lymphatic ECs (LECs), but poorly differentiated. We focus on recent insights into KSHV's ability to exploit the normal differentiation pathway and intrinsic plasticity of ECs, through manipulation of EC-specific transcriptional regulators [i.e., prospero homeobox 1 (PROX1) and MAF] and discuss how this may contribute to viral persistence and KS sarcomagenesis.

The JAK-STAT transcriptional regulator, STAT-5, activates the ATM DNA damage pathway to induce HPV 31 genome amplification upon epithelial differentiation

High-risk human papillomavirus (HPV) must evade innate immune surveillance to establish persistent infections and to amplify viral genomes upon differentiation. Members of the JAK-STAT family are important regulators of the innate immune response and HPV proteins downregulate expression of STAT-1 to allow for stable maintenance of viral episomes. STAT-5 is another member of this pathway that modulates the inflammatory response and plays an important role in controlling cell cycle progression in response to cytokines and growth factors. Our studies show that HPV E7 activates STAT-5 phosphorylation without altering total protein levels. Inhibition of STAT-5 phosphorylation by the drug pimozide abolishes viral genome amplification and late gene expression in differentiating keratinocytes. In contrast, treatment of undifferentiated cells that stably maintain episomes has no effect on viral replication. Knockdown studies show that the STAT-5β isoform is mainly responsible for this activity and that this is mediated through the ATM DNA damage response. A downstream target of STAT-5, the peroxisome proliferator-activated receptor γ (PPARγ) contributes to the effects on members of the ATM pathway. Overall, these findings identify an important new regulatory mechanism by which the innate immune regulator, STAT-5, promotes HPV viral replication through activation of the ATM DNA damage response.

Over 120 types of human papillomavirus (HPV) have been identified, and approximately one-third of these infect epithelial cells of the genital mucosa. A subset of HPV types are the causative agents of cervical and other anogenital cancers. The infectious life cycle of HPV is dependent on differentiation of the host epithelial cell, with viral genome amplification and virion production restricted to differentiated suprabasal cells. While normal keratinocytes exit the cell cycle upon differentiation, HPV-positive suprabasal cells are able to re-enter S-phase to mediate productive replication. HPV induces an ATM-dependent DNA damage response in differentiating cells that is essential for viral genome amplification. Our studies describe an important mechanism by which human papillomaviruses activate a member of the JAK/STAT innate immune signaling pathway to induce the ATM DNA damage pathway. This is necessary for differentiation-dependent productive viral replication. HPVs must suppress the transcription of one member of the JAK/STAT pathway, STAT-1, while at the same time activating STAT-5 to regulate genome amplification in suprabasal cells. The E7 protein activates STAT-5 leading to induction of ATM phosphorylation through the PPARγ pathway. Our study identifies important links between innate immune signaling, the ATM DNA damage pathway and productive HPV replication that may lead to the characterization of new targets for the development of therapeutics to treat HPV-induced infections.