Targeting Kaposi's Sarcoma-Associated Herpesvirus ORF21 Tyrosine Kinase and Viral Lytic Reactivation by Tyrosine Kinase Inhibitors Approved for Clinical Use

Polypharmacology-based kinome screen identifies new regulators of KSHV reactivation

Kaposi's sarcoma-associated herpesvirus (KSHV) causes several human diseases including Kaposi's sarcoma (KS), a leading cause of cancer in Africa and in patients with AIDS. KS tumor cells harbor KSHV predominantly in a latent form, while typically <5% contain lytic replicating virus. Because both latent and lytic stages likely contribute to cancer initiation and progression, continued dissection of host regulators of this biological switch will provide insights into fundamental pathways controlling the KSHV life cycle and related disease pathogenesis. Several cellular protein kinases have been reported to promote or restrict KSHV reactivation, but our knowledge of these signaling mediators and pathways is incomplete. We employed a polypharmacology-based kinome screen to identify specific kinases that regulate KSHV reactivation. Those identified by the screen and validated by knockdown experiments included several kinases that enhance lytic reactivation: ERBB2 (HER2 or neu), ERBB3 (HER3), ERBB4 (HER4), MKNK2 (MNK2), ITK, TEC, and DSTYK (RIPK5). Conversely, ERBB1 (EGFR1 or HER1), MKNK1 (MNK1) and FRK (PTK5) were found to promote the maintenance of latency. Mechanistic characterization of ERBB2 pro-lytic functions revealed a signaling connection between ERBB2 and the activation of CREB1, a transcription factor that drives KSHV lytic gene expression. These studies provided a proof-of-principle application of a polypharmacology-based kinome screen for the study of KSHV reactivation and enabled the discovery of both kinase inhibitors and specific kinases that regulate the KSHV latent-to-lytic replication switch.

Mouse Models for Human Herpesviruses

More than one hundred herpesviruses have been isolated from different species so far, with nine infecting humans. Infections with herpesviruses are characterized by life-long latency and represent a significant challenge for human health. To investigate the consequences of infections and identify novel treatment options, in vivo models are of particular relevance. The mouse has emerged as an economical small animal model to investigate herpesvirus infections. However, except for herpes simplex viruses (HSV-1, HSV-2), human herpesviruses cannot infect mice. Three natural herpesviruses have been identified in mice: mouse-derived cytomegalovirus (MCMV), mouse herpesvirus 68 (MHV-68), and mouse roseolovirus (MRV). These orthologues are broadly used to investigate herpesvirus infections within the natural host. In the last few decades, immunocompromised mouse models have been developed, allowing the functional engraftment of various human cells and tissues. These xenograft mice represent valuable model systems to investigate human-restricted viruses, making them particularly relevant for herpesvirus research. In this review, we describe the various mouse models used to study human herpesviruses, thereby highlighting their potential and limitations. Emphasis is laid on xenograft mouse models, covering the development and refinement of immune-compromised mice and their application in herpesvirus research.

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.

Polypharmacology-based kinome screen identifies new regulators of KSHV reactivation

Kaposi's sarcoma-associated herpesvirus (KSHV) causes several human diseases including Kaposi's sarcoma (KS), a leading cause of cancer in Africa and in patients with AIDS. KS tumor cells harbor KSHV predominantly in a latent form, while typically <5% contain lytic replicating virus. Because both latent and lytic stages likely contribute to cancer initiation and progression, continued dissection of host regulators of this biological switch will provide insights into fundamental pathways controlling the KSHV life cycle and related disease pathogenesis. Several cellular protein kinases have been reported to promote or restrict KSHV reactivation, but our knowledge of these signaling mediators and pathways is incomplete. We employed a polypharmacology-based kinome screen to identifiy specific kinases that regulate KSHV reactivation. Those identified by the screen and validated by knockdown experiments included several kinases that enhance lytic reactivation: ERBB2 (HER2 or neu ), ERBB3 (HER3), ERBB4 (HER4), MKNK2 (MNK2), ITK, TEC, and DSTYK (RIPK5). Conversely, ERBB1 (EGFR1 or HER1), MKNK1 (MNK1) and FRK (PTK5) were found to promote the maintenance of latency. Mechanistic characterization of ERBB2 pro-lytic functions revealed a signaling connection between ERBB2 and the activation of CREB1, a transcription factor that drives KSHV lytic gene expression. These studies provided a proof-of-principle application of a polypharmacology-based kinome screen for the study of KSHV reactivation and enabled the discovery of both kinase inhibitors and specific kinases that regulate the KSHV latent-to-lytic replication switch.

Author Summary

Kaposi's sarcoma-associated herpesvirus (KSHV) causes Kaposi's sarcoma, a cancer particularly prevalent in Africa. In cancer cells, the virus persists in a quiescent form called latency, in which only a few viral genes are made. Periodically, the virus switches into an active replicative cycle in which most of the viral genes are made and new virus is produced. What controls the switch from latency to active replication is not well understood, but cellular kinases, enzymes that control many cellular processes, have been implicated. Using a cell culture model of KSHV reactivation along with an innovative screening method that probes the effects of many cellular kinases simultaneously, we identified drugs that significantly limit KSHV reactivation, as well as specific kinases that either enhance or restrict KSHV replicative cycle. Among these were the ERBB kinases which are known to regulate growth of cancer cells. Understanding how these and other kinases contribute to the switch leading to production of more infectious virus helps us understand the mediators and mechanisms of KSHV diseases. Additionally, because kinase inhibitors are proving to be effective for treating other diseases including some cancers, identifying ones that restrict KSHV replicative cycle may lead to new approaches to treating KSHV-related diseases.

Kaposi's Sarcoma-Associated Herpesvirus ORF21 Enhances the Phosphorylation of MEK and the Infectivity of Progeny Virus

Kaposi's sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus-8, is the causative agent of Kaposi's sarcoma, Castleman's disease, and primary effusion lymphoma. Although the functions of the viral thymidine kinases (vTK) of herpes simplex virus-1/2 are well understood, that of KSHV ORF21 (an ortholog of vTK) is largely unknown. Here, we investigated the role of ORF21 in lytic replication and infection by generating two ORF21-mutated KSHV BAC clones: ORF21-kinase activity deficient KSHV (21KD) and stop codon-induced ORF21-deleted KSHV (21del). The results showed that both ORF21 mutations did not affect viral genome replication, lytic gene transcription, or the production of viral genome-encapsidated particles. The ORF21 molecule-dependent function, other than the kinase function of ORF21, was involved in the infectivity of the progeny virus. ORF21 was expressed 36 h after the induction of lytic replication, and endogenously expressed ORF21 was localized in the whole cytoplasm. Moreover, ORF21 upregulated the MEK phosphorylation and anchorage-independent cell growth. The inhibition of MEK signaling by U0126 in recipient target cells suppressed the number of progeny virus-infected cells. These suggest that ORF21 transmitted as a tegument protein in the progeny virus enhances the new infection through MEK up-regulation in the recipient cell. Our findings indicate that ORF21 plays key roles in the infection of KSHV through the manipulation of the cellular function.

Kaposi Sarcoma, a Trifecta of Pathogenic Mechanisms

Kaposi’s sarcoma is a rare disease with four known variants: classic, epidemic, endemic and iatrogenic (transplant-related), all caused by an oncogenic virus named Human Herpes Virus 8. The viral infection in itself, along with the oncogenic properties of HHV8 and with immune system dysfunction, forms the grounds on which Kaposi’s Sarcoma may develop. Infection with HHV8 occurs through saliva via close contacts, blood, blood products, solid organ donation and, rarely, vertical transmission. Chronic inflammation and oncogenesis are promoted by a mix of viral genes that directly promote cell survival and transformation or interfere with the regular cell cycle and cell signaling (of particular note: LANA-1, v-IL6, vBCL-2, vIAP, vIRF3, vGPCR, gB, K1, K8.1, K15). The most common development sites for Kaposi’s sarcoma are the skin, mucocutaneous zones, lymph nodes and visceral organs, but it can also rarely appear in the musculoskeletal system, urinary system, endocrine organs, heart or eye. Histopathologically, spindle cell proliferation with slit-like vascular spaces, plasma cell and lymphocyte infiltrate are characteristic. The clinical presentation is heterogenic depending on the variant; some patients have indolent disease and others have aggressive disease. The treatment options include highly active antiretroviral therapy, surgery, radiation therapy, chemotherapy, and immunotherapy. A literature search was carried out using the MEDLINE/PubMed, SCOPUS and Google Scholar databases with a combination of keywords with the aim to provide critical, concise, and comprehensive insights into advances in the pathogenic mechanism of Kaposi’s sarcoma.

Whole-genome sequencing of Kaposi sarcoma-associated herpesvirus (KSHV/HHV8) reveals evidence for two African lineages

Kaposi sarcoma (KS)-associated herpesvirus (KSHV/HHV-8) was first sequenced from the body cavity (BC) lymphoma cell line, BC-1, in 1996. Few other KSHV genomes have been reported. Our knowledge of sequence variation for this virus remains spotty. This study reports additional genomes from historical US patient samples and from African KS biopsies. It describes an assay that spans regions of the virus that cannot be covered by short read sequencing. These include the terminal repeats, the LANA repeats, and the origins of replication. A phylogenetic analysis, based on 107 genomes, identified three distinct clades; one containing isolates from USA/Europe/Japan collected in the 1990s and two of Sub-Saharan Africa isolates collected since 2010. This analysis indicates that the KSHV strains circulating today differ from the isolates collected at the height of the AIDS epidemic. This analysis helps experimental designs and potential vaccine studies.

Therapeutic Perspectives in the Systemic Treatment of Kaposi’s Sarcoma

Simple Summary

Alternative systemic treatments are needed for patients who develop chemotherapy-refractory KS. Anti-angiogenic therapies constitute interesting therapeutic targets in this context, due to the central role of angiogenesis in KS pathogenesis, and could represent attractive alternatives. Immune checkpoints blockade could also be an interesting therapeutic approach in order to restore anti-HHV-8 immunity and tumor control.

Abstract

In patients with Kaposi’s sarcoma (KS), the therapeutic goal is to achieve a durable remission in the size and number of skin and visceral lesions. Although most patients show tumor regression in response to standard systemic chemotherapy regimens, alternative systemic treatments are needed for patients who develop refractory KS. Anti-angiogenic therapies represent attractive therapeutic targets in this context, due to the central role of angiogenesis in KS pathogenesis. Pomalidomide, which exhibits such anti-angiogenic activity through inhibition of VEGF, currently constitutes the most promising agent of this class and has been recently approved by the FDA. In addition, immune checkpoint blockade also represents an interesting alternative therapeutic approach through the restoration of immunity against HHV-8, the causative agent of KS, and improvement of tumor control. Although small series of cases treated successfully with these drugs have been reported, there is no marketing approval for anti-immune checkpoint antibodies for KS to date. In the present review, we will discuss potential therapeutic options for patients with recurrent or refractory KS, including systemic chemotherapies, immune checkpoint inhibitors, anti-herpesvirus agents, and anti-angiogenic drugs. Well-conducted clinical trials in this population are urgently needed to correctly address the efficacy of targeted agents and immunomodulators, while monitoring for adverse effects.

Recent Advances in Developing Treatments of Kaposi's Sarcoma Herpesvirus-Related Diseases

Kaposi-sarcoma-associated herpesvirus (KSHV) or human herpesvirus 8 (HHV-8) is the causative agent of several malignancies, including Kaposi's sarcoma (KS), primary effusion lymphoma (PEL), and multicentric Castleman's disease (MCD). Active KSHV replication has also been associated with a pathological condition called KSHV inflammatory cytokine syndrome (KICS), and KSHV may play a role in rare cases of post-transplant polyclonal lymphoproliferative disorders. Several commonly used herpesviral DNA polymerase inhibitors are active against KSHV in tissue culture. Unfortunately, they are not always efficacious against KSHV-induced diseases. To improve the outcome for the patients, new therapeutics need to be developed, including treatment strategies that target either viral proteins or cellular pathways involved in tumor growth and/or supporting the viral life cycle. In this review, we summarize the most commonly established treatments against KSHV-related diseases and review recent developments and promising new compounds that are currently under investigation or on the way to clinical use.

Optimal control application to a Kaposi’s sarcoma treatment model

Kaposi’s sarcoma (KS) is a malignant disorder of lymphatic endothelial origin that can have two main variants: AIDS-related KS (AKS) and non-AIDS related KS (NAKS) that all share a causal relationship with the human herpesvirus-8 (KSHV or HHV-8). We develop a mathematical model that accounts for B-cells latently and lytically infected with HHV-8 as well as the innate and adaptive arms of the immune system. As a sequel to numerous studies that have investigated the inhibition of HHV-8 endocytosis and reactivation of HHV-8 replication, we employ optimal control strategy to obtain treatment efficacies for these two therapeutic approaches. We have shown that when [Formula: see text] of the B-cell infections result in latency, administration of high efficacy drugs that inhibit entry and reactivation of latently infected B-cells leads to the clearance of KS as the population of infected cells cannot be sustained. Our results also reveal that at [Formula: see text] latency of B-cells, the therapy could produce similar results if the drug that targets viral entry is of moderate efficacy but the efficacy of the drug inhibiting reactivation is considerably more than [Formula: see text] Administration of the same drugs but both at moderate efficacy levels leads to the depletion of both uninfected B- and progenitor cells, a scenario which can lead to the growth of KS variants. When [Formula: see text] of the B-cell infections result in latency, administration with high efficacy drugs reduces the viral entry of HHV-8 but as [Formula: see text] of the infected B-cells are productive, this event leads to production of HHV-8 which ultimately results in more progenitor cells getting infected and the growth of KS. Our findings have the potential to offer more effective therapeutic approaches in the treatment of NAKS.

Manipulation of Focal Adhesion Signaling by Pathogenic Microbes

Focal adhesions (FAs) serve as dynamic signaling hubs within the cell. They connect intracellular actin to the extracellular matrix (ECM) and respond to environmental cues. In doing so, these structures facilitate important processes such as cell-ECM adhesion and migration. Pathogenic microbes often modify the host cell actin cytoskeleton in their pursuit of an ideal replicative niche or during invasion to facilitate uptake. As actin-interfacing structures, FA dynamics are also intimately tied to actin cytoskeletal organization. Indeed, exploitation of FAs is another avenue by which pathogenic microbes ensure their uptake, survival and dissemination. This is often achieved through the secretion of effector proteins which target specific protein components within the FA. Molecular mimicry of the leucine-aspartic acid (LD) motif or vinculin-binding domains (VBDs) commonly found within FA proteins is a common microbial strategy. Other effectors may induce post-translational modifications to FA proteins through the regulation of phosphorylation sites or proteolytic cleavage. In this review, we present an overview of the regulatory mechanisms governing host cell FAs, and provide examples of how pathogenic microbes have evolved to co-opt them to their own advantage. Recent technological advances pose exciting opportunities for delving deeper into the mechanistic details by which pathogenic microbes modify FAs.

3D culture conditions support Kaposi's sarcoma herpesvirus (KSHV) maintenance and viral spread in endothelial cells

Kaposi's sarcoma-associated herpesvirus (KSHV) is a human tumorigenic virus and the etiological agent of an endothelial tumor (Kaposi's sarcoma) and two B cell proliferative diseases (primary effusion lymphoma and multicentric Castleman's disease). While in patients with late stage of Kaposi's sarcoma the majority of spindle cells are KSHV-infected, viral copies are rapidly lost in vitro, both upon culture of tumor-derived cells or from newly infected endothelial cells. We addressed this discrepancy by investigating a KSHV-infected endothelial cell line in various culture conditions and in tumors of xenografted mice. We show that, in contrast to two-dimensional endothelial cell cultures, KSHV genomes are maintained under 3D cell culture conditions and in vivo. Additionally, an increased rate of newly infected cells was detected in 3D cell culture. Furthermore, we show that the PI3K/Akt/mTOR and ATM/γH2AX pathways are modulated and support an improved KSHV persistence in 3D cell culture. These mechanisms may contribute to the persistence of KSHV in tumor tissue in vivo and provide a novel target for KS specific therapeutic interventions. KEY MESSAGES: In vivo maintenance of episomal KSHV can be mimicked in 3D spheroid cultures 3D maintenance of KSHV is associated with an increased de novo infection frequency PI3K/Akt/mTOR and ATM/ γH2AX pathways contribute to viral maintenance.