Role of heme oxygenase-1 in the pathogenesis and tumorigenicity of Kaposi's sarcoma-associated herpesvirus

Roles of Human Endogenous Retrovirus-K-Encoded Np9 in Human Diseases: A Small Protein with Big Functions

Human Endogenous Retrovirus Sequences (HERVs) constitute up to 8% of the human genome, yet not all HERVs remain silent passengers within our genomes. Some HERVs, especially HERV type K (HERV-K), have been found to be frequently transactivated in a variety of inflammatory diseases and human cancers. Np9, a small protein translated from the HERV-K env reading frame, has been reported as an oncogenic protein and is present in a variety of tumors and transformed cells. The Np9 protein can crosstalk with many cellular factors and is involved in the pathogenicity of various diseases, including some oncogenic virus infections. In the current review, we summarize recent findings about Np9 clinical relevance/implications, its mediated cellular functions/mechanisms, and potential targeted therapies in development.

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.

The role of endogenous retroviruses-K in human cancer

It is known that human endogenous retroviruses (HERVs) constitute almost 8% of the human genome. Although the expression of HERVs from the human genome is tightly regulated, different exogenous and endogenous factors could trigger HERV activation. Aberrant expression of different HERVs may potentially cause a variety of diseases such as neurological and autoimmune diseases as well as cancer. It is suggested that HERV-K can induce cancer through different mechanisms that are discussed. The interplay between some tumor viruses and HERV-K seems to be a key player in progression of viral-associated cancers because elevated levels of Rec and Np9 proteins are observed in several cancers. The frequent over expression of HERV proteins and some specific antibodies in cancer cells could be considered as suitable prognostic and therapeutic biomarkers in diagnosis and treatment of cancers. The expression of HERV proteins in cancers and development of immune responses against them may also be used as targets for cancer immunotherapy. Further studies are warranted to evaluate the role of HERVs in cancer formation and use of different HERV proteins in developing new diagnostic and therapeutic approaches for cancer treatments.

Modulation of Angiogenic Processes by the Human Gammaherpesviruses, Epstein-Barr Virus and Kaposi's Sarcoma-Associated Herpesvirus

Angiogenesis is the biological process by which new blood vessels are formed from pre-existing vessels. It is considered one of the classic hallmarks of cancer, as pathological angiogenesis provides oxygen and essential nutrients to growing tumors. Two of the seven known human oncoviruses, Epstein–Barr virus (EBV) and Kaposi’s sarcoma-associated herpesvirus (KSHV), belong to the Gammaherpesvirinae subfamily. Both viruses are associated with several malignancies including lymphomas, nasopharyngeal carcinomas, and Kaposi’s sarcoma. The viral genomes code for a plethora of viral factors, including proteins and non-coding RNAs, some of which have been shown to deregulate angiogenic pathways and promote tumor growth. In this review, we discuss the ability of both viruses to modulate the pro-angiogenic process.

Towards Better Understanding of KSHV Life Cycle: from Transcription and Posttranscriptional Regulations to Pathogenesis

Kaposi’s sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus-8 (HHV-8), is etiologically linked to the development of Kaposi’s sarcoma, primary effusion lymphoma, and multicentric Castleman’s disease. These malignancies often occur in immunosuppressed individuals, making KSHV infection-associated diseases an increasing global health concern with persistence of the AIDS epidemic. KSHV exhibits biphasic life cycles between latent and lytic infection and extensive transcriptional and posttranscriptional regulation of gene expression. As a member of the herpesvirus family, KSHV has evolved many strategies to evade the host immune response, which help the virus establish a successful lifelong infection. In this review, we summarize the current research status on the biology of latent and lytic viral infection, the regulation of viral life cycles and the related pathogenesis.

Role of iron in cancer development by viruses

Increased levels of iron in body are attributed to higher cancer risk. Given the fact that 16% of all human cancers are caused by viral infections, iron is suggested to play an important role in carcinogenesis particularly those induced by viral infections. The present study provides an updated summary of the literature and the plausible mechanisms of iron involvement in cancer development by viruses. Our understanding about the interplay between viral infections and iron in different settings particularly cancer development is yet to be improved as it may shed a new light in development of new therapeutic strategies.

An endothelial cell line infected by Kaposi's sarcoma-associated herpes virus (KSHV) allows the investigation of Kaposi's sarcoma and the validation of novel viral inhibitors in vitro and in vivo

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiological agent of Kaposi's sarcoma (KS), a tumor of endothelial origin predominantly affecting immunosuppressed individuals. Up to date, vaccines and targeted therapies are not available. Screening and identification of anti-viral compounds are compromised by the lack of scalable cell culture systems reflecting properties of virus-transformed cells in patients. Further, the strict specificity of the virus for humans limits the development of in vivo models. In this study, we exploited a conditionally immortalized human endothelial cell line for establishment of in vitro 2D and 3D KSHV latency models and the generation of KS-like xenograft tumors in mice. Importantly, the invasive properties and tumor formation could be completely reverted by purging KSHV from the cells, confirming that tumor formation is dependent on the continued presence of KSHV, rather than being a consequence of irreversible transformation of the infected cells. Upon testing a library of 260 natural metabolites, we selected the compounds that induced viral loss or reduced the invasiveness of infected cells in 2D and 3D endothelial cell culture systems. The efficacy of selected compounds against KSHV-induced tumor formation was verified in the xenograft model. Together, this study shows that the combined use of anti-viral and anti-tumor assays based on the same cell line is predictive for tumor reduction in vivo and therefore allows faithful selection of novel drug candidates against Kaposi's sarcoma. KEY MESSAGES: Novel 2D, 3D, and xenograft mouse models mimic the consequences of KSHV infection. KSHV-induced tumorigenesis can be reverted upon purging the cells from the virus. A 3D invasiveness assay is predictive for tumor reduction in vivo. Chondramid B, epothilone B, and pretubulysin D diminish KS-like lesions in vivo.

Inhibition of Heme Oxygenase-1 enhances hyperthermia-induced autophagy and antiviral effect

Hyperthermia has been clinically utilized as an adjuvant therapy in the treatment of cervical carcinoma. However, thermotolerance induced by heme oxygenase-1 (HO-1), a stress-inducible cytoprotective protein, limits the efficacy of hyperthermic therapy, for which the exact mechanism remains unknown. In the present study, we found that heat treatment induced HO-1 expression and decreased copy number of HPV16 in cervical cancer cells and tissues from cervical cancer and precursor lesions. Knockdown of HO-1 stimulated autophagy accompanied by downregulation of X-linked inhibitor of apoptosis protein. Furthermore, silencing of HO-1 led to cell intolerance to hyperthermia, as manifested by inhibition of cell viability and induction of autophagic apoptosis. Moreover, HO-1 modulated hyperthermia-induced, autophagy-dependent antiviral effect. Thus, the findings indicate that blockade of HO-1 enhances hyperthermia-induced autophagy, an event resulting in apoptosis of cervical cancer cells through an antiviral mechanism. These observations imply the potential clinical utility of hyperthermia in combination with HO-1 inhibition in the treatment of cervical cancer.

Transactivation of human endogenous retrovirus K (HERV-K) by KSHV promotes Kaposi's sarcoma development

Kaposi's sarcoma-associated herpesvirus (KSHV) is the causative agent of several human cancers such as Kaposi's sarcoma (KS), which represents the most common AIDS-associated malignancy that lacks effective treatment options. Despite its clear role in AIDS malignancies, the fact that only a small set of KSHV-infected patients will eventually develop these tumors implies that additional co-factors are required for the development of KSHV-related cancers. In the current study, we demonstrate for the first time that KSHV de novo infection or viral latent proteins are able to transactivate human endogenous retrovirus K (HERV-K) through a variety of cellular signaling pathways and transcriptional factors. Moreover, we found that HERV-K transactivation, particularly activation of its encoded oncogenic NP9 protein, plays an important role in KSHV pathogenesis and tumorigenesis in vitro and in vivo. Our data provide innovative insights into the mechanisms of HERV-K transactivation contributing to viral oncogenesis, which may represent a promising target for KS treatment.

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.