Kaposi's sarcoma herpesvirus activates the hypoxia response to usurp HIF2α-dependent translation initiation for replication and oncogenesis

Hypoxia and HIF-1α promote lytic de novo KSHV infection

IMPORTANCE

The current view is that the default pathway of Kaposi's sarcoma-associated herpesvirus (KSHV) infection is the establishment of latency, which is a prerequisite for lifelong infection and viral oncogenesis. This view about KSHV infection is supported by the observations that KSHV latently infects most of the cell lines cultured in vitro in the absence of any environmental stresses that may occur in vivo. The goal of this study was to determine the effect of hypoxia, a natural stress stimulus, on primary KSHV infection. Our data indicate that hypoxia promotes euchromatin formation on the KSHV genome following infection and supports lytic de novo KSHV infection. We also discovered that hypoxia-inducible factor-1α is required and sufficient for allowing lytic KSHV infection. Based on our results, we propose that hypoxia promotes lytic de novo infection in cells that otherwise support latent infection under normoxia; that is, the environmental conditions can determine the outcome of KSHV primary infection.

Hypoxia and hypoxia-inducible factors in Kaposi sarcoma-associated herpesvirus infection and disease pathogenesis

Kaposi sarcoma-associated herpesvirus (KSHV) is the causative agent of Kaposi sarcoma and several other tumors and hyperproliferative diseases seen predominantly in human immunodeficiency virus-infected and other immunocompromised persons. There is an increasing body of evidence showing that hypoxia and hypoxia-inducible factors (HIFs) play important roles in the biology of KSHV and in the pathogenesis of KSHV-induced diseases. Hypoxia and HIFs can induce lytic activation of KSHV and KSHV can in turn lead to a hypoxic-like state in infected cells. In this review, we describe the complex interactions between KSHV biology, the cellular responses to hypoxia, and the pathogenesis of KSHV-induced diseases. We also describe how interference with HIFs can lead to decreased tumor growth and/or death of infected cells and KSHV-induced tumors. Finally, we show how these observations may lead to novel strategies for the treatment of KSHV-induced diseases.

Hypoxia in Skin Cancer: Molecular Basis and Clinical Implications

Skin cancer is one of the most prevalent cancers in the Caucasian population. In the United States, it is estimated that at least one in five people will develop skin cancer in their lifetime, leading to significant morbidity and a healthcare burden. Skin cancer mainly arises from cells in the epidermal layer of the skin, where oxygen is scarce. There are three main types of skin cancer: malignant melanoma, basal cell carcinoma, and squamous cell carcinoma. Accumulating evidence has revealed a critical role for hypoxia in the development and progression of these dermatologic malignancies. In this review, we discuss the role of hypoxia in treating and reconstructing skin cancers. We will summarize the molecular basis of hypoxia signaling pathways in relation to the major genetic variations of skin cancer.

Survival Mechanisms of Metastatic Melanoma Cells: The Link between Glucocorticoids and the Nrf2-Dependent Antioxidant Defense System

Circulating glucocorticoids increase during stress. Chronic stress, characterized by a sustained increase in serum levels of cortisol, has been associated in different cases with an increased risk of cancer and a worse prognosis. Glucocorticoids can promote gluconeogenesis, mobilization of amino acids, fat breakdown, and impair the body's immune response. Therefore, conditions that may favor cancer growth and the acquisition of radio- and chemo-resistance. We found that glucocorticoid receptor knockdown diminishes the antioxidant protection of murine B16-F10 (highly metastatic) melanoma cells, thus leading to a drastic decrease in their survival during interaction with the vascular endothelium. The BRAF^V600E mutation is the most commonly observed in melanoma patients. Recent studies revealed that VMF/PLX40-32 (vemurafenib, a selective inhibitor of mutant BRAF^V600E) increases mitochondrial respiration and reactive oxygen species (ROS) production in BRAF^V600E human melanoma cell lines. Early-stage cancer cells lacking Nrf2 generate high ROS levels and exhibit a senescence-like growth arrest. Thus, it is likely that a glucocorticoid receptor antagonist (RU486) could increase the efficacy of BRAF-related therapy in BRAF^V600E-mutated melanoma. In fact, during early progression of skin melanoma metastases, RU486 and VMF induced metastases regression. However, treatment at an advanced stage of growth found resistance to RU486 and VMF. This resistance was mechanistically linked to overexpression of proteins of the Bcl-2 family (Bcl-xL and Mcl-1 in different human models). Moreover, melanoma resistance was decreased if AKT and NF-κB signaling pathways were blocked. These findings highlight mechanisms by which metastatic melanoma cells adapt to survive and could help in the development of most effective therapeutic strategies.

Better late than never: A unique strategy for late gene transcription in the beta- and gammaherpesviruses

During lytic replication, herpesviruses express their genes in a temporal cascade culminating in expression of "late" genes. Two subfamilies of herpesviruses, the beta- and gammaherpesviruses (including human herpesviruses cytomegalovirus, Epstein-Barr virus, and Kaposi's sarcoma-associated herpesvirus), use a unique strategy to facilitate transcription of late genes. They encode six essential viral transcriptional activators (vTAs) that form a complex at a subset of late gene promoters. One of these vTAs is a viral mimic of host TATA-binding protein (vTBP) that recognizes a strikingly minimal cis-acting element consisting of a modified TATA box with a TATTWAA consensus sequence. vTBP is also responsible for recruitment of cellular RNA polymerase II (Pol II). Despite extensive work in the beta/gammaherpesviruses, the function of the other five vTAs remains largely unknown. The vTA complex and Pol II assemble on the promoter into a viral preinitiation complex (vPIC) to facilitate late gene transcription. Here, we review the properties of the vTAs and the promoters on which they act.

Differential HIF2α Protein Expression in Human Carotid Body and Adrenal Medulla under Physiologic and Tumorigenic Conditions

Hypoxia-inducible factors (HIF) 2α and 1α are the major oxygen-sensing molecules in eukaryotic cells. HIF2α has been pathogenically linked to paraganglioma and pheochromocytoma (PPGL) arising in sympathetic paraganglia or the adrenal medulla (AM), respectively. However, its involvement in the pathogenesis of paraganglioma arising in the carotid body (CB) or other parasympathetic ganglia in the head and neck (HNPGL) remains to be defined. Here, we retrospectively analyzed HIF2α by immunohistochemistry in 62 PPGL/HNPGL and human CB and AM, and comprehensively evaluated the HIF-related transcriptome of 202 published PPGL/HNPGL. We report that HIF2α is barely detected in the AM, but accumulates at high levels in PPGL, mostly (but not exclusively) in those with loss-of-function mutations in VHL and genes encoding components of the succinate dehydrogenase (SDH) complex. This is associated with upregulation of EPAS1 and the HIF2α-regulated genes COX4I2 and ADORA2A. In contrast, HIF2α and HIF2α-regulated genes are highly expressed in CB and HNPGL, irrespective of VHL and SDH dysfunctions. We also found that HIF2α and HIF1α protein expressions are not correlated in PPGL nor HNPGL. In addition, HIF1α-target genes are almost exclusively overexpressed in VHL-mutated HNPGL/PPGL. Collectively, the data suggest that involvement of HIF2α in the physiology and tumor pathology of human paraganglia is organ-of-origin-dependent and HIF1α-independent.