Latent Kaposi's Sarcoma-Associated Herpesvirus Infection of Endothelial Cells Activates Hypoxia-Induced Factors

Abstract

Kaposi's sarcoma-associated herpesvirus (KSHV or HHV-8) is the etiological agent of Kaposi's sarcoma, a highly vascularized, endothelial-derived tumor. A direct role for KSHV-mediated induction of angiogenesis has been proposed based upon the nature of the neoplasia and various KSHV gene overexpression and infection model systems. We have found that KSHV infection of endothelial cells induces mRNA of hypoxia-induced factor 1␣ (HIF1␣) and HIF2␣, two homologous alpha subunits of the heterodimeric transcription factor HIF. HIF is a master regulator of both developmental and pathological angiogenesis, composed of an oxygensensitive alpha subunit and a constitutively expressed beta subunit. HIF is classically activated posttranscriptionally with hypoxia, leading to increased protein stability of HIF1␣ and/or HIF2␣. However, we demonstrate that both alpha subunits are up-regulated at the transcript level by KSHV infection. The transcriptional activation of HIF leads to a functional increase in HIF activity under normoxic conditions, as demonstrated by both luciferase reporter assay and the increased expression of vascular endothelial growth factor receptor 1 (VEGFR1), an HIF-responsive gene. KSHV infection synergizes with hypoxia mimics and induces higher expression levels of HIF1␣ and HIF2␣ protein, and HIF1␣ is increased in a significant proportion of the latently infected endothelial cells. Src family kinases are required for the activation of HIF and the downstream gene VEGFR1 by KSHV. We also show that KS lesions, in vivo, express elevated levels of HIF1␣ and HIF2␣ proteins. Thus, KSHV stimulates the HIF pathway via transcriptional up-regulation of both HIF alphas, and this activation may play a role in KS formation, localization, and progression.

Induction of the Warburg effect by Kaposi's sarcoma herpesvirus is required for the maintenance of latently infected endothelial cells

“…This increase is in line with many cancer cells in culture (13). We previously showed that HIF transcriptional activity is increased by KSHV infection of endothelial cells (6). Hypoxia increases the stability of HIFs and HIF protein levels and activity are synergistically increased by KSHV infection combined with the hypoxia mimic, desferoxamine (Dfo) (6).…”

“…We previously showed that HIF transcriptional activity is increased by KSHV infection of endothelial cells (6). Hypoxia increases the stability of HIFs and HIF protein levels and activity are synergistically increased by KSHV infection combined with the hypoxia mimic, desferoxamine (Dfo) (6). When KSHV-infected cells were treated with Dfo, there was also a large synergistic increase in glucose uptake (Fig.…”

Kaposi's Sarcoma–Associated Herpesvirus Viral IFN Regulatory Factor 3 Stabilizes Hypoxia-Inducible Factor-1α to Induce Vascular Endothelial Growth Factor Expression

“…The expression and transcriptional activity of HIF-1a are up-regulated in KSHV-infected endothelial cells (33). To identify KSHV proteins that affect HIF1a activity, we surveyed several KSHV proteins for their potential effects on HIF-1a activity using the luciferase construct containing six repeats of HRE.…”

Hypoxia inducible factor (HIF) function in innate immunity and infection

“…Latent infection with HHV-8 is associated with the endothelial tumor Kaposi's sarcoma (KS) in AIDS patients and others with lowered immunity, and KS lesions in vivo are associated with high levels of both the HIF-1α and HIF-2α protein [45]. HHV-8 infection of endothelial cells in vitro leads to increased stabilization of the two HIF α subunits and increased HIF-responsive gene expression [45].…”

“…Latent infection with HHV-8 is associated with the endothelial tumor Kaposi's sarcoma (KS) in AIDS patients and others with lowered immunity, and KS lesions in vivo are associated with high levels of both the HIF-1α and HIF-2α protein [45]. HHV-8 infection of endothelial cells in vitro leads to increased stabilization of the two HIF α subunits and increased HIF-responsive gene expression [45]. The HHV-8 latency-associated nuclear antigen (LANA), which plays a critical role in modulating viral and target cell gene expression, increases HIF-1α mRNA levels, and also physically interacts with the transcription factor to enhance its promoter activities [46].…”