The impact of physiological stress conditions on Kaposi's sarcoma-associated herpesvirus (KSHV) infection remains poorly understood. One such stressor, hypoxia, is regulated by the transcription factor HIF-1α. We recently reported that hypoxia, or HIF-1α expression alone, can promote lytic infection in cells that typically support latent infection under normoxia. Here, we show that hypoxia-induced lytic infection is reversible, leading to an abortive lytic cycle if the hypoxic condition ceases. Additionally, we found that HIF-1α induces lytic de novo infection only if expressed within the first 24 h post-infection (hpi). We show that HIF-1α can bind to viral promoters and induce lytic genes only during this early window of infection, before the KSHV genome undergoes heterochromatinization and establishes latency. In contrast, regardless of the timing of HIF-1α expression during KSHV infection, the induction of HIF-1α host target genes remains unaffected. These results indicate that the heterochromatinized KSHV DNA becomes resistant to HIF-1α-mediated activation after latency is established. These findings may explain why, despite the expression of HIF-1α in Kaposi's sarcoma tumors, KSHV remains in latency, because HIF-1α cannot induce lytic genes once the viral DNA is heterochromatinized. Importantly, we also demonstrate that inhibition of the epigenetic repressor PRC2, which associates with lytic promoters after 24 hpi, restores HIF-1α's ability to bind viral promoters and induce lytic gene expression post-latency. Collectively, our results indicate that not only the presence of HIF-1α, but also the timing and duration of its expression during KSHV infection, are critical determinants of its ability to drive lytic infection.IMPORTANCEThe current view is that the default pathway of KSHV infection is the establishment of latency, however, how this is altered under physiological stress conditions remains largely unknown. We previously showed that hypoxia, or the expression of its transcription factor HIF-1α alone, promotes the establishment of lytic rather than latent KSHV infection. In this study, we show that the duration of hypoxia, as well as the timing and duration of HIF-1α expression, are crucial determinants in facilitating lytic de novo KSHV infection. Notably, we found that PRC2-mediated heterochromatin inhibits the HIF-1α-mediated upregulation of lytic genes as chromatinization of the KSHV genome progresses during infection. Our findings offer a deeper understanding of how epigenetic regulation intersects with host stress responses to influence viral pathogenesis.
Lee et al. (Tue,) studied this question.