PROJECT SUMMARY Kaposi sarcoma (KS) is a multifocal angioproliferative tumor affecting the skin, oral mucosa, lymph nodes and visceral organs. Common factors for KS development include infection with the oncogenic herpesvirus Kaposi sarcoma herpesvirus (KSHV/HHV8) and immune deregulation. While ART has reduced KS incidence in the Western world, it remains the most common AIDS-associated malignancy worldwide. Growing knowledge of KS has improved treatment options, but it remains an incurable cancer causing significant morbidity and mortality, particularly in resource-limited regions. Thus, there is an urgent need for effective and affordable novel therapies for use in combination with, or instead of, conventional therapies. KSHV profoundly alters the transcriptome of the host cell, significantly influencing the function of multiple cellular homeostatic and adaptive pathways. Interrogation of these pathways has identified a number of components as potential therapeutic targets for KS, but significant gaps in our knowledge of the virus-host interaction hamper progress. In this application, we focus specifically on two important and inter-connected host cell pathways that are influenced by KSHV infection, iron metabolism and the antioxidant pathway, and investigate why KSHV-infected EC are resistant to ferroptosis, an iron-dependent form of regulated cell death that is characterized by accumulation of ROS and lethal oxidative damage to phospholipids. To support abnormal growth, tumor cells typically have a higher iron need, which is satisfied through altered expression of genes regulating iron uptake, utilization and storage. Our preliminary data suggest that KS resembles other cancers in this regard and directly implicate KSHV in this phenotype. Notably, in vitro infection of lymphatic endothelial cells (LEC) results in deregulation of iron metabolism genes and development of an iron-responsive growth phenotype. While iron fuels tumor growth, iron addiction presents a paradox: how to maintain redox homeostasis and resist ferroptotic death? Many tumor cells achieve this balance with an enhanced antioxidant response that includes activation of ferroptosis suppressor pathways to resist lethal lipid peroxidation. Our data show that KSHV upregulates SLC7A11 (xCT), a critical initiator of the canonical xCT/GSH/GPX4 ferroptosis suppressor pathway, and that KSHV-infected cells are uniquely susceptible to ferroptotic death when xCT is inhibited. In addition, KSHV upregulates the newly-identified anti-ferroptotic gene, ferroptosis suppressor protein 1 (FSP1), which functions through an independent, CoQ-dependent, pathway to suppress ferroptosis, thus presenting an alternate target for the selective elimination of KSHV-infected cells. In this application, we will test the innovative premise that KSHV reprograms host cell metabolism both to acquire iron for growth and to activate two independent but complementary ferroptosis suppressor pathways. We hypothesize ...