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Hypoxic Induction of Exosome Uptake through Proteoglycan-Dependent Endocytosis Fuels the Lipid Droplet Phenotype in Glioma

As an adaptive response to hypoxic stress, aggressive tumors rewire their metabolic phenotype into increased malignant behavior through extracellular lipid scavenging and storage in lipid droplets (LD). However, the underlying mechanisms and potential lipid source retrieved in the hypoxic tumor microenvironment remain poorly understood. Here, we show that exosome-like extracellular vesicles (EV), known as influential messengers in the tumor microenvironment, may also serve anabolic functions by transforming hypoxic, patient-derived human glioblastoma cell lines into the LD+ phenotype. EVs were internalized via a hypoxia-sensitive, endocytic mechanism that fueled LD formation through direct lipid transfer, and independently of fatty acid synthase activity. EVs can enter cells through multiple and yet ill-defined pathways. On a mechanistic level, we found that hypoxia-mediated EV uptake depends on increased heparan sulfate proteoglycan (HSPG) endocytosis that preferentially followed the lipid raft pathway. The functional relevance of HSPG was evidenced by the reversal of EV-mediated LD loading by targeting of HSPG receptor function. Implications: Together, our data extend the multifaceted role of EVs in cancer biology by showing their LD-inducing capacity in hypoxic glioma cells. Moreover, these findings highlight a potential function for HSPG-mediated endocytosis as a salvage pathway for EV retrieval during tumor stress conditions.