Hemin Triggers Cytoprotective Lipid Droplet Formation in Pulmonary Microvascular Endothelial Cells

Sickle cell disease (SCD) is characterized by chronic and episodic hemolysis, inducing endothelial cell dysfunction and death. Lipid droplets (LDs) accumulate intracellularly in response to stress but the role of LDs in hemin-stressed pulmonary microvascular cells remain unexplored. We hypothesized that LD formation is an adaptive response that improves endothelial cell survival during hemin-induced stress. We utilized cultures of human lung microvascular endothelial cells (hLMVECs) to test this hypothesis. Hemin alone was able to induce LD biogenesis in culture. In response to hemin treatment, we found an increase in cellular death by lactate dehydrogenase (LDH) assay when LD biogenesis was inhibited with a small molecule inhibitor of diacylglycerol acyltransferase 1 (DGAT1), A922500. Induction of LDs by treatment with oleic acid (OA) coupled with hemin resulted in improved endothelial cell survival. Hemin treatment was able to activate p-JNK and p-P38 signaling, while treatment with OA alone could not induce either signaling pathway. Hemin treatment combined with DGAT1 inhibition resulted in an increase in endothelial cell secreted IL-6 and IL-8 levels as measured by enzyme linked immunosorbent assay (ELISA), demonstrating an increase in endothelial cell dysfunction and stress. Induction of LD formation with oleic acid (OA) treatment resulted a reduction in secreted IL-6 and IL-8. Mitochondrial reactive oxygen species (ROS) production was decreased by OA treatment, as evidenced by MitoSOXRed assay. These data suggest that hLMVECs are protected from hemin-induced inflammation and death by the presence of LDs. Further studies are needed to explore the Hemin-LD-Cell Death axis for translational therapeutic use.