Lipid droplets are intracellular organelles that provide a depot for triglyceride storage in almost all cells and are a central point for the control of cellular energy homeostasis. Defective lipid droplet metabolism is a common occurrence in the pathogenesis of obesity, non-alcoholic fatty liver disease (NAFLD), and type 2 diabetes (T2D). Perilipin-5 (PLIN5) regulates lipid metabolism by coordinating protein-protein interactions at the surface of lipid droplets. We rationalised that other proteins located at the lipid droplet might interact with PLIN5 to regulate lipid metabolism. To test this, we affinity-purified PLIN5 tagged with 3XHA from cell homogenates and identified potential interacting proteins using mass spectrometry analysis. Steroid hormone enzyme 17-β-hydroxysteroid dehydrogenase type 11 (HSD17β11) was the most abundant interacting partner of PLIN5 and was subsequently confirmed as a bone fide lipid droplet localised protein using confocal microscopy.
We next employed coimmunoprecipitation, live cell imaging and fluorescence lifetime imaging microscopy (FLIM) to spatially map protein-protein interactions at the lipid droplet. Together, these studies showed that HSD17β11 interacts with PLIN5, and this interaction is an important regulator of PKA-stimulated lipolysis. CRISPR/Cas9 gene editing of HSD17β11 in both HEK293T and human liver THLE-2 cells resulted in the accumulation of enlarged lipid droplets, which was mediated by reduced lipolysis and fatty acid β-oxidation. Re-expression of HSD17β11 in HSD17β11-null cells reversed these effects on lipid metabolism. Gene expression analysis in human liver biopsies revealed downregulation of HSD17β11 mRNA expression in patients with non-alcoholic steatohepatitis and T2D compared with weight and age-matched individuals, and HSD17β11 expression was negatively associated with hepatic steatosis. Altogether, these results demonstrate an important role of HSD17β11 in regulating cellular lipid metabolism and indicate a potential role for HSD17β11 in the pathogenesis of fatty liver disease. Further studies in mice with HSD17β11 deletion and overexpression aim to delineate HSD17β11’s metabolic functions in vivo.