Perilipin-1 autoantibodies linked to idiopathic lipodystrophy in the setting of two distinct breaks in immune tolerance

Background: Acquired lipodystrophy is often characterized as an idiopathic subtype of lipodystrophy. Despite suspicion of an immune-mediated pathology, biomarkers such as autoantibodies are generally lacking. Methods: Here, we used an unbiased proteome-wide screening approach to identify autoantibodies to the adipocyte specific lipid droplet protein Perilipin-1 in a murine model of Autoimmune Polyendocrine Syndrome 1 (APS1). We then tested for PLIN1 autoantibodies in human subjects with lipodystrophy with two independent severe breaks in immune tolerance (including APS1) along with controls using a specific Radioligand Binding Assay and indirect immunofluorescence on fat tissue. Results: We identified autoantibodies to the lipid-droplet protein Perilipin-1 (PLIN1) in two human case reports including the first reported case of human APS1 with acquired lipodystrophy. Further, we extend PLIN1 autoantibodies to a second non-APS1 patient who acquired lipodystrophy as an immune-related adverse event following cancer immunotherapy. Conclusion: These data suggest that PLIN1 autoantibodies may represent a unifying marker of autoimmune lipodystrophy.

Proteome and Phosphoproteome Analysis of Brown Adipocytes Reveals That RICTOR Loss Dampens Global Insulin/AKT Signaling

Stimulating brown adipose tissue (BAT) activity represents a promising therapy for overcoming metabolic diseases. mTORC2 is important for regulating BAT metabolism, but its downstream targets have not been fully characterized. In this study, we apply proteomics and phosphoproteomics to investigate the downstream effectors of mTORC2 in brown adipocytes. We compare wild-type controls to isogenic cells with an induced knockout of the mTORC2 subunit RICTOR (Rictor-iKO) by stimulating each with insulin for a 30-min time course. In Rictor-iKO cells, we identify decreases to the abundance of glycolytic and de novo lipogenesis enzymes, and increases to mitochondrial proteins as well as a set of proteins known to increase upon interferon stimulation. We also observe significant differences to basal phosphorylation because of chronic RICTOR loss including decreased phosphorylation of the lipid droplet protein perilipin-1 in Rictor-iKO cells, suggesting that RICTOR could be involved with regulating basal lipolysis or droplet dynamics. Finally, we observe mild dampening of acute insulin signaling response in Rictor-iKO cells, and a subset of AKT substrates exhibiting statistically significant dependence on RICTOR.

The lipid droplet protein Pgc1 controls the subcellular distribution of phosphatidylglycerol

ABSTRACTThe biosynthesis of yeast phosphatidylglycerol (PG) takes place in the inner mitochondrial membrane. Outside mitochondria, the abundance of PG is low. Here, we present evidence that the subcellular distribution of PG is maintained by the locally controlled enzymatic activity of the PG-specific phospholipase, Pgc1. A fluorescently labeled Pgc1 protein accumulates on the surface of lipid droplets (LD). We show, however, that LD are not only dispensable for Pgc1-mediated PG degradation, but do not even host any phospholipase activity of Pgc1. Our in vitro assays document the capability of LD-accumulated Pgc1 to degrade PG upon entry to the membranes of the endoplasmic reticulum, mitochondria and even of artificial phospholipid vesicles. Fluorescence recovery after photobleaching analysis confirms the continuous exchange of GFP-Pgc1 within the individual LD in situ, suggesting that a steady-state equilibrium exists between LD and membranes to regulate the immediate phospholipase activity of Pgc1. In this model, LD serve as a storage place and shelter Pgc1, preventing its untimely degradation, while both phospholipase activity and degradation of the enzyme occur in the membranes.