Mesoderm-specific transcript localization in the ER and ER-lipid droplet interface supports a role in adipocyte hypertrophy.

Highly variable expression of mesoderm-specific transcript (Mest) in adipose tissue among genetically homogeneous mice fed an obesogenic diet, and its positive association with fat mass expansion, suggests that Mest is an epigenetic determinant for the development of obesity. Although the mechanisms by which MEST augments fat accumulation in adipocytes have not been elucidated, it has sequence homology and catalytic peptide motifs which suggests that it functions as an epoxide hydrolase or as a glycerol- or acylglycerol-3-phosphate acyltransferase. To better understand MEST function, detailed studies were performed to precisely define the intracellular organelle localization of MEST using immunofluorescence confocal microscopy. Lentiviral-mediated expression of a C-terminus Myc-DDK-tagged MEST fusion protein expressed in 3T3-L1 preadipocytes/adipocytes, and ear-derived mesenchymal stem cells (EMSC) from mice was observed in the endoplasmic reticulum (ER) membranes and is consistent with previous studies showing endogenous MEST in the membrane fraction of adipose tissue. MEST was not associated with the Golgi apparatus or mitochondria; however, frequent contacts were observed between MEST-positive ER and mitochondria. MEST-positive domains were also shown on the plasma membrane (PM) of non-permeabilized cells but they did not co-localize with ER-PM bridges. Post-adipogenic differentiated 3T3-L1 adipocytes and EMSC showed significant co-localization of MEST with the lipid droplet surface marker perilipin at contact points between the ER and lipid droplet. Identification of MEST as an ER-specific protein that co-localizes with lipid droplets in cells undergoing adipogenic differentiation supports a function for MEST in the facilitation of lipid accumulation and storage in adipocytes.