Endocrine Abstracts

Adipocytes are cells highly specialized in storing excess energy as neutral lipids in lipid droplets (LDs). These organelles act not only as mere fat reservoirs, but also as active modulators of lipid storage and mobilization in response to hormonal signals. This role is accomplished through a plethora of proteins associated with the LD surface that spatially and temporally coordinate lipid metabolism and traffic on demand. Recently, we have shown that the LD-associated protein Rab18 is involved in insulin-mediated lipogenesis and in β-adrenergic-induced lipolysis in 3T3-L1 adipocytes. Given the involvement of the endoplasmic reticulum (ER) in the biogenesis, maintenance, and regression of LDs, herein we investigated whether Rab18 modulates LD mobilization and LD/ER relationship. Double immunocytochemistry and confocal microscopy revealed that both insulin and the β-adrenergic agonist isoproterenol increased Rab18 recruitment to LDs and the rapprochement of these organelles to ER membranes. However, unlike isoproterenol, insulin treatment induced Rab18-labeled LDs to contact specific regions of the ER membrane enriched in lipogenic enzymes (i.e. diacylglicerol-acyltransferase 2 (DGAT2)). Furthermore, disruption of ER traffic impaired insulin-induced Rab18 association with LDs, whereas isoproterenol effect was unaffected. These data suggest that, depending on the nature of the extracellular stimuli reaching the adipocytes, Rab18 is recruited to LDs from the ER membrane (upon insulin treatment) or, likely, from the cytosol (upon isoproterenol treatment). We also analyzed the role of microtubules in Rab18 association to LDs and LD/ER apposition. Thus, we found that microtubule depolymerization by nocodazole did not affect insulin- and isoproterenol-induced Rab18 mobilization towards LDs but increased LD rapprochement to the ER. Finally, we explored Rab18-interacting proteins by proteomic analysis. Altogether, our data suggest that Rab18 plays a role in the control of lipogenesis- and lipolysis-induced LD/ER association, probably through its interaction with the cytoskeleton.

Declaration of interest: The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.

Funding: This work was supported, however funding details unavailable.