Lipid-droplet specific autophagy (LD-Autophagy) is an integral component of energy homeostasis, in which cholesterol and acyl-glycerols are mobilized to free-fatty acids to meet metabolic demands during states of nutrient deprivation. Dysregulations in this process can lead to an array of metabolic disorders including obesity. Previous animal and in vitro studies have indicated that the melanocortin 3 receptor (MC3R) is a key player in lipid metabolism and obesity, and we have reported that its specific agonist, (D-Trp8) ƴ-melanocyte stimulating hormone (ƴ-MSH), induces the autophagy process via conversion of LC3I to LC3II in cultured primary hepatocytes; this suggests a potential regulatory role of MC3R in LD-autophagy.
To determine if we could replicate our findings in vivo, 12-week-old C57BL/6J mice were injected intraperitoneally with vehicle control saline or ƴ-MSH (200μg/kg bodyweight) and then sacrificed at either 30, 60, 90, or 120 minutes post injection. Liver samples were collected, and hepatic expression of LC3I/II was examined via western blot analysis. The absolute absorbance of LC3II, corrected for β-actin, was then used to determine activation of autophagy.
ƴ-MSH injected mice collected after 120 minutes had a significant increase in the activation of LC3II relative to saline injected mice at the same timepoint (ƴ-MSH: n=4, 0.61±0.61; Saline: n=4, 0.23±0.16; p=0.026). There were no significant differences in saline and ƴ-MSH groups for the other timepoints.
The increase in LC3II observed at 120 minutes post injection suggests that activation of MC3R by ƴ-MSH activates hepatic autophagy in vivo. Although no effect was observed at the other time points this could indicate there is a slow onset of action of ƴ-MSH after intraperitoneal injections. Further studies are required to better understand the effects and duration of ƴ-MSH treatment on LD-autophagy and to increase understanding of the mechanistic role of MC3R in LD-Autophagy.