Abstract
Cellular lipid droplets (LD) are organelles involved in cellular lipid metabolism. When liver cellular components were fractionated using sucrose density gradient centrifugation, adipose differentiation-related protein (ADRP) was distributed in both the top and bottom fractions, which correspond to the LD and membranous fractions, respectively, in the mouse liver under normal feeding conditions. After overnight fasting, triacylglycerol and ADRP increased nearly 2.5-fold in the mouse liver, and a portion appeared in the intermediate-density LD (iLD) fractions. ADRP in the iLD fractions was also increased in a mouse nonalcoholic steatohepatitis model induced by methione/choline-deficient diet. When HuH-7 human hepatoma cells were incubated with oleic acid for 24 h, the amount of ADRP increased, and it was distributed in both the LD and membrane fractions. However, ADRP appeared in the iLD fractions upon treatment of HuH-7 cells with glucagon. This behavior of ADRP was cAMP-dependent, as the ADRP-positive iLD fractions were induced by dibutylyl cAMP and were blocked by protein kinase A inhibitors. A portion of ADRP colocalized microscopically with calnexin, which is present in the iLD fractions, by treatment of HuH-7 cells or human primary hepatocytes with oleic acid and glucagon, but not by treatment with oleic acid alone. Glucagon has a role in the reorganization of endoplasmic reticulum membranes to generate ADRP-associated lipid-poor particles in hepatic cells, which is related to LD formation during lipid storage.
Highlights
Cellular lipid droplets (LD) are organelles involved in cellular lipid metabolism
When the blood glucose level decreases during the fasting period, glucagon stimulates glycogenolysis and adrenalin upregulates TG hydrolysis in white adipose tissue (WAT) to increase the free fatty acids (FFA) released into the circulation, and excess amounts of FFA are converted to TG in the liver [30, 31]
Such distribution patterns of adipose differentiation-related protein (ADRP) and TG in oleic acid (OA)-treated HuH-7 cells do not correspond to those observed in the fasted liver and the methionine/ choline-deficient (MCD)-induced fatty liver, suggesting that factors other than fatty acids are involved in formation of intermediate-density LD (iLD) fractions in hepatocytes, even though fatty acid is sufficient to induce the accumulation of buoyant LDs
Summary
Cellular lipid droplets (LD) are organelles involved in cellular lipid metabolism. When liver cellular components were fractionated using sucrose density gradient centrifugation, adipose differentiation-related protein (ADRP) was distributed in both the top and bottom fractions, which correspond to the LD and membranous fractions, respectively, in the mouse liver under normal feeding conditions. We investigated the factors responsible for appearance of ADRP-positive iLD fractions using the hepatoma cell line HuH-7, and we found that glucagon induces the change in ADRP distribution.
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