ARS | Publication request: Hepatic triacylglycerol hydrolysis regulates peroxisome proliferator-activated receptor alpha activity

Submitted to: Journal of Lipid Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 23, 2009
Publication Date: August 29, 2009
Citation: Sapiro, J.M., Mashek, M.T., Greenberg, A.S., Mashek, D.G. 2009. Hepatic triacylglycerol hydrolysis regulates peroxisome proliferator-activated receptor alpha activity. Journal of Lipid Research. 50(8):1621-1629.

Interpretive Summary: Obesity is a worldwide problem. Increased storage of fat in the body results in obesity and increases the risk of developing diabetes and heart disease. When fat is broken down in body cells it will be burned, which lead to a reduction in body fat. We are interested in identifying pathways that promote fat breakdown and fat burning. By identifying pathways that promote fat breakdown and burning we can then develop nutritional regimens that act on these pathways in humans to ameliorate obesity. We studied two proteins found in cells, one that facilitates the storage of fat in cells, called adipose differentiation-related protein (ADRP), and another protein that breaks down stored fat, called adipose triglyceride lipase (ATGL). We hypothesize that ADRP blocks ATGL’s ability to break down fat and promote burning. We investigated whether a reduction in ADRP protein expression in cells increased fat breakdown, and also whether increased expression of ATGL could also increase fat breakdown. When we reduced expression of ADRP in cells, we found that fat was broken down and burned. When we increased expression of ATGL in cells, we found that increased expression overcame ADRP’s actions to block fat breakdown and increased burning of fat. These results suggest that novel pathways which increase the breakdown of stored fat in cells results in activation of pathways that burn fat.

Technical Abstract: Fatty acids have gained attention for their important roles in regulating transcription factors involved in hepatic energy metabolism. Intracellular fatty acids are derived from multiple sources, including exogenous uptake, chylomicron remnant uptake, de novo synthesis, and glycerolipid hydrolysis. However, little is known about how fatty acids from different sources affect the activity of transcription factors such as PPAR-alpha. To determine if fatty acids derived from triacylglycerol (TAG) hydrolysis will regulate PPAR-alpha in rat primary hepatocytes, we either over expressed or knocked down adipose differentiation-related protein (ADRP) or over expressed adipose TAG lipase (ATGL). Over expressing ADRP attenuated TAG hydrolysis, whereas siRNA-mediated knockdown of ADRP resulted in enhanced TAG hydrolysis. Results from PPAR-alpha reporter activity assays demonstrated that decreasing TAG hydrolysis by ADRP over expression resulted in a 35-60% reduction in reporter activity under basal conditions. As expected, PPAR-alpha target genes were also decreased in response to ADRP over expression. However, manipulating TAG hydrolysis did not affect PPAR-gamma activity. Enhancing TAG hydrolysis through ADRP knockdown or ATGL over expression increased PPAR-alpha activity. Following the removal of exogenous fatty acids, PPAR-alpha activity decreased more rapidly in hepatocytes over expressing ADRP compared to control cells. Finally, the PPAR-alpha ligand, WY-14643, was able to restore PPAR-alpha activity following ADRP over expression. These results indicate that TAG hydrolysis, and the consequential release of fatty acids that act as PPAR-alpha ligands, are important physiological processes that regulate PPAR-alpha activity.