Consequences of lipid droplet coat protein downregulation in liver cells: abnormal lipid droplet metabolism and induction of insulin resistance.
OBJECTIVE
Accumulation of intracellular lipid droplets (LDs) in non-adipose tissues is recognized as a strong prognostic factor for the development of insulin resistance in obesity. LDs are coated with perilipin, adipose differentiation-related protein, tail interacting protein of 47 kd (PAT) proteins that are thought to regulate LD turnover by modulating lipolysis. Our hypothesis is that PAT proteins modulate LD metabolism and therefore insulin resistance.


METHODS
We used a cell culture model (murine AML12 loaded with oleic acid) and small interfering RNA to directly assess the impact of PAT proteins on LD accumulation, lipid metabolism, and insulin action. PAT proteins associated with excess fat deposited in livers of diet-induced obese (DIO) mice were also measured.


RESULTS
Cells lacking PAT proteins exhibited a dramatic increase in LD size and a decrease in LD number. Further, the lipolytic rate increased by approximately 2- to 2.5-fold in association with increased adipose triglyceride lipase (ATGL) at the LD surface. Downregulation of PAT proteins also produced insulin resistance, as indicated by decreased insulin stimulation of Akt phosphorylation (P < 0.001). Phosphoinositide-dependent kinase-1 and phosphoinositide 3-kinase decreased, and insulin receptor substrate-1 307 phosphorylation increased. Increased lipids in DIO mice livers were accompanied by changes in PAT composition but also increased ATGL, suggesting a relative PAT deficiency.


CONCLUSIONS
These data establish an important role for PAT proteins as surfactant at the LD surface, packaging lipids in smaller units and restricting access of lipases and thus preventing insulin resistance. We suggest that a deficiency of PAT proteins relative to the quantity of ectopic fat could contribute to cellular dysfunction in obesity and type 2 diabetes.
Bell
Ming
M
Geriatric Research, Education and Clinical Center, Baltimore Veterans Affairs Health Care Center, Division of Gerontology, Department of Medicine, School of Medicine, University of Maryland, Baltimore, Maryland, USA.
Wang
Hong
H
Chen
Hui
H
McLenithan
John C
JC
Gong
Da-Wei
DW
Yang
Rong-Zee
RZ
Yu
Daozhan
D
Fried
Susan K
SK
Quon
Michael J
MJ
Londos
Constantine
C
Sztalryd
Carole
C
D000818:Animals; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D002478:Cells, Cultured; D002855:Chromatography, Thin Layer; D015536:Down-Regulation; D005230:Fatty Acids, Nonesterified / Q000378:metabolism; D022781:Hepatocytes / Q000166:cytology / Q000378:metabolism*; D015151:Immunoblotting; D007150:Immunohistochemistry; D007333:Insulin Resistance / Q000502:physiology; D050356:Lipid Metabolism / Q000502:physiology; D008066:Lipolysis / Q000502:physiology; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D051379:Mice; D008810:Mice, Inbred C57BL; D018613:Microscopy, Confocal; D000070780:Perilipin-2; D000070796:Perilipin-3; D034741:RNA, Small Interfering / Q000235:genetics
D002352:Carrier Proteins; D005230:Fatty Acids, Nonesterified; D008565:Membrane Proteins; D000070780:Perilipin-2; D000070796:Perilipin-3; C000602983:Plin2 protein, mouse; C523440:Plin3 protein, mouse; D034741:RNA, Small Interfering