Free fatty acids sensitise hepatocytes to TRAIL mediated cytotoxicity.
BACKGROUND
Elevated circulating free fatty acids (FFA) contribute to the development of hepatic steatosis and promote hepatocyte apoptosis by incompletely defined mechanisms. Although the death ligand TRAIL has been implicated in a variety of pathological liver diseases, the role of TRAIL in mediating apoptosis of FFA induced steatotic hepatocytes is unknown.


OBJECTIVE
We examined TRAIL cytotoxicity in an in vitro model of hepatocyte steatosis induced by FFA.


METHODS
Hepatocytes (Huh 7 cells, HepG2 cells, and primary rat hepatocytes) were rendered steatotic by incubation with oleic acid. Apoptosis was assessed morphologically and biochemically by caspase activity. TRAIL receptor regulation was examined using immunoblot analysis and siRNA for targeted knockdown. c-jun N-terminal kinase (JNK) inhibition was attained with SP600125.


RESULTS
Oleic acid sensitised the cells to TRAIL but not TNF-alpha cytotoxicity. FFA sensitisation to TRAIL occurred at much lower concentrations than required for FFA mediated sensitisation to Fas, or FFA induced lipoapoptosis. Oleic acid treatment led to upregulation of the cognate TRAIL receptor death receptor 5 (DR5) but not death receptor 4 (DR4). The upregulation of DR5 was JNK dependent. siRNA targeted knockdown of either DR5 or DR4 demonstrated that DR5 was responsible for FFA sensitisation to TRAIL killing. DR5 expression was enhanced in steatotic human liver samples.


CONCLUSIONS
Our results suggest that FFA induced hepatocyte steatosis sensitises to TRAIL by a DR5 mediated JNK dependent mechanism.
Malhi
Harmeet
H
Miles and Shirley Fiterman Center for Digestive Diseases, Mayo Clinic College of Medicine, Rochester, MN 55905, USA.
Barreyro
Fernando J
FJ
Isomoto
Hajime
H
Bronk
Steven F
SF
Gores
Gregory J
GJ
D000818:Animals; D017209:Apoptosis / Q000502:physiology*; D051017:Apoptosis Regulatory Proteins / Q000378:metabolism; D002478:Cells, Cultured; D005230:Fatty Acids, Nonesterified / Q000009:adverse effects*; D005234:Fatty Liver / Q000503:physiopathology*; D022781:Hepatocytes / Q000187:drug effects* / Q000502:physiology; D006801:Humans; D007150:Immunohistochemistry / Q000379:methods; D048031:JNK Mitogen-Activated Protein Kinases / Q000378:metabolism; D019301:Oleic Acid / Q000494:pharmacology; D051381:Rats; D053220:Receptors, TNF-Related Apoptosis-Inducing Ligand / Q000378:metabolism; D011994:Recombinant Proteins / Q000494:pharmacology; D013229:Stearic Acids / Q000494:pharmacology; D053221:TNF-Related Apoptosis-Inducing Ligand / Q000494:pharmacology*; D014409:Tumor Necrosis Factor-alpha / Q000378:metabolism
D051017:Apoptosis Regulatory Proteins; D005230:Fatty Acids, Nonesterified; D053220:Receptors, TNF-Related Apoptosis-Inducing Ligand; D011994:Recombinant Proteins; D013229:Stearic Acids; D053221:TNF-Related Apoptosis-Inducing Ligand; C505653:TNFSF10 protein, human; D014409:Tumor Necrosis Factor-alpha; D019301:Oleic Acid; C031183:stearic acid; D048031:JNK Mitogen-Activated Protein Kinases