Abstract
Nonalcoholic fatty liver disease (NAFLD) is associated with obesity, insulin resistance, type 2 diabetes, and dyslipidemia. The purpose of this study was to identify novel proteins and pathways that contribute to the pathogenesis and complications of NAFLD. C57BL/6J male mice were fed a 60% (HFD) or 10% (LFD) high or low fat diet. HFD induced obesity, hepatic steatosis and insulin resistance (euglycemic clamps, glucose infusion rate: LFD 50.5 ± 6.4 vs. HFD 14.2 ± 9.5 μg/ (g·min); n = 12). Liver proteins were analyzed by mass spectrometry-based proteomics analysis. Numerous hepatic proteins were altered in abundance after 60% HFD feeding. Nine down-regulated and nine up-regulated proteins were selected from this list for detailed analysis based on the criteria of 1.5-fold difference, consistency across replicates, and having at least 2 spectra assigned. Proteins that decreased in abundance were acyl-coA desaturase-I (SCD-1), acetyl-CoA carboxylase (ACC), fatty acid synthase (FAS), pyruvate kinase isozymes R/L (PKLR), NADP-dependent malic enzyme (ME-1), ATP-citrate synthase (ACL), ketohexokinase (KHK), long-chain-fatty acid-CoA ligase-5 (ACSL-5) and carbamoyl-phosphate synthase-I (CPS-1). Those that increased were KIAA0564, apolipoprotein A-I (apoA-1), ornithine aminotransferase (OAT), multidrug resistance protein 2 (MRP-2), liver carboxylesterase-I (CES-1), aminopeptidase N (APN), fatty aldehyde dehydrogenase (FALDH), major urinary protein 2 (MUP-2) and KIAA0664. KIAA0564 and KIAA0664 proteins are uncharacterized and are novel proteins associated with NAFLD. The decreased abundance of normally highly abundant proteins like FAS and CPS-1 was confirmed by Coomassie Blue staining after bands were identified by MS/MS, and immunoblot analysis confirmed the increased abundance of KIAA0664 after 60% HFD feeding. In conclusion, this study shows NAFLD is characterized by changes in abundance of proteins related to cell injury, inflammation, and lipid metabolism. Two novel and uncharacterized proteins, KIAA0564 and KIAA0664, may provide insight into the pathogenesis of NAFLD induced by lipid oversupply.
Highlights
Type 2 diabetes mellitus is the most common metabolic disease and is one of the leading causes of death in the United States
High fat diet feeding results in obesity and insulin resistance in mice In order to identify hepatic proteins that respond to the consumption of a high dietary fat, and which may potentially be responsible for the development of insulin resistance, six week old male C57BL/6J mice were fed a 10% low fat diet (LFD, control) or a 60% high fat diet (HFD) for 9 weeks
In order to rule out any potentially spurious results arising from stress due to surgeries and clamp procedures, two additional control groups of six week old male C57BL/6J mice were fed a 10% low fat diet (LFD, control) or a 60% high fat diet (HFD) for 10 weeks, but were not subjected to surgeries or clamps before being weighed, analyzed for body fat percentage, and sacrificed for tissue collection
Summary
Type 2 diabetes mellitus is the most common metabolic disease and is one of the leading causes of death in the United States. Insulin resistance plays an important role in the development of the disease. Of several organs that exhibit insulin resistance, liver is one of the most important. Excessive ectopic lipid accumulation in the liver, or non-alcoholic fatty liver disease (NAFLD), is closely associated with obesity, insulin resistance and type 2 diabetes [1,2,3]. Stefan et al [4] noted that, in a cohort of obese people, insulin-sensitive and insulin-resistant individuals could be distinguished on the basis of lipid accumulation in muscle and liver, rather than subcutaneous or visceral adiposity. Fabbrini et al [5] reported that intrahepatic triglyceride content, not visceral adiposity was associated with insulin resistance. Some hypotheses, including the involvement of inflammatory mechanisms [6,7] and the DAG-PKC pathway [8] have been raised to explain the mechanism of hepatic insulin resistance, they remain to be fully explored
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