The metabolic pathways in nonalcoholic steatohepatitis (NASH) that contribute to fatty liver?s progression to inflammation are being intensely studied. One pathway which has been overlooked is the CYP27A1 initiated ?alternative pathway.? This pathway is responsible for the metabolism of cholesterol to intracellular regulatory oxysterols. The pathway?s subsequent 7?- hydroxylation of these oxysterols and their lipotoxic metabolites by CYP7B1 is what is believed to eliminate their regulatory effects. However, what controls cell levels of these oxysterols and their known metabolites remains incompletely defined. Furthermore, the potential for these alternative pathway cholesterol metabolites in eliciting cytotoxicity and inflammation in fatty liver has not been studied. A preliminary study in a ?fatty liver? mouse model showed that overexpression of the mitochondrial cholesterol delivery protein, StARD1, increased mitochondrial CYP27A1 cholesterol metabolism, and, led to a dramatic reduction in hepatic cholesterol, triglycerides, and free fatty acids levels. Unanticipated, we found significant down regulation of hepatic CYP7B1 expression coupled with high levels of the regulatory oxysterols, 25- hydroxycholesterol(25HC), 27-hydroxycholesterol(27HC), and 24(S)-hydroxycholesterol(24HC) and a marked increase in LFTs. Based upon these observations, we postulated that low CYP7B1 and increased oxysterol levels may represent a pathway to inflammation in fatty liver. More specifically, chronic down-regulation of CYP7B1 leading to chronically increased oxysterol levels may play a role in the transition from fatty liver to inflammation as seen in NASH. Supportive of this hypothesis we found significant suppression of CYP7B1 expression correlated to an increase in 27HC levels in human steatotic livers obtained from the NIH liver tissue cell distribution system. Furthermore, in Western diet fed mice, we have now shown that a low CYP7B1 is associated with an increase in 27HC levels and subsequent inflammasome activation as determined by increased IL-1B levels; outlining a pathway from fatty liver to inflammation as occurs in NASH. These observations also provided evidence for CYP7B1 being a key regulator of the levels and ratio of the three CYP27A1 generated oxysterols (24HC/25HC/27HC), and their hydroxylated metabolites In conclusion: we hypothesize that in nonalcoholic fatty liver (NAFL) there is chronic suppression of CYP7B1. The chronic effects of increased levels of cholesterol metabolites as controlled by down-regulated CYP7B1 have injurious effects, and represent the major driving force for transition from NAFL to steatohepatitis via inflammasome activation. The means by which CYP7B1 is suppressed in NAFL is unknown. We propose three specific aims to define the pathway:
Aim1 : To define the CYP27A1 initiated ?alternative pathway? of oxysterol/BAS. More specifically, more clearly define the pathway metabolites and quantitate their levels/ratios under different metabolic conditions to elucidate their role in mediating liver inflammation.
Aim2 : To investigate the regulation of CYP7B1 in NAFL, and its control over intracellular levels of regulatory and potentially toxic cholesterol metabolites Aim3: To demonstrate that chronically increased hepatic levels of oxysterols and their metabolites that occur in fatty liver represent a driving force to hepatic inflammation.

Public Health Relevance

Nonalcoholic steatohepatitis (NASH) is an inflammatory response to the accumulation of toxic lipids within hepatocytes. In the United States, in Veterans and non Veterans, the progression of fatty liver disease to NASH, and ultimately to liver fibrosis and cirrhosis, will soon overtake Hepatitis C as the leading cause of liver transplantation; creating one of the largest near future financial challenges for our country?s Health Care Budget. The metabolic pathways that contribute to fatty liver?s progression to inflammation (NASH) and fibrosis are under intense study. This proposal provides a basis for the study of a novel pathway from fatty liver to liver inflammation; one which is rooted in a known human genetic model of inflammation/fibrosis. Our preliminary findings have uncovered a previously unappreciated key regulatory step responsible for the initiation of an inflammatory response. It includes basic with translational research.

Agency
National Institute of Health (NIH)
Institute
Veterans Affairs (VA)
Type
Non-HHS Research Projects (I01)
Project #
5I01BX000197-10
Application #
9679222
Study Section
Gastroenterology (GAST)
Project Start
2009-10-01
Project End
2022-03-31
Budget Start
2019-04-01
Budget End
2020-03-31
Support Year
10
Fiscal Year
2019
Total Cost
Indirect Cost
Name
VA Veterans Administration Hospital
Department
Type
DUNS #
146678115
City
Richmond
State
VA
Country
United States
Zip Code
23249
Nagahashi, Masayuki; Takabe, Kazuaki; Liu, Runping et al. (2015) Conjugated bile acid-activated S1P receptor 2 is a key regulator of sphingosine kinase 2 and hepatic gene expression. Hepatology 61:1216-26
Sharon, Chetna; Baranwal, Somesh; Patel, Nirmita J et al. (2015) Inhibition of insulin-like growth factor receptor/AKT/mammalian target of rapamycin axis targets colorectal cancer stem cells by attenuating mevalonate-isoprenoid pathway in vitro and in vivo. Oncotarget 6:15332-47
Calderon-Dominguez, Maria; Gil, Gregorio; Medina, Miguel Angel et al. (2014) The StarD4 subfamily of steroidogenic acute regulatory-related lipid transfer (START) domain proteins: new players in cholesterol metabolism. Int J Biochem Cell Biol 49:64-8
Bajaj, J S; Heuman, D M; Hylemon, P B et al. (2014) Randomised clinical trial: Lactobacillus GG modulates gut microbiome, metabolome and endotoxemia in patients with cirrhosis. Aliment Pharmacol Ther 39:1113-25
Ren, Shunlin; Kim, Jin Koung; Kakiyama, Genta et al. (2014) Identification of novel regulatory cholesterol metabolite, 5-cholesten, 3?,25-diol, disulfate. PLoS One 9:e103621
Liu, Runping; Zhao, Renping; Zhou, Xiqiao et al. (2014) Conjugated bile acids promote cholangiocarcinoma cell invasive growth through activation of sphingosine 1-phosphate receptor 2. Hepatology 60:908-18
Kakiyama, Genta; Hylemon, Phillip B; Zhou, Huiping et al. (2014) Colonic inflammation and secondary bile acids in alcoholic cirrhosis. Am J Physiol Gastrointest Liver Physiol 306:G929-37
Kakiyama, Genta; Muto, Akina; Takei, Hajime et al. (2014) A simple and accurate HPLC method for fecal bile acid profile in healthy and cirrhotic subjects: validation by GC-MS and LC-MS. J Lipid Res 55:978-90
Kakiyama, Genta; Pandak, William M; Gillevet, Patrick M et al. (2013) Modulation of the fecal bile acid profile by gut microbiota in cirrhosis. J Hepatol 58:949-55