Hepatic Stress Response Mechanisms in Progressive Human Nonalcoholic Fatty Liver Disease

Abstract

Nonalcoholic fatty liver disease (NAFLD) has become a worldwide, chronic liver disease of increasing clinical significance. It is closely associated with the rising epidemics of obesity and insulin resistance. Up to 17% of the United States population may progress from the disease stage characterized as simple, benign steatosis to the more severe, inflammatory stage of nonalcoholic steatohepatitis (NASH). This progression occurs through 2nd 'hits' of increased oxidative stress and inflammation to a liver that has been sensitized by lipotoxic stress. NASH is also characterized by increased collagen deposition resulting in fibrosis and architectural rearrangement of the liver. Progressive NAFLD is currently recognized as an important contributor to the development of cryptogenic cirrhosis and subsequent liver-related mortalities (estimated at 30-40% in these patients).The pathological progression of NAFLD, as described by the 'two hit' hypothesis, characterizes the different stages of liver injury. However, the mechanism(s) responsible for the progression to NASH are unknown. Profiling global gene expression and metabolite patterns in human liver samples representing the full spectrum of progressive human NAFLD may reveal potential mechanisms of progressive disease. Human liver samples representing each stage of NAFLD progression were analyzed by methodologies such as high-throughput microarrays, high resolution mass spectrometry, and protein immunoblot techniques. Bioinformatics tools and gene expression/regulation database software were utilized in several studies to characterize the altered hepatic profiles of these patients. Hepatic transcriptomic profiles of ADME (absorption, distribution, metabolism and elimination) and ER (endoplasmic reticulum) stress response genes exhibited initiated hepatoprotective responses in patients with NASH. The endogenous pathways of BA (bile acid) synthesis and BCAA (branched chain amino acid) metabolism also showed evidence of coordinately regulated alterations in response to disease-induced stress in NASH. The transcriptional regulation of the investigated pathways was confirmed by transcription factor binding sites enrichment analysis. The collective response to hepatic stress in human NAFLD, demonstrates a coordinated, hepatoprotective intent that may be utilized for future therapeutics in the battle against progressive liver disease.