A Computational Model of Hepatic Energy Metabolism: Understanding Zonated Damage and Steatosis in NAFLD

Abstract

In non-alcoholic fatty liver disease (NAFLD), lipid build-up and the resulting damage is known to occur more severely in pericentral cells. Due to the complexity of studying individual regions of the sinusoid, the causes of this zone specificity and its implications on treatment are largely ignored. In this study, a computational model of liver glucose and lipid metabolism is presented which treats the sinusoid as the repeating unit of the liver rather than the single hepatocyte. This allows for inclusion of zonated enzyme expression by splitting the sinusoid into periportal to pericentral compartments. By simulating insulin resistance (IR) and high intake diets leading to the development of steatosis in the model, we identify key differences between periportal and pericentral cells accounting for higher susceptibility to pericentral steatosis. Secondly, variation between individuals is seen in both susceptibility to steatosis and in its development across the sinusoid. Around 25% of obese individuals do not show excess liver fat, whilst 16% of lean individuals develop NAFLD. Furthermore, whilst pericentral cells tend to show higher lipid levels, variation is seen in the predominant location of steatosis from pericentral to pan-sinusoidal or azonal. Sensitivity analysis was used to identify the processes which have the largest effect on both total hepatic triglyceride levels and on the sinusoidal location of steatosis. As is seen in vivo, steatosis occurs when simulating IR in the model, predominantly due to increased uptake, along with an increase in de novo lipogenesis. Additionally, concentrations of glucose intermediates including glycerol-3-phosphate increased when simulating IR due to inhibited glycogen synthesis. Several differences between zones contributed to a higher susceptibility to steatosis in pericentral cells in the model simulations. Firstly, the periportal zonation of both glycogen synthase and the oxidative phosphorylation enzymes meant that the build-up of glucose intermediates was less severe in the periportal hepatocyte compartments. Secondly, the periportal zonation of the enzymes mediating β-oxidation and oxidative phosphorylation resulted in excess fats being metabolised more rapidly in the periportal hepatocyte compartments. Finally, the pericentral expression of de novo lipogenesis contributed to pericentral steatosis when additionally simulating the increase in sterol-regulatory element binding protein 1c (SREBP-1c) seen in NAFLD patients in vivo. The hepatic triglyceride concentration was predicted to be most sensitive to inter-individual variation in the activity of enzymes which, either directly or indirectly, determine the rate of free fatty acid (FFA) oxidation. The concentration was most strongly dependent on the rate constants for β-oxidation and oxidative phosphorylation. It also showed moderate sensitivity to the rate constants for processes which alter the allosteric inhibition of β-oxidation by acetyl-CoA. The predominant sinusoidal location of steatosis meanwhile was most sensitive variations in the zonation of proteins mediating FFA uptake or triglyceride release as very low density lipoproteins (VLDL). Neither the total hepatic concentration nor the location of steatosis showed strong sensitivity to variations in the lipogenic rate constants.

Fig 1. The structure of the model.

The porto-central axis of the sinusoid is considered to be the repeating unit of the liver. Cells and blood are compartmentalized into groups according to their position along the liver sinusoid. This allows the model to include changes in blood oxygenation, hormone concentrations and substrate and product concentrations across the sinusoid as well as differences in hepatic enzyme expression between compartments. Blood exits the sinusoid into a larger compartment representing the rest of the body. Blood in this compartment interacts with the pancreas, lungs and adipose tissue. Glucose and FFA inputs and consumption also occur in this compartment.

Fig 2. Variables and conversions included in each hepatic compartment.

In addition to the hormonal regulation, almost all of the glucose and lipid metabolism conversions show some form of allosteric regulation. Lipolysis is also included but it occurs at very slow rate in hepatocytes.

Fig 3

(a) The effects of simulating IR on total hepatic metabolism (averaged across the sinusoid). (b) The heterogeneity in the effects of simulating IR across the sinusoid. (c) The effects of simulating increasing severities of IR on plasma triglyceride, glucose and FFA concentrations compared with experimental data from Sindelka et al [65] and Burnt et al. [66].

Fig 4. Metabolite concentrations when simulating NAFLD.

(a) The average triglyceride (b) FFA, (c) ATP, (d) glycerol-3-phosphate and (e) glycogen concentration and (f) the difference between postprandial peak and pre-prandial trough glycogen concentrations in the different regions of the sinusoid when simulating (purple) a MH individual, (orange) IR alone and (red) IR with increase SREBP-1c expression using a moderate intake diet.

Fig 5. Metabolic rates when simulating NAFLD.

The average rates of (a) triglyceride synthesis, (b) lipolysis, (c) triglyceride release as VLDL, (d) β-oxidation (e) FFA uptake, (f) lipogenesis and (g) ATP synthesis from acetyl-CoA in the different regions of the sinusoid when simulating (purple) a MH individual, (orange) IR alone and (red) IR with increase SREBP-1c expression using a moderate intake diet.

Fig 6. The simulated effects of varying dietary intake in the model.

(a-d) The average hepatic lipid content across the sinusoid when simulating varying glucose and FFA intake diets in individuals with (a) MH insulin sensitivity (100%, K_IR = 1), (b) developing IR (5%, K_IR = 0.05), (c) severe IR (1.5%, K_IR = 0.015) and (d) severe IR in combination with increased SREBP-1c expression. (e-f) The effects of high fat and carbohydrate (glucose) intake on (e) ATP concentrations and (f) FFA concentrations across the sinusoid when simulating a MH, insulin sensitive individual. High and low intakes correspond to a sustained 12.5% difference in intake relative to the moderate intake det. Very high and raised intakes correspond to 25% and 5% increases respectively. The average concentrations over a 4 hours intake/output cycle are depicted.