Obesity-induced hepatic steatosis is mediated by endoplasmic reticulum stress in the subfornical organ of the brain

Abstract

Nonalcoholic fatty liver disease (NAFLD), characterized by an excess accumulation of hepatic triglycerides, is a growing health epidemic. While ER stress in the liver has been implicated in the development of NAFLD, the role of brain ER stress - which is emerging as a key contributor to a number of chronic diseases including obesity - in NAFLD remains unclear. These studies reveal that chemical induction of ER stress in the brain caused hepatomegaly and hepatic steatosis in mice. Conversely, pharmacological reductions in brain ER stress in diet-induced obese mice rescued NAFLD independent of body weight, food intake, and adiposity. Evaluation of brain regions involved revealed robust activation of ER stress biomarkers and ER ultrastructural abnormalities in the circumventricular subfornical organ (SFO), a nucleus situated outside of the blood-brain-barrier, in response to high-fat diet. Targeted reductions in SFO-ER stress in obese mice via SFO-specific supplementation of the ER chaperone 78-kDa glucose-regulated protein ameliorated hepatomegaly and hepatic steatosis without altering body weight, food intake, adiposity, or obesity-induced hypertension. Overall, these findings indicate a novel role for brain ER stress, notably within the SFO, in the pathogenesis of NAFLD.

Keywords: Metabolism; Neuroscience.

Figure 1. Short-term induction of brain ER stress promotes hepatic steatosis independent of changes in body weight, food intake, or adiposity.

Liver mass (A) and hepatic triglyceride levels (B) following 3 days of daily intracerebroventricular (ICV) vehicle or thapsigargin (to induce brain ER stress) administration. n = 6–10. (C) H&E staining of the liver in mice that underwent ICV vehicle or thapsigargin dosing. Representative of n = 4. Scale bar: 100 μm. Body weight (D), food intake (E), and cumulative food intake (F) at baseline and during 3 days of daily ICV vehicle or thapsigargin administration. Regional adipose tissue mass (G) following 3 days of ICV vehicle or thapsigargin. n = 6–10. *P < 0.05 vs. ICV vehicle with two-tailed unpaired t-test or 2-way repeated measures ANOVA. Box-and-whisker plots represent the median (line within box), upper and lower quartile (bounds of box), and maximum and minimum values (bars).

Figure 2. Short-term reductions in brain ER stress reduce obesity-induced hepatic steatosis, hypertension, and tachycardia independent of body weight, food intake, or adiposity.

Liver mass (A) and hepatic triglyceride levels (B) following 3 days of daily intracerebroventricular (ICV) administration of the ER chemical chaperone TUDCA (to reduce ER stress) or vehicle in normal chow–fed and HFD-fed mice. n = 6–8. (C) H&E staining of the liver in mice that underwent ICV vehicle or TUDCA dosing. Representative of n = 4. Scale bar: 100 μm. Body weight (D), food intake (E), and cumulative food intake (F) in normal chow–fed and HFD-fed mice at baseline and during 3 days of ICV administration of TUDCA or vehicle. Regional adipose tissue mass (G) following 3 days of ICV vehicle or TUDCA administration. n = 6–8. Radiotelemetric measurements of mean arterial blood pressure (H) and heart rate (I) at baseline and during daily ICV TUDCA or vehicle administration. The respective change in blood pressure and heart rate is shown on the right (H and I). ^#P < 0.05 vs. normal chow groups; *P < 0.05 vs. high fat ICV TUDCA. One-way or two-way repeated measures ANOVA. Box-and-whisker plots represent the median (line within box), upper and lower quartile (bounds of box), and maximum and minimum values (bars).

Figure 3. Obesity elicits robust UPR activation and ER ultrastructural alterations in the SFO.

(A) Schematic highlighting the location of the SFO at the base of the lateral ventricle (LV). (B) Real-time PCR measurements of ER stress biomarkers p58^IPK, CHOP, and XBP1s from micropunches of the SFO in normal chow–fed or HFD-fed mice. n = 5. Two brains pooled per sample. (C) Western blot and quantitative summary of the ER chaperones GRP78 and PDI in SFO homogenates from normal chow–fed and HFD-fed mice. n = 4. Two brains pooled per sample. *P < 0.05 vs. normal chow with a two-tailed unpaired t-test. Box-and-whisker plots represent the median (line within box), upper and lower quartile (bounds of box), and maximum and minimum values (bars). (D) Representative electron micrographs of rough ER (arrows) in SFO neurons of a normal chow–fed (left) and HFD-fed mouse (right). The electron micrographs are representative of 20–28 neuronal and dendritic fragments evaluated from 3 mice in each group. Scale bar = 2 μm

Figure 4. Selective reductions in SFO-ER stress rescue obesity-induced hepatic steatosis, independent of body weight, food intake, or adiposity.

Body weight (A), food intake (B), and cumulative food intake (C) in normal chow–fed and HFD-fed mice following chronic SFO-targeted viral overexpression of the ER chaperone GRP78 (AdGRP78) or control vector (AdLacZ). Regional adipose tissue (D) mass 5 weeks after SFO-targeted AdGRP78 or AdLacZ. Representative liver images in HFD-fed mice (E), as well as liver mass (F) and hepatic triglyceride levels (G) in HFD-fed and normal chow–fed mice following SFO-targeted AdGRP78 or AdLacZ. n = 6–8. H&E staining (H) of the liver in mice 5 weeks after SFO-targeted AdGRP78 or AdLacZ. Representative of n = 4. Scale bar: 100 μm. Radiotelemetric measurements of mean arterial blood pressure (I) and heart rate (J) following adenoviral overexpression of GRP78, or control vector, in the SFO. ^#P < 0.05 vs. normal chow groups; *P < 0.05 vs. high fat AdLacZ. One-way or two-way repeated measures ANOVA. Box-and-whisker plots represent the median (line within box), upper and lower quartile (bounds of box), and maximum and minimum values (bars).