TGF-Beta-Guided Inflammatory Signaling Originating in Visceral Adipose Tissue Is Intrinsically Involved in the Etiology of Depression in Obese Patients with Non-alcoholic Fatty Liver Disease (NAFLD)

Abstract

Purpose: Recent studies have provided compelling evidence linking depression and cytokine imbalance. Numerous studies have shown that the visceral adipose tissue (VAT) of obese NAFLD patients is heavily involved in cytokine release, and particularly overall pathway commitment to Th1/Th2/Th17 type signaling. Our aim was to demonstrate a role for visceral adipose signaling, particularly of adipocytic release of TGF-b, one of the master regulators of Th1/Th2/Th17, in the cytokine imbalance associated with depression. Methods: Patients undergoing bariatric surgery for morbid obesity were included. Diagnosis of depression was based on clinical diagnosis or with documented use of anti-depressants. Diagnosis of NAFLD was based on liver biopsy. Visceral adipose tissue was collected at the time of bariatric surgery, and was used for gene expression analysis. 1 μg of RNA was extracted from frozen VAT of patients with liver biopsy-proven NAFLD, Bio-Rad's aurum total RNA fatty and fibrous tissue kit was used. cDNA was synthesized with the SABiosciences' RT2 first strand kit. cDNA samples were used in 10 μL q-RTPCR reactions. Frozen serum from patients were tested with the Biorad Bio-Plex Pro TGF-b1 and 17-plex cytokine assays. Serum levels of serotonin and BDNF were analyzed using the ALPCO human serotonin ELISA kit and the human BDNF quantikine ELISA kit. Group differences were assessed using Mann-Whitney tests and chi-square test. Co-expression patterns were analyzed using Spearman's rank-sum correlation. Results: A total of 241 biopsy-proven NAFLD patients (39.1% NASH, 20.8% with type 2 diabetes, age 43.61+/-11.41 years, BMI 46.39+/-10.91, depression/anxiety 42.6%) were assayed. In the group-wise comparison of VAT-derived mRNAs, TGFb1 (p<0.007) and UCP1 (p<0.004) transcripts were significantly higher in the depressed versus non-depressed cohort, whereas the levels of serum serotonin (p<0.00017), IL-1B (p<0.04), IL-7 (p<0.023), IL-13 (p<0.008), and G-CSF (p<0.034) were significantly lower. Analysis of correlations further confirmed these findings, and showed a positive association between serotonin levels and degree of ballooning in liver biopsies (r=0.14 p<0.03428). Interestingly, there was no correlation between serum levels of TGFb1, serotonin, or BDNF and depression/anxiety. Additionally, there was no correlation between serum levels of TGFb1 and TGFb1 receptor transcript levels in VAT, suggesting VAT as the site of origin for TGFb1 signal, rather than its target. Conclusion: Inflammatory signaling originating in VAT seems to be intrinsically involved in the etiology of depression, particularly in morbidly obese NAFLD patients. The molecular networks leading to these mood disorders need to be further elucidated.