A Critical Role of Hepatic GABA in The Metabolic Dysfunction and Hyperphagia of Obesity

Abstract

Hepatic lipid accumulation is a hallmark of type II diabetes (T2D) and associated with hyperinsulinemia, insulin resistance, and hyperphagia. Hepatic synthesis of GABA, catalyzed by GABAtransaminase (GABA-T), is upregulated in obese mice. To assess the role of hepatic GABA production in obesity-induced metabolic and energy dysregulation, we treated mice with two pharmacologic GABA-T inhibitors and knocked down hepatic GABA-T expression using an antisense oligonucleotide. Hepatic GABA-T inhibition and knockdown decreased basal hyperinsulinemia and hyperglycemia, and improved glucose intolerance. GABA-T knockdown improved insulin sensitivity assessed by hyperinsulinemiceuglycemic clamps in obese mice. Hepatic GABA-T knockdown also decreased food intake and induced weight loss without altering energy expenditure in obese mice. Data from people with obesity support the notion that hepatic GABA production and transport are associated with serum insulin, HOMA-IR, T2D, and BMI. These results support a key role for hepatocyte GABA production in the dysfunctional glucoregulation and feeding behavior associated with obesity.However, in the liver, GABA-T mediates GABA synthesis 12 . We have proposed that hepatic lipids activate reversed GABA shunt activity in hepatocytes, and that hepatic GABA and glucose production are metabolically linked ). It remains completely untested whether manipulating this GABA shunt can prevent hepatic steatosis derived metabolic disease. Thus, GABA-T represents a promising target to decrease hyperinsulinemia and insulin resistance by limiting hepatic GABA production. Accordingly, in the current manuscript, we employed two novel models to limit hepatic GABA production: 1) pharmacologic inhibition of GABA-T activity, and 2) antisense oligonucleotide (ASO) mediated knockdown of hepatic specific GABA-T expression. Using these models for the first time we assessed systemic glucose homeostasis to strengthen the causative role between hepatic GABA production and hyperinsulinemia / insulin resistance. We also assessed food intake and energy expenditure to understand the role of hepatic GABA production in the dysregulation of energy homeostasis in obesity. Results GABA-Transaminase Inhibition Improves Glucose Homeostasis in ObesityTo directly assess the effect of GABA-T in obesity-induced metabolic dysfunction we treated high fat diet-induced obese mice with one of two irreversible GABA-T inhibitors, ethanolamine-Osulphate (EOS) or vigabatrin (8 mg/day). Both reduce hepatic GABA-T activity by over 90% within two days 13 . Through 5 days of treatment, body weight remained similar among EOS, vigabatrin, and saline injected mice (Fig. 1A). Four days of EOS or vigabatrin treatment decreased serum insulin and glucose concentrations and increased the glucose:insulin ratio relative to pre-treatment ( Figs. 1B-1D). Two-weeks washout from EOS or vigabatrin resulted in a return of serum insulin and the glucose:insulin ratio to pretreatment levels (Figs. 1B-1D). EOS treatment (5 days) decreased serum glucagon relat...

Obesity-Associated Alterations in Lung Function in Mice Measured with Head-Out Plethysmography

“…To assess if obesity-associated alterations in breathing are driven by hepatic GABA production, we treated diet-induced obese male mice for four weeks with an ASO targeted to GABA-Transaminase (ABAT ASO) or a scrambled control (Control) and assessed tidal breathing after treatment using the head-out plethysmography system. This treatment paradigm reduces GABA-Transaminase mRNA expression by >98% and eliminates the obesity-induced rise in ex vivo liver slice GABA release(19, 22). Knocking down GABA-Transaminase did not alter tidal breathing (Figure 5) for any variable assessed.…”

“…We treated diet-induced obese mice (6-8 month age) with a control antisense oligonucleotide (ASO; 12.5 mg/kg bi-weekly for 4 weeks) or an ASO targeted against GABA-Transaminase (ABAT ASO) to limit hepatic GABA production (19,22). To assess the role of the OBESITY-ASSOCIATED ALTERATIONS IN LUNG FUNCTION 6 hepatic vagal nerve, we performed sham or hepatic vagotomy surgeries in 12-week old male mice and assessed lung function at 0, 3, 6, and 9 weeks on HFD (22).…”

“…We recently established that hepatic gamma aminobutyric acid (GABA) inhibits the hepatic vagal afferent nerve (HVAN) resulting in insulin resistance and hyperinsulinemia (19,20).…”

“…We recently established that hepatic gamma aminobutyric acid (GABA) inhibits the hepatic vagal afferent nerve (HVAN) resulting in insulin resistance and hyperinsulinemia(19, 20). Knocking down GABA-transaminase (GABA-T) mRNA expression or pharmacologically inhibiting GABA-T decreases hepatic GABA production and limits obesity induced hyperinsulinemia and insulin resistance(20).…”

New Insights Into Mitochondrial Dysfunction at Disease Susceptibility Loci in the Development of Type 2 Diabetes

“…Examples include CISD2, a causal gene for Wolfram syndrome which manifests with early-onset diabetes (59,60) and ABAT, which encodes Gamma-aminobutyric acid transaminase (GABA-T). GABA-T knockdown improves insulin sensitivity in obese mice, causing weight loss and decreased food intake (61). As another example, the T2D cis-gene GPAM encodes glycerol-3-phosphate acyltransferase and its overexpression has been shown to induce IR (62)(63)(64).…”