Glutathione S‐transferase‐P and JNK Activation in Glucose Intolerance in Mice
Authors: Dastidar Shubha Ghosh , Jagatheesan Ganapathy , Bhatnagar Aruni , Conklin Daniel J 
Published: November 2023

Rationale

Glutathione S‐transferases (GSTs) are dysregulated in obesity, non‐alcoholic fatty liver disease (NAFLD) and diabetes. The GST pi isoform (GSTP) catalyzes unsaturated aldehyde conjugation and also binds and inhibits JNK (c‐Jun NH2‐terminal kinase). Although JNK activation has been linked to obesity and diabetes, the contribution of GSTP‐JNK in the causal progression of obesity and insulin resistance has not been examined.

Objective

Herein, we test whether GSTP has a selective role in glucose homeostasis and obesity via JNK activation.

Methods and Results

In male wild‐type (WT) mice, high fat diet (HFD) (4 weeks; 60% Kcal) led to hepatic (but not skeletal muscle or epididymal fat) GSTP protein down‐regulation, glucose intolerance, obesity and hepatic JNK activation. Similarly, hepatic GSTP was downregulated in obese db/db mice compared with age‐matched WT mice (p<0.05; n=3). Because HFD‐induced hepatic GSTP downregulation was an early and potentially causal change in diet‐induced glucose intolerance, we used GSTP deficient mice. GSTP‐null mice on normal chow diet were similar to WT mice in body weight, lean mass and fat mass but were significantly more glucose intolerant (p<0.001, n=30) by intraperitoneal glucose tolerance test (GTT). Yet, GSTP‐null mice had basal blood glucose and insulin levels (WT: 0.306±0.050 ng/ml, n=19; GSTP‐null: 0.253±0.030 ng/ml, n=20) as well as insulin signaling in skeletal muscle, liver and adipose similar to WT mice. Interestingly, a short‐term HFD (4 weeks; 60% Kcal) increased adiposity in WT and GSTP‐null mice to a similar degree, but HFD significantly increased GTT area under the curve (AUC) in WT but not in GSTP‐null mice. In exploring the basal difference in glucose handling, an intraperitoneal pyruvate tolerance test (PTT) revealed greater PTT AUC in GSTP‐null mice compared with WT mice (p=0.02, n=14), and indicated enhanced hepatic gluconeogenesis. Moreover, there was a significant linear correlation between the GTT AUC and PTT AUC (r2=0.3, p=0.02) in GSTP‐null mice supporting a hepatic locus for GSTP‐dependent glucose intolerance, however, no differences were found in fasting mRNA levels of the rate‐limiting enzymes of hepatic gluconeogenesis: phosphoenolpyruvate carboxykinase (PEPCK) and glucose‐6‐phosphatase (G6Pase). Yet, when JNK activation and inflammatory markers were assessed as potential hepatic‐based mechanisms, the PTT AUC was positively correlated with levels of both hepatic JNK phosphorylation and MIP1α mRNA (increased ≈5‐fold vs WT: p=0.001, n=6). To probe a direct role of JNK activation, GSTP‐null mice were treated with a JNK inhibitor (JNKI; SP600125; 5 mg/kg bwt; 7 days). The PTT AUC was decreased significantly by JNKI treatment (45±7.16 %; p=0.003; n=11) compared with vehicle‐treated littermate controls.

Conclusion

Collectively, these data indicate that hepatic GSTP modulates blood glucose excursions by inhibiting hepatic JNK activation and a JNK‐mediated hepatic gluconeogenesis that may involve hepatic MIP1α upregulation. The relevance of this relationship in human obesity, NAFLD and diabetes requires further investigation.

Support or Funding Information

Diabetes and Obesity Center Grant‐DOC NIHRRO24489