Glucagon-Receptor Signaling Reverses Hepatic Steatosis Independent of Leptin Receptor Expression

Abstract

Glucagon (GCG) is an essential regulator of glucose and lipid metabolism that also promotes weight loss. We have shown that glucagon-receptor (GCGR) signaling increases fatty acid oxidation (FAOx) in primary hepatocytes and reduces liver triglycerides in diet-induced obese (DIO) mice; however, the mechanisms underlying this aspect of GCG biology remains unclear. Investigation of hepatic GCGR targets elucidated a potent and previously unknown induction of leptin receptor (Lepr) expression. Liver leptin signaling is known to increase FAOx and decrease liver triglycerides, similar to glucagon action. Therefore, we hypothesized that glucagon increases hepatic LEPR, which is necessary for glucagon-mediated reversal of hepatic steatosis. Eight-week-old control and liver-specific LEPR-deficient mice (LeprΔliver) were placed on a high-fat diet for 12 weeks and then treated with a selective GCGR agonist (IUB288) for 14 days. Liver triglycerides and gene expression were assessed in liver tissue homogenates. Administration of IUB288 in both lean and DIO mice increased hepatic Lepr isoforms a-e in acute (4 hours) and chronic (72 hours,16 days) (P < 0.05) settings. LeprΔliver mice displayed increased hepatic triglycerides on a chow diet alone (P < 0.05), which persisted in a DIO state (P < 0.001), with no differences in body weight or composition. Surprisingly, chronic administration of IUB288 in DIO control and LeprΔliver mice reduced liver triglycerides regardless of genotype (P < 0.05). Together, these data suggest that GCGR activation induces hepatic Lepr expression and, although hepatic glucagon and leptin signaling have similar liver lipid targets, these appear to be 2 distinct pathways.

Glucagon-receptor signaling regulates weight loss via central KLB receptor complexes

“…Concordant with our previous observations ( 13 , 14 , 39 ), chronic IUB288 administration stimulated robust weight loss in control mice ( P < 0.0001, % change from day 0, Figure 3A ; and body weight change in grams, Figure 3B ). In line with our earlier findings in Fgf21 Δliver mice ( 13 ), Klb ΔCNS mice displayed a partial reduction in body weight ( P < 0.0001, Figure 3, A and B ; and interaction of time, genotype, and treatment P < 0.0001 for absolute body weight loss, Supplemental Figure 2G ) as compared with littermate controls, while IUB288 reduced food intake independent of central Klb ( Figure 3C ).…”

“…As expected, the cannula/pump placement resulted in a slight body weight decrease in all groups; however, we observed no 1153-associated differences ( Supplemental Figure 3C ). Congruent with our previous findings ( 13 , 14 , 39 ), IUB288 stimulated a significant decrease in body weight in control mice ( P < 0.0001, day 12 compared with day 1 IUB288 start, Figure 4A ). Mice receiving the combination of central 1153 and subcutaneous IUB288 also lost a significant amount of body weight in comparison with their respective control; however, this weight loss was significantly less than control IUB288-treated mice ( P < 0.0001, Figure 4A ).…”

“…Consistent with results in Klb ΔCNS mice, IUB288 increased plasma FGF21 independent of 1153 (treatment effect P < 0.001, Figure 5A ), suggesting this regulation is independent of central KLB antagonism. Chronic GCGR agonism is a potent regulator of lipid metabolism, including reducing plasma cholesterol and liver TG ( 13 , 14 , 39 ). We observed a significant and KLB-independent reduction of plasma cholesterol with IUB288 treatment ( P < 0.0001, Figure 5B , middle panel) and unaltered plasma TG ( Figure 5B , left panel).…”

“…Central Klb contributes to GCGR-stimulated weight loss. Chronic GCGR agonism via IUB288 promotes weight loss and improves lipid homeostasis in DIO mice (13,14,39). We previously published that mice deficient for whole-body Fgf21 (Fgf21 -/-) are refractory to GCGR-mediated prevention of diet-induced obesity (14), and DIO mice deficient for liver Fgf21 (Fgf21 Δliver ) exhibit partial reductions in body weight (13) in response to GCGR agonism, suggesting that FGF21 contributes to the antiobesity effects of GCGR signaling.…”

“…Lipids in plasma and tissue samples from 2-hour fasted mice were determined using Infinity Triglycerides (Thermo Fisher Scientific TR22421) and Infinity Cholesterol (Thermo Fisher Scientific TR13421). Hepatic lipid measurements were conducted following extraction as previously described ( 39 , 72 ). Briefly, liver (40–80 mg) was homogenized for 2 minutes at 30 Hz (×2) using a TissueLyser II (Qiagen), and lipid was extracted using chloroform/methanol (2:1, v/v ) and ultrapure water.…”