Absence of the glucagon-like peptide-1 receptor does not affect the metabolic phenotype of mice with liver-specific G(s)α deficiency

Abstract

The stimulatory G protein α-subunit (G(s)α) couples hormone and other receptors to the generation of intracellular cAMP. We previously showed that mice with liver-specific G(s)α deficiency [liver-specific G(s)α knockout (LGsKO) mice] had reduced adiposity and improved glucose tolerance associated with increased glucose-stimulated insulin secretion, pancreatic islet hyperplasia, and very high serum glucagon and glucagon-like peptide 1 (GLP-1) levels. Because GLP-1 is known to stimulate insulin secretion and to have effects on energy balance, we mated LGsKO mice with germline GLP-1 receptor (GLP-1R) knockout mice (Glp1r(-/-)) and compared LGsKO to double-knockout (LGs/Glp1r(-/-)) mice to determine the contribution of excess GLP-1R signaling to the LGsKO phenotype. Loss of the GLP-1R failed to reverse most of the metabolic features of LGsKO mice, including reduced fat mass, increased glucose tolerance, and second-phase glucose-stimulated insulin secretion, islet cell hyperplasia, and very high glucagon and GLP-1 levels. However, loss of GLP-1R impaired first-phase insulin secretion in mice with or without liver-specific G(s)α deficiency. Thus, excess GLP-1 action (or at least through GLP-1R) does not contribute to the LGsKO metabolic phenotype, and other unknown factors involved in the cross talk between the liver G(s)α/cAMP pathway and pancreatic islet function need to be further elucidated.

Fig. 1.

Body weight, body composition, and organ weights of 6- to 7-month-old male mice. A, Body weight; B, body composition; C, organ weights expressed as percentage of total body weights (n = 5–10 per group). BAT, Brown adipose tissue; WAT, epididymal white adipose tissue. Results are expressed as mean ± sem. *, P < 0.05 vs. controls; #, P < 0.05 vs. Glp1r^−/− mice; x, P < 0.05 vs. LGsKO mice.

Fig. 2.

Energy expenditure and activity studies in 5- to 6-month-old male mice. A, Resting and total O₂ consumption (vO₂); B, total and ambulatory (Amb) activity levels; C, resting and total respiratory exchange ratio (RER, vCO₂/vO₂) measured over 24 h at 22 C (n = 6–10 per group). Results are expressed as mean ± sem.

Fig. 3.

Glucose and insulin tolerance tests on overnight-fasted 2- to 3-month-old mice. A, Intraperitoneal glucose tolerance test after administration of glucose (2 mg/g body weight ip); B, oral glucose tolerance test after oral glucose administration by gavage (2 mg/g body weight); C, insulin tolerance test after insulin administration (0.75 mU/g body weight ip). Panels on the left show glucose levels at each time point of the studies, and panels on the right show AUC of glucose using time 0 glucose as baseline. For insulin tolerance tests, each time point is the percent decrease from 100% as baseline. Results are expressed as mean ± sem (n = 4–8 per group). *, P < 0.05 vs. controls; #, P < 0.05 vs. Glp1r^−/− mice.

Fig. 4.

Glucose-stimulated insulin responses. A, Short-term insulin response to high-dose glucose (3 mg/g body weight ip); B, longer-term insulin response to glucose (2 mg/g body weight ip) showing glucose (upper panels), insulin levels (middle panels), and insulin to glucose ratios (lower panels) for each study. Results are expressed as mean ± sem (n = 4–11 per group). Two-way ANOVA showed that both the LGsKO and Glp1r^−/− independently affected first-phase insulin secretion (insulin levels and insulin to glucose ratios at 2.5 min), whereas at later time points representing second-phase insulin secretion, only the LGsKO mutation had a significant effect.

Fig. 5.

Serum glucagon and active GLP-1 levels and islet histology in 3- to 4-month-old male mice. A and B, Serum glucagon (A) and active GLP-1 (B) levels (in log scale) in fed mice (n = 5–10 per group); C, representative hematoxylin- and eosin-stained sections showing islets at ×200 and ×400 original magnification; D and E, average islet area (D) and total islet area (E) as percentage of total pancreatic area (n = 4–6 per group); F, representative immunohistochemistry of islets stained for glucagon (green, left column), insulin (red, middle column), and merged images (right column). Results are expressed as mean ± sem. *, P < 0.05 vs. controls; #, P < 0.05 vs. Glp1r^−/− mice.