BMAL1 and CLOCK, two essential components of the circadian clock, are involved in glucose homeostasis

Abstract

Circadian timing is generated through a unique series of autoregulatory interactions termed the molecular clock. Behavioral rhythms subject to the molecular clock are well characterized. We demonstrate a role for Bmal1 and Clock in the regulation of glucose homeostasis. Inactivation of the known clock components Bmal1 (Mop3) and Clock suppress the diurnal variation in glucose and triglycerides. Gluconeogenesis is abolished by deletion of Bmal1 and is depressed in Clock mutants, but the counterregulatory response of corticosterone and glucagon to insulin-induced hypoglycaemia is retained. Furthermore, a high-fat diet modulates carbohydrate metabolism by amplifying circadian variation in glucose tolerance and insulin sensitivity, and mutation of Clock restores the chow-fed phenotype. Bmal1 and Clock, genes that function in the core molecular clock, exert profound control over recovery from insulin-induced hypoglycaemia. Furthermore, asynchronous dietary cues may modify glucose homeostasis via their interactions with peripheral molecular clocks.

Figure 1. Circadian Variation of Glucose, Triglyceride, and Hormone Levels in Circulating Blood

Plasma from whole blood isolated from unchallenged WT mice at different CTs was analyzed for glucose (A), triglyceride (B), corticosterone (C), and adiponectin levels (D) (n = 12 per time point). Results for Bmal1 ^−/− and Clock^mut mice are shown in Table 1.

Figure 2. Disruption of Genes in the Core Molecular Clock Alters the Response to Insulin

(A) Insulin tolerance (IT) was examined in WT mice on CT7, CT13, CT19, and CT25 at 30 min, 60 min, and 90 min after insulin injection (n = 12 per time point, *p < 0.01). (B) IT was examined in WT (black line), Bmal1 ^−/− (blue line), and Clock^mut mice (green line) at CT1 (i) and CT13 (ii) (n = 6–10, *p < 0.05, †p < 0.01, ††p < 0.001). (C and D) Plasma levels of the counterregulatory hormones corticosterone (C) and glucagon (D) were assessed 60 min after insulin injection in Bmal1 ^−/− and Clock^mut mice (n = 7, corticosterone assay; samples were pooled for glucagon assay, *p < 0.05).

Figure 3. Impaired Gluconeogenesis in Mice with a Disrupted Circadian Clock

(A) Pyruvate tolerance was compared among WT (black line), Bmal1^+/− (blue line), and Bmal1^−/− (purple line), and Clock^mut (green line) mice at CT7 (n = 6–10). (B) Relative PEPCK activity (units are expressed as luciferase activity × 10³) was measured in liver (i), aorta (ii), and kidney (iii) from WT (white bars) and Clock^mut mice (green bars).

Figure 4. The Molecular Clock Conditions HF-Induced Circadian Variation in Glucose Homeostasis

(A) Glucose tolerance (GT) in RC-fed WT mice (i), HF-fed WT mice (ii), and HF-fed Clock^mut mice (iii). (B) IT in RC-fed WT mice (i), HF-fed WT mice (ii), and HF-fed Clock^mut mice (iii) at respective times (n = 6–8; *p < 0.05, †p < 0.01).