Blood-Brain Barrier Integrity and Transport of Major Hormones are Unchanged in Mice With Euglycemic Hyperinsulinemia

Abstract

High-fat diet (HFD) consumption increases the risk of metabolic syndrome as manifested by insulin resistance, fatty liver, hypertriglyceridemia, and diabetes mellitus type 2. Blood-brain barrier (BBB) disruptions and impaired BBB transport of metabolic hormones, including leptin, insulin, and ghrelin, occur in diabetes mellitus type 2 and contribute to metabolic dysregulation and cognitive impairment. However, it is unclear whether the BBB changes are caused by the HFD, obesity, insulin resistance, elevated glucose or triglyceride levels, or other aspects of the metabolic syndrome. This study examined the effects of chronic HFD and an early stage of metabolic syndrome on BBB disruption and transport of insulin, leptin, and ghrelin. Mice on the HFD demonstrated obesity, increases in insulin, leptin, plasminogen activator inhibitor-1, and resistin, fatty liver and hyperglycerolemia, without elevations in glucose, triglycerides, ghrelin, glucagon, gastric inhibitory polypeptide, or glucagon-like peptide. The vascular markers of sucrose and albumin did not show BBB disruption. HFD did not alter the rate of insulin, leptin, or ghrelin transport across the BBB. However, leptin binding to the luminal surface of the BBB was greater in the hypothalamus and reduced for the rest of the brain with HFD treatment. The liver uptake of insulin, leptin, and ghrelin was reduced in the HFD group. Overall, our findings indicate that chronic HFD consumption with concomitant obesity and insulin resistance in the absence of hyperglycemia does not result in BBB disruption or altered BBB permeability to key metabolic hormones but may selectively affect vascular binding of important metabolic hormones in the brain and liver.

Keywords: blood-brain barrier; diet-induced obesity; hyperglycemia; insulin; leptin.

Figure 1.

Effects of chronic HFD consumption on body weight and serum glucose levels. Body weight (A) and liver weight (B) were significantly higher in the HFD group compared to Chow, with no difference in blood glucose (C). There was a significant correlation between blood glucose and body weight at P < .01 (D) and blood glucose and liver weight at P < .01 (E) in the HFD group but not in the Chow group. n = 43-58. Data are presented as mean ± SEM and analyzed by unpaired t-test (A-C) or simple linear regression (D, E). Asterisks indicate significant difference (****P < .0001).

Figure 2.

Serum protein levels were measured between the HFD and Chow groups. Insulin (A), leptin (B), PAI-1 (C), and resistin (D) levels were significantly higher in the HFD group compared to the Chow group. Ghrelin (E), GIP (F), GLP-1 (G), and glucagon (H) levels were not significantly different. n = 10-11. Data are presented as mean ± SEM and analyzed by unpaired t-test. Asterisks indicate significant difference (**P < .01, ***P < .001, ****P < .0001).

Figure 3.

Serum glycerol and triglycerides were measured between the HFD and Chow groups. Free glycerol (A) was significantly higher in the HFD group compared to the Chow group. Serum triglycerides (B) was unchanged by HFD. n = 10-11. Data are presented as mean ± SEM and analyzed by unpaired t-test. Asterisks indicate significant difference (*P < .05, **P < .01).

Figure 4.

The effect of chronic HFD on BBB permeability in various brain regions was assessed by measuring the entry of IV administered (A) radiolabeled sucrose and (B) radiolabeled albumin after washout of the vascular space. The brain/serum ratio of sucrose or albumin did not significantly differ between the HFD group and Chow group. Brain/serum ratios of sucrose in the hypothalamus of the HFD group paradoxically tended to be significantly lower than in the Chow group. n = 9-10. Data are presented as mean ± SEM and analyzed by unpaired t-test.

Figure 5.

The effect of chronic HFD on insulin transport through the BBB was measured by injecting mice IV with radiolabeled insulin. The brain/serum ratio of insulin was measured over 15 minutes in the (A) hypothalamus, (B) hippocampus, (C) cerebellum, (D) remainder of the brain, (E) whole brain (all regions combined), and (F) liver. The rate of insulin transport into the brain did not differ between the HFD group and the Chow group. n = 11-16. Data were analyzed by simple linear regression.

Figure 6.

The effect of chronic HFD on leptin transport through the BBB was measured by injecting mice IV with radiolabeled leptin. The brain/serum ratio of leptin was measured over 15 minutes in the (A) hypothalamus, (B) hippocampus, (C) cerebellum, (D) remainder of the brain, (E) whole brain (all regions combined), and (F) liver. The rate of leptin transport did not differ between the HFD group and the Chow group, although leptin binding to the luminal surface was greater in the hypothalamus in the HFD group. n = 11-16. Data were analyzed by simple linear regression.

Figure 7.

The effect of chronic HFD on ghrelin transport across the BBB was measured by injecting mice IV with radiolabeled ghrelin. The brain/serum ratio of ghrelin was measured over 15 minutes in the (A) hypothalamus, (B) hippocampus, (C) cerebellum, (D) remainder of the brain, (E) whole brain (all regions combined), and (F) liver. The rate of ghrelin transport into the brain did not differ between the HFD group and the Chow group. n = 11-16. Data were analyzed by simple linear regression.