α-Lipoic Acid Reduces Ceramide Synthesis and Neuroinflammation in the Hypothalamus of Insulin-Resistant Rats, While in the Cerebral Cortex Diminishes the β-Amyloid Accumulation

Abstract

Background: Oxidative stress underlies metabolic diseases and cognitive impairment; thus, the use of antioxidants may improve brain function in insulin-resistant conditions. We are the first to evaluate the effects of α-lipoic acid (ALA) on redox homeostasis, sphingolipid metabolism, neuroinflammation, apoptosis, and β-amyloid accumulation in the cerebral cortex and hypothalamus of insulin-resistant rats.

Methods: The experiment was conducted on male cmdb/outbred Wistar rats fed a high-fat diet (HFD) for 10 weeks with intragastric administration of ALA (30 mg/kg body weight) for 4 weeks. Pro-oxidant and pro-inflammatory enzymes, oxidative stress, sphingolipid metabolism, neuroinflammation, apoptosis, and β-amyloid level were assessed in the hypothalamus and cerebral cortex using colorimetric, fluorimetric, ELISA, and HPLC methods. Statistical analysis was performed using three-way ANOVA followed by the Tukey HSD test.

Results: ALA normalizes body weight, food intake, glycemia, insulinemia, and systemic insulin sensitivity in HFD-fed rats. ALA treatment reduces nicotinamide adenine dinucleotide phosphate (NADPH) and xanthine oxidase activity, increases ferric-reducing antioxidant power (FRAP) and thiol levels in the hypothalamus of insulin-resistant rats. In addition, it decreases myeloperoxidase, glucuronidase, and metalloproteinase-2 activity and pro-inflammatory cytokines (IL-1β, IL-6) levels, while in the cerebral cortex ALA reduces β-amyloid accumulation. In both brain structures, ALA diminishes ceramide synthesis and caspase-3 activity. ALA improves systemic oxidative status and reduces insulin-resistant rats' serum cytokines, chemokines, and growth factors.

Conclusion: ALA normalizes lipid and carbohydrate metabolism in insulin-resistant rats. At the brain level, ALA primarily affects hypothalamic metabolism. ALA improves redox homeostasis by decreasing the activity of pro-oxidant enzymes, enhancing total antioxidant potential, and reducing protein and lipid oxidative damage in the hypothalamus of HFD-fed rats. ALA also reduces hypothalamic inflammation and metalloproteinases activity, and cortical β-amyloid accumulation. In both brain structures, ALA diminishes ceramide synthesis and neuronal apoptosis. Although further study is needed, ALA may be a potential treatment for patients with cerebral complications of insulin resistance.

Keywords: brain; ceramide; inflammation; insulin resistance; oxidative stress; α-lipoic acid.

Figure 1

Effects of α-lipoic acid (ALA) on brain pro-oxidant enzymes [NADPH oxidase (NOX), (A) xanthine oxidase (XO), (B)], antioxidant status [ferric-reducing antioxidant power (FRAP), (C)], and oxidative stress [total thiols, (D) thiobarbituric acid reactive substances (TBARS), (E)] in the hypothalamus and cerebral cortex of rats fed a control (CD) and high-fat diet (HFD). Values are presented as mean ± SD. Three-way ANOVA followed by post hoc Tukey HSD test was performed. *p < 0.05, **p < 0.005, ***p < 0.0005, ****p < 0.0001.

Figure 2

Effects of α-lipoic acid (ALA) on brain sphingolipid metabolism [sphingosine (SFO), (A) sphinganine (SFA), (B) sphingosine-1-phosphate (S1P), (C) sphinganine-1-phosphate (SFA1P), (D) ceramide (CER), (E) neutral sphingomyelinase (nSMase), Figure 2F] in the hypothalamus and cerebral cortex of rats fed a control (CD) and high-fat diet (HFD). Values are presented as mean ± SD. Three-way ANOVA followed by post hoc Tukey HSD test was performed. *p < 0.05, **p < 0.005, ***p < 0.0005, ****p < 0.0001.

Figure 3

Effects of α-lipoic acid (ALA) on brain pro-inflammatory enzymes [myeloperoxidase (MPO), (A) β-glucuronidase (GLU), (B)] and cytokines [interleukin 1 beta (IL-1β), (C) interleukin 6 (IL-6), (D)] in the hypothalamus and cerebral cortex of rats fed a control (CD) and high-fat diet (HFD). Values are presented as mean ± SD. Three-way ANOVA followed by post hoc Tukey HSD test was performed. *p < 0.05, ** < 0.005, ***p < 0.0005, ****p < 0.0001.

Figure 4

Effects of α-lipoic acid (ALA) brain apoptosis [caspase-3 (CAS-3), (A) caspase-8 (CAS-8), (B)] in the hypothalamus and cerebral cortex of rats fed a control (CD) and high-fat diet (HFD). Values are presented as mean ± SD. Three-way ANOVA followed by post hoc Tukey HSD test was performed. *p < 0.05, **p < 0.005, ***p < 0.0005.

Figure 5

Effects of α-lipoic acid (ALA) on brain metalloproteinases [metalloproteinase-2 (MMP-2), (A) metalloproteinase-9 (MMP-9), (B)] and their ratio [MMP-2/MMP-9 ratio (MMP-2/MMP-9), (C)] in the hypothalamus and cerebral cortex of rats fed a control (CD) and high-fat diet (HFD). Values are presented as mean ± SD. Three-way ANOVA followed by post hoc Tukey HSD test was performed. * p < 0.05, ** p < 0.005.

Figure 6

Effects of α-lipoic acid (ALA) on brain β-amyloid level in the hypothalamus and cerebral cortex of rats fed a control (CD) and high-fat diet (HFD). Values are presented as mean ± SD. Three-way ANOVA followed by post hoc Tukey HSD test was performed. **p < 0.005, ***p < 0.0005, ****p < 0.0001.