Pioglitazone ameliorates Aβ42 deposition in rats with diet-induced insulin resistance associated with AKT/GSK3β activation

Abstract

Pioglitazone may have potential benefits as an alternative therapeutic treatment for patients with Alzheimer's disease (AD), particularly in individuals that also have comorbid diabetes; however, the mechanisms of action remain unclear. The present study aimed to explore the effects of pioglitazone on amyloid β, isoform 42 (Aβ42) deposition in rats with diet‑induced insulin resistance (IR). Diet‑induced IR model rats were established in the presence or absence of pioglitazone. Plasma glucose and insulin levels, and cerebrospinal fluid insulin levels were measured; in addition, hippocampal tissues were collected for immunohistochemical analysis of Aβ42 expression. The levels of insulin‑degrading enzyme (IDE) and peroxisome proliferator‑activated receptor γ (PPARγ) mRNA and protein expression were analyzed by reverse transcription‑quantitative polymerase chain reaction and western blotting, respectively. In addition, the activation of glycogen synthase kinase 3β (GSK3β) induced by phosphatidylinositol 3‑kinase (PI3K) /protein kinase B (AKT) signaling was detected by western blotting. Results from the present study demonstrated that pioglitazone may enhance peripheral and brain insulin sensitivity in diet‑induced IR model rats. Treatment with pioglitazone ameliorated Aβ42 deposition in the hippocampus by increasing IDE and PPARγ expression. Notably, activation of the PI3K/AKT/GSK3β pathway was also demonstrated to serve a role in pioglitazone‑induced Aβ42 degradation, which was abrogated by the PPARγ antagonist GW9662. Results from the present study indicated that pioglitazone may improve insulin sensitivity and ameliorate Aβ42 accumulation in rats with diet‑induced IR by regulating AKT/GSK3β activation, suggesting that pioglitazone may be a promising drug for AD treatment.

Figure 1.

Pioglitazone treatment improves insulin sensitivity in HD-induced IR rats. Rats were assessed following 16 weeks (12 weeks to establish the IR model plus 4 weeks treatment/HD-diet) on a normal or HD diet for different indices of HD-induced IR, including: (A) Body weight, (B) HOMA-IR, (C) plasma insulin, (D) plasma glucose and (E) CSF insulin. Data are presented as the mean ± standard deviation. Control group, n=10; IR model rat groups, n=8/group. *P<0.05 and **P<0.01. CSF, cerebrospinal fluid; HD, high glucose, high fat and high protein diet; HOMA-IR, homeostasis model assessment-insulin resistance; IR, insulin resistance.

Figure 2.

Pioglitazone treatment alleviates Aβ42 accumulation in the hippocampus of IR model rats. (A) Immunohistochemical analysis of Aβ42 in the hippocampus of the different rat groups. Scale bar, 100 µm. (B) Aβ42-positive cells were increased in IR model rats compared with control, but decreased following pioglitazone treatment. Data are presented as the mean ± standard deviation. Control group, n=10; IR model rat groups, n=8/group. *P<0.05 and ***P<0.001. Aβ42, amyloid β, isoform 42; IR, insulin resistance.

Figure 3.

Pioglitazone treatment increases the expression of IDE and PPARγ mRNA. Reverse transcription-quantitative polymerase chain reaction was used to assay the expression of APP, IDE and PPARγ mRNA in the different experimental rat groups. Control group, n=10; IR model rat groups, n=8/group. Data are presented as the mean ± standard deviation. *P<0.05, **P<0.01 and ***P<0.001. APP, amyloid precursor protein; IDE, insulin-degrading enzyme; IR, insulin resistance; PPARγ, peroxisome proliferator-activated receptor γ.

Figure 4.

IDE and PPARγ are involved in the inhibitory effect of pioglitazone on Aβ42 accumulation. (A) Western blot analysis of the protein expression levels of Aβ42, IDE and PPARγ in the experimental rat groups. (B) Densitometric analysis of the bands in (A), normalized to β-actin. Control group, n=10; IR model rat groups, n=8/group. Data are presented as the mean ± standard deviation. *P<0.05, **P<0.01 and ***P<0.001. IDE, insulin-degrading enzyme; PPARγ, peroxisome proliferator-activated receptor γ.

Figure 5.

GW9662 reverses inhibiting effect of Pioglitazone on Aβ42 deposition in IR model rats via AKT/GSK3β signaling pathway. (A) Western blot analysis of the protein expression of Aβ42 and components of the AKT/GSK3β signaling pathway in the different experimental rat groups. (B) Densitometric analysis of the bands in (A), all normalized to β-actin. Control group, n=10; IR model rat groups, n=8/group. Data are presented as the mean ± standard deviation. *P<0.05, **P<0.01 and ***P<0.001. Aβ42, amyloid β, isoform 42; p-, phosphorylated; AKT, protein kinase B; GSK3β, glycogen synthase kinase 3β; GW9662, peroxisome proliferator-activated receptor γ inhibitor; IR, insulin resistance.