Shen-Zhi-Ling oral liquid ameliorates cerebral glucose metabolism disorder in early AD via insulin signal transduction pathway in vivo and in vitro

Background Shen-Zhi-Ling oral liquid (SZL) is an herbal formula known for its efficacy of nourishing “heart and spleen”, and is used for the treatment and prevention of middle- and early-stage dementia. This study investigated the effects of SZL on amelioration of AD, and examined whether the underlying mechanisms from the perspective of neuroprotection are related to brain glucose metabolism. Methods Firstly, LC–MS/MS was used to analysis the SZL mainly enters the blood component. Then, the effects of SZL on cognitive and behavioral ability of APP/PS1 double transgenic mice and amyloid protein characteristic pathological changes were investigated by behavioral study and morphological observation. The effects of SZL on the ultrastructure of mitochondria, astrocytes, and micrangium related to cerebral glucose metabolism were observed using transmission electron microscopy. Then, micro-PET was also used to observe the effects of SZL on glucose uptake. Furthermore, the effects of SZL on insulin signaling pathway InR/PI3K/Akt and glucose transporters (GLUT1 and GLUT3) were observed by immunohistochemistry, Western-blot and RT-qPCR. Finally, the effects of SZL on brain glucose metabolism and key enzyme were observed. In vitro, the use of PI3K and/or GSK3β inhibitor to observe the effects of SZL drug-containing serum on GLUT1 and GLUT3. Results In vivo, SZL could significantly ameliorate cognitive deficits, retarded the pathological damage, including neuronal degeneration, Aβ peptide aggregation, and ultrastructural damage of hippocampal neurons, improve the glucose uptake, transporters and glucolysis. Beyond that, SZL regulates the insulin signal transduction pathway the insulin signal transduction pathway InR/PI3K/Akt. Furthermore, 15% SZL drug-containing serum increased Aβ42-induced insulin signal transduction-pathway related indicators and GLUT1 and GLUT3 expression in SH-SY5Y cells. The improvement of GLUT1 and GLUT3 in the downstream PI3K/Akt/GSK3β signaling pathway was reversed by the use of PI3K and/or GSK3β inhibitor. Conclusions In summary, our results demonstrated that improving glucose uptake, transport, and glycolysis in the brain may underlie the neuroprotective effects of SZL, and its potential molecular mechanism may be related to regulate the insulin signal transduction pathway.

Insulin Signal Transduction Perturbations in Insulin Resistance

Type 2 diabetes mellitus is a widespread medical condition, characterized by high blood glucose and inadequate insulin action, which leads to insulin resistance. Insulin resistance in insulin-responsive tissues precedes the onset of pancreatic β-cell dysfunction. Multiple molecular and pathophysiological mechanisms are involved in insulin resistance. Insulin resistance is a consequence of a complex combination of metabolic disorders, lipotoxicity, glucotoxicity, and inflammation. There is ample evidence linking different mechanistic approaches as the cause of insulin resistance, but no central mechanism is yet described as an underlying reason behind this condition. This review combines and interlinks the defects in the insulin signal transduction pathway of the insulin resistance state with special emphasis on the AGE-RAGE-NF-κB axis. Here, we describe important factors that play a crucial role in the pathogenesis of insulin resistance to provide directionality for the events. The interplay of inflammation and oxidative stress that leads to β-cell decline through the IAPP-RAGE induced β-cell toxicity is also addressed. Overall, by generating a comprehensive overview of the plethora of mechanisms involved in insulin resistance, we focus on the establishment of unifying mechanisms to provide new insights for the future interventions of type 2 diabetes mellitus.

Role of Chitinase 3-Like 1 Protein in the Pathogenesis of Hepatic Insulin Resistance in Nonalcoholic Fatty Liver Disease

A recently discovered human glycoprotein, chitinase 3-like 1 (Chi3L1), may play a role in inflammation, tissue remodeling, and visceral fat accumulation. We hypothesize that Chi3L1 gene expression is important in the development of hepatic insulin resistance characterized by the generation of pAKT, pGSK, and pERK in wild type and Chi3L1 knockout (KO) murine liver following insulin stimulation. The Chi3L1 gene and protein expression was evaluated by Real Time PCR and ELISA; lipid accumulation in hepatocytes was also assessed. To alter Chi3L1 function, three different anti-Chi3L1 monoclonal antibodies (mAbs) were administered in vivo and effects on the insulin signaling cascade and hepatic lipid deposition were determined. Transmission of the hepatic insulin signal was substantially improved following KO of the CHi3L1 gene and there was reduced lipid deposition produced by a HFD. The HFD-fed mice exhibited increased Chi3L1 expression in the liver and there was impaired insulin signal transduction. All three anti-Chi3L1 mAbs partially restored hepatic insulin sensitivity which was associated with reduced lipid accumulation in hepatocytes as well. A KO of the Chi3L1 gene reduced lipid accumulation and improved insulin signaling. Therefore, Chi3L1 gene upregulation may be an important factor in the generation of NAFLD/NASH phenotype.

Regulation of basal expression of hepatic PEPCK and G6Pase by AKT2

Hepatic glucose metabolism signaling downstream of insulin can diverge to multiple pathways including AKT. Genetic studies suggest that AKT is necessary for insulin to suppress gluconeogenesis. To specifically address the role of AKT2, the dominant liver isoform of AKT in the regulation of gluconeogenesis genes, we generated hepatocytes lacking AKT2 (Akt2−/−). We found that, in the absence of insulin signal, AKT2 is required for maintaining the basal level expression of phosphoenolpyruvate carboxyl kinase (PEPCK) and to a lesser extent G6Pase, two key rate-limiting enzymes for gluconeogenesis that support glucose excursion due to pyruvate loading. We further showed that this function of AKT2 is mediated by the phosphorylation of cyclic AMP response element binding (CREB). Phosphorylation of CREB by AKT2 is needed for CREB to induce the expression of PEPCK and likely represents a priming event for unstimulated cells to poise to receive glucagon and other signals. The inhibition of gluconeogenesis by insulin is also dependent on the reduced FOXO1 transcriptional activity at the promoter of PEPCK. When insulin signal is absent, this activity appears to be inhibited by AKT2 in manner that is independent of its phosphorylation by AKT. Together, this action of AKT2 on FOXO1 and CREB to maintain basal gluconeogenesis activity may provide fine-tuning for insulin and glucocorticoid/glucagon to regulate gluconeogenesis in a timely manner to meet metabolic needs.

Insulin signal transduction is impaired in the type 2 diabetic retina

Rates of type 2 diabetes are reaching epidemic levels. Yet, the tissue specific alterations due to insulin resistance are only recently being investigated. The goal of the present study was to evaluate retinal insulin signal transduction in a common mouse model of type 2 diabetes, the db/db mouse. Retinal lysates from five month old male db/db and db/+ (control) mice were collected and processed for Western blotting or ELISA analyses for insulin receptor, insulin receptor substrate-1 (IRS-1), Akt, tumor necrosis factor alpha (TNFα) and caspase 3 levels. Data demonstrate increased TNFα and IRS-1 phosphorylation on serine 307. This led to decreased Akt phosphorylation on serine 473 and increased cleavage of caspase 3. Taken together, the data suggest dysfunctional insulin signaling in the retina of the db/db mouse.

BMP7 improves insulin signal transduction in the liver via inhibition of mitogen-activated protein kinases

Bone morphogenetic protein 7 (BMP7), a member of the transforming growth factor-β (TGF-β) family, plays pivotal roles in energy expenditure. However, whether and how BMP7 regulates hepatic insulin sensitivity is still poorly understood. Here, we show that hepatic BMP7 expression is reduced in high-fat diet (HFD)-induced diabetic mice and palmitate (PA)-induced insulin-resistant HepG2 and AML12 cells. BMP7 improves insulin signaling pathway in insulin resistant hepatocytes. On the contrary, knockdown of BMP7 further impairs insulin signal transduction in PA-treated cells. Increased expression of BMP7 by adenovirus expressing BMP7 improves hyperglycemia, insulin sensitivity and insulin signal transduction. Furthermore, BMP7 inhibits mitogen-activated protein kinases (MAPKs) in both the liver of obese mice and PA-treated cells. In addition, inhibition of MAPKs recapitulates the effects of BMP7 on insulin signal transduction in cultured hepatocytes treated with PA. Activation of p38 MAPK abolishes the BMP7-mediated upregulation of insulin signal transduction both in vitro and in vivo. Together, our results show that hepatic BMP7 has a novel function in regulating insulin sensitivity through inhibition of MAPKs, thus providing new insights into treating insulin resistance-related disorders such as type 2 diabetes.