Type 2 diabetes is the third most common non-infectious disease in the world. It is characterized by poor blood glucose control due to insulin resistance and progression apoptotic death of insulin-producing islet β-cells.31,32 As T2DM progresses, the loss of insulin production creates an absolute insulin deficiency that can only be addressed by exogenous replacement. Therefore, enhancing insulin sensitivity and reducing β-cell apoptosis are the ultimate strategies for the prevention and treatment of T2DM.33 Several studies have found that quercetin can reduce blood glucose, peripheral insulin resistance, and indicators of oxidative stress induced by high glucose in rats, a response profile similar to the widely used antidiabetic drug metformin.
Alzheimer’s disease is the most common neurodegenerative disorder. Clinically, it is characterized by progressive cognitive impairment, memory loss, and marked changes in personality and behavior. The two primary hypotheses proposed to explain the progressive neurodegeneration and neurological impairments of AD are cytotoxic -amyloid deposition (plaque formation) due to abnormal processing and hyperphosphorylation of tau as these are the pathological hallmarks of the AD brain. In turn, these plaques and NFTs may cause oxidative stress and inflammation, leading to synaptic failure, neuronal apoptosis (especially basal forebrain cholinergic neurons and various hippocampal and cortical neurons), and brain atrophy.34,35 These pathomechanisms are not mutually exclusive and indeed may interact, creating pathogenic cascades. Thus, many protein signaling pathways and gene regulation pathways may contribute to the pathological changes in AD.
Many natural components found in traditional herbal preparations have documented antioxidant, anti-inflammatory, antidiabetic, and anti-apoptotic activities, suggesting potential utility for AD and T2DM treatment. For instance, quercetin can improve insulin production by rat insulinoma INS-1 cells in response to high glucose, as well as downregulate the relative expression levels of pro-apoptotic Bax and PDX-1 mRNAs, inhibit oxidative stress caused by H2O2, and reduce INS-1 cell apoptosis.36 Quercetin can also clear reactive oxygen species (ROS) and reactive nitrogen species and inhibit the level of low density lipoprotein peroxidase, thereby reducing the incidence of cardiovascular disease.37 Further, quercetin was shown to reduce ischemic injury and peroxynitrate injury by inhibiting nitric oxide synthase and xanthine dehydrogenase activities.38
The current study revealed several potential molecular mechanisms for AD and T2DM as well as possible treatment targets by combining network pharmacology with molecular docking. By integrating and collating information from several databases, we identified 95 potential quercetin targets involved in T2DM and AD pathology, and further defined a potential protein functional module of five proteins and 25 core genes from the PPI network. The biological processes mediated by these functional module proteins include protein kinase signaling and cell proliferation, consistent with pathogenic pathways of T2DM and AD identified in previous studies.
CytoHubba and MCODE identified AKT1, JUN, MAPK, TNF, VEGFA, and EGFR as core targets of quercetin that may contribute to AD and T2DM. The PI3K/Akt signaling pathway is anti-apoptotic, and AKT1 inhibits Akt phosphorylation (activation), which in turn upregulates the activity of GSK3, a kinase implicated in the pathogenesis of both T2DM and AD. JUN is a member of the mitogen-activated protein kinase (MAPK) family implicated in the onset, progression, and reversal of vascular diseases through modulation of vascular cell protein expression, proliferation, oxidative stress, and apoptosis. Downregulation of JUN can significantly reduce the expression of inflammatory factors and inhibit endothelial cell apoptosis.39 Tumor necrosis factor (TNF) is a multifunctional cytokine that can directly damage islet -cells and induce insulin resistance by inhibiting the transduction of insulin signals. MAPK is a serine/threonine protein kinase involved in cell proliferation, differentiation, apoptosis, and inflammatory responses.40 Activation of MAPK can also increase tau phosphorylation in neurons,41 while inhibition of MAPK expression in hippocampus can significantly improve memory, cognitive function, synaptic plasticity, and neuronal metabolism.42 Suppression of MAPK can also improve insulin resistance by upregulating the expression of the insulin-dependent glucose transporter GLUT-4 in striatal muscle and adipose tissue, and by enhancing the sensitivity of insulin receptors, thereby promoting the absorption and utilization of glucose. Therefore, MAPK may be a core target for the treatment of T2DM and AD.43
To better understand the interactions among these target genes, we conducted GO and KEGG pathway analyses. The target genes were mainly related to cell apoptosis, regulation of inflammatory response, regulation of ROS, metallic process, and other biological processes. KEGG pathway enrichment analysis revealed that target proteins are involved in the MAPK, TNF, HIF-1, IL-17, PI3K, NF-kappa B, Toll-like receptor, and VEGF signaling pathways, AGE-RAGE signaling pathway in diabetic complications, pathways in cancer, proteoglycans in cancer, EGFR tyrosine kinase inhibitor resistance, cytokine-cytokine receptor interaction, and Alzheimer’s disease. We hypothesize that quercetin may play a therapeutic role in T2DM and AD through the regulation of these pathways, but especially through modulation of MAPK signaling (Figure 8).
A variety of extracellular signaling molecules (growth factors, neurotransmitters, cytokines, hormones, etc.) stimulate MAPK signaling via the MAPKKK-MAPKK cascade, ultimately regulating both cytoplasmic processes and gene expression pathways underlying cell proliferation, differentiation, apoptosis, and stress responses. Three parallel MAPK signaling pathways are found in mammalian cells, extracellular signal regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 mitogen-activated protein kinase (p38 MAPK). Inhibiting MAPK signaling can reduce oxidative stress and ensure the normal proliferation, differentiation, and insulin secretion capacity of islet cells.44 Further, inhibition of the MAPK pathway can improve learning and memory45 and suppress apoptosis of hippocampal CA1 neurons by downregulating the transcription factor c-Fos. Activation of the MAPK/ERK pathway may be an important mechanism underlying early and medium-term cognitive impairment in rats, while inhibition can reduce damage and promote the repair of damaged neurons. Quercetin acts on membrane EGFRs and TNFR1 by promoting the release of VEGFA and TNF, thereby affecting the activation of Ras, Raf, MEK, and phosphorylation of MAPK1 and MAPK3, leading to changes in angiogenesis, cell apoptosis, and cell proliferation. More in-depth studies on these mechanisms and preclinical studies in T2DM and AD animal models are warranted to provide a foundation for future clinical studies targeting the MAPK signaling pathway as a novel therapeutic strategy for T2DM and AD.