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Repurposing Diabetes Drugs to Treat Insulin Resistance in Alzheimer’S Disease

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Repurposing Diabetes Drugs to Treat Insulin Resistance in Alzheimer’S Disease The Science Journal of the Lander College of Arts and Sciences Volume 10 Number 2 Spring 2017 - 2017 Repurposing Diabetes Drugs to Treat Insulin Resistance in Alzheimer’s Disease Yael Lazarus Touro College Follow this and additional works at: https://touroscholar.touro.edu/sjlcas Part of the Nervous System Diseases Commons, Nutritional and Metabolic Diseases Commons, and the Therapeutics Commons Recommended Citation Lazarus, Y. (2017). Repurposing Diabetes Drugs to Treat Insulin Resistance in Alzheimer’s Disease. The Science Journal of the Lander College of Arts and Sciences, 10(2). Retrieved from https://touroscholar.touro.edu/sjlcas/vol10/iss2/4 This Article is brought to you for free and open access by the Lander College of Arts and Sciences at Touro Scholar. It has been accepted for inclusion in The Science Journal of the Lander College of Arts and Sciences by an authorized editor of Touro Scholar. For more information, please contact [email protected]. Repurposing Diabetes Drugs to Treat Insulin Resistance in Alzheimer’s Disease Yael Lazarus Yael Lazarus graduated with a BS in Biology in January 2017 and will attend Touro Manhattan’s PA program in September 2017. Abstract Alzheimer’s disease is a neurodegenerative condition which results in a significant decline in cognitive status. Novel treatment approaches for Alzheimer’s are sorely needed, as current medications for the disease offer only marginal clinical benefit. Research has discovered a connection between the pathology of Alzheimer’s and Type 2 Diabetes, two serious and seemingly unrelated disorders. Clinical studies have shown that Alzheimer’s disease is associated with brain insulin resistance similar to the pathology of Type 2 Diabetes. This observation has led to the notion that drugs devel- oped for the treatment of Type 2 Diabetes may be beneficial in modifying the cognitive function and pathophysiology of individuals suffering from Alzheimer’s disease. This paper offers a comprehensive review of the clinical studies demon- strating the potential of using diabetes medications as an effective therapeutic method for the prevention and treatment of Alzheimer’s disease. Special focus is given towards the metabolic hormones insulin, amylin and leptin. Introduction brain cells by blocking cell-to-cell signaling at synapses (Reitz, Alzheimer’s and Type 2 Diabetes are two of the most common 2012). Amyloid beta plaques have also been shown to cause diseases afflicting the elderly population today. A 2016 Report apoptosis. The presence of amyloid plaques in the brain gener- by the Alzheimer’s Association found that of the 5.4 million ates the production of harmful oxidative free radicals which in Americans with Alzheimer’s, an estimated 5.2 million people turn activates the c-Jun N-terminal kinase (JNK) pathway. The are age 65 and older (Alzheimer’s Association, 2014). Similarly, JNK pathway stimulates the transcription of several key target the American Diabetes Association reported that in 2012, of genes, including the death inducer Fas ligand. The binding of the 29.1 million Americans with Diabetes, an estimated 11.8 Fas ligand to its receptor Fas then induces a cascade of events million people were age 65 and older (American Diabetes that lead to caspase activation and ultimately neuronal cell Association, 2012). The two conditions have traditionally been death. (Yoshiyuki et al., 2001). treated as independent disorders. However, recent studies link- ing Alzheimer’s to Type 2 Diabetes have led to the proposal that The tau protein also plays an important role in Alzheimer’s dis- Alzheimer be referred to as “Type 3 Diabetes”. Discoveries of ease (Lasagna-Reeves CA et al., 2012). This protein is integral common pathological mechanisms between these two diseas- to maintenance of internal support and transport systems be- es have given rise to novel clinical trials incorporating diabetes tween brain cells. Hyperphosphorylation, which is the addition therapies to treat Alzheimer’s disease. of phosphate to too many amino acids, leads to the collapse of tau proteins into twisted strands referred to as neurofibrillary Methods tangles (Jack et al., 2013). Without the support of the tau pro- A variety of literary reviews and research papers on the subject tein, the transport system in the brain collapses. Consequently, were collected through use of the PubMed database. Keywords essential nutrients are unable to reach brain cells and cell death such as Alzheimer’s disease, insulin resistance and Type 2 Diabetes ensues (Gong and Iqbal, 2008). were used to search for relevant material. Access to PubMed was provided by the Touro College online library system. Diabetes is a metabolic disorder which develops when the body does not make enough insulin or is unable to use insulin effec- Pathophysiology of Alzheimer’s and Type 2 tively. (Yarchoan and Arnold, 2014). If left untreated, diabetes can Diabetes cause many complications, including kidney failure, diabetic ke- Alzheimer’s disease is a neurodegenerative condition, which toacidosis, heart disease and stroke (Schnell et al., 2016). Type results in nerve cell death and tissue loss throughout the brain 2 Diabetes is characterized by an insulin resistant state which (Li, Z et al., 2015). Scans of brains of individuals suffering from is most commonly caused by obesity (Smith and Kahn, 2016). Alzheimer’s demonstrate severe shrinkage of the hippocampus In insulin resistance, muscle, fat, and liver cells do not respond and cerebral cortex, as well as the enlargement of the ventricles properly to insulin and thus cannot easily absorb glucose from (Querfurth et al., 2010). the bloodstream. As a result, the body needs higher levels of insulin to help glucose enter cells. The pancreatic beta cells ini- The pathophysiology of Alzheimer’s disease is described by tially increase their insulin output but fail over time to keep up the amyloid cascade hypothesis. Cleavage of amyloid precur- with the body’s increased demands for insulin. Type 2 diabetes sor protein (APP) leads to the formation of the protein amy- develops when insulin production is inadequate to overcome loid beta. Excessive cleavage of APP coupled with inefficient insulin resistance, resulting in the drastic rise in blood glucose removal of amyloid beta can lead to the formation of amyloid levels (Yarchoan and Arnold, 2014). beta plaques in the brain. These plaques damage and destroy 23 Yael Lazarus Discovery of Insulin Resistance in Alzheimer’s In this case the many signaling chemicals produced by the in- Disease flammatory pathway stop the activity of the insulin receptor Insulin receptors are widely distributed in brain regions known to (Talbot et al., 2012). be involved in memory function, including the hippocampus, cere- bral cortex and cerebellum (Werther et al., 2015). Clinical studies Insulin resistance may promote Alzheimer pathology through have found evidence for central insulin resistance in Alzheimer’s several mechanisms. Firstly, abnormal insulin signaling may brains. Post-mortem studies of brain tissues from people with promote amyloid beta and hyperphosphorylated tau formation Alzheimer’s disease have discovered extensive abnormalities in through the kinase GSK-3. The activity of GSK-3 is normally insulin and insulin-like growth factor signaling mechanisms in regulated through inhibition by the protein AKT, which is an im- the brain (Steen et al., 2005). A study of post-mortem human portant kinase in the insulin signaling cascade referred to as the hippocampus Alzheimer’s tissue was done by exposing the tissue IRS-1 –AKT pathway (Yarchoan and Arnold, 2014). Therefore, to different concentrations of insulin. This allowed researchers dysfunctional insulin signaling caused by disturbances in the to study the activation of insulin pathways in the brain tissue of IRS-1 –AKT pathway leads to increased GSK-3 activity. Studies people with Alzheimer’s compared to the brain tissue of those of GSK-3 found that its activity increases tau protein phosphor- with normal cognitive function. In normal brains, activation of ylation (Li, X et al., 2006) and that it is involved in amyloid beta the insulin receptor initiated a signaling cascade which led to the production (Takashima, 2006). production of many downstream insulin signaling proteins. The blunted insulin response observed in the Alzheimer’s brain tissues Abnormal insulin signaling also interferes with memory and was similar to the insulin resistance observed in Type 2 diabetes learning pathways. Insulin is a direct regulator of the ERK/MAP peripheral tissues (Talbot et al., 2012). kinase pathway. This pathway is essential to the induction of longer term potentiation (LTP) and memory consolidation in The formation of amyloid beta plaques has been implicated as the hippocampus (Winder 1999). LTP is responsible for the a possible impetus for the removal of insulin receptors in brain maintenance of long term memory, which is the ability to recall cells. Studies of Alzheimer’s brains have demonstrated that the episodes which are not part of the immediate past (Kelleher et binding of amyloid beta oligomers to hippocampal neurons trig- al., 2004). Disruptions of LTP and memory consolidation con- gers the removal of dendritic insulin receptor substrates from tribute to the impaired cognitive function typified by Alzheimer’s the outer membrane of the cell (Xie et al., 2002). These studies disease (Dineley et al., 2014). prove that elevated amyloid beta levels induce the removal of cell surface insulin receptors, thereby furthering insulin resistance. Discussion: New Role for Diabetes Drugs as a Treatment for Alzheimer’s A molecular model has been proposed to explain how the am- Given the role of insulin resistance and deficiency in the patho- yloid beta plaques found in Alzheimer’s disease promote insulin genesis of Alzheimer’s disease, it is possible that a drug current- resistance (Dineley et al., 2014). This model is based on the in- ly prescribed for Type 2 Diabetes may able also be useful for flammatory pathway observed in Type 2 Diabetes, where inflam- Alzheimer’s.
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