Bhumsoo Kim

How does diabetes accelerate Alzheimer disease pathology

Diabetes and Alzheimer disease (AD)—two age-related diseases—are both increasing in prevalence, and numerous studies have demonstrated that patients with diabetes have an increased risk of developing AD compared with healthy individuals. The underlying biological mechanisms that link the development of diabetes with AD are not fully understood. Abnormal protein processing, abnormalities in insulin signaling, dysregulated glucose metabolism, oxidative stress, the formation of advanced glycation end products, and the activation of inflammatory pathways are features common to both diseases. Hypercholesterolemia is another factor that has received attention, owing to its potential association with diabetes and AD. This Review summarizes the mechanistic pathways that might link diabetes and AD. An understanding of this complex interaction is necessary for the development of novel drug therapies and lifestyle guidelines aimed at the treatment and/or prevention of these diseases.

Insulin-like growth factor-I-mediated neurite outgrowth in vitro requires mitogen-activated protein kinase activation.

Insulin-like growth factor-I (IGF-I) induces neuronal differentiation in vitro. In the present study, we examined the signaling pathway underlying IGF-I-mediated neurite outgrowth. In SH-SY5Y human neuroblastoma cells, treatment with IGF-I induced concentration- and time-dependent tyrosine phosphorylation of the type I IGF receptor (IGF-IR) and extracellular signal-regulated protein kinases (ERK) 1 and 2. These effects of IGF-I were blocked by a neutralizing antibody against IGF-IR. Whereas IGF-IR phosphorylation was observed within 1 min, maximal phosphorylation of ERKs was not reached for 30 min. Both IGF-IR and ERK phosphorylation were maintained for at least 24 h. Also, the concentration dependence of IGF-I-stimulated IGF-IR and ERK tyrosine phosphorylation paralleled that of IGF-I-mediated neurite outgrowth. We further examined the role of mitogen-activated protein kinase activation in IGF-I-stimulated neuronal differentiation using the mitogen-activated protein kinase/ERK kinase inhibitor PD98059. Whereas PD98059 had no effect on IGF-IR phosphorylation, PD98059 reduced IGF-I-mediated ERK tyrosine phosphorylation and ERK phosphorylation of the substrate Elk-1. PD98059 also produced a parallel reduction of IGF-I-stimulated neurite outgrowth. Finally, consistent with its ability to block neuronal differentiation, PD98059 inhibited IGF-I-dependent changes of GAP-43 and c-myc gene expression. Together these results suggest that activation of ERKs is essential for IGF-I-stimulated neuronal differentiation.

Increased tau phosphorylation and cleavage in mouse models of type 1 and type 2 diabetes.

As the population of the United States ages, the incidence of age-related neurodegenerative and systemic diseases including Alzheimers disease (AD) and diabetes is increasing rapidly. Multiple studies report that patients with diabetes have a 50-75% increased risk of developing AD compared with age- and gender-matched patients without diabetes. Abnormally phosphorylated tau is a major building block of neurofibrillary tangles, a classic neuropathological characteristic of AD. In addition, proteolytic tau cleavage promotes AD progression due to cleaved tau serving as a nucleation center for the pathological assembly of tau filaments. The current study examines tau modification in type 1 (streptozotocin-injected) and type 2 (db/db) mouse models of diabetes. Tau phosphorylation is increased in the cortex and hippocampus of db/db mice compared with db+ control mouse brain. Interestingly, there is an age-dependent increase in tau cleavage that is not observed in age-matched control db+ animals. Streptozotocin injection also increased tau phosphorylation; however, the increase was less significant compared with the type 2 mouse model, and more importantly, no tau cleavage was detected. Our results suggest tau modification caused by insulin dysfunction and hyperglycemia may contribute to the increased incidence of AD in diabetes. We hypothesize that type 1 and type 2 diabetes may contribute to AD through different mechanisms; in type 2 diabetes, hyperglycemia-mediated tau cleavage may be the key feature, whereas insulin deficiency may be the major contributing factor in type 1 diabetes.

TYROSINE PHOSPHORYLATION OF PAXILLIN AND FOCAL ADHESION KINASE DURING INSULIN-LIKE GROWTH FACTOR-I-STIMULATED LAMELLIPODIAL ADVANCE

In the current studies, we examined whether focal adhesion kinase (FAK) and paxillin play a role in insulin-like growth factor-I (IGF-I)-stimulated morphological changes in neuronal cells. In SH-SY5Y human neuroblastoma cells, 10 nM IGF-I enhanced the extension of lamellipodia within 30 min. Scanning electron microscopy and staining with rhodamine-phalloidin showed that these lamellipodia displayed ruffles, filopodia, and a distinct meshwork of actin filaments. Immunofluorescent staining identified focal concentrations of FAK, paxillin, and phosphotyrosine within the lamellipodia. Immunoprecipitation experiments revealed that FAK and paxillin are tyrosine-phosphorylated during IGF-I-stimulated lamellipodial extension. Maximal phosphorylation of FAK and paxillin was observed 15-30 min after the addition of 10 nM IGF-I, whereas maximal IGF-I receptor phosphorylation occurred within 5 min. FAK, paxillin, and IGF-I receptor tyrosine phosphorylation had similar concentration-response curves and were inhibited by the receptor blocking antibody αIR-3. These results indicate that FAK and paxillin are tyrosine-phosphorylated during IGF-I-stimulated lamellipodial advance and suggest that the tyrosine phosphorylation of these two proteins helps mediate IGF-I-stimulated cell and growth cone motility. These responses contrast directly with recent reports showing insulin-stimulated dephosphorylation of FAK and paxillin.

Insulin resistance in the nervous system

Metabolic syndrome is a cluster of cardiovascular risk factors including obesity, diabetes and dyslipidemia. Insulin resistance (IR) is at the core of metabolic syndrome. In adipose tissue and muscle, IR results in decreased insulin signaling, primarily affecting downstream phosphatidylinositol 3-kinase (PI3K)/Akt signaling. It was recently proposed that neurons can develop hyperinsulinemia-induced IR, which in turn results in injury to the peripheral and central nervous systems and is probably pathogenic in common neurological disorders such as diabetic neuropathy and Alzheimers disease (AD). This review presents evidence indicating that, similarly to insulin-dependent metabolically active tissues such as fat and muscle, neurons also develop IR and thus cannot respond to the neurotrophic properties of insulin, resulting in neuronal injury, subsequent dysfunction and disease states.

Insulin resistance as a key link for the increased risk of cognitive impairment in the metabolic syndrome

Metabolic syndrome (MetS) is a cluster of cardiovascular risk factors that includes obesity, diabetes, and dyslipidemia. Accumulating evidence implies that MetS contributes to the development and progression of Alzheimers disease (AD); however, the factors connecting this association have not been determined. Insulin resistance (IR) is at the core of MetS and likely represent the key link between MetS and AD. In the central nervous system, insulin plays key roles in learning and memory, and AD patients exhibit impaired insulin signaling that is similar to that observed in MetS. As we face an alarming increase in obesity and T2D in all age groups, understanding the relationship between MetS and AD is vital for the identification of potential therapeutic targets. Recently, several diabetes therapies that enhance insulin signaling are being tested for a potential therapeutic benefit in AD and dementia. In this review, we will discuss MetS as a risk factor for AD, focusing on IR and the recent progress and future directions of insulin-based therapies.

Insulin-like growth factor-I signaling in human neuroblastoma cells

Neuroblastoma is a heterogeneous tumor consisting of N (neuronal) and S (stromal) cells. We report that more tumorigenic and motile N cells express higher levels of IGF-I receptor (IGF-IR) than less tumorigenic, more adherent S cells. Shc, one of the two major docking partners of IGF-IR, is equally expressed in N and S cell lines. IGF-I treatment phosphorylates Shc in N cells, but only weakly activates Shc in S cells. Expression of the second partner, insulin receptor substrate (IRS), is cell type specific. S cells exclusively express IRS-1 that undergoes sustained phosphorylation by IGF-I. In contrast, N cells express IRS-2 that is transiently phosphorylated by IGF-I. Downstream of IRS-2 and Shc, IGF-I treatment results in strong activation of Akt and MAPK in N cells and activation of both pathways is required for IGF-I-mediated differentiation. Only IGF-IR activation of phosphatidylinositol-3 kinase is required for tumor edge ruffling in N and S cells, with stimulation of focal adhesion kinase (FAK) and paxillin. This detailed understanding of the ‘biochemical signature’ of N and S cells provides the background needed to target and disrupt specific IGF signaling pathways in an attempt to develop more effective therapies.

Insulin-like growth factor I stimulates motility in human neuroblastoma cells.

Motility is an important process that contributes to cancer cell spread. Growth factors are key regulators of motility in many cell types. Insulin-like growth factor I (IGF-I) causes SH-SY5Y human neuroblastoma cells to undergo dynamic morphological changes, leading to the extension of lamellipodia. IGF-I stimulated lamellipodia extension requires signaling through both phosphatidylinositol 3-kinase (PI3-K) and MAP kinase pathways. IGF-I, over a period of hours, stimulates SH-SY5Y and SHEP neuroblastoma cells to become more motile. While SH-SY5Y and SHEP cells use different insulin receptor substrate (IRS) isoforms to transduce signals from the IGF-I receptor, IGF-I has the same relative effect on the motility of both cell lines. Blocking the PI3-K and MAP kinase pathways attenuates the ability of IGF-I to increase motility. Overexpression of PTEN also attenuates IGF-I mediated motility. These results delineate some of the proximal events in the signaling mechanism utilized by IGF-I to stimulate cell motility.

Vascular endothelial growth factor prevents G93A-SOD1-induced motor neuron degeneration.

Amyotrophic lateral sclerosis (ALS) is an adult‐onset neurodegenerative disorder characterized by selective loss of motor neurons (MNs). Twenty percent of familial ALS cases are associated with mutations in Cu2+/Zn2+ superoxide dismutase (SOD1). To specifically understand the cellular mechanisms underlying mutant SOD1 toxicity, we have established an in vitro model of ALS using rat primary MN cultures transfected with an adenoviral vector encoding a mutant SOD1, G93A‐SOD1. Transfected cells undergo axonal degeneration and alterations in biochemical responses characteristic of cell death such as activation of caspase‐3. Vascular endothelial growth factor (VEGF) is an angiogenic and neuroprotective growth factor that can increase axonal outgrowth, block neuronal apoptosis, and promote neurogenesis. Decreased VEGF gene expression in mice results in a phenotype similar to that seen in patients with ALS, thus linking loss of VEGF to the pathogenesis of MN degeneration. Decreased neurotrophic signals prior to and during disease progression may increase MN susceptibility to mutant SOD1‐induced toxicity. In this study, we demonstrate a decrease in VEGF and VEGFR2 levels in the spinal cord of G93A‐SOD1 ALS mice. Furthermore, in isolated MN cultures, VEGF alleviates the effects of G93A‐SOD1 toxicity and neuroprotection involves phosphatidylinositol 3‐kinase/protein kinase B (PI3K/Akt) signaling. Overall, these studies validate the usefulness of VEGF as a potential therapeutic factor for the treatment of ALS and give valuable insight into the responsible signaling pathways and mechanisms involved.

Insulin-Like Growth Factor-I Receptor Expression Regulates Neuroblastoma Metastasis to Bone

Neuroblastoma is a pediatric tumor that preferentially metastasizes to bone. Patients with bone metastases have a mortality rate >93%, indicating a need for novel treatment targets. Our laboratory has shown that type I insulin-like growth factor receptor (IGF-IR) expression and activation regulate neuroblastoma cell proliferation, motility, invasion, and survival, and that expression of the IGF-IR correlates with neuroblastoma tumorigenicity. Bone expresses large amounts of IGF ligands, and the IGF system is required for normal bone physiology. The current study addresses the role of the IGF system in neuroblastoma metastasis to bone. Upon reaching the bone marrow through the circulation, neuroblastoma cells must dock at the bone marrow endothelium, extravasate into the bone microenvironment, and destroy bone tissue to allow for tumor growth. This report examines the effects of high IGF-IR expression on neuroblastoma cell interaction with bone. The current data show that neuroblastoma cells with high IGF-IR expression, either endogenously or through transfection, adhere to human bone marrow endothelial cells and subsequently migrate toward both IGF-I and human bone stromal cells. High IGF-IR-expressing neuroblastoma cells adhere tightly to bone stromal cells, flatten, and extend processes. When neuroblastoma cells are injected directly into the tibiae of mice, those cells with increased IGF-IR form both osteolytic lesions within the tibiae and secondary tumors within other sites. These results support the hypothesis that IGF-IR expression in neuroblastoma cells increases tumor cell interaction with the bone microenvironment, resulting in greater formation of metastases.