Diabetes Mellitus occurs in more than 165 million individuals worldwide and leads to both acute and long-term cardiovascular complications that can be tied to cellular oxidative stress. Three exciting novel therapeutic strategies offer significant promise to extend vascular longevity.
By the year 2030, it is predicted that more than 360 million individuals will be affected by the complications of Diabetes Mellitus (DM) 1. Type 1 DM, also known as insulin dependent DM, affects approximately 10 per cent of diabetics while type 2 DM, non-insulin dependent DM, is found in the remaining majority of diabetic individuals. Furthermore, the significance of DM and its complications in the vascular system should not be underestimated, since it is believed that the incidence of undiagnosed DM in the population worldwide is increasing. Individuals with impaired glucose tolerance have a risk of developing diabetic complications two times greater than those with normal glucose tolerance. Both type 1 and type 2 DM can lead to complications in the cardiac and vascular systems, such as impairment of vascular integrity and alter cardiac output that may ultimately affect brain cognitive function2,3 . DM can increase the risk of vascular dementia in elderly subjects and can potentially alter the course of Alzheimer's disease. Some studies suggest a modest adjusted relative risk of Alzheimer's disease in patients with DM compared to those without diabetes to be 1.34 . Costs to care for cognitive impairments resulting from DM that can mimic Alzheimer's disease can approach US$ 100 billion per year.
Both type 1 and type 2 DM can lead to complications in the cardiac and vascular systems.
Diabetes Mellitus and oxidative cell damage
Although a number of pathways in the body can lead to DM most of its complementory arise due to cellular oxidative stress. During oxidative stress, the release of Reactive Oxygen Species (ROS) occurs, that is associated with mitochondrial DNA mutations. These processes together can lead to apoptotic cellular injury.
Apoptosis, also known as programmed cell death, can contribute to several disease states such as DM, dementia, stroke, and trauma. At the cellular level, apoptosis has an early occurrence that leads to the exposure of Phosphatidylserine (PS) residues, which can attract immune system cells for the phagocytosis of injured cells and a later event that involves the cleavage of genomic DNA into fragments. Mitochondrial membrane transition pore permeability is also increased during oxidative stress that results in a significant loss of mitochondrial NAD+ stores and the further generation of ROS5 .
In disorders such as DM, elevated levels of ceruloplasmin have been suggested to represent increased ROS and acute glucose fluctuations have been described as a potential source of oxidative stress3. Elevated serum glucose also has been shown to lead to increased production of rROS in endothelial cells. However, prolonged duration of hyperglycemia does not necessarily lead to oxidative stress injury, since even short periods of hyperglycemia can generate ROS in vascular cells. Recent clinical research correlates support these experimental studies to show that acute glucose swings in addition to chronic hyperglycemia can trigger oxidative stress mechanisms in DM . The maintenance of cellular energy reserves and mitochondrial integrity also becomes a significant factor in DM, since insulin resistance in the elderly also has been associated with reduction in mitochondrial oxidative and phosphorylation activity1,7 .
Identifying novel strategies to tackle vascular injury
A precursor for the coenzyme ß-nicotinamide adenine dinucleotide (NAD+)
One potential pathway to consider for the maintenance of cellular metabolism in DM is nicotinamide, a precursor of the coenzyme ß-nicotinamide adenine dinucleotide (NAD+). Oral nicotinamide protects ß-cell function and prevents clinical disease in islet-cell antibody-positive first-degree relatives of type 1 DM. Furthermore, treatment with nicotinamide in patients with recent onset type 1 DM, combined with intensive insulin therapy for up to two years after diagnosis, can significantly reduce HbA1c levels. In addition, nicotinamide has been shown to reduce intestinal absorption of phosphate and prevent the development of hyperphosphatemia and progressive renal dysfunction, which would be of significant medical assistance to diabetics with renal compromise. During periods of oxidative stress, nicotinamide can improve glucose utilisation and prevent excessive lactate production in ischemic animal models8 . Nicotinamide is believed to be responsible for the preservation of endothelial cell integrity during periods of oxidative stress and it assists left ventricular cardiac function. Nicotinamide also employs the modulation of unique transcription factor pathways, such as with the forkhead family member Foxo3a, to promote cellular protection. The NAD+ precursor may derive its protective capacity through two separate mechanisms of post-translational modification of Foxo3a. Nicotinamide can not only maintain phosphorylation of Foxo3a and inhibit its activity, but also can preserve the integrity of the Foxo3a protein to block Foxo3a proteolysis that can yield pro-apoptotic amino-terminal fragments9 .
Cysteine-rich glycosylated Wnt proteins
Wnt proteins, derived from the Drosophila Wingless (Wg) and the mouse Int-1 genes, are secreted cysteine-rich glycosylated proteins that can control cell proliferation, differentiation, survival and death10 . Abnormalities in the Wnt signaling pathways, such as with transcription factor 7-like 2 gene, may yield an increased risk for DM in some populations and have increased association with obesity11 . Other studies demonstrate an increased expression of Wnt10 family members in adipose tissue, the pancreas, and the liver in diabetic patients, illustrating a potential regulation of adipose cell function by Wnt. Impaired Wnt function has also been observed in patients with the combined metabolic syndrome of hypertension, hyperlipidemia and DM12 .
Interestingly, Wnt may offer glucose tolerance and increased insulin sensitivity and also protect kidney cells from elevated glucose injury and apoptosis. New work shows that Wnt is sufficient for cellular protection during elevated glucose exposure and is a vital component for vascular protection provided by growth factors such as erythropoietin (EPO)13 . Wnt prevents apoptosis through the inhibition of glycogen synthase kinase-3ß (GSK-3ß) and ß-catenin. Inactivation of GSK-3ß by small molecule inhibitors or RNA interference, prevents toxicity from high concentrations of glucose to suggest a possible targeting of GSK-3ß during DM14 . Clinical applications for GSK-3ß that are tied to EPO are also worthy of consideration. The benefits of EPO to improve cardiovascular function in diabetic patients15 and the positive effects of exercise to improve glycemic control during DM appear to rely upon the inhibition of GSK-3ß activity. EPO can prevent GSK-3ß activity and combined with exercise may offer synergistic benefits, since physical exercise also has been shown to phosphorylate and inhibit GSK-3ß activity.
A growth factor and cytokine
EPO is a 30.4 kDa glycoprotein with approximately 50 per cent of its molecular weight derived from carbohydrates. As a growth factor and cytokine, EPO is considered to be ubiquitous in the body, since it can be detected in the breath of healthy individuals. Although EPO is currently approved for the treatment of anemia, the role of EPO is far-reaching beyond the need for erythropoiesis. Plasma EPO is often low in diabetic patients whether or not anemia is present and is believed to have limited response to progressive anemia onset in diabetics. However, EPO secretion is regulated in diabetic pregnancies that may suggest the body's effort to protect against the complications of DM. Treatment with EPO has been shown in diabetic patients with severe, resistant congestive heart failure to improve vitality, increase cardiac output, and remarkably decrease the number of required days in the hospital. In studies that examine the toxic effects of elevated glucose upon vascular cells, EPO is protective and prevents early apoptotic membrane PS exposure and late DNA degradation in vascular cells at concentrations that are clinically relevant.
Vascular protection by EPO is closely tied to the maintenance of mitochondrial membrane potential to prevent cell injury and the subsequent blockade of apoptotic cascades. Yet, similar to nicotinamide, EPO may require the forkhead transcription factor Foxo3a to prevent vascular injury during DM. Foxo3a is involved in pathways responsible for cell metabolism, DM onset, and diabetic complications. Administration of a high-fat diet in animals induced with hyperinsulinemic insulin-resistant obesity was associated with an increased expression of Foxo3a.
Additional studies have linked diabetic nephropathy to Foxo3a, by demonstrating that phosphorylation of Foxo3a increases in rat and mouse kidney cells, after the induction of diabetes by streptozotocin. Interestingly, prevention of Foxo3a activation by EPO during oxidative stress, also protects against vascular cell injury.
Considerations for the future
Although application of novel agents and pathways may offer great promise to extend vascular longevity during conditions such as DM, nicotinamide, Wnt and EPO can raise potential concerns similar to the consideration of any new therapeutic strategy. For example, in some reports nicotinamide has resulted in impaired ß-cell function. Although nicotinamide can protect cells against oxidative stress in millimole concentrations, lower concentrations of nicotinamide can inhibit sirtuin function that may be beneficial and that has been tied to increased lifespan in yeast and metazoans. In addition, both Wnt and EPO, under certain conditions, have been associated with malignancy. For example, EPO may sometimes enhance tumour progression by assisting with tumour angiogenesis. EPO has also been associated with increased incidence of thrombotic vascular effects, progression of cardiac insufficiency, potential vascular stenosis, elevation in mean arterial pressure, and increased metabolic rate and blood viscosity. It is, therefore, evident that for novel therapeutic strategies to effectively and safely extend vascular cell longevity, future studies that involve basic and clinical research must carefully and systematically address both the potential benefits and disadvantages of new therapies. As a result, enthusiasm for developing new therapeutic agents to preserve vascular longevity during debilitating conditions such as DM will continue to grow at an exponential pace and avoid clinical complications to offer patients the best available care.
This work was supported by the following grants (KM): American Diabetes Association, American Heart Association (National), Bugher Foundation Award, Janssen Neuroscience Award, LEARN Foundation Award, MI Life Sciences Challenge Award, Nelson Foundation Award, NIH NIEHS (P30 ES06639), and NIH NINDS/NIA.
Kenneth Maiese is a physician-scientist. At present, he is the Chief of the Division of Cellular and Molecular Cerebral Ischemia and is Professor in Neurology, Anatomy & Cell Biology, Barbara Ann Karmanos Cancer Institute, Molecular Medicine, and the Institute of Environmental Health Sciences at Wayne State University School of Medicine. His investigations are designed to translate basic science into successful therapeutic treatments for conditions such as metabolic disorders, cancer, cardiovascular disease, diabetes, stroke, and Alzheimer's disease.