User:Omid Alizadeh25/sandbox/The Biochemistry of Diabetes

The biochemistry of Diabetes

INTRODUCTION:

The hallmark of diabetes mellitus is an inability to control blood glucose. There are two major clinical syndromes: one characterized by insulin dependence and early age of onset with weight loss and ketonuria, and the second characterized by relatively later onset, insensitivity to insulin and partial insulin deficiency.

Insulin and Metabolism

Insulin is a major metabolism regulating hormone secreted by β-cells of the islets of Langerhans of the pancreas. The major function of insulin is to counter the concerted actions of a number of hyperglycemia-generating hormones and to maintain low blood glucose levels. In addition to its role in regulating glucose metabolism, insulin stimulates lipogenesis, diminishes lipolysis, and increases amino acid transport into cells. Because there are numerous hyperglycemic hormones, untreated disorders associated with insulin generally lead to severe hyperglycemia and shortened life span.

Type 1

In Type 1 diabetes, glucose doesn’t get taken up into tissue nor phosphorylated in the liver. The liver continues to mobilize glycogen even right after a meal, and plasma glucose levels get very high. Glucose starts to be lost into urine, alone with water and electrolytes, resulting in excessive urination and excessive thirst. And because your tissues haven’t taken up glucose, your body thinks you’re starving and you get increased appetite and food intake. (Note that the brain constitutively uses glucose, and is not insulin-sensitive, so the brain is still able to take up glucose even under these conditions). Type 1 is also associated with elevated glucagon because insulin, if it were present, would have suppressed glucagon. The body accordingly fails to store lipids in adipocytes, fails to synthesize TAGs in hepatocytes. Instead it mobilizes TAGs from adipocytes, oxidizes them and produces acetyl-CoA and ketone bodies, and also catabolizes proteins. The ketone bodies lower blood pH (ketoacidosis). Low blood pH is a common cause of death in untreated Type 1 diabetes. This disorder is described as “starvation in the midst of plenty.” Type 1 is treated by insulin injections. This causes tissues to take up glucose, thus lowering blood glucose levels. However, insulin also suppresses glucagon release, thus preventing the liver from breaking down glycogen. In other words, dosing is really important, and if you get a little too much insulin you can have hypoglycemia.

Type 2

The rise in T2D prevalence can be attributed to a couple factors: (1) overnutrition, obesity, inactivity; (2) treatment of T2D has improved, and therefore people live longer with it. T2D risk factors include obesity (particularly abdominal fat), age (> 45), blood pressure (high is bad), inactivity (even if not obese), genetics (family history), and ethnic background. Two hours after you eat a meal, your blood glucose should be back to fasting levels. In the 10 years leading up to T2D onset, your post-meal glucose gradually rises, and more and more insulin is secreted in response, but you develop insulin resistance. Fasting glucose also starts to rise (I think because the liver performs glycogen breakdown or gluconeogenesis but cells are slow to take this glucose up too, and/or because the liver has to produce higher levels of glucose in order to get cells to take up the glucose). Around year 0 (where onset is defined), beta cells start to fail, and insulin secretion goes back down, declining gradually over 30 years. “Prediabetes” is a marker for risk. Impaired Glucose Tolerance (IGT) is when your glucose levels ~2 hours after a meal are higher than they should be. Impaired Fasting Glucose (IFG) is elevated hepatic glucose production. HbA1c is glycosylated hemoglobin – hemoglobin is ordinarily unglycosylated but becomes glycosylated if there is too much glucose in the blood. HbA1c is another risk marker. Increased risk for heart disease and stroke, as measured by low HDL, high LDL, and/or high blood TAGs, are also considered risk factors.

Mechanisms in T2D

There are 3 hypotheses about how insulin resistance arises: 1 Adipose tissue dysfunction – the lipid burden hypothesis. PPARγ expression (which determines ability to store TAGs) is reduced in obese individuals’ adipose tissue but elevated in their liver and muscle, meaning that now liver and muscle can now store fat (which they shouldn’t normally; this is ectopic). Meanwhile, all tissues (adipose, liver and muscle) become less insulin sensitive for reasons this lecture didn’t explain. 2 Adipose tissue dysfunction – the role of inflammation. When adipocytes get too large (in overweight people), the overloading of TAG causes release of MCP-1, a chemoattractant. The MCP-1 attracts macrophages which release TNFα and other cytokines, reducing insulin signaling. TAG storage becomes impaired, resulting in lipolysis and increasing the circulating TAGs and free fatty acids, allowing them to accumulate ectopically in muscle. 3 Insulin resistance in liver and muscle – the role of mitochondria. In either case 1 or 2 above, fatty acids accumulate in the liver. Overnutrition also increases malonyl-CoA in the liver, and malonyl-CoA inhibits CPT1 (carnitine palomitoyltransferase 1) (which inhibits fatty acid oxidation. In lieu of oxidation, TAG storage increases. Fatty acid metabolism during CPT1 inhibition also produces diacylglycerol and ceramide. DAG activates stress-induced kinases, and both these and ceramide reduce insulin signaling. Meanwhile in the muscle, fatty acid accumulation increases beta oxidation thereof, and reduces citric acid cycle activity. Products of incomplete fat oxidation (acylcarnitines and reactive oxygen species) activate stress-induced kinases, which reduce insulin signaling.

After insulin resistance, impaired insulin secretion and beta cell failure follow next. There are likewise a few hypotheses about these: 1 Role of mitochondria and pyruvate cycling. Pyruvate cycling refers to how pyruvate generated in the cytosol enters the mitochondria, is converted to intermediates such as malate and then exported back to the cytosol where they are converted back to pyruvate. During overnutrition, increased fatty acids lead to increased acetyl-CoA which activates pyruvate carboxylase. Meanwhile there is also additional pyruvate due to additional glucose from overnutrition. Together, the added glucose and the activated pyruvate carboxylase increase the volume of pyruvate cycling. Because metabolites from pyruvate cycling are responsible for insulin secretion, an increase in cycling leads to insulin hypersecretion by beta cells. 2 Role of ER stress. Insulin hypersecretion means you have to produce a ton of insulin, which stresses out the ER, leading to protein misfolding and ultimately apoptosis. 3 Role of amyloid fibrils. When insulin is secreted, amylin (gene: IAPP) is also secreted. Amylin helps slow gastic emptying, thus resulting in a slower release of glucose and of insulin into the bloodstream. If amylin production gets too high, it forms amylin fibrils, which can kill the cell.

Mitochondrial Dysfunction in Type 2 Diabetes

Well established data demonstrate that mitochondrial dysfunction, particularly as it relates to the processes of oxidative phosphorylation (oxphos), is contributory to the development of encephalomyopathy, mitochondrial myopathy, and several age-related disorders that include neurodegenerative diseases, the metabolic syndrome, and diabetes. Indeed, with respect to diabetes, several mitochondrial diseases manifest with diabetic complications such as mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) and maternally inherited diabetes and deafness (MIDD). Normal biogenesis of mitochondria is triggered in response to changes in the ATP/ADP ratio and to activation of AMPK which in turn results in increased expression of PPARγ co-activator 1α (PGC-1α) and nuclear respiratory factor-1 (NRF1). PGC-1α is a master transcriptional co-activator of numerous genes involved in mitochondrial biogenesis. NRF1 is a transcription factor that regulates the expression of mitochondrial transcription factor A (TFAM, for transcription factor A, mitochondrial; also designated mtTFA) which is a nuclear transcription factor essential for replication, maintenance, and transcription of mitochondrial DNA. NRF1 also controls the expression of nuclear genes required for mitochondrial respiration and heme biosynthesis. Evidence has shown that both PGC-1α and NRF1 expression levels are lower in diabetic patients as well as in non-diabetic subjects from families with type 2 diabetes. The expression of NRF1 is highest in skeletal muscle which is also the tissue that accounts for the largest percentage of glucose disposal in the body and, therefore, is the tissue that is most responsible for the hyperglycemia resulting from impaired insulin signaling. Mitochondrial dysfunction results in increased production of ROS which activates stress responses leading to increased activity of MAPK and JNK. Both of these serine/threonine kinases phosphorylate IRS1 and IRS2 resulting in decreased signaling downstream of the insulin receptor. Inhibited IRS1 and IRS2 activity results in decreased activation of PI3K. PI3K activation is involved in the translocation of GLUT4 to the plasma membrane resulting in increased glucose uptake. Therefore, inhibition of PI3K results in reduced glucose uptake in skeletal muscle and adipose tissue. Mitochondrial dysfunction results in a reduction in the level of enzymes involved in β-oxidation leading to increases in intramyocellular lipid content. Indeed, skeletal muscle metabolism of lipids has been shown to be impaired in type 2 diabetics. An increased delivery of fatty acids to skeletal muscle, as well as diminished mitochondrial oxidation, results in increased intracellular content of fatty acid metabolites such as diacylglycerol (DAG), fatty acyl-CoAs, and ceramides. These metabolites of fatty acids are all known to induce the activity of protein kinase C isoforms (PKCβ and PKCδ) that phosphorylate IRS1 and IRS2 on serine residues resulting in impaired insulin signaling downstream of the insulin receptor. Because skeletal muscle consumes the largest amount of serum glucose, mitochondrial dysfunction in this tissue will have the greatest impact on glucose disposal. However, adipose tissue also plays an important role in glucose homeostasis and mitochondrial dysfunction in this tissue has been shown to result in impaired glucose homeostasis resulting in diabetes. For example, when animals are treated with inhibitors of mitochondrial oxidation insulin-stimulated glucose uptake in adipose tissue is significantly impaired. Adipose tissue secretes a number of proteins classified as adipokines. Adiponectin is an adipokine that promotes insulin-sensitivity in insulin-responsive tissues, such as skeletal muscle. When plasma levels of adiponectin are measured in obese or type 2 diabetic subjects it is found to be significantly lower than in age and sex matched control subjects that are of normal weight or that do not have diabetes. In animal studies, the enhancement of adipocyte mitochondrial biogenesis results in increased adiponectin release from adipose tissue. Conversely, expression of adiponectin expression is decreased in adipocytes with mitochondrial dysfunction. Given that impaired mitochondrial function is clearly associated with obesity and type 2 diabetes, it is not surprising that there is great interest in the use of pharmacology to augment mitochondrial function in the treatment of these disorders. Of significance is the fact that the thiazolidinedione (TZD) class of drugs used to treat the hyperglycemia of type 2 diabetes (see the next section) activate PPARγ which in turn increases the level of activity of PGC-1α. Although the TZDs were first marketed due to their ability to improve insulin sensitivity, they have since been shown to increase mitochondrial functions both in vitro and in vivo. Antioxidants have also been shown to enhance mitochondrial function by reducing the production of ROS. Resveratrol (found in grape skins and red wine) is a potent antioxidant whose activity is, in part, due to its ability to activate the deacetylase SIRT1 (see below). Activated SIRT1 deacetylates PGC-1α resulting in increased transcriptional activity and thus, enhanced mitochondrial biogenesis.

References

1) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1148905/pdf/biochemj00235-0011.pdf 2) https://themedicalbiochemistrypage.org/diabetes.php#iddm 3) http://www.cureffi.org/2013/12/13/biochemistry-12-diabetes/