Structural biochemistry/Endoplasmic Reticulum Stress and Type 2 Diabetes

Overview
The Endoplasmic Reticulum (ER) is a complex cellular organelle that hosts a multitude of functions including organelle folding, structural modification, intracellular and extracellular secretion associated with Golgi compartments, and protein synthesis. Although protein synthesis is popularly recognized as its central function in a eukaryotic cell, another function involving secretion is also highlighted--secretion. In cases involving ER homeostasis, the ER maintains insulin-secreting Beta-cells. Disruption of this balance have evolutionarily led to the creation of the unfolded protein response or UPR, which continuously monitors and maintains ER homeostasis. This adaptive signaling pathway is also present in other cellular organelles; however, when such adaptive measures fail to resolve an issue in homeostasis, the ER initiates pathways that resemble apoptosis: death signaling pathways. These pathways have recently emerged as some of the main causes for Type 2 Diabetes through the alteration of UPR and ultimately Beta-cell deterioration.

Type 2 Diabetes Frequency
As over-consumption of food and a reduction of physical activity throughout the modern world, the development of type 2 diabetes (T2D) in the population have tremendously increased. T2D has statistically affected approximately 285 million people in 2010 and has a shockingly projected 439 million people affected by 2030. However, T2D frequency is deeply embedded in the definition of the disease itself. T2D is a cellular representation of multiple groups of metabolic states or conditions that all showcase an increase in blood glucose levels due to insulin resistance. These elevated glucose levels are present in muscle tissues, liver tissues, adipose tissues, and most importantly, pancreatic tissues. While obesity is usually linked to the manifestation of T2D, the disease itself only develops in insulin-resistant patients with β-cell dysfunction. Thereafter, pancreatic islets generate increased amounts of β-cells through β-cell proliferation and neogenesis and through the enhancement of the β-cell function itself. However, this ultimately leads to the deterioration of β-cell function generation and present longevity and an impaired glucose tolerance is impending with a reduction of β-cell mass. While mechanistic formulas have yet to be discovered regarding β-cell death, β-cell loss caused by T2D have been attributed to ER stress responses caused by gluco/lipotoxicity and amyloid accumulation.



Endoplasmic Reticulum Stress by Glucotoxicity
Given that T2D causes deficiencies in insulin levels, blood glucose levels effectively remain heightened. This consistent elevated glucose level is known as hyperglycemia. In hyperglycemia, β-cells are overstimulated and progressively show a decrease in glucose-induced insulin secretion and insulin gene expression. This can eventually lead to impaired β-cell functions and a drop in β-cell longevity, a process known as glucotoxicity. Mediation for glucotoxicity exists in the accumulation of excess reactive oxygen species or ROS generated by numerous metabolic pathways that include oxidative phosphorylation in the mitochondria and other metabolic pathways. These pathways include hexosamine metabolism, glucose autoxidation, sorbitol metabolism, and increased protein glycation. In order to describe the mediation for glucotoxicity, there needs to be a clear understanding of excess reactive oxygen. ROS will produce oxidative stress that will ultimately lead to reduced proinsulin generation by decreasing mRNA expression of the β-cell transcription factors, MafA and PDX1. These transcription factors are necessary tools in the regulation of proinsulin genetic expression and downstream genes that are necessary for β-cell variety, spread, and survival. Hence, oxidative stress has been blamed as one of the main factors causing β-cell failure in cellular processes.

The previous description highlights but one of the mechanisms that may result in the manifestation of unnecessary oxidative stress. One other mechanism has been discovered in recent years and it involves the protein folding pathways in the endoplasmic reticulum and ROS production. The ER and ROS generation have shown clear linkage in the research involving causes for T2D. Their relationship stems from prolonged UPR activation which ultimately leads to the over-production of ROS. This ROS accumulation can be seen through two scenes. (1) UPR-regulated oxidative protein folding machinery in the ER itself and (2) oxidative phosphorylation in the mitochondria. The former affects hyperglycemic β-cells that show an enhanced desire for insulin. This demand requires an increased formation of disulfide bonds which consequently leads to the generation of more ROS. An estimated 25% of the ROS generated can be due to this process alone. The latter involves ROS production through the disruption of the electron transport chain in the mitochondria which usually induces mitochondrial apoptotic pathways. In this line of production, approximately 50% of synthesized proteins in the endoplasmic reticulum during the stimulation of glucose is proinsulin requiring intermolecular disulfide bond formation. Given that hyperglycemia results in elevated proinsulin synthesis, the ER would be stressed by the increase of its protein folding. This is most likely the nature of ER Stress and its link to T2D. This stress leads to misfolded proinsulin by UPR-regulated oxidative protein folding machinery and the use of a calcium leak from the ER itself. Consequently, glucotoxicity depicts how it causes ER stress by ROS production which naturally results in lack of insulin genetic expression, β-cell disruption, and ultimately apoptosis.

When the body is stressed under constant hyperglycemia, enhanced proinsulin bio-generation may cause the ER protein folding capacity to be overwhelmed. When overwhelmed, this capacity will result in the activation of the UPR. The hyper-activation of IRE1α in splicing XBP1 mRNA results from chronic high-glucose exposure. Non-chronic exposure only has an activation of IRE1α without mRNA splicing. When these high blood glucose levels are kept elevated for a week, hyper activated IRE1α with its plentiful activation conditions, deteriorates ER proinsulin mRNA and ER-localized mRNA. This will then lead to the reduction of proinsulin biosynthesis and β-cell failure.

Endoplasmic Reticulum Stress by Lipotoxicity
As studies suggest that obesity is related to heightened levels of plasma free fatty acids (FFAs), they are now known as central mediators of β-cell deterioration and apoptosis in T2D. Elongated saturated FFAs can mediate β-cell death in vitro and in vivo even though both unsaturated and saturated FFAs will usually stop proinsulin synthesis and glucose-stimulated insulin secretion in β-cells. The mechanistic nature of causing β-cell deterioration and programmed cell death through the accumulation of saturated FFAs is known as lipotoxicity. These FFAs cause ER stress and consequently the cellular failure later on while unsaturated long chain FFAs only leads to slight β-cell failure. The mechanisms involved in saturated FFAs show that palmitate treatment of β-cells activates PERK pathway and the expression of ATF4 and CHOP by means of eIF2α phosphorylation. Despite these effects, the discovery of IRE1α activation and the splicing XBp1 mRNAs and proteins were dependent on palmitate preparation and β-cell lines. However, controversy exists in the ability of ATF6α activation by palmitate.

The specific mechanisms used to describe the contribution to ER stress and β-cell apoptosis by lipotoxicity must be highlighted. First and foremost, the blocking of PKCδ translocation by the phsopholipase C inhibitor will cause substantially reduced palmitate-induced apoptosis. Hence, it is probable that FFA-induced activation of PKCδ may result in β-cell death in T2D. The overexpression of dominant-negative PKCδ blocked palmitate-induced nuclear accumulation of the FoxO1 protein in islets; however, the findings considering the confirmation of whether apoptosis by PKCδ is a result of changes in FoxO1 nuclear localization is still unclear. FoxO1 expression was enhanced by an ER stress inducer known as thapsigargin and dominant-negative FoxO1 expression protected β-cells cause cell death. Secondly, other research suggests that small toxicity between unsaturated and saturated FFAs is related to their inherent ability to cause triglyceride accumulation and fatty acid oxidation. More specifically, in β-cells, oleate treatment results in the accumulation of triglycerides and is usually well tolerated. However, palmitate is not well placed into the triglyceride which causes unnecessary ER stress and programmed cell death-apoptosis. A third viewpoint suggests that ER calcium ion homeostasis is vital for β-cell function. The elevated intraluminal calcium ion concentration in the ER is key in the maintenance of calcium ion-dependent ER chaperone functions. In this case, calcium ions released into the cytosol becomes an important signaling molecule. Finally, the concentration of calcium ions in the cytosol is duly kept low by active calcium ion transport into the ER by a process known as SERCA. This process creates a large calcium ion gradient between the cytosol and the ER lumen (0.1 micromolar versus 400 micromolar). Calcium ions bind to chaperones and consequently regulates their activity. Hence these ions affect posttranslational protein folding, modification, and trafficking. However, more studies are necessary to uncover the relative contribution of reduced ER calcium ion contents in palmitate-induced ER stress and apoptosis in β-cells.

Endoplasmic reticulum stress by Islet Amyloid
Islet hyalinosis or the deposition of hyaline in β-cells have shown a progressive role in T2D. These hyaline deposits are located in pancreatic β-cells that are present in close to 90% of patients with T2D. They are usually associated with reduced β-cell volume and these deposits are known as amyloids. Amyloids are insoluble fibrous proteins aggregates that share certain structural characteristics. Alzheimer's disease, Huntington's disease, and even Parkinson's disease are human neurodegenerative diseases that are closely associated with the generation of amyloids or pseudo-amyloid fibrils. The amyloid present in patients with T2D was discovered to be an islet amyloid polypeptide (IAPP). This polypeptide is expressed with insulin in pancreatic β-cells which travels through the insulin secretory pathway and is released with insulin directly after food consumption. In order to progress further, information regarding IAPP must be known. Mature IAPP is a 37-amino acid polypeptide that is derived from an 89-amino acid precursor through proteolytic processing.

In vitro IAPP in humans, primates, and cats are conserved; however, IAPP in rodents show sequential differences between the 20th and 29th amino acids. Hence, in vitro IAPP in humans, primates, and cats forms amyloid fibrils that send cellular toxicity to pancreatic β-cells. It is this sequential difference in IAPP that shows the formation of a fibrillar amyloid in aqueous conditions and confirms the toxicity of β-cells. Finally, this amyloid hypothesis for T2D was directly tested by β-cell-specific overexpression of human IAPP or hIAPP in transgenic mice. When hIAPP is inserted in transgenic mice, the mice showed signs of metabolic T2D. These characteristics included impaired insulin secretion, insulin resistance and hyperglycemia. However, recent research have shown that amyloid toxicity in proteins do not appear to be extracellular or intracellular lage amyloid deposits detected under light microscopy. They are rather smaller intracellular nonfibrillar oligomers that can be seen by specific antibodies when placed against amyloid β protein toxic HIAPP oligomers. Thus, experiments revealing designs using chemical chaperones in hIAPP in transgenic mice should demonstrate more insight in the full extent of their involvement in other apoptotic pathways. Lastly, patients with T2D and obesity have hIAPP oligomer-mediated ER stress that is a central part of increased β-cell apoptosis.