2.4.3.1 Pathophysiological induction of ER stress
The main function of pancreatic β-cells is the production, storage and secretion of insulin. Increasing blood glucose levels, for example after meal intake, stimulate the biosynthesis of insulin (Andrali et al. 2008). Elouli et al. (2007) demonstrated that stimulation of rat islets with different glucose concentrations (2 - 30 mM) activates the UPR, in a protein biosynthesis dependent manner (Elouil et al. 2007). Elevated insulin demand in insulin resistant and/or hyperglycaemic diabetic individuals leads to increased proinsulin biosynthesis and proinsulin folding, which can induce ER stress (Scheuner and Kaufman 2008).
In Type 1 diabetes, cytokines are produced during the autoimmune destruction of β-cells (2.2.1 Diabetes type 1). It was demonstrated that exposure of insulin-producing insulinoma-1E ((INS)-1E) cells to cytokines like IL-1β and interferon-γ (IFNγ) results in an indirect and time-dependent activation of ER stress, via nitric oxide (•NO) production (Kharroubi et al. 2004). Il-1β alone or in combination with IFNγ, but not IFNγ alone, reduced the expression of sarcoendoplasmic reticulum pump Ca2+ ATPase 2b (SERCA2b), which resulted in the depletion of ER Ca2+ and activation of ER stress (Cardozo et al. 2005).
Type 2 diabetes is often associated with obesity and elevated triglyceride levels (2.2.2 Diabetes type 2). Incubation of insulin-secreting (INS)-1E cells, MIN6 cells and isolated islets of obese insulin-resistant db/db mice with the free fatty acid (FFA) palmitate induced ER stress and caused apoptosis in a time-dependent mode (Kharroubi et al. 2004; Laybutt et al. 2007). After exposure to the FFA oleate, ER stress response and β-cell apoptosis were lower (Kharroubi et al. 2004) or virtually not detectable (Laybutt et al. 2007) compared to palmitate incubation.
In type 2 diabetics, human islet amyloid polypeptide (hIAPP) can aggregate and form toxic oligomers, whereas rodent IAPP (rIAPP) doesn’t show this characteristic. In experiments with homozygous transgenic mice overexpressing either rIAPP or hIAPP, hIAPP, but not rIAPP induced ER stress and β-cell apoptosis, leading to the developement of diabetes mellitus. These findings indicate that the toxic effects of hIAPP on β-cells are mainly
due the propensity of hIAPP to aggregate and less due to protein overexpression (Huang et al. 2007).
2.4.3.2 Gene alterations in animal models and men
Several human diseases and animal models serve to investigate the relationship between ER stress and diabetes mellitus.
PERK
PKR-like endoplasmic reticulum kinase (PERK) is essential for reducing the protein folding load, for activating the expression of BiP and CHOP, for reducing oxidative stress and for amino acid metabolism (2.4.2.1 Signal transduction).
Mutations in the EIF2AK3 gene, which encodes for PERK, were detected in patients suffering from the Wolcott-Rallison syndrome (WRS). This autosomal recessive disease shows different clinic manifestations including early-infancy diabetes mellitus. A slight residual enzyme activity was associated with a delayed onset of WRS compared to patients with complete PERK inactivation (Senee et al. 2004).
PERK-/- mice were born with a normally developed endo- and exocrine pancreas and physiological blood glucose concentrations. Within less than 4 weeks they showed growth retardation, severe hyperglycaemia, reduced insulin secretion and increased β-cell loss. In addition to the progressive diabetic phenotype, PERK-/- mice developed exocrine pancreas insufficiency. IRE1α levels were increased, indicating ER stress (Harding et al. 2001).
ATF6
Activating transcription factor 6 (ATF6) participates in protein folding and degradation under ER stress conditions (2.4.2.1 Signal transduction).
Double knockout (ATF6α-/- and ATF6β-/-) resulted in embryonic death,
whereas mice carrying single null mutations (ATF6α-/- or ATF6β-/-) developed
normally (Yamamoto et al. 2007). Upon ER stress, ATF6α-/- mouse embryonic
fibroblasts demonstrated a reduced activation of ER stress-associated chaperones like BiP, Grp94 as well as folding enzymes (ERp72 and P5) and a decreased induction of various ERAD components, resulting in diminished
viability of ATF6α-/- MEFs compared to ATF6α+/+ MEFs under ER stress
conditions.
Amino acid variants in the ATF6 gene were associated with type 2 diabetes in Pima Indians (Thameem et al. 2006).
CHOP
The transcription factor C/EBP homologous protein (CHOP) is involved in ER stress-mediated apoptosis (2.4.2.2 Apoptotic pathways).
The targeted disruption of Chop reduced β-cell apoptosis in 4-week-old heterozygous Akita mice and delayed the onset of diabetes (2.3.3.3 ER stress in the Akita mouse). Song et al. (2008) reviewed distinct mouse models for diabetes (e.g. Lepr-/-,eIF2αS/A and STZ-treated mice) in which Chop deletion
improved glycaemic control, β-cell function and β-cell survival upon ER stress (Song et al. 2008).
Insulin gene
Different mutations in the insulin gene were described in men and mice. Mutations which disturb disulfide bond formation result in a severe diabetic phenotype, with early onset and profound β-cell loss, as it is described in human patients with PNDM (2.2.3.2 Neonatal diabetes mellitus (NDM)), in the Munich Ins2C95S mutant mouse (2.3.2 The Munich Ins2C95S mutant mouse), and in the Akita mouse (2.3.3 The Akita mouse). It was demonstrated that misfolded (pro-)insulin 2 induces ER stress in the Akita mouse (2.3.3.3 ER stress in the Akita mouse).
Wolfram syndrome
Wolfram syndrome is a rare autosomal rezessive disorder causing juvenile onset diabetes mellitus, optic atrophy and often sensorineural hearing loss or diabetes insipidus (Fonseca et al. 2005; Ueda et al. 2005). WFS1 encodes for a protein located in the ER membrane and plays an important role in maintaining homeostasis in the ER of pancreatic β-cells. WFS1 might be involved in the UPR, and ER stress activates WFS1 expression. Inactivation of WFS1 in β-cells resulted in chronic ER stress (Fonseca et al. 2005; Ueda et al. 2005).
2.4.3.3 Therapeutic strategies for reducing ER stress
Chemical chaperones were demonstrated to stabilise protein conformation and increase ER folding capacity. Oral treatment of ob/ob mice with 4-phenyl butyric acid (PBA) or tauroursodeoxycholic acid (TUDCA) improved blood glucose concentrations, glucose tolerance and insulin sensitivity and reduced ER stress in liver and fatty tissue (Özcan et al. 2006).
Oxidative stress can induce ER stress and vice versa, and antioxidative treatment reduced ER stress in vitro (2.6 Linkage between oxidative and ER stress). Therefore, treatment with antioxidants possibly contributes to the reduction of ER stress (Malhotra et al. 2008).
CHOP is involved in apoptosis upon ER stress, and deletion of Chop delayed the onset of diabetes in heterozygous Akita mice. Chop-/- mice were
phenotypically inconspicuous and presented a normal lifespan. Therefore specific CHOP inhibitors would be another promising therapeutic approach to preserve β-cells mass upon ER stress (Scheuner and Kaufman 2008).