**5. Onset mechanism of obesity and diabetes in IRS-2 deficient mice**

#### **5.1 Onset mechanism of FT1DM in IRS-2 deficient mice**

Figure 5 summarizes onset mechanism of obesity and diabetes in IRS-2 deficient mice. IRS-2 deficient mice tend to fall in insulin resistance. Excess calorie and physical inactivity induce hyperglycemia followed by increased insulin secretion, which accelerates fatty acid synthesis via activation of transcriptional factor, SREBP-1c etc. Acceleration of fatty acid synthesis induces heterotopic accumulation of lipid, and visceral fat accumulation is increased. This situation is defined as obesity. Adiponectin exerts antidiabetic effects on muscles and the liver through AMP-activated protein kinase (AMPK) activation (Yamauchi et al., 2002) and antiatherosclerotic effects by inhibiting monocyte adhesion to endotherial cells and lipid accumulation into macrophages (Ouchi et al., 2001). Thus adiponectin increases glucose uptake and fatty acid oxidation in muscles via the type 1 adiponectin receptor (Yamauchi et al., 2003), and hepatic gluconeogensis via type 2 adiponectin receptor. Moreover adiponectin protects against oxidative stress in skeletal muscle by activating nuclear factor (NF)-κB target genes, manganese superoxide dismutase and inducible nitric oxide synthase (Ikegami et al., 2009). Decreased adiponectin secretion and increased inflammatory cytokines secretion from swelling adipose tissue deteriorate insulin resistance in obese animals (1st stage). Decreased adiponectin causes depression of activity of AMPK which increases glucose utilization and fatty acid β-oxidation in skeletal muscle and adipose tissues (Whitehead et al., 2006). Then hyperglycemia, hyperinsulinemia and accelerated lipid synthesis are maintained and hyper-secretion of insulin force excessively heavy work on pancreatic β cells. In over functional pancreatic islets, β-oxidation of fatty acid is accelerated resulting in excess amount of reactive oxygen species (ROS) production, which induces ROS stress leading to mitochondrial dysfunction and apoptosis of β-cells with low scavenging activity of ROS (2nd stage). It has been reported that adiponectin inhibits fatty acid-induced apoptosis by suppression of ROS generation via both the cAMP/PKA and AMPK pathway in endothelial cells (Kim et al, 2010). Macrophages (but not T cells) infiltration is observed frequently in FT1DM (Shibasaki et al., 2010). In IRS-2 deficient mice with FT1DM macrophage infiltration induced by MCP-1 was observed. Infiltrated macrophages may participate in destruction process of pancreatic islets leading to T1DM. The β cell deficit is believed to be due to autoimmune induced β cell apoptosis mediated by the release of inflammatory cytokines, such as IL-1β and TNF-α, from T lymphocytes and macrophages (Donath et al., 2003). Cytokine-induced β cell death preferentially affects newly forming beta cells, which implies that replicating beta cells might be more vulnerable to cytokine destruction. Efforts to expand beta cell mass in type 1 diabetes by fostering β cell replication are likely to fail unless cytokine-induced apoptosis is concurrently suppressed (Meier et al., 2006). Inflammatory cytokines from corpulent adipocytes appear to participate in destruction of islets β cells leading to T1DM. In autoimmune T1DM, β cells are assumed to be destroyed through a long-standing autoimmune process, whereas in FT1DM, β cells seem to be destroyed very rapidly, probably by a destructive process triggered by viral infection (Hanafusa & Imagawa, 2008). Since IRS-2 deficient mice were maintained under specific pathogen free conditions (Hashimoto et al., 2006), viral infection was deleted from the causes of β cell destruction. Adipocyte-secreted factors associate the pancreatic β cells destructions. Chronic exposure of human islets to leptin leads to β cell apoptosis (Donath et al., 2003). TNFα, in combination with other cytokines, accelerates dysfunction and destruction of the β cell (Eizirik & Mandrup-Poulsen, 2001). IL-6 released by adipocytes may be responsible for the increases in plasma IL-6 concentrations observed in obesity and

Fulminant Type 1 Diabetes Mellitus in IRS-2 Deficient Mice 175

IRS-2 deficient mice with FT1DM show remarkable body weight loss, polydipsia, polyuria, glycosuria and ketonuria as typical symptoms of T1DM as reported in human FT1DM patients. Laboratory data in IRS-2 deficient mice with FT1DM reveal hyperglycemia, hyperlipidemia and remarkable decrease in insulin secretion as in human FT1DM patients (Table 3). The above symptoms of T1DM were onset abruptly after hyperglycemia was observed in IRS-2 deficient mice. Insulitis with macrophage dominant infiltration was observed in IRS-2 deficient mice and human FT1DM. Destruction mechanism of β cells associated HLA, viral infection and pregnancy were investigated in detail in human FT1DM patients (Kawabata et al., 2009; Murabayashi et al., 2009; Tan & Loh, 2010), whereas association with MHC was not investigated in IRS-2 deficient mice. Since FT1DM was observed in only male IRS-2 deficient mice, pregnancy is not associated with onset of FT1DM. Inflammatory cytokines play a major role in destruction process of pancreatic β cell in both IRS-2 mice and human FT1DM patients. Trigger of the β cell destruction process is different between IRS-2 mice and human. Insulin resistance by increase in inflammatory cytokines seemed to be main cause to lead β cell destruction in IRS-2 deficient mice, whereas viral infection may be a trigger for destruction mechanism in human FT1DM patients.

**5.2 Comparison of pathology of FT1DM between IRS-2 deficient mice and human** 

**patients** 

\*Mean ± SE, \*\*Mean ± SD

Table 3. Characteristics of FT1D in IRS-2 deficient mice and human patients

Type 1 diabetes is a polygenic disease. Approximately 50% of the genetic susceptibility can be explained by allele in HLA class II region, in particular certain DQ alleles. More than 95%

at least in combination with other cytokines, IL-6 has cytotoxic effects on β cell (Eizirik et al., 1994). Increased FFA levels are known to be toxic for β cell, leading to the concept of lipotoxicity (McGarry & Dobbins, 1999). The toxic effect of FFA is mediated via formation of ceramide, increased nitric oxide production and activation of the apoptotic mitochondrial pathway (Maedler et al., 2001). Elevated glucose concentrations induced β cell apoptosis at higher concentration in rodent islet (Efanova et al., 1998). In human islets glucose-induced β cell apoptosis and dysfunction are mediated by β cell production and secretion of IL-1β. Chronic hyperglycemia increases production of ROS, which may cause oxidative damage in β cell (Matsuoka et al., 1997; Laybutt et al., 2002). IL-1β and ROS activate the transcription factor nuclear transcription factor (NF) κB, which plays a critical role in mediating inflammatory responses. A series of inflammatory reaction appear to have important roles in the β cell destruction process in IRS-2 deficient mice with insulin resistance.

SREBP, sterol regulatory element binding protein; AMPK, AMP-activated protein kinase; ROS, reactive oxygen species;

TNF, tumor necrosis factor; IL, interleukins

Fig. 5. Onset mechanism of obesity and diabetes in IRS-2 deficient mice

at least in combination with other cytokines, IL-6 has cytotoxic effects on β cell (Eizirik et al., 1994). Increased FFA levels are known to be toxic for β cell, leading to the concept of lipotoxicity (McGarry & Dobbins, 1999). The toxic effect of FFA is mediated via formation of ceramide, increased nitric oxide production and activation of the apoptotic mitochondrial pathway (Maedler et al., 2001). Elevated glucose concentrations induced β cell apoptosis at higher concentration in rodent islet (Efanova et al., 1998). In human islets glucose-induced β cell apoptosis and dysfunction are mediated by β cell production and secretion of IL-1β. Chronic hyperglycemia increases production of ROS, which may cause oxidative damage in β cell (Matsuoka et al., 1997; Laybutt et al., 2002). IL-1β and ROS activate the transcription factor nuclear transcription factor (NF) κB, which plays a critical role in mediating inflammatory responses. A series of inflammatory reaction appear to have important roles

SREBP, sterol regulatory element binding protein; AMPK, AMP-activated protein kinase; ROS, reactive

Fig. 5. Onset mechanism of obesity and diabetes in IRS-2 deficient mice

oxygen species;

TNF, tumor necrosis factor; IL, interleukins

in the β cell destruction process in IRS-2 deficient mice with insulin resistance.

#### **5.2 Comparison of pathology of FT1DM between IRS-2 deficient mice and human patients**

IRS-2 deficient mice with FT1DM show remarkable body weight loss, polydipsia, polyuria, glycosuria and ketonuria as typical symptoms of T1DM as reported in human FT1DM patients. Laboratory data in IRS-2 deficient mice with FT1DM reveal hyperglycemia, hyperlipidemia and remarkable decrease in insulin secretion as in human FT1DM patients (Table 3). The above symptoms of T1DM were onset abruptly after hyperglycemia was observed in IRS-2 deficient mice. Insulitis with macrophage dominant infiltration was observed in IRS-2 deficient mice and human FT1DM. Destruction mechanism of β cells associated HLA, viral infection and pregnancy were investigated in detail in human FT1DM patients (Kawabata et al., 2009; Murabayashi et al., 2009; Tan & Loh, 2010), whereas association with MHC was not investigated in IRS-2 deficient mice. Since FT1DM was observed in only male IRS-2 deficient mice, pregnancy is not associated with onset of FT1DM. Inflammatory cytokines play a major role in destruction process of pancreatic β cell in both IRS-2 mice and human FT1DM patients. Trigger of the β cell destruction process is different between IRS-2 mice and human. Insulin resistance by increase in inflammatory cytokines seemed to be main cause to lead β cell destruction in IRS-2 deficient mice, whereas viral infection may be a trigger for destruction mechanism in human FT1DM patients.


\*Mean ± SE, \*\*Mean ± SD

Table 3. Characteristics of FT1D in IRS-2 deficient mice and human patients

Type 1 diabetes is a polygenic disease. Approximately 50% of the genetic susceptibility can be explained by allele in HLA class II region, in particular certain DQ alleles. More than 95%

Fulminant Type 1 Diabetes Mellitus in IRS-2 Deficient Mice 177

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of type 1 diabetic patients carry these predisposing alleles, but the occurrence of these alleles in the background population is high, approximately 50%. It is believed that the diabetes predisposing DQ antigens have a shape of the antigen presenting groove of the molecule that leads to more efficient presentation of β cell associated autoantigens (Donath et al., 2003). HLA comment should be in the text. In FT1DM patients, the haplotype frequency of HLA DRB1\*0901-DQB1\*0303 was significantly higher than those in controls (Moreau et al., 2008). HLA phenotyping of these Caucasian patients did not find the specific HLA haplotype (DRB1\*0405-DQB1\*0401) found to be linked to FT1D in Japanese patients. More investigation about haplotype frequency of MHC was necessary for IRS-2 mice in the destruction process of pancreatic β cells.
