Cellular Mechanism of High Glucose/Palmitate-induced INS-1 Beta Cell Death

Type 2 diabetes is considered to be in most cases a polygenic disease with a strong involvement of environmental factors, such as diet and exercise. Chronic exposure of high glucose and free fatty acids (FFAs) induces pancreatic beta cell dysfunction and death, which may contribute to the development of type 2 diabetes. A deficiency of insulin through loss of beta cell mass, and subsequent impaired compensation against insulin resistance is believed to be a pathogenic cause of type 2 diabetes. Pancreatic β-cell mass is regulated by at least four independent mechanisms(i) β-cell replication (i.e., the mitogenic division of existing β cells), (ii) β-cell size, (iii) β-cell neogenesis and (iv) β-cell apoptosis. The sum of the rates of β-cell replication, size, and neogenesis, minus the rate of β-cell apoptosis gives the net rate of β-cell growth(Rhodes et al, 2005). Increased free fatty acid (FFAs), in conjunction with hyperglycemia, have been proposed to provide stimuli triggering the insulin deficiency in type 2 diabetes, since several in vitro studies demonstrated that long-term exposure to saturated FFAs and could induce cell death in cultured beta cells and also in isolated islets. The cellular injury, through excess FFA accompanied by triglyceride accumulation, was termed beta cell glucolipotoxicity. The cellular mechanisms involved in FFA-induced beta cell apoptosis are not fully understood. FFAs are usually non-toxic to beta cells if they are oxidized. However, the accumulation of long chain acyl-CoAs or lipid derivatives, such as, diacylglycerol, lysophosphatidic acid, and sphingolipids, are thought to promote beta cell apoptosis (Assimacopulos et al, 2004). Ceramide has been suggested to be an important mediator of FFA-induced beta cell death (Lupi et al, 2002; Maedler et al, 2001; Shimabukuro et al, 1998; Okuyama et al, 2003). Nuclear translocation and activation of PKC-δ were reported to be necessary for saturated fatty acid-induced beta cell apoptosis (Eitel, 2003). Elevated concentrations of FFAs were found to produce reactive oxygen species (ROS) (Wang et al, 2004) and therefore, ROS and oxidative stress were believed to be involved in FFA-induced beta cell death. On the other hand, inhibition of the IRS/PI3 kinase/Akt signaling pathway was reported to be critical in FFA-induced beta cell apoptosis (Lingohr et al, 2003; Wrede et al, 2002). Recently, endoplasmic reticulum (ER) stress was postulated to be a critical mediator of FFA-induced beta cell apoptosis (Kharroubi et al, 2004; Karaskov et al, 2006) and the involvement of calcium-mediated apoptotic signals was also reported (Choi et al, 2007). To investigate the mechanisms of HG/PA-induced INS-1 beta cells death, we tested the effect of several pharmacological inhibitors. Chronic exposure of free fatty acids (FFAs) in conjunction with high glucose (HG) has been reported to be cytotoxic to beta cells. These works were initiated to elucidate how lipid metabolism elicits glucose/palmitate-induced beta cell cytotoxicity. When the INS-1 beta cells were exposed to 0.4 mM palmitate (PA) in the presence of 25 mM glucose (high glucose: HG) for 24 hrs, cell viability was reduced due to apoptotic death. TOFA and celurenin, inhibitors of acetyl CoA carboxylase (ACC) and fatty acid synthase (FAS), respectively, did not inhibit HG/PA-induced viability reduction and T0901317, a LXR agonist, protected HG/PA-induced INS-1 cell death, suggesting that lipogenesisitself may not be a force for induction of HG/PA-induced cytotoxicity. On the other hand, anaplerotic input through leucine/glutamine, a-ketoisocaproic acid/methyl succinate, or aminobicyclo-heptane-2-carboxylic acid (BCH) and cataplerotic block through benzene tricarboxylate (BTA), an inhibitor of mitochondrial tricarboxylate carrier, significantly protected HG/PA-induced beta cell death. Furthermore, intracellular ATP as well as Krebs cycle intermediates was significantly decreased in HG/PA-treated cells, and the reduced ATP level was restored by anaplerotic input or cataplerotic block. The protective effect by anaplerotic input and cataplerotic block on HG/PA-induced cytoxicity was also observed in primary islet beta cells isolated from Zuker Diabetic Fatty (ZDF) or Otsuka Long-Evans Tokushima Fatty (OLETF) rats. These works suggest that energy depletion, by shortage of Krebs intermediates, may play a critical role for HG/PA-induced beta cell death.

Author dc:creator

• 이, 연정

Contributors dc:contributor

• 강, 엽
• 대학원 의학과
• 200624454

• High Glucose
• Palmitate
• anaplerosis
• energy depletion

Language dc:language

ko

Identifier

http://dcoll.ajou.ac.kr:9080/dcollection/jsp/common/DcLoOrgPer.jsp?sItemId=000000009214
000000009214

OAI identifier oai:identifier

oai:repository.ajou.ac.kr:201003/1815