ABSTRACT
Background@#Protease-activated protein-2 (PAR2) has been reported to regulate hepatic insulin resistance condition in type 2 diabetes mice. However, the mechanism of lipid metabolism through PAR2 in obesity mice have not yet been examined. In liver, Forkhead box O1 (FoxO1) activity induces peroxisome proliferator-activated receptor γ (PPARγ), leading to accumulation of lipids and hyperlipidemia. Hyperlipidemia significantly influence hepatic steatoses, but the mechanisms underlying PAR2 signaling are complex and have not yet been elucidated. @*Methods@#To examine the modulatory action of FoxO1 and its altered interaction with PPARγ, we utilized db/db mice and PAR2-knockout (KO) mice administered with high-fat diet (HFD). @*Results@#Here, we demonstrated that PAR2 was overexpressed and regulated downstream gene expressions in db/db but not in db+ mice. The interaction between PAR2/β-arrestin and Akt was also greater in db/db mice. The Akt inhibition increased FoxO1 activity and subsequently PPARγ gene in the livers that led to hepatic lipid accumulation. Our data showed that FoxO1 was negatively controlled by Akt signaling and consequently, the activity of a major lipogenesis-associated transcription factors such as PPARγ increased, leading to hepatic lipid accumulation through the PAR2 pathway under hyperglycemic conditions in mice. Furthermore, the association between PPARγ and FoxO1 was increased in hepatic steatosis condition in db/db mice. However, HFD-fed PAR2-KO mice showed suppressed FoxO1-induced hepatic lipid accumulation compared with HFD-fed control groups. @*Conclusion@#Collectively, our results provide evidence that the interaction of FoxO1 with PPARγ promotes hepatic steatosis in mice. This might be due to defects in PAR2/β-arrestin-mediated Akt signaling in diabetic and HFD-fed mice.
ABSTRACT
Although replacement therapy with insulin can prevent acute metabolic disorder in patients with IDDM (insulin dependent diabetes mellitus), it does not permanently restore glucose homeostasis. Recently it has been reported that islet transplantation could completely correct the glucose metabolic abnormalities and prevent further progression of the secondary complications of IDDM. For successful transplantation, the isolated islets should be prepared without loss of viability, while their immunogenicity being suppressed to reduce graft rejection. The present study was aimed to determine the optimal condition of islet culture, and to transplant them into the digestive organs including gastroin-testinal wall and salivary gland. For islet culture, pancreatic islets were isolated by a modified collagenase digestion technique from rats and cultured for 24, 48 and 72 hours in RPMI-1640 containing 0, 5.6 and 16.7 mM glucose. The viability of islets was evaluated by detection of insulin mRNA expressed in islet beta-cells using the in-situ hybridization and northern blot analysis, while their insulin content was examined by immunocytochemistry. Insulin mRNA was significantly reduced after 48 hours of culture in the islets incubated with absence of glucose, while distinct immunoreaction for insulin remained in the same islet. On the other hand, the islets cultured with normoglycemic (5.6 mM glucose) and hyperglycemic (16.7 mM glucose) conditions showed a normal or excessive transcription of insulin gene after 72 hours, respectively. These results indicate that biosynthetic activity of islets could be maintained longer than 72 hours without alteration of viability when they were cultured in normoglycemic condition. Therefore, we used islets cultured for 72 hours with 5.6 mM glucose for transplantation. The islets were implanted into the submucosal wall of the stomach and duodenum as well as into the parenchyme of the submandibular gland of the streptozotocin-induced diabetic rats. The transplanted islets in the gastrointestinal wall were abolished in 72 hours, while the islets injected into the submandibular gland retained normal cellular structure with viability for longer period. The beta-cell in the submandibular gland showed similar immunoreactivity for insulin compared to that of normal islets. However, they showed gradual infiltration of lymphocytes and beta-cell destruction at 10~14 days after transplantation. We suggested that the submandibular gland could be recommended as an alternative site for islet transplantation, because it is very easy to access for transplantation and provides the structural and functional similarities to pancreas in which the islets spontaneously grow.