Mitochondrial Dysfunction in the Pathogenesis of Insulin Resistance---Roles of TLR Signaling and Endoplasmic Reticulum Stress

Diabetes is a risk factor for cardiovascular diseases. Deranged adipocyte metabolism and altered body fat distribution are important determinants of insulin resistance (IR), which plays the central role of pathogenesis of DM. The molecular events linking excessive fat to IR have been focused on the oxidative stress and chronic inflammation in accumulated adipose tissue. In our previous study of Chinese adults, we found that the IR index estimated by HOMA correlated with content of intraperitoneal fat and liver fat, but not with the subcutaneous fat. Furthermore, fatty liver index can be a better marker of chronic inflammation than the conventional components of metabolic syndrome. In our current study, we have established the animal model of diet-induced obesity with C57BL/6J mice. Increased visceral fat and glucose intolerance have been noted since 2 months after feeding with high fat diet. How is the inflammation process initiated in the adipose tissue is still not well known. Recent data from experiment model indicate that endoplasmic reticulum stress (ER stress) is crucial to trigger and integrate the pathways of inflammation. Furthermore, observations of involvement of mitochondria in ER stress-induced apoptosis and ER stress secondary to mitochondrial dysfunction have emerged in current publication. Crosstalk between the ER and mitochondria in IR-related tissue is worth studying. Another clue to link obesity and chronic inflammation is the finding of activation of Toll-like receptor 4 (TLR4) signaling in macrophage cell line by lipid component of lipopolysaccharide (LPS). TLRs are a family of pattern-recognition receptors that play a critical role in the innate immune system by activating proinflammatory signaling pathways in response to microbial pathogens. Increased nutritional fatty acids in obesity have been shown to active TLR4 in adipocytes and macrophages. Obese C57BL/6 mice lacking TLR4 are partially protected against high fat diet-induced IR. TLR4 has been shown to recognize endogenous ligands and its expression is enhanced in lipid-rich, macrophage-infiltrated human coronary plaques. Members of some TLRs are known to be localized in the ER and an intimate relationship between TLR signaling and ER stress is considered possible. Taken the above information together, link among TLR4, ER stress, and mitochondrial dysfunction might explain how the innate immune system involved in the regulation of energy balance and IR in response to changes in the nutritional environment. We will conduct a three-year project to prove the hypothesis. The proposal is expected to: In the 1st year: Using C57BL/6J mice fed with high fat diet, to establish an association of TLR signaling (MyD88, TRIF, TRAF3) with mitochondrial dysfunction and ER stress, including changes in mitochondrial transcription factors (PGC-1α, NRF-1, NRF-2, Tfam), mtDNA content, and tissue oxidative damage, as well as ER molecules (GRP78, PERK, IRE1, ATF6) in liver, muscle, and adipose tissue (visceral and subcutaneous) of the mice. Cultured adipocytes will be studied with or without LPS stimulation for this TLR-ER-mitochondria pathway. In the 2nd year: Using mouse 3T3-L1 cell line, confirmation of TLR mediating mitochondrial and ER stress will be conducted by using RNA interference in vivo to silence the expression of TLR. The influence of insulin-sensitizer anti-diabetic drugs (Metformin, Pioglitazone) on this TLR-ER-mitochondria pathway will be compared with placebo with C57BL/6J mice fed with high fat diet. In the 3rd year: Using mouse fat cell line (3T3-L1), confirmation of the role of mitochondria in mediating TLR signaling and ER stress by mitochondrial depletion with ethidium bromide. This project will elucidate the possible mechanism of TLR modulation of ER and mitochondrial stress specific to IR and diabetes. The results may be useful in future implementing innate immunity in therapy of type 2 diabetes.

Project ID:PC9902-1224
External Project ID:NSC98-2314-B182-007-MY3