Dr. Khoo investigates the basic molecular mechanisms underlying the development of metabolic syndrome and the role of electrophilic lipids, particularly nitro-fatty acids (NO2-FAs) in preventing this pathogenesis. His specific research projects include:
1) Determination of the molecular mechanism(s) responsible for the anti-inflammatory cell signaling actions of electrophilic NO2-FAs resulting in insulin senstivity.
Obesity induces chronic inflammatory responses that are characterized by abnormal cytokine production, increased reactive oxygen species (ROS) generation and activation of inflammatory signaling pathways. Preliminary studies demonstrate these inflammatory conditions induce the oxidation and nitration of fatty acids to electrophilic products, specifically NO2-FA derivatives, that serve as potent anti-inflammatory cell signaling mediators. These electrophilic NO2-FA species potently bind peroxisome proliferator-activated receptors (PPARs), inhibit NF-kB activity and induce heme oxygenase (HO)-1. The treatment of NO2-FAs results in improved glucose homeostasis in mouse models of obesity and diabetes. The electrophilic nature of these NO2-FA signaling molecules and their anti-inflammatory properties are being examined using cultured mammalian cells as well as mouse models of obesity and diabetes. Additionally, a mass spectrometer approach is being used to characterize the formation of NO2-FA derivatives in insulin responsive tissues from murine models of obesity and diabetes (ob/ob, db/db or high fat diet). This is being complemented by similar approaches in cell culture.
· Identify molecular signaling pathways modulated by NO2-FA treatment in mice subjected to a high-fat diet. Currently, three putative pathways for the anti-inflammatory actions of electrophilic NO2-FAs are being examined. The signaling pathways of all three PPAR isotypes, NF-kB and HO-1 are being explored in insulin-responsive tissues (adipose, liver and muscle).
· Define mechanistic roles of all three PPAR isotypes, NF-kB and HO-1 in cultured cells. The knockdown of these putative signaling pathways using si-RNA will be tested in cultured adipocytes, hepatocytes and skeletal muscle cells. Additionally, mouse embryonic fibroblasts isolated from Nrf2 knockout mice will explore the potential mechanism(s) of electrophilic NO2-FA-induced HO-1 expression.
2) Determination of the impact of NO2-FA derivatives on ROS and oxidative stress in insulin-responsive cultured cells and tissues of mouse models of obesity and diabetes. While oxidative stress and ROS are emerging as key culprits in the pathogenesis of obesity-induced insulin resistance, the sources of ROS remain unclear. Emerging studies suggest a link between mitochondrial dysfunction, insulin resistance and diabetic complications, suggesting that mitochondrially derived ROS could play a role in pathogenesis. How does this increase in ROS result in oxidative stress? Is there a decrease in antioxidant enzyme expression and activity in insulin-responsive tissues? These questions are currently being addressed by utilizing cutting edge techniques to detect ROS levels and antioxidant enzyme activity/expression in the cell culture and mouse models of obesity and diabetes described above.
3) The PPAR conundrum- Identification of novel PPAR agonists. The activation of PPARs have been shown to regulate glucose and lipid metabolism. These receptors are molecular targets for a number of marketed drugs. The hypolipidemic fibrates activate the isotype PPARa whereas PPAR? is the molecular target of thiazolidinedione (TZD) class of antidiabetic drugs. The activation of PPAR?-dependent downstream signaling has shown to improve insulin sensitivity. Rosiglitazone works as an insulin sensitizer by activating PPAR? and its downstream signaling pathways. Unfortunately, concerns about the severe adverse side effects have drastically limited the use of rosiglitazone despite excellent glycemic control in patients with diabetes. Thus, the development of new therapeutic strategies, such as dual PPARa/? activators or selective PPAR? partial agonists, that retain their antidiabetic efficacy without adverse side effects are appealing, such as NO2-FAs.
In summary, these research interests will generate insights into mechanisms leading to obesity and its associated myriad of health problems and/or diseases such as diabetes, atherosclerosis and other cardiovascular complications, which will hopefully elucidate novel preventative and therapeutic strategies. The potential for electrophilic NO2-FA mediated therapy to prevent obesity-induced type 2 diabetes complications, without the known secondary effects exerted by TZDs, is currently being studied.
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