Open Access Dissertation
Date of Award
Glucose stimulated insulin secretion (GSIS) involves a series of metabolic and cationic events, leading to translocation of insulin-laden secretory granules from a distal site toward the plasma membrane for fusion and release of insulin into circulation. Vesicular transport and fusion events are tightly regulated by signals which coordinate between vesicle- and membrane-associated docking proteins. It is now being accepted that reactive oxygen species [ROS] plays a second messenger role in islet â-cell function. Further, evidence from multiple laboratories suggests a tonic increase in ROS generation is necessary for GSIS and fatty acid-induced insulin secretion. On the other hand, excessive ROS generated during glucolipotoxic / exposures to cytokines and ceramide have proved to be detrimental for islet â-cells. Recent studies have shown activation of phagocyte-like NADPH oxidase [Nox] to be underlying cause for increased ROS generation observed under the above pathological conditions.
The overall objective of the present study is to i) determine potential mechanism[s] underlying nutrient-induced generation of ROS; ii) contributory roles of Tiam1-Rac1-Nox signaling in free fatty acid (e.g., palmitate) and cytokines- induced â-cell dysfunction. Findings from current study suggest that posttranslational prenylation is a requisite for signaling G-proteins involved in the activation of Nox and generation of ROS for nutrient-induced insulin secretion from islet â-cells. Studies with pertussis toxin [Ptx] suggested that glucose-induced Nox-mediated ROS generation is regulated by inhibitory class of G-proteins [Go/Gi]. Our next set of studies, directed towards understanding the mechanism of Nox activation under chronic exposure to high palmitate, cytokines and C2-ceramide implicate increased expression of Nox subunits to precede the functional activation of the holoenzyme and excessive ROS generation resulting in mitochondrial dysfunction. This study also provide first evidence for a critical modulatory role of Tiam1, a guanine nucleotide exchange factor [GEF] in Rac1-Nox signaling axis.
The next set of studies validated the above observations in Zucker Diabetic Fatty [ZDF] rat model, which mimics type2 diabetes in humans, characterized by obesity, hyperinsulinemia, hyperglycemia and gradual decline in â-cell function. The results obtained were comparable with clonal â-cells. Islets derived from ZDF-rats presented high levels of Nox subunit expression [p47phox, gp91phox, Rac1] which constitutively activated Nox-holoenzyme and augmented ROS levels. The increased oxidative stress under conditions of diabetes activated Jun-N-terminal kinases [JNK 1/2, stress-activated kinases] leading to mitochondrial abnormalities and eventual demise of islet â cells. A similar pattern of induction in Nox subunit expression/activation, ROS generation and JNK 1/2 were also observed in type 2 diabetes human islets. Taken together, herein I propose that high levels of oxidative stress, activation of stress-activated kinases [JNK1/2] and mitochondrial abnormalities underlies pancreatic â-cell dysfunction[s] during diabetes. Additional studies are needed to understand the precise regulatory roles for Tiam1-Rac1-Nox-ROS-JNK1/2 signaling to develop therapeutic strategies in the treatment of metabolic disorder.
Syed, Ismail, "Mechanisms of regulation of islet function by nadph oxidase" (2011). Wayne State University Dissertations. 332.