Access Type

Open Access Dissertation

Date of Award

January 2013

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Pharmaceutical Sciences

First Advisor

Anjaneyulu Kowluru

Abstract

MECHANISMS OF CYTOKINE-INDUCED METABOLIC DYSFUNCTION OF THE PANCREATIC BETA-CELL

by

ABIY MUSSA MOHAMMED

August 2013

Advisor: Dr. Anjaneyulu Kowluru

Major: Pharmaceutical Sciences

Degree: Doctor of Philosophy

Type I diabetes is characterized by an absolute insulin deficiency due to loss of pancreatic â-cell mass by autoimmune aggression. During the progression of the disease proinflammatory cytokines such as IL-1â, TNFá and INFã are secreted by infiltrated and activated T-cells and macrophages which ultimately damage the pancreatic â-cell. However, the signaling mechanisms involved in cytokine-induced damage are only partially understood. Phagocyte-like NADPH oxidase [NOX2] has been shown to play regulatory roles in the metabolic dysfunction of the islet â-cell under the duress of glucolipotoxic conditions and exposure to proinflammatory cytokines. However, the precise mechanisms underlying NOX2 activation by these stimuli remain less understood. Herein, I determined some of the putative cellular mechanisms underlying proinflammatory cytokine-induced metabolic dysfunction and demise of the islet â-cell. Some of the novel findings of my study are: [i] cytokines induce ROS generation and oxidative stress via activation of phagocyte-like NADPH-oxidase [NOX2] such effects are comprised of Rac1 activation, p47phox phosphorylation, and gp91phox expression. I further confirmed that NOX2 is one of the sources for ROS generation under proinflammatory cytokines and glucotoxic conditions as demonstrated by activation of NOX2 activity which is sensitive to apocynin under those conditions; [ii] 2-Bromopalmitate, a classic inhibitor of protein palmitoylation, markedly attenuated cytokine-induced Rac1 activation, NOX2-mediated reactive oxygen species generation and inducible nitric oxide synthase-mediated nitric oxide release indicating that palmitoylation of specific G-proteins [e.g., H-Ras and Rac1] is a key regulatory step involved in cytokine-induced nitrosative and oxidative stress.In addition to oxidative and nitrosative stress, the effect of cytokines in other stress related signaling pathways were also examined. Cytokines activated JNK1/2 and p38 MAPK kinases. They also increased CHOP [C/EBP homologous protein] expression, a marker for endoplasmic reticulum stress, caused caspase-3 activation and FTase and GGTase degradation which leads to defective activation of key G-proteins, defective nuclear membrane assembly and loss in cell viability. Pharmacological inhibitors such as 2-bromopalmitate [inhibitor of palmitoylation], EHop-016 [inhibitor of Vav2-Rac1 axis] and NSC23766 [inhibitor of Tiam1-Rac1 axis] attenuated cytokine-induced JNK1/2 activation implying that Rac1 is upstream to cytokine-induced JNK1/2 activation. Based on the results obtained from my studies, I propose that protein palmitoyl transferase is a novel therapeutic target for the prevention of cytokine-induced metabolic dysfunction of the islet â-cell. As a logical extension of the in vitro studies, a preliminary work has been done in the NOD [non obese diabetic] mice, an animal model of Type I diabetes, and the results showed that both the islets of NOD and control mice express the NOX2 subunits namely, p47phox, p67phox, Rac1 and gp91phox.

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