The Role of Arachidonic Acid - Cyclooxygenase Pathway in the Anti-leukemic Properties of Selenium
Open Access
- Author:
- Gandhi, Ujjawal Hitendra
- Graduate Program:
- Molecular Toxicology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 29, 2012
- Committee Members:
- Kumble Sandeep Prabhu, Dissertation Advisor/Co-Advisor
Robert Paulson, Committee Member
Dr Jack Vanden Heuvel, Committee Member
Shantu G Amin, Committee Member
Dr Ramesh Ramachandran, Committee Member - Keywords:
- Selenium
leukemia
15d-prostaglandin J2
gambogic acid
arachidonic acid
inflammation
leukemia stem cell
BCR-ABL
H-PGDS - Abstract:
- The trace mineral selenium (Se) is an essential micronutrient of fundamental importance to human biology. Findings are unequivocal in linking the deficiency of selenium to conditions with increased oxidative stress and inflammation. Epidemiological studies suggest that supranutritional intake of selenium may be associated with a reduced risk of cancers. Many of these beneficial effects of selenium are thought to be due to the action of selenoproteins, which collectively form a part of the cellular antioxidant defense system. But the exact molecular mechanisms involved in these actions are still to be defined. The primary objective of this study is to understand the mechanisms associated with the anti-inflammatory and anti-carcinogenic effects of selenium, which would form the basis for exploring the role of supplemental selenium as a potential therapeutic agent. Using a combination of in vitro and ex vivo models, the anti-inflammatory action of selenium was dissected with a focus on the regulation of the arachidonic acid (AA) - cyclooxygenase (COX) metabolism pathway. Supplementation of macrophages with supra-physiological levels of selenium showed that the AA was preferentially metabolized to electrophilic metabolites of prostaglandin D2 (PGD2). These include Δ12-PGJ2 and 15d-PGJ2, which are structurally classified as cyclopentenone PGs (CyPGs). Interestingly, these lipid metabolites impacted the expression of downstream PG synthases via the differential modulation of two transcription factors, nuclear factor-kappa B (NF-κB) and peroxisome proliferator-activated receptor γ (PPARγ) to activate the expression of hematopoietic PGD2 synthase (H-PGDS), and suppress that of microsomal PGE synthase-1 (mPGES-1). As a result, LPS-stimulation of Se-supplemented macrophages produced high levels of CyPGs when compared to the Se-deficient macrophages, while the levels of pro-inflammatory PGE2 were suppressed. To relate these effects in vivo, well-established models of murine leukemia were used, where the infection of progenitor cells with Friend erythroleukemia virus (FV), or expression of a fusion oncogene, BCR-ABL, in the hematopoietic stem cells (HSCs) led to the development of the disease through the formation of leukemic stem cells (LSCs). Se-supplemented mice were completely resistant to the disease, while Se-deficient and Se-adequate mice succumbed to leukemia. Treatment of Se-supplemented mice with a COX inhibitor, indomethacin, blunted the protective effect of selenium, while the administration of 15d-PGJ2 to these mice rescued them. Further analysis showed that supplementation with selenium selectively targeted the LSCs for apoptosis via p53 pathway activation, without any effects on normal HSCs. In a related study to examine the role of electrophilic compounds of natural origin, a botanical with anti-inflammatory properties, gambogic acid (GA), was demonstrated to inhibit NF-κB activation by forming covalent Michael adduct with 179Cys in a key enzyme, inhibitor of kappa B kinase β (IKKβ). This is similar to that previously reported with the endogenously produced 15d-PGJ2. Taken together, the studies included in this thesis suggest that anti-oxidant and anti-inflammatory compounds can activate pathways of resolution by shunting AA-COX metabolism towards increased production of CyPGs, which can potentially affect the survival and/or proliferation of LSCs.