Date of Graduation

2003

Document Type

Dissertation/Thesis

Abstract

Reports that estrogen (E2) treatment modulates arachidonic acid (AA) metabolism by bone and bone cells are found in the literature. However, conflicting indications of the relationship that exists between E2 and AA metabolism emerge from the analysis of those reports. The present studies were undertaken to determine if E2 effected the production of prostaglandins (PG) in human osteoblast-like cells (hOB), and if this effect is due to the regulation of key enzymes involved in PG production, namely cytosolic phospholipase A2 (cPLA2) and the cyclooxygenases (COX-1, COX-2). We determined that hOB cells pretreated with E2 and co-stimulated with tumor growth factor-β (TGFβ) and tumor necrosis factor-α (TNFα) showed a significant increase in PG production as compared to cytokine stimulation alone, particularly in the generation of prostaglandin E2 (PGE2). An E2-dependent increase in the activity of cPLA2 was also seen. E2 did not, however, effect the activity of cyclooxygenase. We further examined the role E2 might have in regulating the expression of cPLA2, COX-1, and COX-2 mRNA and/or protein levels in hOB cells. hOB cells pretreated with E2 followed by co-stimulation with TGFβ and TNFα showed an increase in cPLA2 protein levels as compared to cytokine stimulation alone. However, cPLA2 mRNA, COX-1 mRNA, and COX-2 mRNA and protein levels were not effected by E2 stimulation. Regulation of cPLA2 activity often involves the phosphorylation of key amino-acid residues. Therefore, we stimulated hOB cells with E2 and analyzed immunoprecipitated 32P-labelled cPLA2 for changes in protein phosphorylation. E2 treated hOB cells showed an increase in overall cPLA2 phosphorylation, as compared to vehicle treatment. Taken as a whole, the increase in cPLA2 phosphorylation could explain the increase in cPLA2 activity seen in hOB cells stimulated with E2. The increase in cPLA2 protein observed in hOB cells pretreated with E2 and co-treated with TGFβ and TNFα might represent a mechanism for replacing depleted pools of cPLA2 protein in the proper physiological environment, as might be the case in bone remodeling induced local inflammation.

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