Inactivation of tumor suppressors LKB1 and PTEN by arachidonic acid metabolites

This dissertation describes the modification and inactivation of tumor suppressors by arachidonic acid (AA) metabolites. Cyclooxygenase (COX) and lipoxygenase (LOX) enzymes are AA metabolizing enzymes involved in inflammation as well as cancer. In this work, a novel mechanism linking inflammation and cancer is presented. LKB1 is a serine/threonine kinase that activates AMP-Kinase, thereby regulating anabolic and catabolic processes in the cell. LKB1 has a nucleophilic cysteine in its activation loop. Chapter 2 demonstrates the covalent modification of LKB1 by clectrophilic, cyclopcntcnone prostaglandins. Formation of this covalent adduct in the activation loop inhibits the kinase activity of LKB1 and prevents the phosphorylation and activation of its substrate AMPK. This affects downstream signaling to important enzymes such as Aeetyl-CoA Carboxylase and S6-Kinase, preventing an appropriate response to low energy levels. PTEN is a phosphoinositol phosphatase and tumor suppressor involved in regulating the PI3K-Akt pathway. Chapter 3 illustrates the oxidation of PTEN by reactive oxygen lipid species generated during AA metabolism by COX-2 and 5-LOX. Oxidation of PTEN decreases its phosphatase activity, favoring increased PIP3 production, activation of Akt, and phosphorylation of downstream Akt targets including GSK-3[3 and S6K. These effects are reeapitulated with pancreatic PLA2, which hydrolyzes the release of membrane-bound AA. Chapter 4 describes the covalent modification and inactivation of PTEN by cyclopentenone prostaglandins. Chapter 3 and 4 illustrate that covalent modification and oxidation of tumor suppressors during AA metabolism are not exclusive, but may both occur and contribute tumor suppressor inactivation. In summary, this work investigates two novel mechanisms involving the chemical inactivation of tumor suppressors at the protein level. Interference with the physiological roles of tumor suppressors may confer risk for hypertrophic or neoplastic diseases associated with chronic inflammation or unwarranted oxidative metabolism of essential fatty acids. These events likely occur as the results of constant exposure to eicosanoid metabolites and byproducts during chronic inflammation or during COX/LOX overexpression, giving rise to decreased sensitivity to anti-growth signals and the promotion of tumorigenesis.