Probing the Role of Sulfenylation in Redox Regulation of Protein Kinases.

Abstract

Hydrogen peroxide (H2O2) functions as a secondary messenger to regulate intracellular signaling cascades, largely through the modification of specific cysteine residues within redox-sensitive targets such as protein kinases. Oxidation can influence kinase activity in physiology and pathology, but the molecular mechanisms underlying these events remain largely unknown. In the present study, we present the development and application of chemical tools to detect and quantify changes in sulfenic acid formation (sulfenylation). These tools were used to demonstrate epidermal growth factor (EGF)-mediated signaling induces global and dynamic changes in protein sulfenylation. Three protein tyrosine phosphatases (PTPs) were shown to undergo EGF-dependent oxidation and exhibit a unique sulfenylation profile in cells, suggesting the extent of sulfenylation may be related to differences in subcellular locations of target proteins. In addition, epidermal growth factor receptor (EGFR) was identified as a direct target of signal-derived H2O2 at its active site cysteine (Cys797). Sulfenylation of EGFR Cys797 enhances its intrinsic tyrosine kinase activity, and subsequent work has demonstrated mutation of this residue abrogates EGFR autophosphorylation, sulfenylation, and decreases its affinity for ATP. These results highlight the importance of Cys797 with respect to EGFR function during normal cell signaling. Nine additional kinases harbor a cysteine residue that is structurally homologous to Cys797, and we present evidence that these kinases may be similarly regulated by oxidation of this residue. EGFR is mutated or amplified in a number of human carcinomas, and the proximity of Cys797 in the kinase domain potentially renders it more sensitive to oxidation in oncogenic mutants as shown by our data. Interestingly, EGFR Cys797 serves as a therapeutic target for covalent inhibitors that react with the reduced residue through electrophilic Michael addition. Oxidation of Cys797 alters the potency of current EGFR inhibitors that cannot recognize the kinase in its sulfenyl form. We report the development of a panel of first-generation nucleophilic RSOH-targeted inhibitors and evaluate compound potency towards EGFR Cys797 in response to concomitant oxidation. Collectively, the results of this thesis provide novel insight into how oxidative modification of kinases affects signal transduction pathways and have broad implications for therapeutics in disease states.