Hsp72 Is an Intracellular Target of the α,β-Unsaturated Sesquiterpene Lactone, Parthenolide

The electrophilic natural product parthenolide has generated significant interest as a model for potential chemotherapeutics. Similar to other α,β-unsaturated carbonyl electrophiles, parthenolide induces the heat shock response in leukemia cells, potentially through covalent adduction of heat shock proteins. Other thiol-reactive electrophiles have also been shown to induce the heat shock response as well as to covalently adduct members of the heat shock protein family, such as heat shock protein 72 (Hsp72). To identify sites of modification of Hsp72 by parthenolide, we used high-resolution tandem mass spectrometry to detect 10 lysine, histidine, and cysteine residues of recombinant Hsp72 as modified in vitro by 10 and 100 μM parthenolide. To further ascertain that modification of Hsp72 by parthenolide occurs inside cells and not simply as an in vitro artifact, an alkyne-labeled derivative of parthenolide was synthesized to enable enrichment and detection of protein targets of parthenolide using copper-catalyzed [3 + 2] azide-alkyne cycloaddition. The alkyne-labeled parthenolide derivative displays an half maximal inhibitory concentration (IC₅₀) in undifferentiated acute monocytic leukemia cells (THP-1) of 13.1 ± 1.1 μM, whereas parthenolide has an IC₅₀ of 4.7 ± 1.1 μM. Concentration dependence of protein modification by the alkyne-parthenolide derivative was demonstrated, as well as in vitro adduction of Hsp72. Following treatment of THP-1 cells in culture by the alkyne-parthenolide, adducted proteins were isolated with neutravidin resin and detected by immunoblotting in the enriched protein fraction. Hsp70 proteins were detected in the enriched proteins, indicating that Hsp70 proteins were adducted intracellularly by the alkyne-parthenolide derivative.