Abstract 4760: Inhibition of c-myc by the triterpenoid celastrol

Abstract

Abstract c-Myc is a bHLH-ZIP transcription factor that is deregulated in a variety of human cancers. More specifically, c-Myc heterodimerizes with another bHLH-ZIP partner protein, Max, to bind to specific sequences (E-Boxes: CAC/TGTG) located near the transcriptional start sites of its numerous target genes. Because c-Myc's highly variable downstream effects are difficult to attack individually, directly inhibiting the association between c-Myc and Max association by interfering with their bHLH-ZIP-mediated heterodimerization has emerged as an attractive therapeutic strategy. Thus far, high throughput screens of chemical libraries have identified a number of small molecules that specifically inhibit the c-Myc-Max association. However, these synthetic compounds and their analogs are generally of low potency and therefore have limited clinical utility. Here we report identification of celastrol, a naturally occurring triterpenoid, as a potent c-Myc inhibitor. We sought to further assess celastrol's ability to inhibit c-Myc-Max interaction and cell growth, and determine its structure activity relationship. Surface plasmon resonance and electrophoretic mobility shift studies revealed that celastrol specifically binds to c-Myc and prevents both its interaction with Max, and subsequent DNA binding with a Kd in the low micromolar range. Co-immunoprecipitation studies confirmed that the dose-dependent disruption of c-Myc-Max heterodimers by celastrol could also be achieved in c-Myc-overexpressing human HL60 promyelocytic leukemia cells. Moreover, in vitro data demonstrated that celastrol significantly inhibited both HL60 and Burkitt's lymphoma cell growth with IC50s in the nanomolar range. However, modifications to the C-28 carboxylic acid group of celastrol eliminated c-Myc binding, suggesting that celastrol binds to c-Myc via this functional group. This structure activity relationship may provide a basis for the development of more pharmacologically suitable analogs. Together these results suggest that celastrol is among the most potent Myc-Max disruptors yet identified, and suggest a mechanism of action. Celastrol thus represents a promising new agent with which to target c-Myc and provides a new structural chemical platform for further pharmaceutical development. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4760. doi:1538-7445.AM2012-4760