The anticancer natural product ophiobolin A induces cytotoxicity by covalent modification of phosphatidylethanolamine.

Christopher Chidley,Sunia A Trauger,Erin K O'Shea,Kıvanç Birsoy

doi:10.7554/elife.14601

Abstract

Phenotypic screens allow the identification of small molecules with promising anticancer activity, but the difficulty in characterizing the mechanism of action of these compounds in human cells often undermines their value as drug leads. Here, we used a loss-of-function genetic screen in human haploid KBM7 cells to discover the mechanism of action of the anticancer natural product ophiobolin A (OPA). We found that genetic inactivation of de novo synthesis of phosphatidylethanolamine (PE) mitigates OPA cytotoxicity by reducing cellular PE levels. OPA reacts with the ethanolamine head group of PE in human cells to form pyrrole-containing covalent cytotoxic adducts and these adducts lead to lipid bilayer destabilization. Our characterization of this unusual cytotoxicity mechanism, made possible by unbiased genetic screening in human cells, suggests that the selective antitumor activity displayed by OPA may be due to altered membrane PE levels in cancer cells.

Highlights

Natural products are an important source for the development of pharmaceutical drugs, especially in oncology; half of all anticancer drugs developed since the 1940s are natural products or derivatives of natural products (Newman and Cragg, 2012)
We discovered that inactivation of the pathway for de novo synthesis of phosphatidylethanolamine (PE), named the Kennedy pathway, confers resistance to ophiobolin A (OPA)
Our work illustrates the utility of loss-of-function screens in human cells to identify genes involved in the mechanism of action (MOA) of promising anticancer compounds such as OPA

Summary

Introduction

Natural products are an important source for the development of pharmaceutical drugs, especially in oncology; half of all anticancer drugs developed since the 1940s are natural products or derivatives of natural products (Newman and Cragg, 2012). As unraveling the MOA of bioactive small molecules remains challenging and time consuming (Schenone et al, 2013; Ziegler et al, 2013), the MOA of many natural products that display promising anticancer activities in phenotypic screens remains uncharacterized (Shoemaker, 2006). An example of such a natural product is ophiobolin A (OPA), a plant toxin isolated from pathogenic fungi of the Bipolaris genus which displays cytotoxicity at nanomolar concentrations against a range of cancer cell lines (Au et al, 2000; Bury et al, 2013). The toxicity of OPA to plants is believed to involve calmodulin inhibition via formation of a covalent adduct between OPA and specific lysine side

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