The Bisphenol A analogue Bisphenol S binds to K-Ras4B – implications for ‘BPA-free’ plastics
Miriam Schöpel1, Christian Herrmann1, Jürgen Scherkenbeck2 and Raphael Stoll1,*
1 Faculty of Chemistry and Biochemistry, Ruhr University of Bochum, Germany
2 Faculty of Chemistry, University of Wuppertal, Germany
K-Ras4B is a small GTPase that belongs to the Ras superfamily of guanine nucleotide-binding proteins. GTPases function as molecular switches in cells and are key players in intracellular signalling. Ras has been identified as an oncogene and is mutated in more than 20% of human cancers. Here, we report that Bisphenol S binds into a binding pocket of K-Ras4B previously identified for various low molecular weight compounds. Our results advocate for more comprehensive safety studies on the toxicity of Bisphenol S, as it is frequently used for Bisphenol A-free food containers.
Proper cell signaling is a fundamental for the correct respond of cells to their external microenvironment. Small GTPases like Ras and Ras homologue enriched in the brain (Rheb) are members of Ras superfamily of guanine nucleotide-binding proteins. Ras is as known proto-oncogene that carries mutations in more than 20% of human cancers, such as pancreatic, colon, and lung carcinomas. The ability of these enzymes to hydrolyse guanosine triphosphate (GTP) to guanosine diphosphate (GDP) is often regarded as their physiological hallmark as small GTPase switch between an inactive, GDP-bound state and an active, GTP-bound state (Fig. 1). This enables small GTPases to function as molecular switches in living cells. Thus, these proteins are regarded as key players of the intracellular signaling cascade and – not surprinsingly – have attracted widespread attention in cancer drug development initiatives. The activation/inactivation GTP/GDP cycle of all Ras-like G-proteins is not only negatively regulated by GTPase activating proteins (GAPs) but also positively influenced by guanine nucleotide exchange factors (GEFs). GEFs interact directly with G-proteins and lower the affinity of these G-proteins for its bound nucleotide (Fig. 1). For example, the son of sevenless (Sos) protein serves as a GEF as it catalyses the rate-limiting step of restoring the level of activated, GTP-bound K-Ras4B in the cell, an important protein isoform of the ras gene family members.
In an earlier study  we designed a small fragment library that contained not more than 100 compounds, which was screened by multidimensional NMR spectroscopy to identify structures that bind to the GTPases Rheb and K-Ras. We were indeed able to identify 4,4’-biphenol and bisphenol A as novel small molecular weight compounds for Rheb and K-Ras, respectively. Our study revealed for the first time that the plasticiser bisphenol A is a K-Ras ligand, suggesting an entirely new mode of action for this compound. The family of bisphenols is chemically characterised by an optionally substituted central carbon atom that is substituted by two hydroxyphenyl moieties. The most common member of this family Bisphenol A (BPA, 4,4′-(propane-2,2-diyl)diphenol, CAS 80-05-7), whose central carbon carries two methyl and two phenolic groups, is a topic in heated scientific and public discussion as it is a well-known endocrine disrupting chemical (EDC) that can unsettle the normal activity of hormone receptors (Fig. 2). Bisphenol A is one of the most frequent chemicals humans are exposed to everyday. For instance, it is a building block of polycarbonate plastics and hence is present in many household products, such as plastic food containers and eating utensils, baby bottles, and thermal papers like cashier receipts. It has been suggested that bisphenol A might cause – amongst others – cardiovascular diseases, breast and prostate cancers as well as neuronal disorders. Our results support a new and/or additional mode of action for bisphenol A – its binding to small H- and K-Ras GTPases – that, in turn, advocate a reduced exposure to this compound in our environment. Lately, public pressure and new governmental restrictions led producers to replace BPA by its chemical analogue BPS (Fig. 2). Based on these results we set out to examine as to whether BPS (4,4′-Sulphonyldiphenol, CAS 80-09-1) can also bind to small Ras-like . Unlike BPA, BPS contains a central sulphonyl group instead of a branched sp3-hybridised carbon atom and serves as a developer in thermal paper like in cashier receipts or boarding passes, for instance. It has been suggested that BPS could show a similar or at least comparable physiological effect by functioning as an EDC.
The publication highlighted here reports for the first time that BPS also binds to the small GTPase K-Ras4B (Fig. 2). As previously found for BPA, BPS binds to a small ligand binding site between switch I and switch II of K-Ras4B, which is close to helix α2 and to the core β-sheet (β1–β3). Thus, we could show that BPS is yet another ligand of K-Ras4B, albeit with a lower binding affinity than that of BPA. Furthermore, our new study shows that – unlike BPA – BPS cannot interfere with the Sos-meditated nucleotide exchange of K-Ras4B. Taken together, our findings decipher the binding mode of BPA and BPS to Ras GTPases and provide a molecular basis for further and more comprehensive toxicity studies on bisphenols in the future, since it is a frequently used replacement for BPA in ‘BPA-free’-labeled containers.
- Schöpel M, Herrmann C, Scherkenbeck J, Stoll R 2016 The Bisphenol A analogue Bisphenol S binds to K-Ras4B – implications for ‘BPA-free’ plastics. FEBS Lett. 590:369-75.
- Schöpel M, Jockers KF, Düppe PM, Autzen J, Potheraveedu VN, Ince S, Yip KT, Heumann R, Herrmann C, Scherkenbeck J, Stoll R 2013 Bisphenol A binds to Ras proteins and competes with guanine nucleotide exchange: implications for GTPase-selective antagonists. J Med Chem. 56:9664-72.
Acknowledgements: This study has been supported by the Deutsche Krebshilfe (109776 and 109777), the DFG (SFB 642), and the RUB Research SchoolPlus.