Searching for the protein targets of 4-nonylphenol using a computational approach.

Introduction 4-Nonylphenol (4-NP) is an environmental contaminant originating from the degradation of alkyl phenol ethoxylates, which are common surfactants employed in many industrial applications. This chemically stable molecule is widely diffused and, due to its hydrophobic feature, it can easily accumulate into living organisms, including humans, where it can reach concentrations up to 1000-fold higher than those found in the environment [1]. 4-NP is recognized as a common endocrine-disrupter [2], causing alteration of the reproductive biology and development abnormalities in many species [3, 4]. However, the molecular mechanisms of other 4-NP multiple negative effects have not been fully explored. Therefore, we applied a computational approach in order to find possible unknown protein targets to which 4-NP can bind, exerting a toxic effect in the human organism. Methods We applied a reverse docking approach using the Web server idTarget (http://idtarget.rcas.edu.tw) [5] in order to identify possible targets for the binding of 4-NP among those proteins whose structure is deposited in PDB database. The binding of 4-NP to proteins selected among those identified in the previous step was studied with a direct docking approach using AutoDock 4.2 [6]. Results Among the first 100 results obtained by idTarget with the lower predicted G, we selected for further analyses those human proteins, or proteins homologous of human proteins with known structures, with a predicted energy lower than -7.00 kcal/mol. In addition, further 8 proteins with a negative Z-score but with a higher energy, were taken into account. Among these selected targets, the estrogen receptor was present as expected, but a striking abundance of matrix metalloproteinases (MMP) was also found, with 5 proteins of these families among the first 30, and other 3 targets in the rest of the list. In addition, another putative target found for 4-NP binding is fatty acid binding protein (FABP). The binding of 4-NP to these proteins was simulated with a direct docking approach, and it was found that this pollutant can bind to these pro- teins with relatively high affinity. This interaction can be involved in other toxic effects of this molecule, since these proteins are involved in inflammatory processes and migration of leucocytes, and their alteration can play a role e.g. in tumor diffusion. Moreover, the binding of 4-NP to FABP can explain its accumulation in lipids and the risk of hepatic steatosis in people exposed to this pollutant. Further studies investigating the involvement of these proteins in metabolic and pathogenic pathways will clarify the role of these putative targets in contributing to 4-NP toxic effects. References [1] Soares A. et al., Environ Int, 34(7), 1033–1049, 2008 [2] Shanle. E. K. and Xu W., Chem Res Toxicol, 24(1), 6–19, 2011 [3] Capaldo A. et al, Comp Biochem Physiol C Toxicol Pharmacol, 155(2), 352–358, 2012 [4] Laws S. C. et al., Toxicol Sci, 54(1), 154–167, 2000 [5] Wang J. C. et al., Nucleic Acids Res, 40 (Web Server issue), W393–W399, 2012 [6] Morris G. M. et al., AutoDock4 and AutoDockTools4: Automated docking with selective receptor flexibility, J. Comput. Chem., 30, 2785–2791, 2009