br Conflict of interest br Acknowledgements
Conflict of interest
Acknowledgements We thank Ann Jackman, Mike Ormerod and members of Julian Downward's and Alan Ashworth's groups for helpful discussions. This work was supported by grant number PEM/GME/D391/40 from the Kidani Trust (S. Kaye) and by Keele University.
Introduction Autotaxin (ATX) (EC number: 220.127.116.11), encoded by the ENPP2 gene, is a secreted glycoprotein and the only member of the ectonucleotide pyrophosphatase/phosphodiesterase family (ENPP) that has lysophopholipase D (lysoPLD) activity (Umezu-Goto et al., 2002). ATX hydrolyses lysophosphatidylcholine (LPC) to produce lysophosphatidic daunorubicin (LPA) and choline. LPA is a bioactive lipid, which binds and signals mainly through specific G-protein-coupled lysophosphatidic acid receptors 1 to 6 (LPA1–6) (Kihara et al., 2014). In human, ATX is ubiquitously expressed and is also present in blood (Aoki et al., 2002). For specific spatiotemporal signaling, ATX is recruited on the cell surface by activated β3-integrins (Leblanc et al., 2014, Pamuklar et al., 2009). Upon binding via its somatomedin B-like (SMB) domains to integrins (Hausmann et al., 2011), ATX is tethered to plasma membrane, promoting thereby allocated LPA-signaling. The localized increase in LPA concentration results in specific biological response, as LPA stimulates cell migration, proliferation and survival (Moolenaar et al., 2004), induces platelet aggregation and chemotaxis (Jalink et al., 1993), smooth muscle contraction (Tokumura et al., 1994), neurite remodeling (Fukushima et al., 2002) and ion channel activity (Iftinca et al., 2007). Therefore, ATX contributes to various physiological and pathophysiological processes, such as embryonic development (Moolenaar et al., 2013), wound healing (Lee et al., 2013), inflammation (Knowlden and Georas, 2014), vascular (van Meeteren et al., 2006) and neural development (Fotopoulou et al., 2010), and in tumor growth, metastasis (Leblanc and Peyruchaud, 2015) and chemoresistance (Brindley et al., 2013). Consequently, ATX is a potential therapeutic target in various diseases, including cancer, as the ATX-LPA signaling axis plays a role in various tumor types (Houben and Moolenaar, 2011, Liu et al., 2009). Noteworthy, the ENPP2 gene itself, is amplified in tumors of the breast (8.2%), the ovary (6.9%), the eye (6.3%) and the liver (4.6%) (Fig. 1) (COSMIC, 2016). In addition, genetic silencing of ATX and LPA receptors expression in mouse models reveals the importance of ATX-LPA axis in cancer development (Liu et al., 2009). Being an extracellular enzyme, ATX represents an attractive drug target and there is potential for designing novel small-molecule inhibitors targeting this enzyme. Moreover, pharmacologic inhibition of ATX is well tolerated, at least in adult mice (Katsifa et al., 2015). To date, efforts to target ATX have disclosed small-molecule inhibitors (reviewed in ref. Castagna et al., 2016), as well as an aptamer (Kato et al., 2016). The field is still relatively young, as so far only one ATX inhibitor, GLPG1690 (Fig. 2), has been reported to be in clinical trials (Galapagos NV, 2000) and is currently on phase II for patients with idiopathic pulmonary fibrosis. In general, most of the reported ATX inhibitors display similar chemotype, consisting of three elements: an acidic moiety, a spacer core and a lipophilic tail (Castagna et al., 2016). The acidic headgroup binds to Zn-ions in the active site and the core spacer guides the hydrophobic tail to the lipophilic pocket. However, a few inhibitors have been discovered which do not follow this typical pattern. These inhibitors were identified to target only the lipophilic pocket or the tunnel, excluding the active site (Fells et al., 2013, Stein et al., 2015). With this type of inhibitors, Stein et al. demonstrated inhibitory action only towards ATX lysoPLD activity; moreover, Fells et al. demonstrated that their inhibitors reduced invasion and metastasis in vitro, truly disclosing the potential for inhibitors blocking only the pocket site. Moreover, Shah et al. published imidazo[4,5-c]pyridines series which bind to this site as well (Shah et al., 2016). Most recently, a SAR study identified an inhibitor which binds to the tunnel site (Miller et al., 2017), and benzene-sulfonamide derivatives were reported to reduce melanoma metastasis in vivo (Banerjee et al., 2017). Overall, this type of ATX inhibition, excluding the active-site binding which contains two Zn-ions, will lower the potential risk in off-target metalloprotein binding; for instance, similar tactics has been utilized in selective matrix-metalloproteinase inhibitor design (Jacobsen et al., 2010).