Cyclic guanosine 3 0 ,5 0-monophosphate (cGMP) is an intracellular signalling molecule involved in many sensory and developmental processes. Synthesis of cGMP from GTP is catalysed by guanylate cyclase (GC) in a reaction analogous to cAMP formation by adenylate cyclase (AC). Although detailed structural information is available on the catalytic region of nucleotidyl cyclases (NCs) in various states, these atomic models do not provide a sufficient explanation for the substrate selectivity between GC and AC family members. Detailed structural information on the GC domain in its active conformation is largely missing, and no crystal structure of a GTP-bound wild-type GC domain has been published to date. Here, we describe the crystal structure of the catalytic domain of rhodopsin-GC (RhGC) from Catenaria anguillulae in complex with GTP at 1.7 A resolution. Our study reveals the organization of a eukaryotic GC domain in its active conformation. We observe that the binding mode of the substrate GTP is similar to that of AC-ATP interaction, although surprisingly not all of the interactions predicted to be responsible for base recognition are present. The structure provides insights into potential mechanisms of substrate discrimination and activity regulation that may be common to all class III purine NCs. Database Structural data are available in Protein Data Bank database under the accession number 6SIR. Enzymes EC 4.6.1.2. Abbreviations AC, adenylate cyclase; ATP, adenosine-5 0-triphosphate; cAMP, cyclic 3 0 ,5 0-adenosine monophosphate; cGMP, cyclic 3 0 ,5 0-guanosine monophosphate; GC, guanylate cyclase; GTP, guanosine-5 0-triphosphate; NC, nucleotidyl cyclase; sGC, soluble guanylate cyclase.
Microtubules have been an attractive target of cancer drug discovery due to their highly dynamic nature during mitosis. Griseofulvin, a natural antifungal compound, is known to interfere with microtubule dynamics. In the present study, we prepared and analyzed twenty‐seven novel griseofulvin derivatives. Three of these compounds had GI50 values <10 μM (5.74 to 9.7 μM) in breast cancer cell line CAL‐51. The most promising compound ((2S,6’R)‐4’‐(benzhydrylamino)‐7‐chloro‐4,6‐dimethoxy‐6’‐methyl‐3H‐spiro[benzofuran‐2,1’‐cyclohexan]‐3’‐ene‐2’,3‐dione), was characterized as a microtubule‐stabilizing agent with a GI50 value of 5.74±1.43 μM compared to 10.79±3.06 μM GI50 for parental griseofulvin. It also inhibited the proliferation of other cancer cell lines, including KB‐3‐1 and HCT116, with GI50 values of 1.19±0.34 μM and 2.48±0.40 μM, respectively. Treatment of cancer cells with it resulted in aberrant mitosis causing G2/M arrest. Finally, we show that this compound increased the expression of p53 protein and induced apoptotic cell death.
Aurora kinases (Aurora A, B, and C) are a family of serine/threonine kinases that play critical roles during mitotic initiation and progression. Aurora A and B kinases are ubiquitously expressed, and their overexpression and/or amplification in many cancers have been associated with poor prognosis. Several inhibitors that target Aurora kinases A, B, or both have been developed during the past decade with efficacy in different in vitro and in vivo models for a variety of cancers. Recent studies have also identified Aurora A as a synthetic lethal target for different tumor suppressors, including RB1, SMARCA4, and ARID1A, which signifies the need for Aurora-A-selective inhibitors. Here, we report the screening of a small library of quinones (nine naphthoquinones, one orthoquinone, and one anthraquinone) in a biochemical assay for Aurora A kinase that resulted in the identification of several quinones as inhibitors. IC 50 determination against Aurora A and B kinases revealed the inhibition of both kinases with selectivity toward Aurora A. Two of the compounds, natural quinone naphthazarin (1) and a pseudo anthraquinone, 2-(chloromethyl)quinizarin ( 11), potently inhibited the proliferation of various cancer cell lines with IC 50 values ranging from 0.16 ± 0.15 to 1.7 ± 0.06 and 0.15 ± 0.04 to 6.3 ± 1.8 μM, respectively. Treatment of cancer cells with these compounds for 24 h resulted in abrogated mitosis and apoptotic cell death. Direct binding of both the compounds with Aurora A kinase was also confirmed through STD NMR analysis. Docking studies predicted the binding of both compounds to the ATP binding pocket of Aurora A kinase. We have, therefore, identified quinones as Aurora kinase inhibitors that can serve as a lead for future drug discovery endeavors.
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