Histone acetyltransferases:challenges in targeting bi-substrate enzymes

Histone acetyltransferases (HATs) are epigenetic enzymes that install acetyl groups onto lysine residues of cellular proteins such as histones, transcription factors, nuclear receptors, and enzymes. HATs have been shown to play a role in diseases ranging from cancer and inflammatory diseases to neurological disorders, both through acetylations of histone proteins and non-histone proteins. Several HAT inhibitors, like bi-substrate inhibitors, natural product derivatives, small molecules, and protein-protein interaction inhibitors, have been developed. Despite their potential, a large gap remains between the biological activity of inhibitors in in vitro studies and their potential use as therapeutic agents. To bridge this gap, new potent HAT inhibitors with improved properties need to be developed. However, several challenges have been encountered in the investigation of HATs and HAT inhibitors that hinder the development of new HAT inhibitors. HATs have been shown to function in complexes consisting of many proteins. These complexes play a role in the activity and target specificity of HATs, which limits the translation of in vitro to in vivo experiments. The current HAT inhibitors suffer from undesired properties like anti-oxidant activity, reactivity, instability, low potency, or lack of selectivity between HAT subtypes and other enzymes. A characteristic feature of HATs is that they are bi-substrate enzymes that catalyze reactions between two substrates: the cofactor acetyl coenzyme A (Ac-CoA) and a lysine-containing substrate. This has important-but frequently overlooked-consequences for the determination of the inhibitory potency of small molecule HAT inhibitors and the reproducibility of enzyme inhibition experiments. We envision that a careful characterization of molecular aspects of HATs and HAT inhibitors, such as the HAT catalytic mechanism and the enzyme kinetics of small molecule HAT inhibitors, will greatly improve the development of potent and selective HAT inhibitors and provide validated starting points for further development towards therapeutic agents.</p

DNMT3B PWWP mutations cause hypermethylation of heterochromatin

The correct establishment of DNA methylation patterns is vital for mammalian development and is achieved by the de novo DNA methyltransferases DNMT3A and DNMT3B. DNMT3B localises to H3K36me3 at actively transcribing gene bodies via its PWWP domain. It also functions at heterochromatin through an unknown recruitment mechanism. Here we find that knockout of DNMT3B causes loss of methylation predominantly at H3K9me3-marked heterochromatin and that DNMT3B PWWP domain mutations or deletion result in striking increases of methylation in H3K9me3-marked heterochromatin. Removal of the N-terminal region of DNMT3B affects its ability to methylate H3K9me3-marked regions. This region of DNMT3B directly interacts with HP1 and facilitates the bridging of DNMT3B with H3K9me3-marked nucleosomes in vitro. Our results suggest that DNMT3B is recruited to H3K9me3 marked heterochromatin in a PWWP-independent mannerthat is facilitated by the protein’s N-terminal region through an interaction with a key heterochromatin protein. More generally, we suggest that DNMT3B plays a role in DNA methylation homeostasis at heterochromatin, a process which is disrupted in cancer, aging and Immunodeficiency, Centromeric Instability and Facial Anomalies (ICF) syndrome

Enzyme kinetics and inhibition of histone acetyltransferase KAT8

Lysine acetyltransferase 8 (KAT8) is a histone acetyltransferase (HAT) responsible for acetylating lysine 16 on histone H4 (H4K16) and plays a role in cell cycle progression as well as acetylation of the tumor suppressor protein p53. Further studies on its biological function and drug discovery initiatives will benefit from the development of small molecule inhibitors for this enzyme. As a first step towards this aim we investigated the enzyme kinetics of this bi-substrate enzyme. The kinetic experiments indicate a ping-pong mechanism in which the enzyme binds Ac-CoA first, followed by binding of the histone substrate. This mechanism is supported by affinity measurements of both substrates using isothermal titration calorimetry (ITC). Using this information, the KAT8 inhibition of a focused compound collection around the non-selective HAT inhibitor anacardic acid has been investigated. Kinetic studies with anacardic acid were performed, based on which a model for the catalytic activity of KAT8 and the inhibitory action of anacardic acid (AA) was proposed. This enabled the calculation of the inhibition constant Ki of anacardic acid derivatives using an adaptation of the Cheng-Prusoff equation. The results described in this study give insight into the catalytic mechanism of KAT8 and present the first well-characterized small-molecule inhibitors for this HAT

DNMT3B PWWP mutations cause hypermethylation of heterochromatin

The correct establishment of DNA methylation patterns is vital for mammalian development and is achieved by the de novo DNA methyltransferases DNMT3A and DNMT3B. DNMT3B localises to H3K36me3 at actively transcribing gene bodies via its PWWP domain. It also functions at heterochromatin through an unknown recruitment mechanism. Here, we find that knockout of DNMT3B causes loss of methylation predominantly at H3K9me3-marked heterochromatin and that DNMT3B PWWP domain mutations or deletion result in striking increases of methylation in H3K9me3-marked heterochromatin. Removal of the N-terminal region of DNMT3B affects its ability to methylate H3K9me3-marked regions. This region of DNMT3B directly interacts with HP1α and facilitates the bridging of DNMT3B with H3K9me3-marked nucleosomes in vitro. Our results suggest that DNMT3B is recruited to H3K9me3-marked heterochromatin in a PWWP-independent manner that is facilitated by the protein’s N-terminal region through an interaction with a key heterochromatin protein. More generally, we suggest that DNMT3B plays a role in DNA methylation homeostasis at heterochromatin, a process which is disrupted in cancer, aging and Immunodeficiency, Centromeric Instability and Facial Anomalies (ICF) syndrome

Discovery of chromenes as inhibitors of macrophage migration inhibitory factor

Macrophage migration inhibitory factor (MIF) is an essential signaling cytokine with a key role in the immune system. Binding of MIF to its molecular targets such as, among others, the cluster of differentiation 74 (CD74) receptor plays a key role in inflammatory diseases and cancer. Therefore, the identification of MIF binding compounds gained importance in drug discovery. In this study, we aim to discover novel MIF binding compounds by screening of a focused compound collection for inhibition of its tau- tomerase enzyme activity. Inspired by the known chromen-4-one inhibitor Orita-13, a focused collection of compounds with a chromene scaffold was screened for MIF binding. The library was synthesized using versatile cyanoacetamide chemistry to provide diversely substituted chromenes. The screening provided inhibitors with IC50’s in the low micromolar range. Kinetic evaluation suggested that the inhibitors were reversible and did not bind in the binding pocket of the substrate. Thus, we discovered novel inhibitors of the MIF tautomerase activity, which may ultimately support the development of novel therapeutic agents against diseases in which MIF is involved

Kinetics and inhibition of enzymes in early stage drug discovery: A MOF and MIF symphony

Aan de basis van onderzoek naar de behandeling van ziekten zoals kanker en immuunziekten staat onderzoek naar de werking van processen in het lichaam. Een goede manier om deze processen te onderzoeken is door middel van het remmen van enzymen. Door een enzym te remmen kunnen we meer te weten komen over het enzym en over zijn rol in zowel gezonde als zieke toestand. Dit proefschrift beschrijft de ontdekking van remmers voor twee enzymen: Lysine Acetyltransferase 8 (KAT8) en Macrophage Migration Inhibitory Factor (MIF). Voor KAT8 zijn vier verschillende soorten nieuwe remmers beschreven en de manier waarop zij KAT8 remmen (het mechanisme) is onderzocht. Dit resulteerde in verbazingwekkende verschillen in mechanisme tussen de verschillende remmers. Dit mechanisme kan invloed hebben op de manier waarop de remmers gebruikt (of niet gebruikt) kunnen worden in de ontwikkeling van nieuwe medicijnen. Dit laat zien dat het belangrijk is om het mechanisme van enzym remmers te onderzoeken in een vroeg stadium van hun ontwikkeling. Voor MIF zijn remmers met een niet eerder beschreven structuur ontdekt. Deze remmers zijn mogelijk een begin van de ontwikkeling van nieuwe geneesmiddelen tegen ontstekingsziekten

Details in vroege ontwikkelfase enzymremmers onderschat: Aandacht voor remmingsmechanisme misschien tijdrovend, maar moeite waard

It is important to pay attention to details such as the inhibitor mechanism at an early stage of drug development, such as enzyme inhibitors. Even though this is seen as time-consuming and complicated, this is certainly important for the further development of these resources. This is what Hannah Wapenaar argues on the basis of her doctoral research on inhibitors of lysine acetyltransferase 8

Underestimated details early stage enzyme inhibitors:Attention to the mechanism of inhibition may be time-consuming, but worth while

It is important to pay attention to details such as the inhibitor mechanism at an early stage of drug development, such as enzyme inhibitors. Even though this is seen as time-consuming and complicated, this is certainly important for the further development of these resources. This is what Hannah Wapenaar argues on the basis of her doctoral research on inhibitors of lysine acetyltransferase 8