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Histone mimics target BET bromodomains Small molecules that perturb chromatin proteins are an emerging focus of current biomedical research. Two groups reporting in this issue have targeted bromodomain-containing BET proteins that bind acetylated lysine residues during gene activation, arriving at cell-permeable small molecule compounds with similar structures based on fused triazole-diazepine rings. James Bradner and colleagues report the development of a compound named JQ1. The BET protein BRD4, with two bromodomains, is implicated in human squamous cell carcinoma. JQ1 inhibits the growth of BRD4-dependent tumours in mouse models. Alexander Tarakhovsky and colleagues' inhibitor, I-BET, is shown to interfere with the binding of certain BET family members to acetylated histones. It inhibits activation of pro-inflammatory genes in macrophages and has immunomodulatory activity in a mouse model of inflammatory disease. Post-translationally modified histones are recognized by effector proteins which contain specific binding modules; for example, the bromodomain-containing BET proteins bind acetylated lysine residues during gene activation. Here a synthetic small molecule is described that interferes with the binding of certain BET family members to acetylated histones. The compound inhibits activation of pro-inflammatory genes in macrophages and has activity in a mouse model of inflammatory disease. Interaction of pathogens with cells of the immune system results in activation of inflammatory gene expression. This response, although vital for immune defence, is frequently deleterious to the host due to the exaggerated production of inflammatory proteins. The scope of inflammatory responses reflects the activation state of signalling proteins upstream of inflammatory genes as well as signal-induced assembly of nuclear chromatin complexes that support mRNA expression 1 , 2 , 3 , 4 . Recognition of post-translationally modified histones by nuclear proteins that initiate mRNA transcription and support mRNA elongation is a critical step in the regulation of gene expression 5 , 6 , 7 , 8 , 9 , 10 . Here we present a novel pharmacological approach that targets inflammatory gene expression by interfering with the recognition of acetylated histones by the bromodomain and extra terminal domain (BET) family of proteins. We describe a synthetic compound (I-BET) that by ‘mimicking’ acetylated histones disrupts chromatin complexes responsible for the expression of key inflammatory genes in activated macrophages, and confers protection against lipopolysaccharide-induced endotoxic shock and bacteria-induced sepsis. Our findings suggest that synthetic compounds specifically targeting proteins that recognize post-translationally modified histones can serve as a new generation of immunomodulatory drugs.

Redox properties of cytochrome b559 (Cyt b559) and cytochrome c550 (Cyt c550) have been studied by using highly stable photosystem II (PSII) core complex preparations from a mutant strain of the thermophilic cyanobacterium Thermosynechococcuselongatus with a histidine tag on the CP43 protein of PSII. Two different redox potential forms for Cyt b559 are found in these preparations, with a midpoint redox potential (E′m) of +390 mV in about half of the centers and +275 mV in the other half. The high-potential form, whose E′m is pH independent, can be converted into the lower potential form by Tris washing, mild heating or alkaline pH incubation. The E′m of the low-potential form is significantly higher than that found in other photosynthetic organisms and is not affected by pH. The possibility that the heme of Cyt b559 in T. elongatus is in a more hydrophobic environment is discussed. Cyt c550 has a higher E′m when bound to the PSII core (–80 mV at pH 6.0) than after its extraction from the complex (–240 mV at pH 6.0). The E′m of Cyt c550 bound to PSII is pH independent, while in the purified state an increase of about 58 mV/pH unit is observed when the pH decreases below pH 9.0. Thus, Cyt c550 seems to have a single protonateable group which influences the redox properties of the heme. From these electrochemical measurements and from EPR controls it is proposed that important changes in the solvent accessibility to the heme and in the acid-base properties of that protonateable group could occur upon the release of Cyt c550 from PSII.