Dual EZH2 and EHMT2 histone methyltransferase inhibition increases biological efficacy in breast cancer cells.

BACKGROUND: Many cancers show aberrant silencing of gene expression and overexpression of histone methyltransferases. The histone methyltransferases (HKMT) EZH2 and EHMT2 maintain the repressive chromatin histone methylation marks H3K27me and H3K9me, respectively, which are associated with transcriptional silencing. Although selective HKMT inhibitors reduce levels of individual repressive marks, removal of H3K27me3 by specific EZH2 inhibitors, for instance, may not be sufficient for inducing the expression of genes with multiple repressive marks. RESULTS: We report that gene expression and inhibition of triple negative breast cancer cell growth (MDA-MB-231) are markedly increased when targeting both EZH2 and EHMT2, either by siRNA knockdown or pharmacological inhibition, rather than either enzyme independently. Indeed, expression of certain genes is only induced upon dual inhibition. We sought to identify compounds which showed evidence of dual EZH2 and EHMT2 inhibition. Using a cell-based assay, based on the substrate competitive EHMT2 inhibitor BIX01294, we have identified proof-of-concept compounds that induce re-expression of a subset of genes consistent with dual HKMT inhibition. Chromatin immunoprecipitation verified a decrease in silencing marks and an increase in permissive marks at the promoter and transcription start site of re-expressed genes, while Western analysis showed reduction in global levels of H3K27me3 and H3K9me3. The compounds inhibit growth in a panel of breast cancer and lymphoma cell lines with low to sub-micromolar IC50s. Biochemically, the compounds are substrate competitive inhibitors against both EZH2 and EHMT1/2. CONCLUSIONS: We have demonstrated that dual inhibition of EZH2 and EHMT2 is more effective at eliciting biological responses of gene transcription and cancer cell growth inhibition compared to inhibition of single HKMTs, and we report the first dual EZH2-EHMT1/2 substrate competitive inhibitors that are functional in cells.

Chiroptical studies on brevianamide B: vibrational and electronic circular dichroism confronted.

Chiroptical spectroscopies, such as electronic circular dichroism (ECD) and vibrational circular dichroism (VCD), are highly sensitive techniques to probe molecular conformation, configuration, solvation, and aggregation. Here we report the application of these techniques to study the fungal metabolite brevianamide B. Comparison of the experimental ECD and VCD spectra with the density functional theory simulated counterparts establishes that VCD is the more reliable technique to assign absolute configuration due to the larger functional and dispersion dependence of computed ECD spectra. Despite a low amount of available material and a relatively unusual example of using VCD carbonyl multiplets, the absolute configuration could be reliably predicted, strengthening the case for application of VCD in the study of complex natural products. Spectral and crystallographic evidence for or against the formation of a dimeric aggregate is discussed; in solution, the VCD spectra strongly suggest only monomeric species are present.

Mechanistic and chiroptical studies on the desulfurization of epidithiodioxopiperazines reveal universal retention of configuration at the bridgehead carbon atoms.

The stereochemistry of the desulfurization products of chiral natural and synthetic 3,6-epidithiodiketopiperazines (ETPs) is specified inconsistently in the literature. Qualitative mechanisms have been put forward to explain apparently divergent stereochemical pathways, but the quantitative feasibility of such mechanistic pathways has not been assessed. We report a computational study revealing that desulfurization of ETPs should occur universally with retention of configuration. While the majority of stereochemically assigned and reassigned cases fit this model, until now desulfurization of the synthetic gliotoxin analogue shown has remained assigned as proceeding via inversion of configuration. Through detailed chiroptical studies comparing experimentally obtained optical rotation values, electronic circular dichroism spectra, and vibrational circular dichroism spectra to their computationally simulated counterparts as well as chemical derivatization studies, we have unambiguously demonstrated that contrary to its current assignment in the literature, the desulfurization of this synthetic ETP also proceeds with retention of configuration.

On the histone lysine methyltransferase activity of fungal metabolite chaetocin.

Histone lysine methyltransferases (HKMTs) are an important class of targets for epigenetic therapy. 1 (chaetocin), an epidithiodiketopiperazine (ETP) natural product, has been reported to be a specific inhibitor of the SU(VAR)3-9 class of HKMTs. We have studied the inhibition of the HKMT G9a by 1 and functionally related analogues. Our results reveal that only the structurally unique ETP core is required for inhibition, and such inhibition is time-dependent and irreversible (in the absence of DTT), ultimately resulting in protein denaturation. Mass spectrometric data provide a molecular basis for this effect, demonstrating covalent adduct formation between 1 and the protein. This provides a potential rationale for the selectivity observed in the inhibition of a variety of HKMTs by 1 in vitro and has implications for the activity of ETPs against these important epigenetic targets.

Perspectives on natural product epigenetic modulators in chemical biology and medicine.

Epigenetic processes are complex regulatory transcriptional pathways that underpin fundamental physiology and are implicated in the aetiology of many diseases. Small molecule modulators of key epigenetic targets will be central in the study of these intricate networks as well as providing highly useful start points for drug discovery efforts. In this review we will give our perspective on the current status of natural product epigenetic modulators, highlighting the limitations, challenges and opportunities for currently identified molecules, as well as potential strategies for novel compound discovery moving forward.