Explore the vast range of titles available.

Squalene epoxidase (SQLE), also known as squalene monooxygenase, catalyzes the stereospecific conversion of squalene to 2,3( S )-oxidosqualene, a key step in cholesterol biosynthesis. SQLE inhibition is targeted for the treatment of hypercholesteremia, cancer, and fungal infections. However, lack of structure-function understanding has hindered further progression of its inhibitors. We have determined the first three-dimensional high-resolution crystal structures of human SQLE catalytic domain with small molecule inhibitors (2.3 Å and 2.5 Å). Comparison with its unliganded state (3.0 Å) reveals conformational rearrangements upon inhibitor binding, thus allowing deeper interpretation of known structure-activity relationships. We use the human SQLE structure to further understand the specificity of terbinafine, an approved agent targeting fungal SQLE, and to provide the structural insights into terbinafine-resistant mutants encountered in the clinic. Collectively, these findings elucidate the structural basis for the specificity of the epoxidation reaction catalyzed by SQLE and enable further rational development of next-generation inhibitors. Squalene epoxidase (SQLE) is a key enzyme in cholesterol biosynthesis and is a target for hypercholesteremia and cancer drug development. Here the authors present the crystal structures of the human SQLE catalytic domain alone and bound with small molecule inhibitors, which will facilitate the development of next-generation SQLE inhibitors.

The consumption of sweeteners, natural as well as synthetic sugars, is implicated in an array of modern-day health problems. Therefore, natural nonsugar sweeteners are of increasing interest. We identify here the biosynthetic pathway of the sweet triterpenoid glycoside mogroside V, which has a sweetening strength of 250 times that of sucrose and is derived from mature fruit of luohan-guo (Siraitia grosvenorii, monk fruit). A whole-genome sequencing of Siraitia, leading to a preliminary draft of the genome, was combined with an extensive transcriptomic analysis of developing fruit. A functional expression survey of nearly 200 candidate genes identified the members of the five enzyme families responsible for the synthesis of mogroside V: squalene epoxidases, triterpenoid synthases, epoxide hydrolases, cytochrome P450s, and UDP-glucosyltransferases. Protein modeling and docking studies corroborated the experimentally proven functional enzyme activities and indicated the order of the metabolic steps in the pathway. A comparison of the genomic organization and expression patterns of these Siraitia genes with the orthologs of other Cucurbitaceae implicates a strikingly coordinated expression of the pathway in the evolution of this species-specific and valuable metabolic pathway. The genomic organization of the pathway genes, syntenously preserved among the Cucurbitaceae, indicates, on the other hand, that gene clustering cannot account for this novel secondary metabolic pathway.

Squalene epoxidase (also known as squalene monooxygenase, EC 1.14.99.7) is a key rate-limiting enzyme in cholesterol biosynthesis. Anil Padyana and colleagues report the long awaited structure of human squalene epoxidase (SQLE). They solved the crystal structure of the catalytic domain of human SQLE alone and in complex with two similar pharmacological inhibitors and elucidate their mechanism of action. SQLE is the target of fungicides and of increasing interest in human health and disease, particularly as a new anti-cancer target. Indeed, in a companion paper, Christopher Mahoney and colleagues performed an inhibitor screen with cancer cell lines and identified SQLE as an unique vulnerability in a subset of neuroendocrine tumours, where SQLE inhibition caused a toxic accumulation of the substrate squalene. The SQLE structure will facilitate the development of improved inhibitors. Here, we comment on these two studies in the wider context of the field and discuss possible future directions.

Cholesterol synthesis is both energy- and oxygen-intensive, yet relatively little is known of the regulatory effects of hypoxia on pathway enzymes. We previously showed that the rate-limiting and first oxygen-dependent enzyme of the committed cholesterol synthesis pathway, squalene monooxygenase (SM), can undergo partial proteasomal degradation that renders it constitutively active. Here, we show hypoxia is a physiological trigger for this truncation, which occurs through a two-part mechanism: (1) increased targeting of SM to the proteasome via stabilization of the E3 ubiquitin ligase MARCHF6 and (2) accumulation of the SM substrate, squalene, which impedes the complete degradation of SM and liberates its truncated form. This preserves SM activity and downstream pathway flux during hypoxia. These results uncover a feedforward mechanism that allows SM to accommodate fluctuating substrate levels and may contribute to its widely reported oncogenic properties. Cells need cholesterol to work properly but too much cholesterol is harmful and can contribute to atherosclerosis (narrowing of blood vessels), cancer and other diseases. Cells therefore carefully control the activity of the enzymes that are involved in making cholesterol, including an enzyme known as squalene monooxygenase. When the level of cholesterol in a cell rises, a protein called MARCHF6 adds molecules of ubiquitin to squalene monooxygenase. These molecules act as tags that direct the enzyme to be destroyed by a machine inside cells, known as the proteasome, thereby preventing further (unnecessary) production of cholesterol. Previous studies found that squalene monooxygenase is sometimes only partially broken down to make a shorter (truncated) form of the enzyme that is permanently active, even when the level of cholesterol in the cell is high. However, it was unclear what triggers this partial breakdown. The process of making cholesterol uses a lot of oxygen, yet many cancer cells thrive in tumours with low levels of oxygen. Here, Coates et al. used biochemical and cell biology approaches to study the effect of low oxygen levels on the activity of squalene monooxygenase in human cells. The experiments revealed that low oxygen levels trigger squalene monooxygenase to be partially degraded to make the truncated form of the enzyme. Firstly, MARCHF6 accumulates and adds ubiquitin to the enzyme to accelerate its delivery to the proteasome. Secondly, as the proteasome starts to degrade the enzyme, a build-up of squalene molecules impedes further breakdown of the enzyme. This mechanism preserves squalene monooxygenase activity when oxygen levels drop in cells, which may compensate for temporary oxygen shortfalls and allow cells to continue to make cholesterol. Squalene monooxygenase is overactive in individuals with a wide variety of diseases including fatty liver and prostate cancer. Drugs that block squalene monooxygenase activity have been shown to stop cancer cells from growing, but unfortunately these drugs are also toxic to mammals. These findings suggest that reducing the activity of squalene monooxygenase in more subtle ways, such as stopping it from being partially degraded, may be a more viable treatment strategy for cancer and other diseases associated with high levels of cholesterol.

Squalene epoxidase (SQLE) is a crucial enzyme in the cholesterol-biosynthesis pathway and a promising target for treating cholesterol-related disorders. This study aimed to repurpose eighteen clinically approved phenothiazine antipsychotics as competitive SQLE inhibitors by integrating structure-based virtual screening, 200 ns molecular-dynamics simulations, MM/PBSA binding-energy calculations and in vitro enzyme assays. We first screened the 18 derivatives using molecular docking, structural/pharmacological diversity and ADMET analysis. Six compounds—ethopropazine, periciazine, piperacetazine, dixyrazine, fluphenazine and trifluoperazine—were prioritized for detailed MD simulations and MM/PBSA evaluation. Potential-energy-landscape analysis revealed that ethopropazine, periciazine and piperacetazine formed the most stable enzyme–ligand complexes, each occupying well-defined energy wells. Corresponding ΔG total values of − 27.05 ± 2.10 kcal mol⁻¹, − 27.84 ± 1.67 kcal mol⁻¹ and − 26.94 ± 1.82 kcal mol⁻¹ indicated high binding affinities. In   vitro assays confirmed potent SQLE inhibition, with IC₅₀ values of 1.69 ± 0.06 µM, 1.55 ± 0.13 µM and 1.44 ± 0.04 µM, respectively; kinetic studies established competitive inhibition with K i values of 0.65–0.69 µM. The strong correlation between computational predictions and experimental data underscores the effectiveness of our integrated approach and identifies ethopropazine, periciazine and piperacetazine as promising lead compounds for further optimization and pre-clinical development as SQLE inhibitors.

Dermatophyte infections, as a significant public health threats, are increasingly associated with antifungal drug resistance, particularly to terbinafine. Mutations in the squalene epoxidase ( SQLE ) gene have been linked to resistance by altering amino acid residues and interfering with drug-protein interactions. This study applied computational tools including I-Mutant, ConSurf, HOPE, DynaMut2, STRING, and molecular docking to assess the structural and functional impact of clinically reported SQLE missense mutations in terbinafine-resistant dermatophyte isolates. Twelve out of fourteen mutations significantly reduced SQLE stability, with L393F, L393S, and F397L identified as the most destabilizing. ConSurf analysis revealed that residues F311, L393S, L393F, F397I, L437P, H440Y, and H440T were highly conserved, structurally buried, and essential for SQLE integrity, while V237I, F397L, and F415S were conserved but less critical. Notably, Q408L was identified as functionally significant and surface-exposed, underscoring its potential as a key contributor to resistance. Conserved regions were found to be more susceptible to functional disruption than non-conserved ones. HOPE analysis highlighted changes in size, charge, and hydrophobicity in the mutant residues, suggesting potential disruption of SQLE’s functional architecture. Also, DynaMut2 analysis predicted decreased flexibility and stability in most mutants. Molecular docking identified altered binding pockets in four variants F397L, L437P, F415V, and Y394N compared to the wild-type, potentially compromising terbinafine binding. STRING network analysis revealed functional interactions between SQLE and ten proteins involved in ergosterol biosynthesis. These findings offer valuable molecular insights into terbinafine resistance mechanisms and identify conserved, mutation-sensitive sites that may guide antifungal drug development and resistance management strategies.

Although fungi do not cause outbreaks or pandemics, the incidence of severe systemic fungal infections has increased significantly, mainly because of the explosive growth in the number of patients with compromised immune system. Thus, drug resistance in pathogenic fungi, including dermatophytes, is gaining importance. The molecular aspects involved in the resistance of dermatophytes to marketed antifungals and other cytotoxic drugs, such as modifications of target enzymes, over-expression of genes encoding ATP-binding cassette (ABC) transporters and stress-response-related proteins are reviewed. Emphasis is placed on the mechanisms used by dermatophytes to overcome the inhibitory action of terbinafine and survival in the host environment. The relevance of identifying new molecular targets, of expanding the understanding about the molecular mechanisms of resistance and of using this information to design new drugs or to modify those that have become ineffective is also discussed.

In plants, phytosterols and triterpenes are major secondary metabolites. In an attempt to reveal the mechanism for synthesis and storage of these compounds, we isolated and characterized cDNA clones for squalene epoxidase (SE), from a succulent shrub, Euphorbia tirucalli. Southern-blot analysis of total DNA using cDNA fragment as a probe showed that the E. tirucalli squalene epoxidase gene (EtSE) is single-copy type in terms of restriction fragment length polymorphism (RFLP). Deduced amino-acid sequence of the cDNA showed 83 and 75% identity to those of rice and ginseng, respectively, in an area excluding a less homologous putative transmembrane region in the N-terminal end. Functional characterization with heterologous expression using an erg1-disrupted yeast mutant KLN1 indicated that the EtSE recovered ergosterol auxotrophy of the mutant, and gave rise to an ergosterol accumulation in the EtSE transformant. RT-PCR analysis showed the EtSE transcripts in leaves and stem internodes accumulated in almost equal amounts, which were more abundant than those in roots. In situ hybridization using EtSE antisense probe revealed prominent EtSE expression on a parenchyma cell adjacent to primary laticifers that were located in a rosary orientation in the inner region of cortex. This is the first report of expression of a gene for a rate-limiting enzyme in mevalonate pathway in organs and tissues of a plant.

Cholesterol is essential for cells to grow and proliferate. Normal mammalian cells meet their need for cholesterol through its uptake or de novo synthesis 1 , but the extent to which cancer cells rely on each of these pathways remains poorly understood. Here, using a competitive proliferation assay on a pooled collection of DNA-barcoded cell lines, we identify a subset of cancer cells that is auxotrophic for cholesterol and thus highly dependent on its uptake. Through metabolic gene expression analysis, we pinpoint the loss of squalene monooxygenase expression as a cause of cholesterol auxotrophy, particularly in ALK + anaplastic large cell lymphoma (ALCL) cell lines and primary tumours. Squalene monooxygenase catalyses the oxidation of squalene to 2,3-oxidosqualene in the cholesterol synthesis pathway and its loss results in accumulation of the upstream metabolite squalene, which is normally undetectable. In ALK + ALCLs, squalene alters the cellular lipid profile and protects cancer cells from ferroptotic cell death, providing a growth advantage under conditions of oxidative stress and in tumour xenografts. Finally, a CRISPR-based genetic screen identified cholesterol uptake by the low-density lipoprotein receptor as essential for the growth of ALCL cells in culture and as patient-derived xenografts. This work reveals that the cholesterol auxotrophy of ALCLs is a targetable liability and, more broadly, that systematic approaches can be used to identify nutrient dependencies unique to individual cancer types. The authors find that loss of squalene monooxygenase expression alters the lipid metabolism of cancer cells, which confers protection from ferroptotic cell death and thus promotes tumour growth.

Aberrant metabolism of cancer cells is well appreciated, but the identification of cancer subsets with specific metabolic vulnerabilities remains challenging. We conducted a chemical biology screen and identified a subset of neuroendocrine tumors displaying a striking pattern of sensitivity to inhibition of the cholesterol biosynthetic pathway enzyme squalene epoxidase (SQLE). Using a variety of orthogonal approaches, we demonstrate that sensitivity to SQLE inhibition results not from cholesterol biosynthesis pathway inhibition, but rather surprisingly from the specific and toxic accumulation of the SQLE substrate, squalene. These findings highlight SQLE as a potential therapeutic target in a subset of neuroendocrine tumors, particularly small cell lung cancers. Cancer cells are metabolically adaptable and the identification of specific vulnerabilities is challenging. Here the authors identify a subset of neuroendocrine cell lines exquisitely sensitive to inhibition of SQLE, an enzyme in the cholesterol biosynthetic pathway, due to the toxic accumulation of pathway intermediate squalene.