Zinc inhibits Hedgehog autoprocessing: linking zinc deficiency with Hedgehog activation

A cell-based bioluminescence reporter assay of human Sonic Hedgehog protein autoprocessing to identify inhibitors and activators

The Sonic Hedgehog (SHh) precursor protein undergoes biosynthetic autoprocessing to cleave off and covalently attach cholesterol to the SHh signaling ligand, a vital morphogen and oncogenic effector protein. Autoprocessing is self-catalyzed by SHhC, the SHh precursor's C-terminal enzymatic domain. A method to screen for small molecule regulators of this process may be of therapeutic value. Here, we describe the development and validation of the first cellular reporter to monitor human SHhC autoprocessing noninvasively in high-throughput compatible plates. The assay couples intracellular SHhC autoprocessing using endogenous cholesterol to the extracellular secretion of the bioluminescent nanoluciferase enzyme. We developed a WT SHhC reporter line for evaluating potential autoprocessing inhibitors by concentration response-dependent suppression of extracellular bioluminescence. Additionally, a conditional mutant SHhC (D46A) reporter line was developed for identifying potential autoprocessing activators by a concentration response-dependent gain of extracellular bioluminescence. The D46A mutation removes a conserved general base that is critical for the activation of the cholesterol substrate. Inducibility of the D46A reporter was established using a synthetic sterol, 2-α carboxy cholestanol, designed to bypass the defect through intramolecular general base catalysis. To facilitate direct nanoluciferase detection in the cell culture media of 1536-well plates, we designed a novel anionic phosphonylated coelenterazine, CLZ-2P, as the nanoluciferase substrate. This new reporter system offers a long-awaited resource for small molecule discovery for cancer and for developmental disorders where SHh ligand biosynthesis is dysregulated.

Hedgehog Autoprocessing: From Structural Mechanisms to Drug Discovery

Hedgehog (Hh) signaling plays pivotal roles in embryonic development. In adults, Hh signaling is mostly turned off but its abnormal activation is involved in many types of cancer. Hh signaling is initiated by the Hh ligand, generated from the Hh precursor by a specialized autocatalytic process called Hh autoprocessing. The Hh precursor consists of an N-terminal signaling domain (HhN) and a C-terminal autoprocessing domain (HhC). During Hh autoprocessing, the precursor is cleaved between N- and C-terminal domain followed by the covalent ligation of cholesterol to the last residue of HhN, which subsequently leads to the generation of Hh ligand for Hh signaling. Hh autoprocessing is at the origin of canonical Hh signaling and precedes all downstream signaling events. Mutations in the catalytic residues in HhC can lead to congenital defects such as holoprosencephaly (HPE). The aim of this review is to provide an in-depth summary of the progresses and challenges towards an atomic level understanding of the structural mechanisms of Hh autoprocessing. We also discuss drug discovery efforts to inhibit Hh autoprocessing as a new direction in cancer therapy.

Inteins as Drug Targets and Therapeutic Tools

Multidrug-resistant pathogens are of significant concern in recent years. Hence new antifungal and anti-bacterial drug targets are urgently needed before the situation goes beyond control. Inteins are polypeptides that self-splice from exteins without the need for cofactors or external energy, resulting in joining of extein fragments. Inteins are present in many organisms, including human pathogens such as Mycobacterium tuberculosis, Cryptococcus neoformans, C. gattii, and Aspergillus fumigatus. Because intein elements are not present in human genes, they are attractive drug targets to develop antifungals and antibiotics. Thus far, a few inhibitors of intein splicing have been reported. Metal-ions such as Zn²⁺ and Cu²⁺, and platinum-containing compound cisplatin inhibit intein splicing in M. tuberculosis and C. neoformans by binding to the active site cysteines. A small-molecule inhibitor 6G-318S and its derivative 6G-319S are found to inhibit intein splicing in C. neoformans and C. gattii with a MIC in nanomolar concentrations. Inteins have also been used in many other applications. Intein can be used in activating a protein inside a cell using small molecules. Moreover, split intein can be used to deliver large genes in experimental gene therapy and to kill selected species in a mixed population of microbes by taking advantage of the toxin-antitoxin system. Furthermore, split inteins are used in synthesizing cyclic peptides and in developing cell culture model to study infectious viruses including SARS-CoV-2 in the biosafety level (BSL) 2 facility. This mini-review discusses the recent research developments of inteins in drug discovery and therapeutic research.

Celecoxib alleviates zinc deficiency-promoted colon tumorigenesis through suppressing inflammation

Accumulating evidence has shown that dietary zinc deficiency (ZD) increases the risk of various cancers including esophageal and gastric cancer. However, the role of ZD in colon tumorigenesis is unknown and the related mechanisms need to be investigated. Apc^min/+ mice, widely used to mimic the spontaneous process of human intestinal tumor, were used to construct a ZD mice model in this study. Inflammatory mediators such as COX-2, TNF-α, CCL, CXCL, and IL chemokines families were evaluated using real-time PCR and Enzyme-linked immunosorbent assay (ELISA). Besides, the immunoreactivities of cyclin D1, PCNA, and COX-2 in the colon were detected by immunohistochemistry. We found that zinc deficiency could promote colon tumorigenesis in Apc^min/+ mice. The mechanisms are involved in the upregulation of inflammatory mediators: COX-2, TNF-α, CCL, CXCL, and IL chemokines families. Administration of celecoxib, a selective COX-2 inhibitor, decreased colon tumorigenesis in Apc^min/+ mice via inhibiting the inflammatory mediators. ZD plays an important role in the process of colon cancers of Apc^min/+ mice. Celecoxib attenuates ZD-induced colon tumorigenesis in Apc^min/+ mice by inhibiting the inflammatory mediators. Our novel finding would provide potential prevention of colorectal tumor-induced by ZD.

Conditional DnaB Protein Splicing Is Reversibly Inhibited by Zinc in Mycobacteria

Inteins, as posttranslational regulatory elements, can tune protein function to environmental changes by conditional protein splicing (CPS). Translated as subdomains interrupting host proteins, inteins splice to scarlessly join flanking sequences (exteins). We used DnaB-intein1 (DnaBi1) from a replicative helicase of Mycobacterium smegmatis to build a kanamycin intein splicing reporter (KISR) that links splicing of DnaBi1 to kanamycin resistance. Using expression in heterologous Escherichia coli, we observed phenotypic classes of various levels of splicing-dependent resistance (SDR) and related these to the insertion position of DnaBi1 within the kanamycin resistance protein (KanR). The KanR-DnaBi1 construct demonstrating the most stringent SDR was used to probe for CPS of DnaB in the native host environment, M. smegmatis We show here that zinc, important during mycobacterial pathogenesis, inhibits DnaB splicing in M. smegmatis Using an in vitro reporter system, we demonstrated that zinc potently and reversibly inhibited DnaBi1 splicing, as well as splicing of a comparable intein from Mycobacterium leprae Finally, in a 1.95 Å crystal structure, we show that zinc inhibits splicing through binding to the very cysteine that initiates the splicing reaction. Together, our results provide compelling support for a model whereby mycobacterial DnaB protein splicing, and thus DNA replication, is responsive to environmental zinc.IMPORTANCE Inteins are present in a large fraction of prokaryotes and localize within conserved proteins, including the mycobacterial replicative helicase DnaB. In addition to their extensive protein engineering applications, inteins have emerged as environmentally responsive posttranslational regulators of the genes that encode them. While several studies have shown compelling evidence of conditional protein splicing (CPS), examination of splicing in the native host of the intein has proven to be challenging. Here, we demonstrated through a number of measures, including the use of a splicing-dependent sensor capable of monitoring intein activity in the native host, that zinc is a potent and reversible inhibitor of mycobacterial DnaB splicing. This work also expands our knowledge of site selection for intein insertion within nonnative proteins, demonstrating that splicing-dependent host protein activation correlates with proximity to the active site. Additionally, we surmise that splicing regulation by zinc has mycobacteriocidal and CPS application potential.

Sterol A-ring plasticity in hedgehog protein cholesterolysis supports a primitive substrate selectivity mechanism

Cholesterolysis of Hedgehog family proteins couples endoproteolysis to protein C-terminal sterylation. The transformation is self-catalyzed by HhC, a partially characterized enzymatic domain found in precursor forms of Hedgehog. Here we explore spatial ambiguity in sterol recognition by HhC, using a trio of derivatives where the sterol A-ring is contracted, fused, or distorted. Sterylation assays indicate that these geometric variants react as substrates with relative activity: cholesterol, 1.000 > A-ring contracted, 0.100 > A-ring fused, 0.020 > A-ring distorted, 0.005. Experimental results and computational sterol docking into the first HhC homology model suggest a partially unstructured binding site with substrate recognition governed in large part by hydrophobic interactions.

Copper and Zinc Homeostasis: Lessons from Drosophila melanogaster

Maintenance of metal homeostasis is crucial for many different enzymatic activities and in turn for cell function and survival. In addition, cells display detoxification and protective mechanisms against toxic accumulation of metals. Perturbation of any of these processes normally leads to cellular dysfunction and finally to cell death. In the last years, loss of metal regulation has been described as a common pathological feature in many human neurodegenerative diseases. However, in most cases, it is still a matter of debate whether such dyshomeostasis is a primary or a secondary downstream defect. In this review, we will summarize and critically evaluate the contribution of Drosophila to model human diseases that involve altered metabolism of metals or in which metal dyshomeostasis influence their pathobiology. As a prerequisite to use Drosophila as a model, we will recapitulate and describe the main features of core genes involved in copper and zinc metabolism that are conserved between mammals and flies. Drosophila presents some unique strengths to be at the forefront of neurobiological studies. The number of genetic tools, the possibility to easily test genetic interactions in vivo and the feasibility to perform unbiased genetic and pharmacological screens are some of the most prominent advantages of the fruitfly. In this work, we will pay special attention to the most important results reported in fly models to unveil the role of copper and zinc in cellular degeneration and their influence in the development and progression of human neurodegenerative pathologies such as Parkinson's disease, Alzheimer's disease, Huntington's disease, Friedreich's Ataxia or Menkes, and Wilson's diseases. Finally, we show how these studies performed in the fly have allowed to give further insight into the influence of copper and zinc in the molecular and cellular causes and consequences underlying these diseases as well as the discovery of new therapeutic strategies, which had not yet been described in other model systems.

PEGylation of zinc nanoparticles amplifies their ability to enhance olfactory responses to odorant

Olfactory responses are intensely enhanced with the addition of endogenous and engineered primarily-elemental small zinc nanoparticles (NPs). With aging, oxidation of these Zn nanoparticles eliminated the observed enhancement. The design of a polyethylene glycol coating to meet storage requirements of engineered zinc nanoparticles is evaluated to achieve maximal olfactory benefit. The zinc nanoparticles were covered with 1000 g/mol or 400 g/mol molecular weight polyethylene glycol (PEG). Non-PEGylated and PEGylated zinc nanoparticles were tested by electroolfactogram with isolated rat olfactory epithelium and odorant responses evoked by the mixture of eugenol, ethyl butyrate and (±) carvone after storage at 278 K (5 ^oC), 303 K (30 ^oC) and 323 K (50 ^oC). The particles were analyzed by atomic force microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and laser Doppler velocimetry. Our data indicate that stored ZnPEG400 nanoparticles maintain physiologically-consistent olfactory enhancement for over 300 days. These engineered Nanoparticles support future applications in olfactory research, sensitive detection, and medicine.

Biophysical characterisation of the novel zinc binding property in Suppressor of Fused

Suppressor of Fused (SUFU) is a highly conserved protein that acts as a negative regulator of the Hedgehog (HH) signalling pathway, a major determinant of cell differentiation and proliferation. Therefore, SUFU deletion in mammals has devastating effects on embryo development. SUFU is part of a multi-protein cytoplasmic signal-transducing complex. Its partners include the Gli family of transcription factors that function either as repressors, or as transcription activators according to the HH activation state. The crystal structure of SUFU revealed a two-domain arrangement, which undergoes a closing movement upon binding a peptide from Gli1. There remains however, much to be discovered about SUFU’s behaviour. To this end, we expressed recombinant, full-length SUFU from Drosophila, Zebrafish and Human. Guided by a sequence analysis that revealed a conserved potential metal binding site, we discovered that SUFU binds zinc. This binding was found to occur with a nanomolar affinity to SUFU from all three species. Mutation of one histidine from the conserved motif induces a moderate decrease in affinity for zinc, while circular dichroism indicates that the mutant remains structured. Our results reveal new metal binding affinity characteristics about SUFU that could be of importance for its regulatory function in HH.

The miR-183 family cluster alters zinc homeostasis in benign prostate cells, organoids and prostate cancer xenografts

The miR-183 cluster, which is comprised of paralogous miRs-183, -96 and -182, is overexpressed in many cancers, including prostate adenocarcinoma (PCa). Prior studies showed that overexpression of individual pre-miRs-182, -96 and -183 in prostate cells decreased zinc import, which is a characteristic feature of PCa tumours. Zinc is concentrated in healthy prostate 10-fold higher than any other tissue, and an >80% decrease in zinc is observed in PCa specimens. Here, we studied the effect of overexpression of the entire 4.8 kb miR-183 family cluster, including the intergenic region which contains highly conserved genomic regions, in prostate cells. This resulted in overexpression of mature miR-183 family miRs at levels that mimic cancer-related changes. Overexpression of the miR-183 cluster reduced zinc transporter and intracellular zinc levels in benign prostate cells, PCa xenografts and fresh prostate epithelial organoids. Microarray analysis of miR-183 family cluster overexpression in prostate cells showed an enrichment for cancer-related pathways including adhesion, migration and wound healing. An active secondary transcription start site was identified within the intergenic region of the miR-183 cluster, which may regulate expression of miR-182. Taken together, this study shows that physiologically relevant expression of the miR-183 family regulates zinc levels and carcinogenic pathways in prostate cells.

Micronutrient status (calcium, zinc, vitamins D and E) in patients with medullary thyroid carcinoma: A cross-sectional study

OBJECTIVE

The aim of this study was to evaluate the micronutrient status of Iranian patients with medullary thyroid carcinoma (MTC) and to analyze potential relationships with respect to MTC risk.

METHODS

This was a cross-sectional study (Tehran Thyroid Cancer Survey 2015-2016). We measured and compared preoperative serum calcium, zinc, and vitamins D and E in patients with MTC and healthy controls. Forty cases with MTC and 40 (age-, sex-, and body mass index-matched) healthy controls voluntarily participated in the project.

RESULTS

Serum calcium, zinc, and vitamin D and E concentrations were lower in the patients with cancer (P_Ca < 0.001, P_Zn = 0.01, P_D = 0.056, P_E = 0.002) than in the healthy controls. We found that serum calcium remarkably associated with enhanced risk for thyroid cancer (odds ratio [OR], 6.5; P = 0.001). Likewise, serum vitamin E was linked to the risk for cancer (OR, 1.31; P = 0.056). Moreover, serum zinc was correlated with vitamin E and calcium (r = +0.23; P = 0.04 and r = +0.25, P = 0.03; respectively). We also observed a correlation between calcium and vitamin E (r = +0.27; P = 0.02).

CONCLUSIONS

A multiple-micronutrient decrease was confirmed in patients with MTC. A low serum calcium level was a potent risk factor for MTC. Findings from the present study suggest that dietary intake and/or supplementation of micronutrients, especially calcium and vitamin E, may be beneficial in reducing the risk for thyroid cancer.

A Single Aspartate Coordinates Two Catalytic Steps in Hedgehog Autoprocessing

Hedgehog (Hh) signaling is driven by the cholesterol-modified Hh ligand, generated by autoprocessing of Hh precursor protein. Two steps in Hh autoprocessing, N-S acyl shift and transesterification, must be coupled for efficient Hh cholesteroylation and downstream signal transduction. In the present study, we show that a conserved aspartate residue, D46 of the Hh autoprocessing domain, coordinates these two catalytic steps. Mutagenesis demonstrated that D46 suppresses non-native Hh precursor autoprocessing and is indispensable for transesterification with cholesterol. NMR measurements indicated that D46 has a pKa of 5.6, ∼2 units above the expected pKa of aspartate, due to a hydrogen-bond between protonated D46 and a catalytic cysteine residue. However, the deprotonated form of D46 side chain is also essential, because a D46N mutation cannot mediate cholesteroylation. On the basis of these data, we propose that the proton shuttling of D46 side chain mechanistically couples the two steps of Hh cholesteroylation.

Hedgehog Proteins Consume Steroidal CYP17A1 Antagonists: Potential Therapeutic Significance in Advanced Prostate Cancer

Abiraterone, a potent inhibitor of the human enzyme CYP17A1 (cytochrome P450c17), provides a last line of defense against ectopic androgenesis in advanced prostate cancer. Herein we report an unprecedented off-target interaction between abiraterone and oncogenic hedgehog proteins. Our experiments indicate that abiraterone and its structural congener, galeterone, can replace cholesterol as a substrate in a specialized biosynthetic event of hedgehog proteins, known as cholesterolysis. The off-target reaction generates covalent hedgehog-drug conjugates. Cell-based reporter assays indicate that these conjugates activate hedgehog signaling when present in the low nanomolar range. Because hedgehog signaling is implicated in prostate cancer progression, and abiraterone is administered to treat advanced stages of the disease, this off-target interaction may have therapeutic significance.

Hedgehog Cholesterolysis: Specialized Gatekeeper to Oncogenic Signaling

Discussions of therapeutic suppression of hedgehog (Hh) signaling almost exclusively focus on receptor antagonism; however, hedgehog's biosynthesis represents a unique and potentially targetable aspect of this oncogenic signaling pathway. Here, we review a key biosynthetic step called cholesterolysis from the perspectives of structure/function and small molecule inhibition. Cholesterolysis, also called cholesteroylation, generates cholesterol-modified Hh ligand via autoprocessing of a hedgehog precursor protein. Post-translational modification by cholesterol appears to be restricted to proteins in the hedgehog family. The transformation is essential for Hh biological activity and upstream of signaling events. Despite its decisive role in generating ligand, cholesterolysis remains conspicuously unexplored as a therapeutic target.