Natural biflavones as novel inhibitors of cathepsin B and K

Review of Cathepsin K Inhibitor Development and the Potential Role of Phytochemicals

Cathepsin K plays a pivotal role in bone resorption and has emerged as a prominent therapeutic target for treating bone-related diseases such as osteoporosis. Despite significant advances in synthetic inhibitor development, none have achieved FDA approval due to safety and efficacy challenges. This review highlights the potential of phytochemicals as alternative inhibitors, emphasizing their natural origin, structural diversity, and minimal adverse effects. Key phytochemicals, including AC-5-1, Cycloaltilisin 6, Cycloaltilisin 7, Nicolaioidesin C, and Panduratin A, were examined for their inhibitory activities against cathepsin K. While these compounds exhibit varying IC50 values, their docking studies revealed significant interactions within Cathepsin K's active site, particularly involving critical residues such as Cys25 and His162. However, challenges such as lower potency compared to synthetic inhibitors and limited in vivo studies underscore the need for structural optimization and comprehensive preclinical evaluations. This review discusses biological insights, current limitations, and future strategies for advancing phytochemical-based inhibitors toward clinical applications in managing Cathepsin K-associated diseases.

Hypoxia Pathway in Osteoporosis: Laboratory Data for Clinical Prospects

The hypoxia pathway not only regulates the organism to adapt to the special environment, such as short-term hypoxia in the plateau under normal physiological conditions, but also plays an important role in the occurrence and development of various diseases such as cancer, cardiovascular diseases, osteoporosis. Bone, as a special organ of the body, is in a relatively low oxygen environment, in which the expression of hypoxia-inducible factor (HIF)-related molecules maintains the necessary conditions for bone development. Osteoporosis disease with iron overload endangers individuals, families and society, and bone homeostasis disorder is linked to some extent with hypoxia pathway abnormality, so it is urgent to clarify the hypoxia pathway in osteoporosis to guide clinical medication efficiently. Based on this background, using the keywords "hypoxia/HIF, osteoporosis, osteoblasts, osteoclasts, osteocytes, iron/iron metabolism", a matching search was carried out through the Pubmed and Web Of Science databases, then the papers related to this review were screened, summarized and sorted. This review summarizes the relationship and regulation between the hypoxia pathway and osteoporosis (also including osteoblasts, osteoclasts, osteocytes) by arranging the references on the latest research progress, introduces briefly the application of hyperbaric oxygen therapy in osteoporosis symptoms (mechanical stimulation induces skeletal response to hypoxic signal activation), hypoxic-related drugs used in iron accumulation/osteoporosis model study, and also puts forward the prospects of future research.

Cathepsin K: A Versatile Potential Biomarker and Therapeutic Target for Various Cancers

Cancer, a common malignant disease, is one of the predominant causes of diseases that lead to death. Additionally, cancer is often detected in advanced stages and cannot be radically cured. Consequently, there is an urgent need for reliable and easily detectable markers to identify and monitor cancer onset and progression as early as possible. Our aim was to systematically review the relevant roles of cathepsin K (CTSK) in various possible cancers in existing studies. CTSK, a well-known key enzyme in the bone resorption process and most studied for its roles in the effective degradation of the bone extracellular matrix, is expressed in various organs. Nowadays, CTSK has been involved in various cancers such as prostate cancer, breast cancer, bone cancer, renal carcinoma, lung cancer and other cancers. In addition, CTSK can promote tumor cells proliferation, invasion and migration, and its mechanism may be related to RANK/RANKL, TGF-β, mTOR and the Wnt/β-catenin signaling pathway. Clinically, some progress has been made with the use of cathepsin K inhibitors in the treatment of certain cancers. This paper reviewed our current understanding of the possible roles of CTSK in various cancers and discussed its potential as a biomarker and/or novel molecular target for various cancers.

Cathepsin B Dipeptidyl Carboxypeptidase and Endopeptidase Activities Demonstrated across a Broad pH Range

Cathepsin B is a lysosomal protease that participates in protein degradation. However, cathepsin B is also active under neutral pH conditions of the cytosol, nuclei, and extracellular locations. The dipeptidyl carboxypeptidase (DPCP) activity of cathepsin B, assayed with the Abz-GIVR↓AK(Dnp)-OH substrate, has been reported to display an acidic pH optimum. In contrast, the endopeptidase activity, monitored with Z-RR-↓AMC, has a neutral pH optimum. These observations raise the question of whether other substrates can demonstrate cathepsin B DPCP activity at neutral pH and endopeptidase activity at acidic pH. To address this question, global cleavage profiling of cathepsin B with a diverse peptide library was conducted under acidic and neutral pH conditions. Results revealed that cathepsin B has (1) major DPCP activity and modest endopeptidase activity under both acidic and neutral pH conditions and (2) distinct pH-dependent amino acid preferences adjacent to cleavage sites for both DPCP and endopeptidase activities. The pH-dependent cleavage preferences were utilized to design a new Abz-GnVR↓AK(Dnp)-OH DPCP substrate, with norleucine (n) at the P3 position, having improved DPCP activity of cathepsin B at neutral pH compared to the original Abz-GIVR↓AK(Dnp)-OH substrate. The new Z-VR-AMC and Z-ER-AMC substrates displayed improved endopeptidase activity at acidic pH compared to the original Z-RR-AMC. These findings illustrate the new concept that cathepsin B possesses DPCP and endopeptidase activities at both acidic and neutral pH values. These results advance understanding of the pH-dependent cleavage properties of the dual DPCP and endopeptidase activities of cathepsin B that function under different cellular pH conditions.

Cathepsin B - A Neuronal Death Mediator in Alzheimer’s Disease Leads to Neurodegeneration

Abstract:

The lysosomal cysteine protease enzyme, named Cathepsin B, mainly degrades the protein and manages its average turnover in our body. The Cathepsin B active form is mostly present inside the lysosomal part at a cellular level, providing the slightly acidic medium for its activation. Multiple findings on Cathepsin B reveal its involvement in neurons' degeneration and a possible role as a neuronal death mediator in several neurodegenerative diseases. In this review article, we highlight the participation of Cathepsin B in the etiology/progress of AD, along with various other factors. The enzyme is involved in producing neurotoxic Aβ amyloid in the AD brain by acting as the β-secretase enzyme in the regulated secretory pathways responsible for APP processing. Aβ amyloid accumulation and amyloid plaque formation lead to neuronal degeneration, one of the prominent pathological hallmarks of AD. Cathepsin B is also involved in the production of PGlu-Aβ, which is a truncated and highly neurotoxic form of Aβ. Some of the findings also revealed that Cathepsin B specific gene deletion decreases the level of PGlu-Aβ inside the brain of experimental mice. Therefore, neurotoxicity might be considered a new pathological indication of AD due to the involvement of Cathepsin B. It also damages neurons present in the CNS region by producing inflammatory responses and generating mitochondrial ROS. However, Cathepsin B inhibitors, i.e., CA-074, can prevent neuronal death in AD patients. The other natural inhibitors are also equally effective against neuronal damage with higher selectivity. Its synthetic inhibitors are specific for their target; however, they lose their selectivity in the presence of quite a few reducing agents. Therefore, a humanized monoclonal antibody is used as a selective Cathepsin B inhibitor to overcome the problem experienced. The use of Cathepsin B for the treatment of AD and other neurodegenerative diseases could be considered a rational therapeutic target.

Functional characterization of cathepsin B and its role in the antimicrobial immune responses in golden pompano (Trachinotus ovatus)

Cathepsin B (CTSB) is one of the typical representatives of cysteine protease family. It has the activity of both exopeptidase and endopeptidase. It plays an important role in antigen presentation, degradation, apoptosis, inflammatory response and physiological process of many diseases. In this study, CTSB of Trachinotus ovatus (TroCTSB) was cloned, and its structure and function were analyzed. The results showed that the coding region of TroCTSB was 993 bp, encoding 330 amino acid residues. The homology analysis showed that the amino acid sequence of TroCTSB was similar to that in other teleosts and mammals (68.69%-88.48%). Under normal physiological conditions, TroCTSB was widely distributed in various tissues with the highest expression level in stomach, followed by liver, and the lowest expression level in blood. The optimal pH and temperature of purified recombinant protein rTroCTSB were 5.5 and 40 °C, respectively. The toxicity test of metal ions showed that Fe²⁺, Cu²⁺, Ca²⁺ and Zn²⁺ could all inhibit the activity of TroCTSB, with Zn²⁺ ranking the first. In addition, after Edwardsiella tarda infection, the expression of TroCTSB was significantly up-regulated in liver, spleen and head kidney. The overexpression of TroCTSB significantly inhibited the infection of E. tarda in golden pompano tissues, and the knockdown of TroCTSB remarkably promoted the reproduction of E. tarda in golden pompano tissues in vivo. This study suggests that TroCTSB was involved in the antibacterial immune response of T. ovatus, and provided a reference for further research in elucidating the resistance mechanism of TroCTSB.

Identification of Novel Cathepsin B Inhibitors with Implications in Alzheimer's Disease: Computational Refining and Biochemical Evaluation

Amyloid precursor protein (APP), upon proteolytic degradation, forms aggregates of amyloid β (Aβ) and plaques in the brain, which are pathological hallmarks of Alzheimer’s disease (AD). Cathepsin B is a cysteine protease enzyme that catalyzes the proteolytic degradation of APP in the brain. Thus, cathepsin B inhibition is a crucial therapeutic aspect for the discovery of new anti-Alzheimer’s drugs. In this study, we have employed mixed-feature ligand-based virtual screening (LBVS) by integrating pharmacophore mapping, docking, and molecular dynamics to detect small, potent molecules that act as cathepsin B inhibitors. The LBVS model was generated by using hydrophobic (HY), hydrogen bond acceptor (HBA), and hydrogen bond donor (HBD) features, using a dataset of 24 known cathepsin B inhibitors of both natural and synthetic origins. A validated eight-feature pharmacophore hypothesis (Hypo III) was utilized to screen the Maybridge chemical database. The docking score, MM-PBSA, and MM-GBSA methodology was applied to prioritize the lead compounds as virtual screening hits. These compounds share a common amide scaffold, and showed important interactions with Gln23, Cys29, His110, His111, Glu122, His199, and Trp221. The identified inhibitors were further evaluated for cathepsin-B-inhibitory activity. Our study suggests that pyridine, acetamide, and benzohydrazide compounds could be used as a starting point for the development of novel therapeutics.

Dual-Mechanism Quenched Fluorogenic Probe Provides Selective and Rapid Detection of Cathepsin L Activity*

Cathepsin L (CTL) is a cysteine protease demonstrating upregulated activity in many disease states. Overlapping substrate specificity makes selective detection of CTL activity difficult to parse from that of its close homologue CTV and the ubiquitous CTB. Current probes of CTL activity have limited applications due to either poor contrast or extra assay steps required to achieve selectivity. We have developed a fluorogenic probe, CTLAP, that displays good selectivity for CTL over CTB and CTV while exhibiting low background fluorescence attributed to dual quenching mechanisms. CTLAP achieves optimum CTL selectivity in the first 10 min of incubation, thus suggesting that it is amenable for rapid detection of CTL, even in the presence of competing cathepsins.

Cysteine Cathepsins: Potential Applications in Diagnostics and Therapy of Malignant Tumors

Cysteine cathepsins are proteolytic enzymes involved in protein degradation in lysosomes and endosomes. Cysteine cathepsins have been also found in the tumor microenvironment during carcinogenesis, where they are implicated in proliferation, invasion and metastasis of tumor cells through the degradation of extracellular matrix, suppression of cell-cell interactions, and promotion of angiogenesis. In this regard, cathepsins can have a diagnostic value and represent promising targets for antitumor drugs aimed at inhibition of these proteases. Moreover, cysteine cathepsins can be used as activators of novel targeted therapeutic agents. This review summarizes recent discovered roles of cysteine cathepsins in carcinogenesis and discusses new trends in cancer therapy and diagnostics using cysteine cathepsins as markers, targets, or activators.

Cell death mechanisms in eukaryotes

Like the organism they constitute, the cells also die in different ways. The death can be predetermined, programmed, and cleanly executed, as in the case of apoptosis, or it can be traumatic, inflammatory, and sudden as many types of necrosis exemplify. Nevertheless, there are a number of cell deaths-some of them bearing a resemblance to apoptosis and/or necrosis, and many, distinct from each-that serve a multitude of roles in either supporting or disrupting the homoeostasis. Apoptosis is coordinated by death ligands, caspases, b-cell lymphoma-2 (Bcl-2) family proteins, and their downstream effectors. Events that can lead to apoptosis include mitotic catastrophe and anoikis. Necrosis, although it has been considered an abrupt and uncoordinated cell death, has many molecular events associated with it. There are cell death mechanisms that share some standard features with necrosis. These include methuosis, necroptosis, NETosis, pyronecrosis, and pyroptosis. Autophagy, generally a catabolic pathway that operates to ensure cell survival, can also kill the cell through mechanisms such as autosis. Other cell-death mechanisms include entosis, ferroptosis, lysosome-dependent cell death, and parthanatos.

Cathepsins: Proteases that are vital for survival but can also be fatal

The state of enzymes in the human body determines the normal physiology or pathology, so all the six classes of enzymes are crucial. Proteases, the hydrolases, can be of several types based on the nucleophilic amino acid or the metal cofactor needed for their activity. Cathepsins are proteases with serine, cysteine, or aspartic acid residues as the nucleophiles, which are vital for digestion, coagulation, immune response, adipogenesis, hormone liberation, peptide synthesis, among a litany of other functions. But inflammatory state radically affects their normal roles. Released from the lysosomes, they degrade extracellular matrix proteins such as collagen and elastin, mediating parasite infection, autoimmune diseases, tumor metastasis, cardiovascular issues, and neural degeneration, among other health hazards. Over the years, the different types and isoforms of cathepsin, their optimal pH and functions have been studied, yet much information is still elusive. By taming and harnessing cathepsins, by inhibitors and judicious lifestyle, a gamut of malignancies can be resolved. This review discusses these aspects, which can be of clinical relevance.

Synthesis and Cytoprotective Characterization of 8-Hydroxyquinoline Betti Products

The 8-hydroxyquinoline pharmacophore scaffold has been shown to possess a range of activities as metal chelation, enzyme inhibition, cytotoxicity, and cytoprotection. Based on our previous findings we set out to optimize the scaffold for cytoprotective activity for its potential application in central nervous system related diseases. A 48-membered Betti-library was constructed by the utilization of formic acid mediated industrial-compatible coupling with sets of aromatic primary amines such as anilines, oxazoles, pyridines, and pyrimidines, with (hetero)aromatic aldehydes and 8-hydroxiquinoline derivatives. After column chromatography and re-crystallization, the corresponding analogues were obtained in yields of 13–90%. The synthesized analogs were optimized with the utilization of a cytoprotection assay with chemically induced oxidative stress, and the most active compounds were further tested in orthogonal assays, a real time cell viability method, a fluorescence-activated cell sorting (FACS)-based assay measuring mitochondrial membrane potential changes, and gene expression analysis. The best candidates showed potent, nanomolar activity in all test systems and support the need for future studies in animal models of central nervous system (CNS) disorders.

Addition of 2-(ethylamino)acetonitrile group to nitroxoline results in significantly improved anti-tumor activity in vitro and in vivo

Lysosomal cysteine peptidase cathepsin B, involved in multiple processes associated with tumor progression, is validated as a target for anti-cancer therapy. Nitroxoline, a known antimicrobial agent, is a potent and selective inhibitor of cathepsin B, hence reducing tumor progression in vitro and in vivo. In order to further improve its anti-cancer properties we developed a number of derivatives using structure-based chemical synthesis. Of these, the 7-aminomethylated derivative (compound 17) exhibited significantly improved kinetic properties over nitroxoline, inhibiting cathepsin B endopeptidase activity selectively. In the present study, we have evaluated its anti-cancer properties. It was more effective than nitroxoline in reducing tumor cell invasion and migration, as determined in vitro on two-dimensional cell models and tumor spheroids, under either endpoint or real time conditions. Moreover, it exhibited improved action over nitroxoline in impairing tumor growth in vivo in LPB mouse fibrosarcoma tumors in C57Bl/6 mice. Taken together, the addition of a 2-(ethylamino)acetonitrile group to nitroxoline at position 7 significantly improves its pharmacological characteristics and its potential for use as an anti-cancer drug.

The current stage of cathepsin B inhibitors as potential anticancer agents

Cathepsin B is a lysosomal cysteine peptidase, with an important role in the development and progression of cancer. It is involved in the degradation of extracellular matrix proteins, a process promoting invasion and metastasis of tumor cells and tumor angiogenesis. Cathepsin B is unique among cathepsins in possessing both carboxypeptidase and endopeptidase activities. While the former is associated with its physiological role, the latter is involved in pathological degradation of the extracellular matrix. Its activities are regulated by different means, the most important being its endogenous inhibitors, the cystatins. In cancer this peptidase/inhibitor balance is altered, leading to harmful cathepsin B activity. The latter can be prevented by exogenous inhibitors. They differ in modes of inhibition, size, structure, binding affinity, selectivity, toxicity and bioavailability. In this article, we review the properties and function of endogenous and exogenous cathepsin B inhibitors and indicate their application as possible anticancer agents.

Synthetic chalcone derivatives as inhibitors of cathepsins K and B, and their cytotoxic evaluation

A series of chalcone derivatives, 1-15, were prepared by Claisen-Schmidt condensation and evaluated for their cytotoxicities on tumor cell lines and also against proteolytic enzymes such as cathepsins B and K. Of the compounds synthesized, (E)-3-(3,4-dimethoxyphenyl)-1-phenylprop-2-en-1-one (12), (E)-3-(4-chlorophenyl)-1-phenylprop-2-en-1-one (13), (E)-3-(4-methoxyphenyl)-1-phenylprop-2-en-1-one (14), and (E)-3-(4-nitrophenyl)-1-phenylprop-2-en-1-one (15) showed significant cytotoxicities. The most effective compound was 15, which showed high cytotoxic activity with an IC50 value lower than 1 μg/ml, and no selectivity on the tumor cells evaluated. Substituents at C(4) of ring B were found to be essential for cytotoxicity. In addition, it was also demonstrated that some of these chalcones are moderate inhibitors of cathepsin K and have no activity against cathepsin B.

Recent advances in natural products from plants for treatment of liver diseases

Liver disease is any condition that may cause liver inflammation or tissue damage and affects liver function. Natural products that are found in vegetables, fruits, plant extracts, herbs, insects, and animals, have been traditionally used for treating liver diseases. They are chemical compounds that usually have biological activities for use in drug discovery and design. Many natural products have been clinically available as potent hepatoprotective agents against commonly occurring liver diseases. This review summarizes the current progress in the basic, clinical, and translational research on natural products in treatment of various liver diseases. Furthermore, we will focus on the discovery and biological evaluation of the natural products, which shows potential as a new therapeutic agent of liver diseases.

Docking-based virtual screening of covalently binding ligands: an orthogonal lead discovery approach

In pharmaceutical industry, lead discovery strategies and screening collections have been predominantly tailored to discover compounds that modulate target proteins through noncovalent interactions. Conversely, covalent linkage formation is an important mechanism for a quantity of successful drugs in the market, which are discovered in most cases by hindsight instead of systematical design. In this article, the implementation of a docking-based virtual screening workflow for the retrieval of covalent binders is presented considering human cathepsin K as a test case. By use of the docking conditions that led to the best enrichment of known actives, 44 candidate compounds with unknown activity on cathepsin K were finally selected for experimental evaluation. The most potent inhibitor, 4-(N-phenylanilino)-6-pyrrolidin-1-yl-1,3,5-triazine-2-carbonitrile (CP243522), showed a K(i) of 21 nM and was confirmed to have a covalent reversible mechanism of inhibition. The presented approach will have great potential in cases where covalent inhibition is the desired drug discovery strategy.

ent-Kaurane diterpenoids from Croton tonkinensis stimulate osteoblast differentiation

Four new ent-kaurane diterpenoids (1-4) were isolated from the leaves of Croton tonkinensis by bioactivity-guided fractionation using an in vitro osteoblast differentiation assay. Their structures were identified as ent-11β-acetoxykaur-16-en-18-ol (1), ent-11α-hydroxy-18-acetoxykaur-16-ene (2), ent-14β-hydroxy-18-acetoxykaur-16-ene (3), and ent-7α-hydroxy-18-acetoxykaur-16-ene (4). Compounds 1-4 significantly increased alkaline phosphatase activity and osteoblastic gene promoter activity. Compounds 1-3 also increased the levels of ALP and collagen type I alpha mRNA in C2C12 cells in a dose-dependent manner. These results suggest that ent-kaurane diterpenoids from C. tonkinensis have a direct stimulatory effect on osteoblast differentiation and may be potential therapeutic molecules against bone diseases such as osteoporosis.

Inhibitors of osteoclast differentiation from Cephalotaxus koreana

Three new flavonoid glycosides ( 1- 3), 11-hydroxyhainanolidol ( 4), and a new dibenzylbutyrolactone lignan glycoside ( 5) were isolated from the aerial parts of Cephalotaxus koreana Nakai, along with 19 known flavonoids. The structures of the new compounds were elucidated using spectroscopic evidence, primarily NMR and MS. Twenty-four compounds were isolated, and among these isoscutellarein 5-O-beta-D-glucopyranoside ( 3), apigenin ( 6), kaempferol 3-O-alpha-L-rhamnopyranosyl(1'''-->6'')-beta-D-glucopyranoside ( 7), tamarixetin 3-O-alpha-L-rhamnopyranosyl(1'''-->6'')-beta-D-glucopyranoside ( 8), quercetin 3-O-[6''-O-acetyl]-beta-D-glucopyranoside ( 9), and quercetin 3-O-alpha-L-rhamnopyranoside ( 10) showed significant inhibitory activities against osteoclast differentiation at concentrations of 0.1 and 1.0 microg/mL.