Phytocompound inhibitors of caspase 3 as beta-cell apoptosis treatment development option: An In-silico approach

1.        Introduction

Beta-cell apoptosis is a critical event in the pathogenesis of type 1 diabetes mellitus (DM). Aside from being the primary mechanism by which cells are destroyed, beta-cell apoptosis has been linked to the onset of type 1 DM via antigen cross-presentation mechanisms that result in beta-cell-specific T-cell activation (1). Apoptosis can be activated via the extrinsic death receptor or intrinsic mitochondrial pathway, activating effector caspases (2). Apoptosis is also a critical process in the development of atherosclerosis (2).

Caspases are endoproteases and genes crucial for preserving homeostasis by controlling cell death and inflammation. A phylogenetically conserved death program that is essential for the homeostasis and growth of higher organisms carefully regulates their activation. Numerous human diseases are primarily pathogenetic due to the dysregulation of apoptosis. Caspases are potential therapeutic targets because they are part of the apoptotic machinery (3, 4).

Caspases are classified broadly according to their known roles in apoptosis (caspase-3, -6, -7, -8, and -9 in mammals) and inflammation (caspase-1, -4, -5, -12 in humans and caspase-1, -11, and -12 in mice). Caspase-2, -10, and -14 functions are more difficult to classify. Caspases involved in apoptosis have been divided into two groups based on their mechanism of action: initiator caspases (caspases - 8 and -9) and executioner caspases (caspase-3, -6, and -7) (3).

In a study titled "Caspase-3-Dependent -Cell Apoptosis in the Initiation of Autoimmune Diabetes Mellitus", the authors used a genetic approach to show that this process is necessary for cross-presentation of beta-cell antigen to activate beta-cell-specific T cells (1). They proved that mice lacking caspase-3 do not experience the onset of autoimmune diabetes, which is indicated by normal level of glucose concentration in the blood, unaffected beta-cells revealing high insulin content, and absence of beta-cell specific T-cell activation in the pancreatic draining lymph nodes. In a different study titled "Immunocytochemical localization of caspase-3 in pancreatic islets from type 2 diabetic subjects", the author reported finding more cleaved caspase-3 immunostained islets from type 2 diabetics, which may indicate an accelerated apoptotic cascade in the islets, along with increasing amyloid deposition before ultimate cell death (5).

The improper control of caspase-mediated cell death and inflammation is linked to various illnesses, including inflammatory, neurological, and other metabolic diseases and cancer. It may be necessary to therapeutically target caspase-3 activity in cells to stop the onset of autoimmune diabetes (1). Numerous natural and synthetic caspase inhibitors have been discovered and created to be used therapeutically. Only a few synthetic caspase inhibitors have progressed into clinical trials due to their lacklustre efficacy or harmful side effects. They have yet to prove compelling enough for patient use (6).

The aim of this study is to detect phytocompounds with drug like properties in African plants that could inhibit Caspase-3 through in-silico analysis.

2.        Materials and Methods

2.1     Materials

The materials used are personal computer, African Natural Compounds Database, PubChem (http://Pubchem.ncbi.nlm.nih.gov) (7), Linux operating system (Ubuntu desktop 18.04), Protein data bank (https://www.rcsb.org/) (8), DataWarrior software (9), PyMOL software (10), AutoDockTools-1.5.6 software (11), AutoDock Vina 1.1.2 software (12), on Ubuntu operating system, and Molinspiration Chemoinformatics web tool (https://www.molinspiration.com/cgi-bin/properties) (13).

2.2     Literature Mining

To find essential targets and receptors for apoptotic processes, literatures were explored. This was done to examine the role of the target and receptors in the pathophysiology and initiation of cell apoptosis. This provides more details regarding the receptor's characteristics, activities, and druggability.

2.3     Selection and Preparation of the Receptor

Caspase 3 in 3D format was retrieved from Protein Data Bank (PDB) with the PDB ID: 3KJF after various targets and receptors had been identified, literature had been mined, and the target and receptor had been analyzed. PyMOL program was initially used to prepare the pdb file by selecting the necessary chains and deleting multiple ligands. To understand how the ligands attach to receptors, PyMOL software was used. The AutoDockTools was used to get the receptor ready for molecular docking simulations. The receptors were prepared by adding polar hydrogens and Kollman's charges before storing them in the pdbqt file format, the structural format needed for performing molecular docking simulation on Autodock vina. As shown in Table 1, the electrostatic grid boxes and the three-dimensional affinity with various sizes and centers were formed around the protein's active region.

Table 1. Grid box parameters used for the molecular docking simulations

2.4     Selection of the Ligands

In this study, 6511 phytocompounds were examined, which were obtained from the African Natural Products Database (African-compounds.org) (14, 15). The compounds were downloaded as 3D-structure data files for analysis. Various parameters such as partition coefficient (Log P), topological polar surface area (TPSA), molecular weight, hydrogen bond donor, and hydrogen bond acceptor were used to assess the phytocompounds. Some of the phytocompounds were found to infringe Lipinski's rule. Those that did not breach the rule underwent toxicological assessment for mutagenicity, carcinogenicity, tumorigenicity, and reproductive effect.

2.5     Preparation of the Ligands

Phytocompounds with no Lipinski’s rule of five infarction and no predicted toxicity (mutagenicity, carcinogenicity, tumorigenicity, and reproductive effect) in-silico were prepared for the molecular docking simulation. Reference ligands were identified from the literature, including the compound co-crystallized with the receptor/protein on the PDB database. In preparation for the ligands for molecular docking simulation, all rotatable bonds, torsions, and Gasteiger charges were assigned and saved in the pdbqt file format.

2.6     Validation of Docking Protocol

The PDB structure of the 3KJF (Caspase 3) protein, in association with a reference inhibitor as was downloaded from the PDB, was replicated in-silico to validate the molecular docking simulations procedure for this protein. Other known inhibitors of Caspase 3 were also used for the validation, including Flubendazole, Fenoprofen, Pranoprofen, and Diflunisal (16). The AutoDockTools-1.5.6 was used to calculate polar hydrogen, Kollman charges, grid box sizes, and centers at a grid space of 1.0 (11, 12). The protein was stored as a pdbqt file. AutoDockTools-1.5.6 was used to prepare the reference chemicals for molecular docking simulation. Torsion-free bonds, as well as any other rotatable bonds, were permitted. After that, files with the pdbqt extension were generated as output. On a Linux environment, a virtual screening shell script was used to locally implement the AutoDockVina® molecular docking simulation of the protein and reference chemical utilizing the centers and sizes (12). Co-crystal inhibitor binding interaction was compared with the re-docked co-crystalized compounds, Flubendazole, Fenoprofen, Pranoprofen, and Diflunisal using PyMol-1.4.1 software and Discovery studio visualizer.

2.7     Molecular Docking of the Phytocompounds on Caspase 3

The phytocompounds were prepared in batches for molecular docking simulations using virtual screening scripts against the Caspase 3. Following the validation of docking methods, four replicates of Molecular Docking Simulations were performed on a Linux platform using AutoDockVina® and related tools. To determine the leading phytocompounds, binding free energy values (kcal/mol SD) were ranked.

2.8     Bioactivity Prediction of Phytocompounds

The online Molinspiration web tool version 2011.06 (www.molinspiration.com) was supplied SMILES notations of the leading phytocompounds to forecast the bioactivity scores for protease inhibition.

2.9     Computational Pharmacokinetics of Frontrunner Phytocompounds

The top phytocompounds underwent a thorough pharmacokinetics evaluation using SwissADME, a web-based tool that assesses the druglikeness, physicochemical, ADME properties, and medicinal chemistry compatibility of small molecules (17). The assessment was conducted to examine the pharmacokinetics of the lead phytocompounds in detail.

2.10  In-depth Toxicity Assessment of Frontrunner Phytocompounds

An in-depth toxicity prediction of the frontrunner phytocompounds for AMES toxicity, Max. tolerated dose (human), hERG I inhibitor, hERG II inhibitor, Oral Rat Acute Toxicity (LD₅₀), Oral Rat Chronic Toxicity (LOAEL), Hepatotoxicity, Skin Sensitization, T. Pyriformis toxicity and Minnow toxicity on the pkCSM platform (18).

2.11  Analysis of the Frontrunner Phytocompounds-Caspase 3 Binding Interactions

The amino acids of Caspase 3 binding interactions with each frontrunner phytocompounds were analyzed using Discovery Studio Visualizer v20.1.0.19295, and Maestro 13.3 aided the generation of 2D structures of the interaction for easy observation (19, 20).

3.        Results

3.1     Preliminary Drug-likeness and Toxicity Assessment of the Ligands (Phytocompounds)

The drug-likeness assessment of the 6511 phytocompounds was performed using Lipinski's rule of five to screen out phytocompounds that violated the guidelines on the DataWarrior application. Following the screening, 3814 phytocompounds had no infraction of Lipinski's rule, but 2697 phytocompounds did. Toxicity testing on the 3814 phytocompounds that did not violate Lipinski's criteria was performed using DataWarrior to discover phytocompounds that could be mutagenic, tumorigenic, irritating, or have reproductive implications. In-silico testing revealed that 1897 phytocompounds possessed none of the identified toxicities. The total polar surface area (TPSA) was also calculated for each phytocompounds.

3.2     Validation of docking protocol

The docking procedure was validated to assure the in-silico repeatability of the experimental protein-ligand interactions gathered from the protein data bank and to observe Caspase 3 amino-acids-conventional hydrogen bond interactions with the reference compounds known as inhibitors of caspase 3. Table 2 shows the binding energy of the docked co-crystalized ligand and that of the reference known inhibitors of caspase 3. Figure 2 is the 2D representation of the docked co-crystalized ligand and reference compounds with the specific Caspase 3 amino acids involved in the interaction. Table 3 shows each reference compound, docked co-crystalized ligand, and the docked co-crystalized ligand with the specific amino acids involved in their interaction with caspase 3.

Table 2. Mean binding energies of the docked co-crystalized ligand and reference compounds