7-Ketocholesterol and 7β-Hydroxycholesterol — In Vitro and Animal Models, Biological Activities, and Toxicity-Preventing Molecules

Purpose: 7-Ketocholesterol (7KC) and 7β-hydroxycholesterol (7β-OHC) are major auto-oxidation products of cholesterol that accumulate in cardiovascular disease, age-related macular degeneration (ARMD), neurodegenerative disorders, and inflammatory bowel diseases. While their cytotoxicity (oxidative stress, organelle dysfunction, cell death, inflammation) is well documented in cell culture, the field lacks a consolidated understanding of how these oxysterols behave in vivo and which model systems (in vitro, animal, and emerging alternatives) are most appropriate for identifying pharmacological targets and protective molecules. This review addresses that gap by systematically surveying the models and mechanisms reported to date. ---

Hypothesis: The authors do not test a single hypothesis (this is a comprehensive review), but the organizing premise is: 7KC and 7β-OHC exert conserved cytotoxic effects across cell types and species—characterized by oxiappotophagy (oxidative stress + apoptosis + autophagy) and inflammation—and these effects can be attenuated by natural and synthetic molecules, with emerging alternative models (organoids, organ/body-on-a-chip, iPSCs) offering improved translational relevance over traditional animal models. ---

Aims: - Primary aim: Review the biogenesis, catabolism, and biological activities of 7KC and 7β-OHC, including the signaling pathways underlying oxidative stress, organelle dysfunction, cell death (oxiappotophagy), and inflammation. - Secondary aim: Catalog the in vitro cell models (vascular, ocular, neural, intestinal, yeast, protozoan) and in vivo animal models (mouse, rat, rabbit, pig, monkey, pigeon, hamster, C. elegans) used to characterize these oxysterols. - Tertiary aim: Identify natural and synthetic compounds capable of preventing or attenuating 7KC- and 7β-OHC-induced cytotoxicity. - Quaternary aim: Discuss alternative methods to animal models ("Lab on a Chip"/"Body on Chip"/organoids/iPSCs) as promising integrative approaches for studying oxysterol biology and drug discovery. ---

Delivery system: This is a review, not a primary research article, so there is no engineered delivery system. However, the model systems/platforms reviewed are: | Category | Examples | |----------|----------| | In vitro cell models — vascular | HUVEC, Eahy926, HAEC, ISO-HAS, EOMA, CAEC, RAEC, A7r5, HA-VSMC, CASMC, THP-1, U937, J774, RAW 264.7, P388D1, PBMCs, peritoneal/arterial macrophages | | In vitro cell models — ocular | ARPE-19, fetal human RPE, ox RPE, porcine RPE, R28, 661W, retinal microglia | | In vitro cell models — neural | SH-SY5Y, SK-N-BE, PC12, C6 glioma, 158N oligodendrocytes, BV-2 microglia, organotypic hippocampal slices | | In vitro cell models — intestinal | Caco-2 | | In vitro cell models — microbial | Saccharomyces cerevisiae, Tetrahymena pyriformis | | In vivo animal models | Mouse (transgenic and wild-type), rat, rabbit, pig/mini-pig, monkey (Macaca mulatta), pigeon, hamster, Caenorhabditis elegans | | Emerging alternatives | Organoids (brain, bowel, retina), organ-on-a-chip, body-on-a-chip, iPSCs | ---

Approach: This is a narrative review synthesizing published literature across: - Biogenesis/catabolism: Enzymatic (CYP7A1, 11β-HSD1/2, SULT2B1b, SOAT1/2, LCAT, CYP27A1) and auto-oxidation pathways. - In vitro studies: Cytotoxicity, oxidative stress, organelle dysfunction, cell death (apoptosis/autophagy), inflammation, signaling pathways (MAPK, PI3K/Akt, NF-κB, NLRP3 inflammasome), Ca²⁺/K⁺ signaling. - Compound screening: Natural products (vitamin E, EGCG, resveratrol, indicaxanthin, argan oil, olive oil, milk thistle seed oil, sea urchin egg oil, Caprobrutus edulis extract, mint extracts) and synthetic molecules (DMF, biotin, K-80003, azelnidipine, sterculic acid). - Bioremediation: Bacterial degradation of 7KC (Nocardia nova, Proteobacterium Y-134, Sphingomonas sp. JEM-1, Rhodococcus jostii RHA1, Pseudomonas aeruginosa, Alcanivorax jadensis IP4, Streptomyces auratus IP2, Serratia marcescens IP3, Thermobifida fusca IP1, Lactobacillus casei ATCC334). - Animal models: Atherosclerosis, ocular disease, neurodegeneration, bowel disease. - Alternative methods: Organoids, microfluidic organ/body-on-a-chip. ---

Key methods: | Technique | Application | |-----------|-------------| | Cell viability assays (MTT, flow cytometry) | Cytotoxicity of 7KC/7β-OHC | | ROS detection (DHE, H₂DCFDA) | Oxidative stress | | Mitochondrial membrane potential (ΔΨm) | Mitochondrial dysfunction | | Caspase activity assays (caspase-2, -3, -7, -8, -9) | Apoptosis | | LC3-I/LC3-II conversion, MDC staining | Autophagy | | PARP cleavage, DNA fragmentation (DFF45/ICAD, CAD) | Apoptosis | | Western blotting | Bcl-2, Bax, Bid, cytochrome c, AIF, Endo-G, PKB/Akt, GSK3β, ERK, p38, NF-κB | | Ca²⁺ imaging (fluo-4, fura-2) | Calcium signaling | | Patch-clamp / K⁺ channel analysis | Kv3.1b, P2X7 | | ELISA | Cytokines (IL-1β, IL-8, IL-6), adhesion molecules (VCAM-1, ICAM-1, E-selectin) | | Gas chromatography | Oxysterol quantification in tissues | | Microarray analysis | Gene expression (rat prefrontal cortex) | | Ames test | Mutagenicity | | PET imaging | 11β-HSD1 in monkey brain | | Bacterial degradation assays | 7KC mineralization, CO₂ release | | Organ-on-a-chip / microfluidics | Emerging alternative models | ---

Key results: 1. Oxiappotophagy is the dominant cell death mode: 7KC and 7β-OHC simultaneously induce oxidative stress (ROS overproduction: O₂⁻, H₂O₂), apoptosis (caspase activation, PARP cleavage, DNA fragmentation), and autophagy (LC3-I→LC3-II conversion, myelin figure formation) across multiple cell types and species. 2. 7β-OHC is a more potent inducer of apoptosis and inflammation than 7KC: Higher percentages of apoptotic cells and greater cytokine secretion (IL-8, IL-1β) and adhesion molecule expression (VCAM-1, ICAM-1, E-selectin) were observed with 7β-OHC. 7KC accumulates in lipid rafts; 7β-OHC does not. 3. Organelle dysfunction is central: Mitochondria (ΔΨm loss, cytochrome c release), lysosomes (permeabilization, myelin figures), and peroxisomes (decreased ABCD3, altered β-oxidation, VLCFA accumulation) are all targets of 7KC/7β-OHC toxicity. Peroxisomal dysfunction can trigger oxidative stress, and peroxisome–mitochondria interactions may amplify cell death. 4. Signaling pathways are conserved across cell types and species: - 7KC: Activates Nox-4 via IRE-1/JNK/AP-1; inhibits PDK-1/Akt/PKB; activates P2X7 receptor (Ca²⁺/Na⁺ influx, K⁺ efflux); increases Kv3.1b; triggers NLRP3 inflammasome; signals through TLR4 → NF-κB. - 7β-OHC: Inhibits PI3K/PKB/Akt; activates GSK3β; dephosphorylates PKB/Akt; activates PKC/P38/MEK/ERK; triggers Ca²⁺ oscillations via CD38/cADPr/NAADP; downregulates Bcl-2; activates Bid/Bax. 5. Protective compounds identified in vitro: - Natural: Vitamin E (α-tocopherol), EGCG, indicaxanthin, resveratrol, argan oil, olive oil, milk thistle seed oil, sea urchin egg oil, Caprobrutus edulis extract, ethanolic mint extracts, DHA, oleic acid. - Synthetic: Dimethylfumarate (DMF/Tecfidera), biotin (vitamin B8), K-80003 (RXRα modulator), azelnidipine (Ca²⁺ channel blocker), sterculic acid, butyrate/SCFAs, DMSO. 6. Bacterial bioremediation of 7KC is promising: Rhodococcus jostii RHA1, Pseudomonas aeruginosa PseA (88% degradation of 1 g/L 7KC), Rhodococcus erythropolis MTCC 3951 (93% degradation), Alcanivorax jadensis IP4 (100% degradation of 1 mg/L within 12 days). Cholesterol oxidase is the key enzyme (converts 7KC to 4-cholesten-3,7-dione). Lysosome-targeted cholesterol oxidase (pEGFP-COXLI) protected human fibroblasts against 50 µM 7KC. 7. Animal models confirm in vivo toxicity: - Mouse: Transgenic models (ApoE⁻/⁻, CD36⁻/⁻, 11β-HSD1 KO) show reduced atherosclerosis with 7KC accumulation; 7KC implants in rat eye induce TLR4-mediated inflammation and VEGF increase; 7KC/7β-OHC injection into rat prefrontal cortex alters 1365 genes (mostly GPRs); intratumoral 7β-OHC liposomes inhibit C6 glioma growth. - Rabbit: 7KC toxicity demonstrated since 1949; 7KC inhibits cholesterol uptake by arterial wall; 7KC/7β-OHC prevent arterial relaxation. - Pig: Oxidized LDL cytotoxicity associated with 7KC/7β-OHC in aortic smooth muscle cells; 7KC involved in pulmonary inflammation during mechanical ventilation. - Monkey: 11β-HSD1 converts 7KC to 7β-OHC

Interpretation: The authors conclude that the potential implications of 7KC and 7β-OHC in many common and disabling diseases (cardiovascular diseases, cataract, ARMD, neurodegenerative diseases, inflammatory bowel diseases) are well documented, but demonstrating direct or indirect involvement still requires significant work on cells, animal models, and/or alternatives to animal models. These approaches will enable better understanding of the biological activities of 7KC and 7β-OHC and identification of natural or synthetic molecules capable of preventing their deleterious effects in fatal and/or strongly debilitating diseases with important societal impacts. ---

Limitations: - Translational gap: Results obtained in animal models are sometimes difficult to transfer to humans. - Limited animal models for non-atherosclerosis diseases: Only a few animal models are described for eye, neurodegenerative, and bowel diseases, whereas numerous models exist for atherosclerosis. - Incomplete understanding of 7β-OHC catabolism: Little is known about 7β-OHC metabolism compared to 7KC. - Limited data on 7KC-induced inflammation pathways: Currently limited data are available on the metabolic pathway(s) contributing to 7KC-induced inflammation. - No data on 7β-OHC-induced inflammasome activation: No data are available on the relationship between 7β-OHC-induced IL-1β secretion and inflammasome activation. - Ca²⁺ signaling mechanisms unclear: The molecular mechanism(s) by which 7KC or 7β-OHC alter cytoplasmic Ca²⁺ levels are unclear; most responses are too rapid to require gene expression changes, excluding LXR- or SREBP-dependent oxysterol sensing pathways. - Cell-type specificity: Not all cell types respond to oxysterols with Ca²⁺ changes (e.g., nerve cells did not show rapid Ca²⁺ responses). - Lack of complete 7KC degradation pathway: None of the identified bacterial gene clusters code for a complete 7KC degradation pathway. - Organ/body-on-a-chip still in early stages: These alternative methods are promising but not yet validated for routine oxysterol research.