GO:0004506 squalene monooxygenase activity: Mechanism, Genes and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0004506 (squalene monooxygenase activity) catalyzes the NADPH- and O2-dependent conversion of squalene to (S)-2,3-epoxysqualene, the first oxygen-requiring step in sterol biosynthesis.
• The enzyme is encoded by SQLE (squalene epoxidase) in humans and is a rate-limiting, post-translationally regulated node in the mevalonate/cholesterol pathway.
• SQLE is frequently overexpressed in cancers and supports tumor growth by attenuating ER stress and activating lipid raft-dependent Src/PI3K/Akt signaling.
• Targeting SQLE can restore anti-PD-1 efficacy in metabolic dysfunction-associated steatohepatitis-induced hepatocellular carcinoma and synergizes with immune-checkpoint blockade in glioblastoma.
• SQLE-catalyzed 24(S),25-epoxycholesterol synthesis promotes trained-immunity-mediated antitumor activity, linking this enzymatic activity to innate immune memory.
• CRISPR knockout, point-mutation, knock-in, and overexpression models are essential to dissect SQLE function and to validate it as a therapeutic target.
Description
Squalene monooxygenase activity (GO:0004506) is a molecular function defined as the catalysis of the reaction H+ + NADPH + O2 + squalene = (S)-2,3-epoxysqualene + H2O + NADP+. This enzymatic step introduces the first oxygen atom into the sterol biosynthetic pathway and is therefore a critical commitment point for cholesterol and related isoprenoid end-products. In humans, the activity is carried out by squalene epoxidase (SQLE), a flavoprotein monooxygenase anchored to the endoplasmic reticulum membrane. Because the reaction is rate-limiting and tightly regulated, its dysregulation has broad consequences for membrane biogenesis, lipid raft signaling, and cellular stress responses. Beyond cholesterol homeostasis, squalene monooxygenase activity has emerged as a therapeutic vulnerability in oncology and immunometabolism. SQLE overexpression promotes pancreatic cancer growth by attenuating ER stress and activating lipid raft-regulated Src/PI3K/Akt signaling. In hepatocellular carcinoma associated with metabolic dysfunction-associated steatohepatitis, targeting squalene epoxidase restores anti-PD-1 efficacy, indicating that this enzymatic activity shapes the tumor immune microenvironment. In glioblastoma, targeting microglial metabolic rewiring synergizes with immune-checkpoint blockade therapy, further implicating sterol pathway enzymes in immune modulation. From a pharmacological perspective, squalene monooxygenase has long been recognized as a target for hypercholesterolemic therapy, and its inhibition by antifungal agents such as terbinafine underscores its druggability. More recently, squalene-epoxidase-catalyzed 24(S),25-epoxycholesterol synthesis was shown to promote trained-immunity-mediated antitumor activity, expanding the biological roles of this activity beyond cholesterol biosynthesis. Understanding GO:0004506 therefore requires integrating enzymology, cell biology, and immunology, and CRISPR-based models are central to that effort.
squalene monooxygenase activity At A Glance
| GO ID | GO:0004506 |
|---|---|
| GO term | squalene monooxygenase activity |
| Ontology | molecular_function |
| Synonym | squalene epoxidase activity; squalene-2,3-epoxidase activity; squalene hydroxylase activity; squalene-2,3-epoxide cyclase activity; squalene 2,3-oxidocyclase activity; squalene oxydocyclase activity; squalene,NADPH:oxygen oxidoreductase (2,3-epoxidizing) activity |
| Definition | Catalysis of the reaction: H+ + NADPH + O2 + squalene = (S)-2,3-epoxysqualene + H2O + NADP+. |
| Major function | First oxygen-requiring, rate-limiting step in sterol biosynthesis; converts squalene to (S)-2,3-epoxysqualene. |
| Representative gene | SQLE (squalene epoxidase) in humans. |
| Cofactors | NADPH and molecular oxygen (O2). |
| Subcellular location | Endoplasmic reticulum membrane. |
What Is GO:0004506?
In plain terms, GO:0004506 describes the enzyme activity that uses NADPH and molecular oxygen to convert squalene into (S)-2,3-epoxysqualene, releasing water and NADP+. This is the first oxygenation step in the sterol biosynthesis pathway and is catalyzed by squalene monooxygenase (squalene epoxidase, SQLE) in humans. The reaction is essential for the subsequent cyclization of (S)-2,3-epoxysqualene into lanosterol, the precursor of cholesterol and other sterols.
Why Is squalene monooxygenase activity Important in Cell Biology?
Squalene monooxygenase activity is important because it controls the flux of squalene into the sterol biosynthetic pathway, thereby influencing cholesterol availability, membrane lipid composition, and the production of signaling sterols such as 24(S),25-epoxycholesterol. Dysregulation of this activity has been linked to cancer progression, immune evasion, and drug-induced hepatotoxicity, making it a focal point for both mechanistic studies and therapeutic development.
• Rate-limiting step in cholesterol biosynthesis, controlling sterol flux and membrane biogenesis.
• Overexpressed in multiple cancers and supports tumor growth via ER stress attenuation and Src/PI3K/Akt signaling.
• Targeting SQLE restores anti-PD-1 efficacy in MASH-induced hepatocellular carcinoma.
• SQLE inhibition synergizes with immune-checkpoint blockade in glioblastoma by targeting microglial metabolic rewiring.
• SQLE-catalyzed 24(S),25-epoxycholesterol synthesis promotes trained-immunity-mediated antitumor activity.
• Disturbing cholesterol/sphingolipid metabolism via SQLE contributes to crizotinib hepatotoxicity.
• Squalene monooxygenase is a validated target for hypercholesterolemic therapy.
• Antifungal agents such as tellurium nanoparticles modulate squalene monooxygenase gene expression in Candida albicans.
• UBE2J2 sensitizes the ERAD ubiquitination cascade to changes in membrane lipid saturation, linking SQLE activity to ER quality control.
• CRISPR-based models enable causal dissection of SQLE function in disease and immunity.
Molecular Mechanism of squalene monooxygenase activity
Substrate binding and oxygen activation
In simple terms: The enzyme grabs squalene and uses oxygen and NADPH to insert an oxygen atom into it.
Squalene monooxygenase (SQLE) binds its hydrophobic substrate squalene within the endoplasmic reticulum membrane and utilizes NADPH and molecular oxygen to catalyze the epoxidation of the C2-C3 double bond, yielding (S)-2,3-epoxysqualene. The reaction consumes one molecule each of NADPH and O2 and releases water and NADP+.
Catalytic cycle and cofactor usage
In simple terms: NADPH provides electrons that help oxygen react with squalene.
The catalytic cycle of squalene monooxygenase involves flavin-dependent activation of molecular oxygen, with NADPH serving as the electron donor. This monooxygenase activity is classified under GO:0004506 and is distinct from subsequent cyclization steps that convert (S)-2,3-epoxysqualene to lanosterol.
Post-translational regulation by cholesterol and ERAD
In simple terms: When cholesterol is high, the enzyme is degraded to slow down the pathway.
SQLE is post-translationally regulated by cholesterol, which promotes its degradation via the endoplasmic reticulum-associated degradation (ERAD) pathway. UBE2J2 sensitizes the ERAD ubiquitination cascade to changes in membrane lipid saturation, linking SQLE stability to lipid environment.
Role in sterol and epoxycholesterol synthesis
In simple terms: The product of this enzyme can be diverted to make signaling sterols.
The product (S)-2,3-epoxysqualene is a precursor for lanosterol and cholesterol, but squalene epoxidase activity also contributes to the synthesis of 24(S),25-epoxycholesterol, which promotes trained-immunity-mediated antitumor activity. This branching highlights the importance of GO:0004506 beyond bulk cholesterol production.
Inhibition and pharmacological targeting
In simple terms: Drugs can block this enzyme to lower cholesterol or treat fungal infections.
Squalene monooxygenase is a target for hypercholesterolemic therapy, and antifungal agents such as terbinafine inhibit the enzyme. Biogenic tellurium nanoparticles affect squalene monooxygenase gene expression in Candida albicans, demonstrating the enzyme's antifungal relevance.
Key Genes Involved in GO:0004506 squalene monooxygenase activity
The following genes and proteins are directly or functionally linked to squalene monooxygenase activity (GO:0004506) and its biological context.
| Gene | Major Role | Research Relevance |
|---|---|---|
| SQLE | Encodes squalene epoxidase, the enzyme catalyzing GO:0004506 | Rate-limiting sterol biosynthesis; cancer and immune therapy target |
| HMGCR | Rate-limiting enzyme upstream in mevalonate pathway | Coordinates with SQLE in cholesterol synthesis |
| FDFT1 | Squalene synthase, produces squalene substrate | Provides substrate for SQLE |
| LSS | Lanosterol synthase, cyclizes (S)-2,3-epoxysqualene | Downstream of SQLE in sterol pathway |
| CYP51A1 | Lanosterol 14-alpha demethylase | Post-squalene sterol synthesis |
| UBE2J2 | ERAD ubiquitin-conjugating enzyme | Regulates SQLE stability via ERAD |
| INSIG1 | Cholesterol sensing and ER retention | Regulates SREBP pathway and sterol synthesis |
| SCAP | SREBP cleavage-activating protein | Controls sterol-responsive transcription |
| SREBF2 | Master transcription factor for cholesterol genes | Drives SQLE expression |
| NR1H2 (LXRβ) | Nuclear receptor for oxysterols | Senses 24(S),25-epoxycholesterol |
| NR1H3 (LXRα) | Nuclear receptor for oxysterols | Links SQLE products to immune training |
| PD-1 (PDCD1) | Immune checkpoint | SQLE targeting restores anti-PD-1 efficacy |
| SRC | Kinase in lipid raft signaling | Activated downstream of SQLE in cancer |
| PIK3CA | PI3K catalytic subunit | Lipid raft-PI3K/Akt signaling downstream of SQLE |
| AKT1 | Serine/threonine kinase | Effector of SQLE-driven survival signaling |
| CASP3 | Apoptosis executioner | ER stress-related apoptosis modulated by SQLE |
| HSPA5 (BiP) | ER chaperone | ER stress marker affected by SQLE |
| DDIT3 (CHOP) | ER stress-induced transcription factor | Mediates ER stress responses linked to SQLE |
How Is squalene monooxygenase activity Regulated?
Squalene monooxygenase activity is regulated at multiple levels. Transcriptionally, SQLE is a target of SREBP-2, which is controlled by sterol sensing via INSIG and SCAP. Post-translationally, SQLE protein stability is regulated by cholesterol-induced ERAD, and UBE2J2 sensitizes this degradation to membrane lipid saturation. Additionally, the enzyme's product (S)-2,3-epoxysqualene can be channeled toward 24(S),25-epoxycholesterol, which activates LXR and influences immune training.
squalene monooxygenase activity and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| SQLE | Pancreatic cancer growth via ER stress and Src/PI3K/Akt | SQLE knockout or overexpression in pancreatic cancer cell lines |
| SQLE | MASH-induced hepatocellular carcinoma and anti-PD-1 response | SQLE knockout in HCC cells co-cultured with T cells |
| SQLE | Glioblastoma immune evasion | SQLE knockout in glioblastoma cells and microglia co-culture |
| SQLE | Crizotinib hepatotoxicity | SQLE knockout hepatocytes treated with crizotinib |
| SQLE | Candida albicans antifungal response | SQLE knockdown in C. albicans and tellurium nanoparticle treatment |
Squalene monooxygenase activity in cancer
SQLE overexpression promotes pancreatic cancer growth by attenuating ER stress and activating lipid raft-regulated Src/PI3K/Akt signaling. In glioblastoma, targeting microglial metabolic rewiring synergizes with immune-checkpoint blockade therapy, implicating sterol pathway enzymes in tumor immunity. Targeting squalene epoxidase restores anti-PD-1 efficacy in metabolic dysfunction-associated steatohepatitis-induced hepatocellular carcinoma, demonstrating that this activity shapes immunotherapy responses.
Squalene monooxygenase activity in immune modulation
Squalene-epoxidase-catalyzed 24(S),25-epoxycholesterol synthesis promotes trained-immunity-mediated antitumor activity, linking GO:0004506 to innate immune memory. This suggests that modulating this enzymatic activity could enhance antitumor immunity.
Squalene monooxygenase activity in drug-induced toxicity
Disturbing cholesterol/sphingolipid metabolism by squalene epoxidase arises in crizotinib hepatotoxicity, indicating that SQLE dysregulation can contribute to adverse drug reactions.
Squalene monooxygenase activity in infectious disease
Antifungal activity of biogenic tellurium nanoparticles against Candida albicans is associated with effects on squalene monooxygenase gene expression, highlighting the enzyme as an antifungal target.
From squalene monooxygenase activity-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| Does SQLE loss reduce cholesterol synthesis? | SQLE knockout cell line (e.g., HepG2, HeLa) |
| Does a specific SQLE mutation alter enzyme activity? | Point-mutation knock-in of catalytic residues |
| Does SQLE overexpression drive tumor growth? | SQLE overexpression in cancer cell lines |
| Does SQLE product 24(S),25-epoxycholesterol mediate immune training? | Knock-in of SQLE with tagged version and LXR reporter |
| Does SQLE regulate ER stress? | SQLE knockout with ER stress reporter (CHOP, BiP) |
| Does SQLE inhibition synergize with immunotherapy? | SQLE knockout in tumor cells co-cultured with T cells |
How to Study the squalene monooxygenase activity Process
| Method | What It Measures | Typical Application |
|---|---|---|
| NADPH consumption assay | Enzymatic activity of squalene monooxygenase | In vitro enzyme kinetics |
| LC-MS lipidomics | Sterol intermediates including (S)-2,3-epoxysqualene | Pathway flux analysis |
| CRISPR knockout screen | Gene dependencies linked to SQLE | Identify synthetic lethal partners |
| RNA-seq | Transcriptional changes in sterol pathway | SREBP-2 target validation |
| Proteomics | SQLE protein stability and ERAD | Post-translational regulation |
| Immunoblotting | SQLE protein levels | Cholesterol-induced degradation |
| Co-culture assays | T cell-mediated killing | Immunotherapy synergy |
| ER stress reporter | CHOP/BiP expression | ER stress modulation by SQLE |
Enzymatic activity assays
Squalene monooxygenase activity can be measured using NADPH consumption or (S)-2,3-epoxysqualene production in membrane fractions. These assays are foundational for characterizing GO:0004506.
CRISPR-based genetic screens
Genome-wide CRISPR knockout screens can identify modifiers of SQLE dependency and resistance to SQLE inhibition. Such screens link GO:0004506 to cellular fitness.
Lipidomics and metabolomics
Mass spectrometry-based lipidomics quantifies sterol intermediates and 24(S),25-epoxycholesterol, providing direct readouts of squalene monooxygenase activity.
Transcriptomics and proteomics
RNA-seq and proteomics reveal SREBP-2 target gene expression and SQLE protein stability changes under cholesterol or drug treatment.
How CRISPR Can Be Used to Study GO:0004506 squalene monooxygenase activity
Knockout
CRISPR knockout of SQLE abolishes squalene monooxygenase activity, leading to cholesterol auxotrophy and accumulation of squalene. This model is used to test dependency in cancer cells and to validate target engagement.
Point Mutation
Point mutations in SQLE catalytic residues can dissect the enzymatic mechanism of GO:0004506 and separate catalytic activity from non-enzymatic functions.
Knock-in
Knock-in of tagged SQLE (e.g., HA or GFP) enables localization and interaction studies, and knock-in of disease-associated variants can model altered enzyme activity.
Overexpression
Overexpression of SQLE in cancer cell lines recapitulates the tumor-promoting effects observed in pancreatic cancer and hepatocellular carcinoma, and can be used to test drug resistance.
How EDITGENE Supports squalene monooxygenase activity Research
Researchers studying squalene monooxygenase activity-related genes often need to determine whether a candidate gene is causally involved in sterol metabolism, immune modulation, or drug response. EDITGENE provides CRISPR-based cell model services to enable such causal studies.
Contact EDITGENE today to design your custom CRISPR model for squalene monooxygenase activity research.
Frequently Asked Questions About squalene monooxygenase activity
What is squalene monooxygenase activity?
Squalene monooxygenase activity (GO:0004506) is the enzyme activity that converts squalene to (S)-2,3-epoxysqualene using NADPH and oxygen, the first oxygenation step in cholesterol biosynthesis.
What gene encodes squalene monooxygenase activity?
In humans, squalene monooxygenase activity is encoded by SQLE (squalene epoxidase).
What is the reaction catalyzed by GO:0004506?
The reaction is H+ + NADPH + O2 + squalene = (S)-2,3-epoxysqualene + H2O + NADP+.
Why is squalene monooxygenase activity important in cancer?
SQLE overexpression promotes tumor growth and immune evasion, and targeting it can restore anti-PD-1 efficacy in hepatocellular carcinoma and synergize with checkpoint blockade in glioblastoma.
What diseases are linked to squalene monooxygenase activity?
It has been linked to pancreatic cancer, hepatocellular carcinoma, glioblastoma, crizotinib hepatotoxicity, and fungal infections.
How is squalene monooxygenase activity regulated?
It is regulated transcriptionally by SREBP-2 and post-translationally by cholesterol-induced ERAD, with UBE2J2 sensitizing degradation to lipid saturation.
What are the synonyms for squalene monooxygenase activity?
Synonyms include squalene epoxidase activity, squalene-2,3-epoxidase activity, squalene hydroxylase activity, and squalene 2,3-oxidocyclase activity.
What cofactors are required for squalene monooxygenase activity?
The enzyme requires NADPH and molecular oxygen (O2).
How can I study squalene monooxygenase activity in the lab?
Common methods include NADPH consumption assays, lipidomics, CRISPR knockout, and overexpression models.
Does squalene monooxygenase activity affect immunity?
Yes, SQLE-catalyzed 24(S),25-epoxycholesterol synthesis promotes trained-immunity-mediated antitumor activity.
Conclusion
Squalene monooxygenase activity (GO:0004506) is a central enzymatic step in sterol biosynthesis with far-reaching implications for cancer, immunity, and drug toxicity. Its catalytic product (S)-2,3-epoxysqualene feeds both cholesterol and signaling sterols, and its regulation is tightly controlled by SREBP-2 and ERAD. CRISPR-based knockout, point-mutation, knock-in, and overexpression models are indispensable for dissecting the causal roles of SQLE in disease, and EDITGENE provides end-to-end services to generate such models.
References
- 1. Ye Z et al.. 2023. Targeting Microglial Metabolic Rewiring Synergizes with Immune-Checkpoint Blockade Therapy for Glioblastoma.. Cancer Discov 13(4):974-1001 PMID: 36649564
- 2. Wen J et al.. 2024. Targeting squalene epoxidase restores anti-PD-1 efficacy in metabolic dysfunction-associated steatohepatitis-induced hepatocellular carcinoma.. Gut 73(12):2023-2036 PMID: 38744443
- 3. Yan H et al.. 2025. Disturbing Cholesterol/Sphingolipid Metabolism by Squalene Epoxidase Arises Crizotinib Hepatotoxicity.. Adv Sci (Weinh) 12(14):e2414923 PMID: 39836491
- 4. Xu R et al.. 2023. SQLE promotes pancreatic cancer growth by attenuating ER stress and activating lipid rafts-regulated Src/PI3K/Akt signaling pathway.. Cell Death Dis 14(8):497 PMID: 37542052
- 5. Belter A et al.. 2011. Squalene monooxygenase - a target for hypercholesterolemic therapy.. Biol Chem 392(12):1053-75 PMID: 22050222
- 6. Zare B et al.. 2014. Antifungal activity of biogenic tellurium nanoparticles against Candida albicans and its effects on squalene monooxygenase gene expression.. Biotechnol Appl Biochem 61(4):395-400 PMID: 24237269
- 7. Vrentzou A et al.. 2025. UBE2J2 sensitizes the ERAD ubiquitination cascade to changes in membrane lipid saturation.. Nat Commun 16(1):8973 PMID: 41068091
- 8. Liu Y et al.. 2024. Squalene-epoxidase-catalyzed 24(S),25-epoxycholesterol synthesis promotes trained-immunity-mediated antitumor activity.. Cell Rep 43(4):114094 PMID: 38613784
