Affinage

ISX

Intestine-specific homeobox · UniProt Q2M1V0

Length
245 aa
Mass
27.0 kDa
Annotated
2026-04-28
24 papers in source corpus 13 papers cited in narrative 13 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

ISX (intestine-specific homeobox) is a homeodomain transcription factor that serves as a central gatekeeper of dietary carotenoid and vitamin A metabolism and, when aberrantly expressed outside the intestine, drives oncogenic and immunomodulatory transcriptional programs. In intestinal enterocytes, retinoic acid signaling through retinoic acid receptors induces ISX expression, and ISX in turn directly binds and represses the promoters of BCMO1/BCO1 and SR-BI, forming a negative feedback loop that limits β-carotene absorption and retinoid production; loss of ISX in mice leads to uncontrolled BCO1 expression, excess retinoid accumulation, lymphocyte gut-homing defects, and pancreatic β-islet destruction (PMID:20061533, PMID:18093975, PMID:23393141, PMID:29073082, PMID:37225015). In cancer contexts, ISX is acetylated by PCAF at K69, enabling complex formation with BRD4 and nuclear translocation to activate EMT genes (TWIST1, Snail1, VEGF), and ISX also transcriptionally induces IDO1 to drive kynurenine-AHR signaling and PD-L1-mediated immune evasion in hepatocellular carcinoma (PMID:31908141, PMID:28625979, PMID:23221382). ISX additionally functions in the placenta to regulate β-carotene-dependent MTP expression and lipoprotein biogenesis, and its expression can be induced by pathogen-associated signals (H. pylori, HCV, SARS-CoV-2 spike protein) through NF-κB, whereupon it directly binds and activates metabolic enzyme gene promoters including those of the arachidonic acid pathway (PMID:39645027, PMID:37316966, PMID:41417109).

Mechanistic history

Synthesis pass · year-by-year structured walk · 9 steps
  1. 2007 High

    The foundational question of where ISX is expressed and what happens without it was answered: ISX is an intestinal epithelial transcription factor whose absence derepresses Bcmo1 and SR-BI, establishing it as a repressor of carotenoid metabolism genes.

    Evidence LacZ knock-in mouse model with expression mapping and mRNA profiling under dietary vitamin A manipulation

    PMID:18093975

    Open questions at the time
    • Mechanism of transcriptional repression (direct binding vs. indirect) not yet shown
    • Upstream regulators of ISX expression not defined
    • No human genetic evidence at this point
  2. 2010 High

    The upstream trigger for ISX expression was identified: retinoic acid acting through retinoic acid receptors induces ISX, which then represses SR-BI and BCMO1, completing the negative feedback loop model for vitamin A homeostasis.

    Evidence BCMO1-KO mice and human CaCo-2 cells with dietary retinoid supplementation

    PMID:20061533

    Open questions at the time
    • Direct DNA binding by ISX to target promoters not yet demonstrated
    • Whether feedback loop operates identically in humans in vivo unknown
  3. 2013 High

    Direct promoter binding was demonstrated, resolving whether ISX acts as a direct transcriptional repressor: ISX binds the Bcmo1 promoter, and a human SNP in this binding site alters β-carotene conversion, providing the first human genetic link.

    Evidence Promoter-binding assays, luciferase reporter in CaCo-2 cells, ISX-KO mice with β-carotene supplementation

    PMID:23393141

    Open questions at the time
    • Genome-wide binding profile of ISX in enterocytes not available
    • Effect of the human SNP on vitamin A status in population studies not fully characterized
  4. 2016 Medium

    ISX was found to function as a transcriptional activator in cancer cells: it directly binds the E2F1 promoter to drive proliferation and also induces intestinal metaplasia markers (CDX1/2, cyclin D1) in gastric cancer upon H. pylori infection, revealing context-dependent activator versus repressor roles.

    Evidence Promoter binding assays, forced expression/shRNA knockdown in hepatoma and gastric cancer cells, xenograft models

    PMID:26872890 PMID:27175585

    Open questions at the time
    • How the same transcription factor switches between repressor and activator function is mechanistically unexplained
    • Both studies from single labs
    • No ChIP-seq to define genome-wide oncogenic targets
  5. 2017 High

    Two parallel advances established ISX's systemic consequences: (1) ISX-deficient mice on β-carotene showed that uncontrolled retinoid production disrupts lymphocyte homing and causes pancreatic insulitis, linking ISX to immune and metabolic homeostasis; (2) ISX was shown to mediate IL-6-induced IDO1 expression and kynurenine-AHR positive feedback in HCC, driving PD-L1 upregulation and CD8+ T-cell suppression.

    Evidence ISX-KO mice with dietary β-carotene, immunological/histological analysis; RNAi and ectopic expression in HCC cells with kynurenine measurements

    PMID:28625979 PMID:29073082

    Open questions at the time
    • Whether pancreatic insulitis phenotype is reversible with dietary intervention unknown
    • IDO1/PD-L1 axis demonstrated only in HCC cell lines, not validated in primary tumors
    • Relationship between intestinal repressor function and cancer activator function unexplored
  6. 2020 High

    The post-translational mechanism enabling ISX's oncogenic activator function was decoded: PCAF acetylation of ISX at K69 promotes interaction with acetylated BRD4, nuclear translocation, H3 acetylation at target promoters, and EMT gene activation, explaining how ISX switches from repressor to activator.

    Evidence Co-IP, acetylation assays, ChIP, nuclear translocation imaging, site-specific mutagenesis in lung cancer cells

    PMID:31908141

    Open questions at the time
    • Whether PCAF-ISX-BRD4 axis operates in non-lung cancers not tested
    • Crystal structure of ISX or ISX-BRD4 complex unavailable
    • Whether acetylation also modulates ISX's intestinal repressor function unknown
  7. 2023 High

    ISX's gatekeeper role was extended to non-retinoid carotenoids and to pathogen-driven contexts: double-KO mice showed ISX controls zeaxanthin bioavailability via SR-B1, while HCV core protein was found to activate ISX through NF-κB to drive IDO1/kynurenine and fibrosis programs.

    Evidence Isx−/−/Bco2−/− double-KO mice with LC-MS profiling; HCV replicon/transgenic mice with shRNA rescue

    PMID:37225015 PMID:37316966

    Open questions at the time
    • Whether ISX regulates additional carotenoid transporters beyond SR-B1 unknown
    • HCV-ISX axis studied in single lab
    • In vivo immune phenotype from HCV-induced ISX not fully characterized
  8. 2024 Medium

    ISX's tissue scope was broadened beyond the intestine: it was shown to regulate MTP expression in placenta in a β-carotene-dependent manner, establishing ISX as a tissue-specific regulator of placental lipoprotein biogenesis.

    Evidence Isx-KO mouse model with β-carotene administration, MTP expression compared between placenta and intestine

    PMID:39645027

    Open questions at the time
    • How ISX reaches placental expression given its intestinal specification is not explained
    • Downstream consequences for fetal lipid transport not measured
    • Single study, not independently replicated
  9. 2025 Medium

    ChIP-seq revealed a new genome-wide binding repertoire for ISX in pulmonary cells: SARS-CoV-2 spike activates ACE2-MYD88-NF-κB to induce VSIR-ISX signaling, and ISX directly occupies promoters of arachidonic acid metabolism genes, expanding the known ISX target gene set beyond carotenoid and EMT genes.

    Evidence RNA-seq, ChIP-seq, shRNA knockdown, NF-κB inhibitor treatment in pulmonary cells

    PMID:41417109

    Open questions at the time
    • VSIR-MAPK-ISX signaling axis requires independent validation
    • Physiological relevance during actual SARS-CoV-2 infection in vivo not tested
    • How ISX expression is induced in lung tissue not fully resolved

Open questions

Synthesis pass · forward-looking unresolved questions
  • Key unresolved questions include: (1) the structural basis for ISX DNA binding and how acetylation switches its function from repressor to activator; (2) the full genome-wide cistrome of ISX in intestinal versus cancer versus placental contexts; (3) whether ISX-targeted therapies (e.g., BRD4 inhibitors or deacetylase modulation) can selectively block oncogenic ISX without disrupting vitamin A homeostasis.
  • No crystal or cryo-EM structure of ISX
  • No integrative multi-tissue ChIP-seq comparison
  • No therapeutic targeting studies

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0140110 transcription regulator activity 8 GO:0003677 DNA binding 4
Localization
GO:0005634 nucleus 3
Pathway
R-HSA-74160 Gene expression (Transcription) 8 R-HSA-1430728 Metabolism 6 R-HSA-1643685 Disease 5 R-HSA-162582 Signal Transduction 4 R-HSA-168256 Immune System 3
Partners
BCO1BRD4E2F1IDO1PCAFSCARB1
Complex memberships
PCAF-ISX-BRD4

Evidence

Reading pass · 13 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2010 Retinoic acid via retinoic acid receptors induces expression of intestinal transcription factor ISX, which then represses expression of SR-BI (scavenger receptor class B type 1) and the carotenoid-15,15'-oxygenase BCMO1, establishing a negative feedback loop controlling β-carotene absorption and vitamin A production. Mouse knockout models (BCMO1-KO mice) combined with human CaCo-2 cell lines; dietary retinoid supplementation experiments FASEB journal High 20061533
2007 ISX is expressed in intestinal epithelial cells from duodenum to proximal colon; Isx-deficient mice show dramatically increased expression of Bcmo1 and SR-BI, indicating ISX acts as a transcriptional repressor of these genes in the intestine. Vitamin A deficiency regulates Isx expression in a segment-specific manner. LacZ knock-in mouse model (IsxLacZ/LacZ), LacZ staining for expression mapping, quantitative mRNA profiling, dietary vitamin A manipulation The Journal of biological chemistry High 18093975
2013 ISX directly binds to the Bcmo1 promoter and acts as a transcriptional repressor of BCMO1 expression; ISX-deficient mice display increased intestinal BCMO1 expression and produce more vitamin A from β-carotene. A common SNP in the ISX binding site of the human BCMO1 promoter is associated with altered conversion rates. Promoter-binding assays, luciferase reporter assay in CaCo-2 cells, ISX-knockout mice with β-carotene supplementation The Journal of biological chemistry High 23393141
2017 ISX represses Bco1 gene expression in response to retinoic acid signaling; in ISX-deficient mice, uncontrolled Bco1 expression leads to increased retinoid production in the intestine, elevated hepatic retinoid stores, increased expression of retinoic acid-inducible target genes (Aldh1a2, Dhrs3, Ccr9), disruption of gut-homing and differentiation of lymphocytes, and pancreatic insulitis with β-islet cell destruction. ISX-knockout mice with dietary β-carotene supplementation; gene expression profiling; immunological and histological analysis Proceedings of the National Academy of Sciences of the United States of America High 29073082
2020 PCAF acetylates ISX at lysine 69, promoting interaction between ISX and acetylated BRD4 (at lysine 332), and the resulting PCAF-ISX-BRD4 complex translocates into the nucleus where it binds promoters of EMT genes, induces histone 3 acetylation at K9, K14, and K18, and drives transcriptional activation of EMT genes including TWIST1, Snail1, and VEGF while suppressing E-cadherin. Co-immunoprecipitation, acetylation assays, chromatin immunoprecipitation, nuclear translocation imaging, ectopic expression and shRNA knockdown in lung cancer cells EMBO reports High 31908141
2017 ISX mediates IL-6-induced expression of IDO1 and tryptophan 2,3-dioxygenase in hepatocellular carcinoma cells, increasing kynurenine levels; kynurenine activates aryl hydrocarbon receptor (AHR), which feeds back to increase ISX expression. ISX also induces expression of immune modulators CD86 and PD-L1 in an IDO1-dependent manner, suppressing CD8+ T-cell responses. RNAi-mediated knockdown, ectopic ISX expression, metabolite measurement (kynurenine), gene expression assays in HCC cells Cancer research Medium 28625979
2012 ISX promotes hepatocellular carcinoma cell proliferation and tumorigenic activity through CCND1 (cyclin D1) induction; shRNA-mediated ISX knockdown decreases cell proliferation and malignant transformation in vitro and in vivo. Enforced expression and shRNA knockdown of ISX in hepatoma cells; in vitro proliferation assays; xenograft in vivo model Cancer research Medium 23221382
2016 ISX transcriptionally activates E2F1 by directly binding to the E2 site of its promoter; ISX increases E2F1 expression and phosphorylation at serine 332, enabling nuclear translocation of E2F1 and formation of the E2F1-DP-1 complex, promoting cell proliferation and suppressing apoptosis in hepatoma cells. Promoter binding assay (E2 site), forced ISX expression and shRNA knockdown, phosphorylation analysis, in vitro and in vivo hepatoma cell models Oncotarget Medium 27175585
2016 ISX expression induced by H. pylori infection upregulates CDX1/2, cyclin D1, and MUC2 while downregulating MUC5AC in gastric cancer cells, inducing intestinal metaplasia-like features and enhanced tumorigenic potential. Stable ISX-expressing MKN45 cells, spheroid colony formation assay, xenograft model, H. pylori co-culture Journal of gastroenterology Medium 26872890
2023 Genetic deletion of ISX in mice (in combination with Bco2 deletion) further enhances zeaxanthin accumulation in tissues beyond that seen with Bco2 deletion alone, demonstrating that ISX controls SR-B1 expression in enterocytes and thereby acts as a gatekeeper of carotenoid (zeaxanthin) bioavailability. Isx-/-/Bco2-/- double-knockout mice; LC-MS metabolite profiling; tissue accumulation measurements Molecular metabolism High 37225015
2023 HCV core protein activates the NF-κB signaling pathway to upregulate ISX expression; ISX then drives expression of metabolic enzymes (IDO1, kynurenine pathway), fibrosis progenitors, and immune modulators (PD-L1, B7-2); shRNAi knockdown of ISX inhibits HCV core protein-induced metabolic disturbance and immune suppression. HCV JFH-1 replicon cell system, transgenic mice with HCV core protein, shRNA knockdown of ISX, in vivo high-fat diet liver fibrosis model Advanced science Medium 37316966
2024 ISX is expressed in mouse placenta and is regulated by β-carotene availability; genetic deletion of Isx disrupts β-carotene-mediated transcriptional regulation of placental MTP (microsomal triglyceride transfer protein), identifying ISX as a tissue-specific regulator of placental β-carotene-dependent lipoprotein biogenesis (this regulation was not observed in intestinal enterocytes). Isx-knockout mouse model, β-carotene administration, MTP expression analysis in placenta vs. intestine Biochimica et biophysica acta. Molecular and cell biology of lipids Medium 39645027
2025 SARS-CoV-2 spike protein activates the ACE2-MYD88-NF-κB pathway to upregulate the VSIR-ISX signaling axis; ISX, activated via VSIR-MAPK signaling, directly binds promoters of arachidonic acid metabolism enzyme genes (as shown by ChIP-seq) to drive their expression, disrupting metabolic homeostasis; shRNA knockdown of ISX or NF-κB inhibitors mitigates these effects. RNA sequencing, chromatin immunoprecipitation followed by genome sequencing (ChIP-seq), shRNA knockdown, NF-κB inhibitor treatment in pulmonary cells Cell biology and toxicology Medium 41417109

Source papers

Stage 0 corpus · 24 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2010 ISX is a retinoic acid-sensitive gatekeeper that controls intestinal beta,beta-carotene absorption and vitamin A production. FASEB journal : official publication of the Federation of American Societies for Experimental Biology 180 20061533
2013 Genetics and diet regulate vitamin A production via the homeobox transcription factor ISX. The Journal of biological chemistry 107 23393141
2007 Isx participates in the maintenance of vitamin A metabolism by regulation of beta-carotene 15,15'-monooxygenase (Bcmo1) expression. The Journal of biological chemistry 76 18093975
2020 PCAF-mediated acetylation of ISX recruits BRD4 to promote epithelial-mesenchymal transition. EMBO reports 43 31908141
2017 Transcription factor ISX mediates the cross talk between diet and immunity. Proceedings of the National Academy of Sciences of the United States of America 42 29073082
2017 Reduced Mutation Rate and Increased Transformability of Transposon-Free Acinetobacter baylyi ADP1-ISx. Applied and environmental microbiology 34 28667117
2017 Intestine-Specific Homeobox Gene ISX Integrates IL6 Signaling, Tryptophan Catabolism, and Immune Suppression. Cancer research 29 28625979
2012 Proinflammatory homeobox gene, ISX, regulates tumor growth and survival in hepatocellular carcinoma. Cancer research 27 23221382
2020 ISX-9 manipulates endocrine progenitor fate revealing conserved intestinal lineages in mouse and human organoids. Molecular metabolism 18 32180555
2016 Intestine-specific homeobox (ISX) upregulates E2F1 expression and related oncogenic activities in HCC. Oncotarget 15 27175585
2016 ISX-9 can potentiate cell proliferation and neuronal commitment in the rat dentate gyrus. Neuroscience 15 27373772
2023 Genetic deletion of Bco2 and Isx establishes a golden mouse model for carotenoid research. Molecular metabolism 14 37225015
2016 Intestine-specific homeobox (ISX) induces intestinal metaplasia and cell proliferation to contribute to gastric carcinogenesis. Journal of gastroenterology 13 26872890
2022 ISX-9 potentiates CaMKIIδ-mediated BMAL1 activation to enhance circadian amplitude. Communications biology 9 35902736
2020 PCAF, ISX, and BRD4: a maleficent alliance serving lung cancer malignancy. EMBO reports 9 31908099
2023 HCV Core Protein-ISX Axis Promotes Chronic Liver Disease Progression via Metabolic Remodeling and Immune Suppression. Advanced science (Weinheim, Baden-Wurttemberg, Germany) 6 37316966
2017 Effects of Isx-9 and stress on adult hippocampal neurogenesis: Experimental considerations and future perspectives. Neurogenesis (Austin, Tex.) 5 28656155
2022 Vitamin A deficiency during the perinatal period induces changes in vitamin A metabolism in the offspring. The regulation of intestinal vitamin A metabolism via ISX occurs only in male rats severely vitamin A-deficient. European journal of nutrition 4 36178520
2024 ISX-9 Promotes KGF Secretion From MSCs to Alleviate ALI Through NGFR-ERK-TAU-β-Catenin Signaling Axis. Stem cells translational medicine 3 38159248
2019 Characterization of Novel Murine and Human PDAC Cell Models: Identifying the Role of Intestine Specific Homeobox Gene ISX in Hypoxia and Disease Progression. Translational oncology 3 31174057
2024 The intestine-specific homeobox (ISX) modulates β-carotene-dependent regulation of microsomal triglyceride transfer protein (MTP) in a tissue-specific manner. Biochimica et biophysica acta. Molecular and cell biology of lipids 2 39645027
2026 ISX promotes tumor migration and invasion in lung cancer by upregulating COL1A1 in vitro. Molecular medicine reports 0 41480691
2025 Engineered Acinetobacter baylyi ADP1-ISx Cells Are Sensitive DNA Biosensors for Antibiotic Resistance Genes and a Fungal Pathogen of Bats. ACS synthetic biology 0 40579381
2025 Spike protein-induced VSIR-ISX signaling disrupts metabolic homeostasis and promotes COVID-19-related immune dysfunction. Cell biology and toxicology 0 41417109