HACD3

HACD3 is an ER-associated 3-hydroxyacyl-CoA dehydratase that catalyzes the third step of very long-chain fatty acid elongation, acting in complex with ELOVL1-7 condensing enzymes, but its intrinsic dehydratase activity is comparatively weak relative to HACD1/HACD2 (PMID:18554506, PMID:28784662). Beyond this metabolic role, HACD3 functions as a multivalent signaling scaffold whose protein-interaction activities are largely independent of fatty acid metabolism. It associates with activated Rac1 to potentiate JNK and NF-κB signaling (PMID:10747961), is recruited by AdipoR2 to channel polyunsaturated fatty acids into phospholipids (PMID:37164154), and binds prohibitin (PHB) via its middle region (aa 141–178) to drive PHB-Y259 dephosphorylation, disrupting PHB-Raf coupling and suppressing Raf/ERK-mediated EMT in colorectal cancer (PMID:38617530). HACD3 also displays protein kinase activity, promoting CDK2 T160 phosphorylation through an internal domain (aa 298–324) to support cell-cycle progression and tumor growth (PMID:39547639), while a distinct C-terminal segment (aa 231–259) mediates binding to MKK7 and MAPK10 to suppress MAPK signaling and promote lung tumor progression without altering fatty acid profiles (PMID:40817125). During influenza A infection, HACD3 competes with SQSTM1/p62 for binding to the viral PB1 protein, shielding PB1 from autophagic degradation and facilitating viral replication (PMID:38793585). HACD3 expression is itself controlled at its promoter by a TCF7l2-TCF19-H3K4me3 epigenetic complex that dissociates under lipid stress to activate transcription (PMID:40172138).

1. 2000 Medium

   Established HACD3 (B-ind1) as a signaling adaptor, defining its first known role as a Rac1-binding potentiator of stress and inflammatory transcription before any enzymatic function was known.

   Evidence Co-IP with activated Rac1, NF-κB luciferase reporter, JNK kinase assay, and dominant-negative deletion mutant in transfected cells

   PMID:10747961

   Open questions at the time

   • Did not define a direct catalytic activity for HACD3
   • Mechanism of how HACD3 bridges Rac1 to JNK/NF-κB effectors not resolved
2. 2008 High

   Answered what biochemical reaction HACD3 catalyzes, assigning it to the dehydratase step of VLCFA elongation and identifying its physical partners among ELOVL condensing enzymes.

   Evidence In vitro 3-hydroxypalmitoyl-CoA dehydratase assay, PHS1-shutoff yeast complementation, and Co-IP with ELOVL1-7

   PMID:18554506

   Open questions at the time

   • HACD3 showed weaker activity than HACD1/2, leaving its physiological catalytic contribution unclear
   • Which ELOVL partner pairings dominate in vivo not established
3. 2017 High

   Clarified that HACD3 contributes only weakly to canonical elongation, with HACD2 as the dominant dehydratase, prompting the question of what non-metabolic functions HACD3 carries.

   Evidence Yeast complementation assays and HACD2 disruption with elongation activity measurements in HAP1 cells

   PMID:28784662

   Open questions at the time

   • Did not test whether weak activity reflects redundancy or a distinct substrate preference
   • Non-catalytic roles not addressed
4. 2015 Medium

   Linked HACD3 to receptor trafficking, showing it relocates upon insulin stimulation and influences insulin receptor phosphorylation and internalization.

   Evidence Subcellular fractionation with insulin stimulation, siRNA knockdown, and IR phosphorylation/endocytosis assays in HEK293 cells

   PMID:25687571

   Open questions at the time

   • Direct molecular partners in the IR endocytosis pathway not identified
   • Whether the effect is catalytic or scaffolding unknown
5. 2018 Low

   Extended the trafficking role by mapping HACD3-dependent associations of the insulin receptor with cytoskeletal and endosomal sorting machinery.

   Evidence Hepatic Golgi/endosome proteomics and interaction network analysis with validation of selected nodes

   PMID:30300385

   Open questions at the time

   • Proteomics-based associations lack direct validation of HACD3-specific binding
   • Causal direction between HACD3 and the network not established
6. 2023 Medium

   Connected HACD3's enzymatic role to receptor biology by showing AdipoR2 recruits HACD3 to elongate and incorporate polyunsaturated fatty acids into phospholipids.

   Evidence Co-IP of tagged AdipoR2 with MS identification and 13C-labeled fatty acid incorporation assay in HEK293 cells

   PMID:37164154

   Open questions at the time

   • Whether AdipoR2 recruitment alters HACD3 catalytic rate not quantified
   • Single-lab interaction without structural detail
7. 2024 Medium

   Defined a non-catalytic tumor-suppressive mechanism whereby HACD3 binds PHB and triggers PHB-Y259 dephosphorylation to inactivate Raf/ERK signaling and block EMT.

   Evidence Co-IP with domain mapping (aa 141–178), phosphorylation assays, and in vitro/in vivo CRC metastasis models

   PMID:38617530

   Open questions at the time

   • The phosphatase responsible for PHB-Y259 dephosphorylation not identified
   • Whether HACD3 acts directly or recruits a phosphatase unresolved
8. 2024 Medium

   Revealed an unexpected protein kinase activity for HACD3, showing it phosphorylates CDK2 T160 to promote cell-cycle progression and tumorigenesis independent of dehydratase function.

   Evidence Phosphoproteomics, Co-IP, domain mapping (aa 298–324), and in vivo NSG and Hacd3-/- ApcMin/+ mouse tumor models

   PMID:39547639

   Open questions at the time

   • Direct in vitro kinase reconstitution not demonstrated
   • Catalytic residues mediating kinase activity not defined
9. 2024 Medium

   Demonstrated a pro-viral function in which HACD3 competitively shields influenza PB1 from selective autophagy.

   Evidence Reciprocal Co-IP of HACD3 with PB1 and SQSTM1/p62, siRNA knockdown, and lysosome inhibitor rescue experiments

   PMID:38793585

   Open questions at the time

   • Binding interface on PB1 shared with p62 not mapped
   • Whether endogenous HACD3 levels limit viral replication not tested
10. 2025 Medium

    Identified a MAPK-suppressive scaffold role through which HACD3 binds MKK7 and MAPK10 to promote lung cancer progression, dissociating this function from fatty acid metabolism.

    Evidence Co-IP with domain mapping (aa 231–259) and synthetic peptides, Hacd3-/- urethane carcinogenesis model, GC-MS fatty acid profiling, and xenografts

    PMID:40817125

    Open questions at the time

    • How MKK7/MAPK10 binding mechanistically suppresses signaling not detailed
    • Reconciliation with HACD3's Rac1/JNK potentiation in other contexts unresolved
11. 2025 Medium

    Established how HACD3 expression is set, showing a TCF7l2-TCF19-H3K4me3 complex represses the promoter and dissociates under lipid stress to upregulate HACD3 and lipogenesis.

    Evidence ChIP for TCF7l2/TCF19/H3K4me3 at the PTPLAD1 promoter, Co-IP, palmitic-acid cell and mouse models, and NAFLD patient gene expression

    PMID:16516406 PMID:40172138

    Open questions at the time

    • Signal transducing lipid stress to H3K4me3 loss not identified
    • Whether other transcription factors contribute not addressed
12. 2026 Low

    Proposed an additional proliferative mechanism in which HACD3 partners with ATP2A2 to activate NF-κB and induce cyclin D1 transcription.

    Evidence Co-IP, MS, RNA-Seq, NF-κB reporter, p65 nuclear translocation imaging, and cell-cycle assays

    PMID:41905455

    Open questions at the time

    • Mechanistic detail limited; direct vs indirect ATP2A2 cooperation unclear
    • Not independently confirmed

• How HACD3 toggles between a weak ER dehydratase and a diverse set of cytoplasmic/nuclear scaffolding and kinase activities, and what governs context-specific partner selection, remains unresolved.
• No structural basis for HACD3's apparent kinase activity
• No unifying model reconciling pro- and anti-tumor and pro- and anti-MAPK roles across tissues