HELZ2

HELZ2 is a dual-function protein that operates both as a ligand-dependent nuclear-receptor transcriptional coactivator and as an ATP-dependent 3'-5' exoribonuclease controlling RNA turnover (PMID:12189208, PMID:37602378). As a coactivator, it was first isolated as a PPARα-interacting cofactor bearing multiple LXXLL motifs that stimulate PPARα-driven transcription (PMID:12189208), and it assembles with the Mediator-associated factor THRAP3 through its helicase motifs to synergistically enhance PPARγ-mediated transcription, being co-recruited with THRAP3 to PPAR-response elements in adipocyte enhancers in a ligand-dependent manner (PMID:23525231). As an enzyme, HELZ2 possesses bona fide ribonuclease activity from an RNB domain that has lost a canonical catalytic residue, and its ATP-dependent helicase activity unwinds structured RNA for degradation (PMID:37602378); this activity directly targets Apob mRNA to set hepatic apoB and lipid levels, with a gain-of-function helicase mutation depleting Apob mRNA and a loss-of-function state elevating it (PMID:41446920). Consistent with these roles in lipid control, Helz2 deficiency in mice de-represses hepatic Leprb expression, activates AMPK, increases fatty-acid β-oxidation, and protects against diet-induced obesity and insulin resistance (PMID:25004093). HELZ2 is an interferon-stimulated antiviral effector that suppresses dengue virus replication and modulates intracellular triglyceride pools (PMID:28265266), and it restricts LINE-1 retrotransposition by recognizing the L1 5'UTR to lower L1 RNA and ORF1p and to dampen L1-triggered IFN-α induction (PMID:36639706). Beyond its transcriptional and RNA-degrading functions, HELZ2 acts in protein-stability pathways: it promotes HUWE1-dependent K63-linked polyubiquitination of c-Myc to drive retinoblastoma proliferation (PMID:34761308) and stabilizes MYC to upregulate ATG16L1-mediated macrophage autophagy against Mycobacterium tuberculosis (PMID:42262972).

1. 2002 Medium

   Established HELZ2's first molecular identity by showing it is a nuclear-receptor coactivator, answering whether the protein has any defined transcriptional role.

   Evidence Biochemical purification of a PPARα-interacting cofactor complex from rat liver, MS identification, and reporter assays

   PMID:12189208

   Open questions at the time

   • Coactivation effect was only moderate
   • Did not address whether HELZ2 has enzymatic activity
   • Mechanism of recruitment to chromatin not defined
2. 2013 High

   Defined how HELZ2 is integrated into the transcriptional machinery by identifying THRAP3 as a helicase-motif partner co-recruited to PPARγ enhancers, explaining ligand-dependent coactivation.

   Evidence Yeast two-hybrid, reciprocal Co-IP/MS, ChIP, reporter assays and siRNA in 3T3-L1 adipocytes

   PMID:23525231

   Open questions at the time

   • Did not test whether helicase catalytic activity is required for coactivation
   • Endogenous complex stoichiometry unresolved
3. 2014 Medium

   Demonstrated an in vivo metabolic consequence of HELZ2 by showing its loss de-represses hepatic Leprb and protects from obesity, linking the coregulator to systemic lipid homeostasis.

   Evidence Helz2 knockout mice with calorimetry and adenoviral Leprb rescue

   PMID:25004093

   Open questions at the time

   • Did not distinguish transcriptional from RNA-degrading mechanism for Leprb control
   • Direct binding to Leprb locus or transcript not shown
4. 2017 Medium

   Placed HELZ2 in innate antiviral defense as an interferon-stimulated gene restricting dengue virus, expanding its role beyond metabolism.

   Evidence siRNA screen, Co-IP with AHR, ChIP-seq, lipid profiling, and KO mouse macrophage infection assays

   PMID:28265266

   Open questions at the time

   • Mechanistic link between AHR interaction, lipid changes, and viral restriction unresolved
   • Whether RNase activity drives restriction not tested
5. 2021 Low

   Probed HELZ2's role in adipogenesis by identifying SFPQ as a phospho-HELZ2-associated factor, raising a candidate RNA-binding partner in differentiation control.

   Evidence Co-IP of tyrosine-phosphorylated HELZ2 with MS and siRNA in 3T3-L1 cells

   PMID:34052659

   Open questions at the time

   • Single Co-IP/MS without reciprocal validation
   • Functional adipogenesis effects attributed to SFPQ knockdown without direct mechanistic link to HELZ2
   • Significance of HELZ2 tyrosine phosphorylation unknown
6. 2021 Medium

   Revealed a non-transcriptional protein-stability function by showing HELZ2 bridges c-Myc to HUWE1 for activating K63-linked ubiquitination, connecting HELZ2 to oncogenic proliferation.

   Evidence Co-IP, linkage-specific ubiquitination assays, and retinoblastoma xenografts

   PMID:34761308

   Open questions at the time

   • Structural basis of the HELZ2-Myc-HUWE1 bridge unknown
   • Whether helicase/RNase domains participate not addressed
7. 2023 High

   Defined HELZ2 as a genome-defense factor by showing it recognizes the L1 5'UTR to suppress retrotransposition and L1-driven IFN-α, linking RNA recognition to innate immune restraint.

   Evidence IP-LC-MS/MS interactome, retrotransposition assays, RNA/protein quantification, foci imaging, and IFN-α reporters

   PMID:36639706

   Open questions at the time

   • Precise 5'UTR sequence/structure determinant not fully mapped
   • Whether degradation is catalytic or sequestration not resolved here
8. 2023 High

   Resolved HELZ2's enzymatic identity by reconstituting active 3'-5' exoribonuclease and ATP-dependent helicase activities, establishing it as a structured-RNA-degrading enzyme despite a non-canonical RNB catalytic site.

   Evidence Evolutionary analysis, non-canonical start-codon verification, in vitro RNase and helicase assays, and cancer somatic mutation analysis

   PMID:37602378

   Open questions at the time

   • Physiological RNA substrates not defined in this study
   • Structural mechanism of catalysis with substituted residue unresolved
9. 2025 High

   Identified the first defined physiological RNA substrate by showing HELZ2 binds and degrades Apob mRNA to control hepatic lipid levels, unifying its RNase/helicase activity with its metabolic phenotypes.

   Evidence Forward genetic screen in mice, mRNA binding assays, inducible liver overexpression, KO mice, and atherosclerosis models

   PMID:41446920

   Open questions at the time

   • Whether other lipid-related transcripts are direct substrates unknown
   • Relationship between Apob-degrading and coactivator functions unresolved
10. 2026 Medium

    Extended the HELZ2-MYC axis to host defense by showing HELZ2 stabilizes MYC to activate ATG16L1 transcription and macrophage autophagy against M. tuberculosis.

    Evidence Co-IP, ChIP, dual-luciferase reporters, knockdown/overexpression, and bacterial survival assays

    PMID:42262972

    Open questions at the time

    • Mechanism of MYC stabilization (vs. the prior K63-ubiquitination model) not reconciled
    • Direct vs. indirect role of HELZ2 enzymatic domains untested
11. 2026 Medium

    Defined upstream control of HELZ2 by showing SETD1A-deposited H3K4me3 drives HELZ2 transcription, with the HELZ2/PPARα complex feeding into HIF1α-dependent glycolysis and cellular senescence.

    Evidence ChIP-seq/qPCR for H3K4me3, SETD1A knockdown/overexpression, NP tissue and disc degeneration models

    PMID:41917726

    Open questions at the time

    • Direct HELZ2-HIF1α regulatory mechanism not shown
    • Whether RNase activity participates in NP senescence untested

• How HELZ2's distinct activities — nuclear-receptor coactivation, structured-RNA degradation, and MYC stabilization — are coordinated within one protein and selected in different cell types remains unresolved.
• No structural model integrating helicase, RNB, and coactivator modules
• Full physiological RNA substrate repertoire unknown
• Domain requirements for the protein-stability functions undetermined