HECTD2

HECTD2 is a HECT-domain E3 ubiquitin ligase that drives proteasomal degradation of multiple substrates to control inflammatory signaling, antioxidant responses, and cell proliferation (PMID:26157031, PMID:39976163). Its founding substrate is PIAS1: HECTD2 ubiquitinates PIAS1 to target it for degradation and thereby promote inflammation, an event that requires prior GSK3β phosphorylation of PIAS1 to generate a phosphodegron, with a naturally occurring HECTD2(A19P) polymorphism mislocalizing the ligase, preventing its nuclear interaction with PIAS1, and dampening inflammation (PMID:26157031). Beyond PIAS1, HECTD2 degrades the histone methyltransferase EHMT2, reducing repressive H3K9me2 at the TNFAIP1 promoter to derepress TNFAIP1 and engage p38/JNK signaling and apoptotic programs (PMID:34972816, PMID:38688591), and degrades KEAP1 to activate the NRF2 antioxidative stress pathway (PMID:39976163). HECTD2 expression is itself tightly regulated—repressed as a direct target of miR-221 (PMID:23770851) and induced by histone H3K18 lactylation (PMID:39976163)—and it influences proliferative and cell-cycle phenotypes in cancer cells (PMID:34145398). Genetic studies link HECTD2 to prion disease incubation time as a quantitative trait gene (PMID:19214206) and identify rare loss-of-function variants as risk factors for bipolar disorder, placing it in the GSK3β signaling axis (PMID:40133559).

1. 2009 Medium

   Established HECTD2 as a disease-relevant E3 ubiquitin ligase by linking its expression to a neurodegenerative phenotype, raising the question of which substrates it acts on.

   Evidence Heterogeneous stock mouse QTL mapping with brain mRNA expression analysis and human haplotype association in prion disease

   PMID:19214206

   Open questions at the time

   • No substrate identified
   • Mechanism linking degradation activity to prion incubation time unresolved
2. 2013 Medium

   Identified an upstream regulator of HECTD2, showing it is transcriptionally repressed by miR-221 with consequences for androgen receptor signaling.

   Evidence miR-221 target identification by bioinformatics plus stable overexpression in LNCaP and HECTD2 knockdown with AR-mediated transcription readout

   PMID:23770851

   Open questions at the time

   • Direct ubiquitination substrate in this context not defined
   • Mechanism linking HECTD2 to AR transcription unresolved
3. 2015 High

   Defined the first molecular substrate of HECTD2, establishing it as a PIAS1-degrading ligase that requires a GSK3β-generated phosphodegron and is disabled by a localization-altering polymorphism.

   Evidence Biochemical ubiquitination assay, co-IP, A19P variant functional analysis, and small-molecule inhibition in LPS- and P. aeruginosa-induced lung inflammation models

   PMID:26157031

   Open questions at the time

   • Direct demonstration of HECTD2 HECT catalytic residues not detailed
   • Generality of phosphodegron requirement across substrates unknown
4. 2021 Medium

   Connected HECTD2 to cell-autonomous proliferation and immune modulation, broadening its role beyond a single substrate axis.

   Evidence Loss- and gain-of-function in human and murine melanoma cell lines with cell cycle analysis and a murine melanoma model

   PMID:34145398

   Open questions at the time

   • Substrate mediating cell-cycle acceleration not identified
   • Direct mechanism of COX2 pathway regulation unresolved
5. 2022 Medium

   Identified EHMT2 as a HECTD2 substrate and linked its degradation to epigenetic derepression of TNFAIP1, providing a chromatin-to-apoptosis mechanism.

   Evidence Western blot degradation assays, reciprocal co-IP, and ChIP for H3K9me2 in colorectal cancer spheroid models with propionate stimulation

   PMID:34972816

   Open questions at the time

   • Direct ubiquitination of EHMT2 by purified HECTD2 not reconstituted
   • Generalizability beyond CRC at the time uncertain
6. 2022 Low

   Added LPCAT1 as a candidate substrate whose degradation represses proliferation, extending the substrate repertoire.

   Evidence Co-IP detecting ubiquitinated LPCAT1 and HECTD2 overexpression with LPCAT1 rescue measuring proliferation in CRC cells

   PMID:35964314

   Open questions at the time

   • Single co-IP and overexpression rescue without reciprocal or in vitro validation
   • Direct ubiquitination not demonstrated biochemically
7. 2024 Medium

   Confirmed the HECTD2–EHMT2–TNFAIP1 axis in a second cancer context and connected it to p38/JNK inflammatory signaling.

   Evidence Immunoprecipitation, western blot, ChIP, qRT-PCR, ELISA, and p38/JNK inhibitor rescue in renal cell carcinoma

   PMID:38688591

   Open questions at the time

   • Direct enzymatic ubiquitination of EHMT2 still not reconstituted
   • Relative contribution of inflammation versus apoptosis outcomes unclear
8. 2025 Medium

   Identified KEAP1 as a HECTD2 substrate linking the ligase to NRF2-driven antioxidant defense and drug resistance, and defined H3K18 lactylation as an inducer of HECTD2 expression.

   Evidence Unbiased proteomic screening, in vitro/in vivo knockdown-overexpression, patient-derived organoids and xenografts in hepatocellular carcinoma

   PMID:39976163

   Open questions at the time

   • Direct ubiquitination of KEAP1 by purified HECTD2 not reconstituted
   • Interplay with other HECTD2 substrates in the same cell not addressed
9. 2025 Low

   Genetically implicated HECTD2 loss-of-function in bipolar disorder and placed it within the GSK3β signaling axis relevant to lithium response.

   Evidence Whole-genome sequencing LOF burden analysis in Icelandic and UK Biobank cohorts with a noted HECTD2–GSK3β interaction

   PMID:40133559

   Open questions at the time

   • GSK3β interaction stated but not biochemically demonstrated in this study
   • No direct mechanistic experiment on HECTD2 protein function in neurons

• Whether HECTD2 selects its diverse substrates (PIAS1, EHMT2, KEAP1, LPCAT1) through a shared phosphodegron logic and how its subcellular localization governs substrate access remain unresolved.
• No unified substrate-recognition mechanism defined
• Structural basis of HECT catalysis for these substrates not determined
• Determinants of nuclear versus cytoplasmic targeting beyond A19P unknown