MAP3K21

MAP3K21 (MLK4) is a serine/threonine MAP3K-family kinase that integrates upstream signals into NF-κB, MAPK, and CREB transcriptional outputs to control cancer cell identity, survival, and invasion (PMID:26859459, PMID:23319808, PMID:37407566). Its best-defined biochemical activity is direct substrate phosphorylation: MLK4 binds and phosphorylates IKKα to activate NF-κB signaling that drives mesenchymal identity, self-renewal, motility, and radioresistance in glioma stem cells (PMID:26859459), and phosphorylates MEK1 to engage the MEK/ERK axis, with MLK4-mutant alleles acting as constitutively active drivers that cooperate with activated RAS in colorectal cancer (PMID:23319808). MLK4 also phosphorylates the transcription factor CREB to induce PCK1 and reprogram glycolytic metabolism, with its own expression positively controlled by KLF5 (PMID:37407566). The kinase exerts context-dependent and opposing effects on the JNK pathway: loss-of-function mutations in colon cancer suppress JNK signaling, and restoring MLK4 activity reactivates JNK targets cJUN, ATF3, CDKN1A, and CDKN2B to limit tumor growth, a behavior rationalized by a catalytic-domain crystal structure explaining mutant inactivity (PMID:26637668). In breast cancer, MLK4 activates NF-κB to promote mesenchymal, migratory, and invasive phenotypes (PMID:30552384), supports the DNA damage response through ATM/CHK2 phosphorylation and NHEJ-mediated repair (PMID:34839359), and mediates macrophage-induced proliferation and ECM remodeling via NF-κB-driven MMP, CXCL1, and IL-8 expression (PMID:41922320). Beyond its oncogenic kinase activities, MLK4 interacts with TLR4 to negatively regulate LPS-induced JNK/ERK activation and TNF-α production in macrophages (PMID:21602844), and engages IL-17RB through a defined R216–C408 interface that supports pancreatic cancer growth and metastasis (PMID:39913969).

1. 2011 Medium

   Established the first signaling partner and an unexpectedly negative regulatory role, distinguishing MLK4 from other MLK-family MAP3Ks that activate stress kinases.

   Evidence Co-IP of TLR4, plus knockdown/overexpression in macrophages with LPS-induced cytokine and MAPK readouts

   PMID:21602844

   Open questions at the time

   • Did not identify the direct substrate through which MLK4 dampens TLR4 signaling
   • Whether kinase activity is required for the negative-regulatory effect was not resolved
2. 2013 High

   Identified MEK1 as a direct MLK4 substrate and showed cancer-associated alleles are constitutively active, framing MLK4 as a RAS-cooperating oncogenic kinase.

   Evidence Direct MEK1 phosphorylation assay, MLK4 knockout cells, transformation and xenograft assays

   PMID:23319808

   Open questions at the time

   • Did not reconcile activating CRC alleles here with loss-of-function CRC alleles reported elsewhere
   • Structural basis of constitutive activation not defined
3. 2015 High

   Resolved a context where MLK4 is tumor-suppressive via JNK, and provided a catalytic-domain structure explaining how mutations inactivate the kinase.

   Evidence Reconstitution of MLK4 activity in mutant CRC cells, kinase assays, catalytic-domain crystal structure, in vivo tumor growth

   PMID:26637668

   Open questions at the time

   • Did not define which upstream signals select JNK versus MEK/ERK or NF-κB output
   • Mechanistic basis for opposite mutational consequences across cancers unresolved
4. 2016 High

   Defined the IKKα–NF-κB axis as the route by which MLK4 enforces mesenchymal identity and radioresistance in glioma stem cells.

   Evidence Reciprocal Co-IP, direct IKKα phosphorylation assay, MLK4 silencing with mesenchymal-signature phenotypic readout

   PMID:26859459

   Open questions at the time

   • Upstream activators recruiting MLK4 to IKKα not identified
   • Selectivity for IKKα over other IKK subunits not addressed
5. 2018 Medium

   Extended the NF-κB-driven mesenchymal program from glioma to breast cancer invasion.

   Evidence MLK4 silencing, NF-κB reporter, migration/invasion and 3D spheroid assays, xenograft

   PMID:30552384

   Open questions at the time

   • Direct substrate in breast cancer NF-κB activation not confirmed
   • Downstream effector genes not enumerated
6. 2019 Low

   Placed MLK4 upstream of a ROS/MAPK survival axis in hepatocellular carcinoma, with MLK4 loss promoting apoptosis.

   Evidence siRNA knockdown, ROS measurement, phospho-MAPK blots, pharmacological rescue, xenograft/metastasis models

   PMID:31071576

   Open questions at the time

   • Upstream placement is indirect, inferred from pharmacological rescue rather than direct biochemistry
   • No direct MLK4 substrate identified in this pathway
   • Single method type, not independently confirmed
7. 2021 Medium

   Connected MLK4 to genome maintenance, showing it promotes ATM/CHK2-mediated DNA damage response and NHEJ repair to support survival in TNBC.

   Evidence MLK4 knockdown/inhibition, phosphoproteomics, NF-κB reporter, mRNA-seq, in vivo doxorubicin sensitivity

   PMID:34839359

   Open questions at the time

   • Whether MLK4 directly phosphorylates ATM/CHK2 or acts indirectly not resolved
   • Mechanistic link between NF-κB cytokine output and repair not dissected
8. 2023 Medium

   Added a metabolic output by showing MLK4 phosphorylates CREB to drive PCK1 and glycolytic reprogramming, and identified KLF5 as a transcriptional activator of MLK4.

   Evidence CREB phosphorylation assay, promoter/reporter analysis, metabolomic profiling, knockdown

   PMID:37407566

   Open questions at the time

   • Whether CREB phosphorylation is direct versus relayed not fully established
   • Generality of the CREB–PCK1 axis beyond lung adenocarcinoma untested
9. 2025 Medium

   Mapped a druggable IL-17RB–MLK4 interface (IL-17RB C408 / MLK4 R216), establishing a protein-protein interaction route to oncogenic signaling in pancreatic cancer.

   Evidence Site-directed mutagenesis of interface residues, cyclic-peptide disruption, in vitro and orthotopic in vivo growth/metastasis assays

   PMID:39913969

   Open questions at the time

   • Downstream signaling triggered by the IL-17RB–MLK4 interaction not delineated
   • Whether the interaction modulates MLK4 catalytic activity unknown
10. 2026 Medium

    Showed MLK4 mediates tumor–macrophage crosstalk, driving NF-κB-dependent MMP, CXCL1, and IL-8 to promote ECM remodeling and invasion in TNBC.

    Evidence TNBC–macrophage co-culture, MLK4 depletion, NF-κB assays, MMP profiling, migration/invasion assays

    PMID:41922320

    Open questions at the time

    • Direct substrate linking macrophage cues to MLK4 activation not identified
    • Receptor relaying macrophage signal to MLK4 not defined

• It remains unresolved how a single kinase produces opposing outputs—tumor-suppressive JNK activation versus oncogenic NF-κB/ERK signaling—and what upstream receptors and contextual cues dictate substrate selection.
• No unifying model of context-dependent substrate choice
• Physiological (non-cancer) function and endogenous activators largely uncharacterized in the corpus
• Whether IL-17RB and TLR4 engagement converge on the same catalytic outputs unknown