MED28

MED28 is a dual-compartment scaffolding protein that functions both as a subunit of the Mediator transcriptional co-activator complex and as a cytoplasmic signaling adaptor linking receptor-proximal kinases to downstream proliferative and migratory pathways. Within the nucleus, MED28 operates in the Mediator head module together with Med6, Med8, and Med18 to repress smooth muscle cell differentiation, maintain pluripotency through Oct4/Nanog expression, and regulate cell cycle progression at the G1–S transition and mitosis (PMID:17848560, PMID:26445504, PMID:30970566). In the cytoplasm, MED28 associates with the actin cytoskeleton and the tumor suppressor merlin (NF2), is phosphorylated at Y64 by Src-family kinases (principally Lck) to create an SH2-binding motif for Grb2, and promotes cell migration and invasion through a MEK1–MMP2 axis and NF-κB–dependent EMT (PMID:15467741, PMID:16899217, PMID:22495818, PMID:27662245). MED28 also activates Wnt/β-catenin signaling by suppressing the negative regulator HBP1 and promotes AKT/mTOR-dependent proliferation and lipogenesis in cancer cells (PMID:26660958, PMID:38619972).

1. 2004 High

   Establishing that MED28 is a cytoplasmic scaffolding protein that physically links the NF2 tumor suppressor merlin to the adaptor Grb2 at the cortical actin cytoskeleton resolved the question of how merlin connects to growth factor signaling complexes.

   Evidence Co-IP, affinity binding, blot overlay, EM, and subcellular fractionation in mammalian cells

   PMID:15467741

   Open questions at the time

   • Functional consequence of merlin–MED28–Grb2 ternary complex on downstream signaling not determined
   • Whether the cytoplasmic pool of MED28 is stoichiometrically distinct from the Mediator-bound nuclear pool was unknown
2. 2005 Medium

   Demonstrating that MED28 overexpression drives proliferation in vitro and in xenografts while activating ERK1/2, JNK, and p38 MAPK established MED28 as a pro-proliferative factor acting upstream of multiple MAPK cascades.

   Evidence Overexpression and siRNA in cancer cell lines, xenograft model, Co-IP with Src, immunoblot

   PMID:16024617

   Open questions at the time

   • No direct substrate or kinase–substrate relationship defined for MED28 in MAPK activation
   • Correlation between Src interaction and MAPK activation not causally dissected
3. 2006 High

   Identification of Y64 as the Lck phosphorylation site on MED28—using Lck-deficient cells and site-directed mutagenesis—revealed how TCR stimulation creates an SH2 docking site for Grb2 on MED28, providing a direct biochemical mechanism for signal-dependent scaffold assembly.

   Evidence Yeast two-hybrid, in vitro kinase assay, mutagenesis, Lck-deficient J.CaM1.6 cells

   PMID:16899217

   Open questions at the time

   • Downstream signaling events specifically dependent on pY64-Grb2 recruitment not mapped
   • Whether Y64 phosphorylation occurs in non-hematopoietic contexts not tested
4. 2007 High

   Showing that MED28 is a bona fide subunit of the Mediator head module (with Med6/Med8/Med18) that represses smooth muscle differentiation genes established its nuclear transcriptional role, distinct from its cytoplasmic scaffolding function.

   Evidence siRNA knockdown, overexpression, genetic epistasis with head-module subunits in NIH3T3 cells

   PMID:17848560

   Open questions at the time

   • Structural basis for MED28 integration into the head module not resolved
   • Mechanism by which head module occupancy at SMC gene promoters leads to repression not determined
5. 2012 High

   Epistatic rescue experiments placing MED28 upstream of MEK1 and MMP2 in breast cancer cell migration/invasion defined a linear signaling hierarchy (MED28→MEK1→MMP2) through which MED28 promotes invasiveness.

   Evidence siRNA, dominant-negative constructs, MEK1 cDNA rescue, recombinant MMP2 rescue, migration/invasion assays in breast cancer cells

   PMID:22495818

   Open questions at the time

   • Direct biochemical link between MED28 and MEK1 activation (e.g., Raf-level regulation) not identified
   • Whether this pathway operates through cytoplasmic or nuclear MED28 not distinguished
6. 2015 High

   Genetic knockout in mice and conditional inactivation in iPSCs demonstrated that MED28 is essential for peri-implantation development and pluripotency maintenance via Oct4/Nanog, and that it enhances somatic reprogramming—establishing a non-redundant in vivo requirement.

   Evidence Knockout mouse model, Cre-mediated conditional inactivation, iPSC reprogramming assay, gene expression analysis

   PMID:26445504

   Open questions at the time

   • Whether the developmental requirement is mediated by Mediator function, cytoplasmic signaling, or both is unresolved
   • Tissue-specific conditional knockouts beyond peri-implantation not reported
7. 2015 Medium

   Demonstrating that MED28 suppresses HBP1 promoter activity to activate Wnt/β-catenin target genes (cyclin D1, c-Myc) in colorectal cancer cells identified a transcriptional mechanism linking MED28 to Wnt pathway output.

   Evidence siRNA knockdown, overexpression, luciferase reporter assay, immunoblot in colorectal cancer cells

   PMID:26660958

   Open questions at the time

   • Whether MED28 represses HBP1 directly at its promoter via Mediator or indirectly through an intermediary not resolved
   • In vivo relevance of this axis not tested
8. 2016 Medium

   Linking MED28 to EMT through NF-κB/p65 phosphorylation and Snail expression in breast cancer cells expanded its oncogenic repertoire beyond proliferation and migration to include phenotypic plasticity.

   Evidence siRNA knockdown, overexpression, adriamycin-induced EMT model, immunoblot in MCF7 and MDA-MB-231 cells

   PMID:27662245

   Open questions at the time

   • Direct mechanism by which MED28 activates NF-κB (e.g., IKK regulation) not identified
   • Single-lab finding without independent replication
9. 2017 Medium

   Identification of ZNF224 as a direct nuclear interaction partner of MED28—with binding mapped to the KRAB domain of ZNF224 and the MED domain of MED28—revealed a mechanism by which MED28 stabilizes a transcriptional repressor to promote colony formation.

   Evidence Co-IP, surface plasmon resonance, bimolecular fluorescence complementation, colony assay in MCF-7 cells

   PMID:29435049

   Open questions at the time

   • Genomic targets co-regulated by MED28–ZNF224 not identified
   • Whether ZNF224 stabilization requires Mediator association is unknown
10. 2018 Medium

    Showing that the C. elegans orthologue MDT-28 undergoes lysine acetylation that reduces its nuclear localization revealed a conserved post-translational mechanism governing the cytoplasmic-to-nuclear partitioning of MED28.

    Evidence Anti-acetyl-lysine immunoprecipitation of GFP::MDT-28, FLIM imaging, VPA treatment in C. elegans

    PMID:29871732

    Open questions at the time

    • Specific acetylated lysine residues not mapped
    • Whether mammalian MED28 is similarly regulated by acetylation not tested
    • Identity of the acetyltransferase and deacetylase acting on MDT-28/MED28 unknown
11. 2018 Medium

    Demonstrating that MED28 and FOXM1 mutually regulate each other's expression and co-drive MMP2-dependent invasion provided evidence of a feed-forward loop in NSCLC that converges on the same MMP2 effector pathway identified in breast cancer.

    Evidence Co-IP, siRNA, doxycycline-inducible overexpression, FOXM1 rescue of MED28 knockdown, invasion assay in NSCLC cells

    PMID:30499104

    Open questions at the time

    • Whether MED28–FOXM1 interaction occurs within the Mediator complex or independently not distinguished
    • Mechanism of mutual transcriptional regulation not resolved
12. 2019 Medium

    Establishing that MED28 expression is cell-cycle-regulated (peaking at G1–S and mitosis) and that its overexpression shortens both interphase and mitosis while increasing aneuploidy connected MED28 to genomic stability and cell-cycle timing.

    Evidence Cell cycle synchronization, live cell imaging, flow cytometry, fluorescence microscopy, luciferase reporter for promoter analysis

    PMID:30970566

    Open questions at the time

    • Molecular targets through which MED28 accelerates cell cycle phases not identified
    • Whether aneuploidy results from Mediator-dependent transcriptional mis-regulation or cytoskeletal defects is unclear
13. 2024 Medium

    Placing MED28 upstream of AKT/mTOR signaling and SREBP1-dependent lipogenesis in hepatocellular carcinoma cells broadened its oncogenic functions to include metabolic reprogramming.

    Evidence siRNA knockdown, overexpression, immunoblot, subcellular fractionation, cell cycle analysis in HCC cells

    PMID:38619972

    Open questions at the time

    • Direct biochemical mechanism linking MED28 to AKT phosphorylation not determined
    • In vivo validation in liver cancer models not reported

• The mechanism governing MED28 partitioning between its Mediator-associated nuclear pool and its cytoplasmic actin-associated pool, and whether these represent functionally independent or coordinated activities, remains unresolved.
• No structural data for MED28 within the Mediator head module at sufficient resolution to define contacts
• Regulatory signals controlling nuclear–cytoplasmic shuttling in mammalian cells not identified
• Whether cytoplasmic signaling functions of MED28 feed back into its Mediator-dependent transcriptional activity is untested