Bortezomib is called an NF-kB inhibitor, and in MLL leukaemia it does two different things in two lineages — neither of which is reliably NF-kB in the lymphoid one
also called bortezomib lineage split · proteasome inhibition in MLL · the NF-kB inhibitor that is not one
Bortezomib is routinely described as a way to block NF-kB, because it stops the brake protein from being destroyed. In myeloid MLL leukaemia it does look like it works that way. In the B-cell form it works by letting the fusion protein itself build up, which triggers a hidden self-destruct programme — a completely different mechanism with the same drug.
The same thing, for a scientist
**Why this entry sits in an NF-kB folder.** Every argument in this literature that reaches for a clinically available 'NF-kB inhibitor' reaches for bortezomib, and in KMT2A-rearranged leukaemia that reach is unsafe in three distinct ways. **(1) Direction.** Bortezomib has been reported to *induce* NF-kB rather than inhibit it, in several tumour types and in normal PBMCs, through calpain-dependent and caspase-independent routes; other work suggests NF-kB inhibition is not a key mechanism of its anti-cancer activity at all. The founding MLL NF-kB paper raises all of this itself [E1]. **(2) Arm.** Proteasome inhibition blocks IkBa degradation *and* p100-to-p52 processing, so it moves both arms at once, in a system where the arms have opposite signs (`mechanism-canonical-versus-non-canonical-nfkb-in-kmt2a-rearranged-leukaemia`). **(3) Lineage, and this is the substantive finding.** In **MLL-rearranged myeloid** leukaemia, bortezomib suppresses LSC self-renewal and leukaemogenesis by **NF-kB-dependent inhibition of CDK6**, reducing NF-kB recruitment to the CDK6 promoter and abolishing its DNA-binding there, with CDK6 overexpression partially rescuing and with little effect on normal HSPCs or on their CDK6 [E2]. In **MLL pro-B lymphoid** leukaemia, proteasome inhibition works by a mechanism with no NF-kB in it: it reduces turnover of endogenous **MLL-AF4**, and the accumulated fusion protein induces a latent tumour-suppression programme — caspase-8 cleavage of BID and upregulation of p27 — explicitly **'in MLL pro-B lymphoid, but not myeloid, leukemias'** [E3]. **So the same drug, in the same fusion family, has two mechanisms split by lineage, and the lymphoid one is not an NF-kB mechanism.**
The distinction that matters for the index disease
This graph's index disease is infant B-ALL. The proteasome literature that touches it directly is the pro-B mechanism 3, and that mechanism says the therapeutic lever is accumulating the fusion protein, not reducing NF-kB output. That is close to the opposite framing from the rest of this folder, where the goal is to remove the fusion from chromatin.
The two are not strictly contradictory — accumulating a protein and evicting it from a promoter are different variables — but an entry that cited a bortezomib benefit in MLL-AF4 leukaemia as support for the NF-kB retention hypothesis would be citing the wrong mechanism, and this entry exists so that does not happen. Note also that the pro-B paper reports preliminary clinical benefit in patients with MLL-AF4 leukaemia 3, which makes the misattribution risk higher rather than lower, because the result is attractive.
The myeloid mechanism, stated at its actual strength
The CDK6 result is the closest thing in this literature to a clinically available agent acting through NF-kB in MLL-rearranged leukaemia: bortezomib reduced CDK6 expression by inhibiting NF-kB recruitment to the CDK6 promoter and abolishing NF-kB DNA-binding activity there, CDK6 overexpression partially rescued, and normal HSPC CDK6 was unaffected 2. It is a single-laboratory result in a modest journal, the NF-kB species is not specified by subunit, and the arm is not identified. Cited for what it is: a promoter-level mechanism with a rescue, in mouse and human myeloid MLL cells.
| mechanism class | drug-mechanism-ambiguity |
|---|---|
| agent | bortezomib / carfilzomib (26S proteasome inhibition) |
| myeloid mechanism | NF-kB-dependent suppression of CDK6 transcription; LSC self-renewal loss; normal HSPCs and their CDK6 unaffected |
| lymphoid mechanism | reduced MLL-AF4 turnover; accumulated fusion triggers caspase-8/BID cleavage and p27 upregulation; explicitly not seen in myeloid MLL leukaemias |
| nfkb arm selectivity | none - proteasome inhibition blocks IkBa degradation and p100 processing simultaneously |
| direction hazard | bortezomib has been reported to INDUCE NF-kB in several settings including normal PBMCs |
- Citing bortezomib as an NF-kB inhibitor in this disease is a misattribution risk in three directions at once: it may raise NF-kB, it hits both arms, and in the index lineage its documented mechanism is not NF-kB at all.
- A drug that works by accumulating an oncogenic fusion protein is one dose-scheduling error away from accumulating it without the tumour-suppressive consequence. The context dependence is the therapy, and context dependence is fragile.
- Proteasome inhibition carries its own toxicity profile - neuropathy, cytopenias - in a population already receiving intensive chemotherapy.
- The preliminary clinical benefit reported in MLL-AF4 patients is the most quotable line in this entry and the least documented; it should not be repeated without the primary cohort.
- Does proteasome inhibition change MLL-fusion chromatin occupancy? Both mechanisms here are about protein level or about a single target promoter, and neither measures occupancy - the variable the rest of this folder is built on.
- Which NF-kB subunit occupies the CDK6 promoter in MLL-rearranged myeloid cells, and from which arm?
- Is the pro-B tumour-suppression programme intact in infant KMT2A::AFF1 blasts specifically, or only in the pro-B lines tested?
- If accumulating MLL-AF4 is therapeutic in pro-B leukaemia and evicting it from chromatin is therapeutic in myeloid leukaemia, are the two strategies compatible in the same patient, or mutually defeating?
- E1established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
Bortezomib, a 26S proteasome inhibitor that indirectly inhibits NF-kB by preventing IkBa proteasomal degradation, is in clinical testing; however, recent studies indicate bortezomib can induce NF-kB rather than inhibit it, through calpain-dependent or caspase-independent mechanisms, and activation of NF-kB by bortezomib has been observed in various tumour types and in normal peripheral blood mononuclear cells, while other studies suggest NF-kB inhibition may not be a key mechanism of bortezomib's anti-cancer activity.
Epigenetic roles of MLL oncoproteins are dependent on NF-κB. · Kuo HP, Wang Z, Lee DF, Iwasaki M, Duque-Afonso J, Wong SHK, Lin CH, Figueroa ME, Su J, Lemischka IR, Cleary ML · Cancer Cell 2013What this evidence cannot show
This is a literature review inside a discussion section; the primary sources for the calpain and caspase-independent claims were not fetched or read here, so the claim is that this is the field's stated position rather than that this beat verified each underlying result. Full text read this session (PMC3816582).
Evidence that argues against this (1)
- If bortezomib can INDUCE NF-kB rather than inhibit it, then proteasome inhibition is not a clean NF-kB perturbation in either direction, and any lineage split observed with it may be a split in proteasome biology rather than in NF-kB. The drug is being used here as a probe for a pathway it does not cleanly probe. Liu H et al. 2014
- E2strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
In AML with MLL rearrangements, bortezomib suppressed proliferation and colony formation in human and murine leukaemic blasts, reduced the frequency and function of leukaemia stem cells, inhibited progression and extended survival in MLL-AF9-transformed leukaemic mice, and reduced human CD34+CD38- LSC percentage and extended survival in AML-xenografted NSG mice. CDK6 was identified as the target by RNA sequencing: bortezomib reduced CDK6 expression by inhibiting NF-kB recruitment to the CDK6 promoter, abolishing NF-kB DNA-binding activity for that promoter, and CDK6 overexpression partially rescued the anti-leukaemogenic effect. Bortezomib had little effect against normal haematopoietic stem and progenitor cells and did not affect CDK6 expression in normal HSPCs.
Bortezomib suppresses self-renewal and leukemogenesis of leukemia stem cell by NF-ĸB-dependent inhibition of CDK6 in MLL-rearranged myeloid leukemia. · Zhou B, Qin Y, ... Gao S · Journal of Cellular and Molecular Medicine 2021What this evidence cannot show
Abstract read; full text not retrieved. Single laboratory, myeloid only, and the NF-kB species is not identified by subunit or arm anywhere in the abstract, so the mechanism is 'NF-kB' at the level the drug cannot resolve. The rescue is partial. Note the journal's own title renders NF-kB with a non-standard character; the title stored here is copied verbatim from the fetched PubMed record.
Evidence that argues against this (1)
- In MLL pro-B lymphoid leukaemias the same drug class works by a mechanism with no NF-kB in it - accumulation of MLL-AF4 itself triggering a latent tumour-suppression programme - and the authors state the effect is context-dependent and specifically NOT seen in myeloid MLL leukaemias. Liu H et al. 2014
- E3established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
Using the clinical proteasome inhibitors bortezomib and carfilzomib to reduce the turnover of endogenous MLL fusions, accumulated MLL-fusions induce latent, context-dependent tumour suppression programmes: in MLL pro-B lymphoid, but not myeloid, leukaemias, proteasome inhibition triggers apoptosis and cell cycle arrest involving activation cleavage of BID by caspase-8 and upregulation of p27. Proteasome inhibition conferred preliminary benefit to patients with MLL-AF4 leukaemia.
Proteasome inhibitors evoke latent tumor suppression programs in pro-B MLL leukemias through MLL-AF4. · Liu H, Westergard TD, ... Hsieh JJ · Cancer Cell 2014What this evidence cannot show
Abstract read; full text not retrieved. 'Preliminary benefit to patients' is an abstract statement with no cohort size, design or response definition given there, and should not be carried into this graph as a clinical efficacy claim. The mechanism is fusion-protein accumulation, so it is a lymphoid-specific finding about MLL-AF4 dosage and not a statement about NF-kB.
Evidence that argues against this (1)
- Reducing turnover of the endogenous fusion and inducing latent tumour-suppression programmes is a mechanism that runs through fusion-protein stability, not through NF-kB - so this evidence supports a different explanation for the same drug's effect than the one this entry is built on. Kuo HP et al. 2013
same source: PMID 24735925, cited here for a lineage-split proteasome mechanism, is also the origin of PAX5 anti-tag co-IP with mll_af9 and mll_enl
bortezomib is called an NF-kB inhibitor because it blocks IkBa degradation; the label is unsafe here
- related to →KMT2A::AFF1 (MLL-AF4), t(4;11)(q21;q23)
in pro-B MLL leukaemia the mechanism is MLL-AF4 accumulation, not NF-kB
a second, independent lineage split in the same pathway and the same fusion family