Metabolism in acute myeloid leukemia: mechanistic insights and therapeutic targets.

Metabolic rewiring and cellular reprogramming are trademarks of neoplastic initiation and progression in acute myeloid leukemia (AML). Metabolic alteration in leukemic cells is often genotype specific, with associated changes in epigenetic and functional factors resulting in the downstream upregulation or facilitation of oncogenic pathways. Targeting abnormal or disease-sustaining metabolic activities in AML provides a wide range of therapeutic opportunities, ideally with enhanced therapeutic windows and robust clinical efficacy. This review highlights the dysregulation of amino acid, nucleotide, lipid, and carbohydrate metabolism in AML; explores the role of key vitamins and enzymes that regulate these processes; and provides an overview of metabolism-directed therapies currently in use or development.

Vitamin B6 Addiction in Acute Myeloid Leukemia.

Cancer cells rely on altered metabolism to support abnormal proliferation. We performed a CRISPR/Cas9 functional genomic screen targeting metabolic enzymes and identified PDXK-an enzyme that produces pyridoxal phosphate (PLP) from vitamin B6-as an acute myeloid leukemia (AML)-selective dependency. PDXK kinase activity is required for PLP production and AML cell proliferation, and pharmacological blockade of the vitamin B6 pathway at both PDXK and PLP levels recapitulated PDXK disruption effects. PDXK disruption reduced intracellular concentrations of key metabolites needed for cell division. Furthermore, disruption of PLP-dependent enzymes ODC1 or GOT2 selectively inhibited AML cell proliferation and their downstream products partially rescued PDXK disruption induced proliferation blockage. Our work identifies the vitamin B6 pathway as a pharmacologically actionable dependency in AML.

SCF-mediated degradation of p100 (NF-κB2): mechanisms and relevance in multiple myeloma.

On the basis of differential analysis of affinity purifications by mass spectrometry, we identified the nuclear factor κB (NF-κB) protein p100 (NF-κB2) as an interactor of the F-box protein FBXW7α. The NF-κB pathway is important for cell growth, differentiation, and survival. p100, which shuttles between the cytoplasm and nucleus, functions as the primary inhibitor of the noncanonical NF-κB pathway by sequestering NF-κB heterodimers in the cytoplasm. In the absence of NF-κB stimulation, the nuclear pool of p100 is constitutively targeted for degradation by FBXW7α, which recognizes a conserved motif that is phosphorylated by glycogen synthase kinase 3 (GSK3). Efficient activation of noncanonical NF-κB signaling depends on the clearance of nuclear p100, either through FBXW7α-mediated degradation or nuclear export mediated by a signal in the C terminus of p100. Upon prolonged stimulation of the NF-κB pathway, p100 is stabilized and retained in the nucleus, contributing to the cessation of noncanonical NF-κB signaling. The molecular mechanism of p100 degradation has implications in multiple myeloma, a disease with constitutive activation of the noncanonical NF-κB pathway. Accordingly, expression of a stable p100 mutant, FBXW7α depletion, or chemical inhibition of GSK3 in multiple myeloma cells results in cell death in vitro and in a xenotransplant model. Thus, the FBXW7α-dependent degradation of p100 functions as a prosurvival mechanism through control of NF-κB activity.

Fbw7γ-mediated degradation of KLF13 prevents RANTES expression in resting human but not murine T lymphocytes.

RANTES (CCL5) is a chemokine implicated in many human diseases. We previously showed that the transcription factor Kruppel-like factor 13 (KLF13) controls the late (3-5 days after activation) expression of RANTES in T lymphocytes and that KLF13 itself is translationally regulated through the 5'-untranslated region of its mRNA. Here, we show that KLF13 levels are further regulated by ubiquitination and degradation. KLF13 protein is undetectable in resting human T lymphocytes, but treatment with either proteosomal or lysosomal inhibitors increases KLF13 protein levels. Glycogen synthase kinase 3ß (GSK3ß)-mediated phosphorylation of KLF13 triggers the ubiquitination of KLF13 by the E3 ligase Fbw7γ, resulting in KLF13 protein degradation. Knockdown of either Fbw7γ or GSK3ß by small interfering RNA increases KLF13 expression in resting human T lymphocytes. In contrast, in murine T lymphocytes, KLF13 protein is abundant because of the absence of Fbw7γ. Treatment of unactivated human lymphocytes with lysosomal inhibitors stabilizes KLF13 protein, resulting in an increase of RANTES mRNA and protein. Taken together, these studies found that tightly regulated control of both synthesis and degradation allows rapid changes in the level of KLF13 in human T lymphocytes.

Fbxw7α- and GSK3-mediated degradation of p100 is a pro-survival mechanism in multiple myeloma.

Fbxw7α is a member of the F-box family of proteins, which function as the substrate-targeting subunits of SCF (Skp1/Cul1/F-box protein) ubiquitin ligase complexes. Using differential purifications and mass spectrometry, we identified p100, an inhibitor of NF-κB signalling, as an interactor of Fbxw7α. p100 is constitutively targeted in the nucleus for proteasomal degradation by Fbxw7α, which recognizes a conserved motif phosphorylated by GSK3. Efficient activation of non-canonical NF-κB signalling is dependent on the elimination of nuclear p100 through either degradation by Fbxw7α or exclusion by a newly identified nuclear export signal in the carboxy terminus of p100. Expression of a stable p100 mutant, expression of a constitutively nuclear p100 mutant, Fbxw7α silencing or inhibition of GSK3 in multiple myeloma cells with constitutive non-canonical NF-κB activity results in apoptosis both in cell systems and xenotransplant models. Thus, in multiple myeloma, Fbxw7α and GSK3 function as pro-survival factors through the control of p100 degradation.