Dietary serine restriction increases venetoclax efficacy in Acute Myeloid Leukemia

Abstract

Abstract Acute myeloid leukemia (AML) is a highly aggressive hematologic malignancy with relapse rates exceeding 50% and long-term survival below 30% at 5 years. The anti-apoptotic protein BCL-2 is upregulated in over 80% of patients at diagnosis and about 90% at relapse, making it a potent therapeutic target. Venetoclax (VEN), a selective BCL-2 inhibitor, is FDA-approved for AML treatment. However, nearly 25% of patients do not respond to VEN-based therapy, and more than half of initial responders eventually relapse, often exhibiting venetoclax resistance. Mechanisms of this resistance include upregulation of alternate anti-apoptotic proteins (e.g., MCL-1, BCL-xL) and metabolic adaptations, including increased glycolysis and fatty acid oxidation. Thus, identifying targetable metabolic vulnerabilities that can potentiate VEN efficacy is essential for improving AML outcomes. One such vulnerability is serine and glycine metabolism, which supports nucleotide biosynthesis and cell proliferation in various cancers. We found that AML cells are highly sensitive to exogenous serine depletion; MV4-11, MOLM-13, NOMO-1, and THP-1 cells exhibited reduced proliferation rates of 0.50, 0.81, 0.61, and 0.74, respectively, after 72 hours of culture in serine-free media. This was accompanied by upregulation of serine synthesis pathway (SSP) enzymes, including phosphoglycerate dehydrogenase (PHGDH). In contrast, glycine depletion had no effect on proliferation or SSP activation. Notably, we observed that AML cells preferentially imported serine and exported glycine under regular media conditions, indicating active SSP and reliance on one-carbon metabolism. Importantly, isotope tracing analysis with mass spectrometry revealed that VEN markedly impaired SSP flux, as evidenced by reduced fractional enrichment of serine (m+3) and glycine (m+2) in both MV4-11 and MOLM-13 cells cultured in [U-13C] glucose media. This effect may be mediated by ATF4, a key transcriptional activator of SSP enzymes, which was downregulated by VEN and accompanied by decreased mRNA levels of PHGDH. These findings suggest that VEN-treated cells become increasingly dependent on exogenous serine. Consistent with this hypothesis, serine deprivation significantly sensitized AML cells to VEN. For instance, the IC50 for MOLM-13 decreased from 50.93 μM to 11.92 μM, and for MV4-11 from 63.36 μM to 9.03 μM under serine-depleted conditions. This sensitization was reflected in increased cell death and early apoptosis across both VEN-responsive (MV4-11, MOLM-13) and less responsive (NOMO-1, THP-1) cell lines. To investigate the mechanism underlying this synergy, we examined metabolic adaptations under combined treatment. VEN impaired mitochondrial respiration, with a compensatory increase in glycolysis, as evidenced by elevated glycolytic ATP production. Serine depletion further increased mitochondrial stress, resulting in loss of membrane potential and reduced maximal respiratory capacity. Mechanistically, loss of serine exacerbates VEN-induced energy stress by diverting glycolytic intermediates toward SSP. This disruption of cellular energy homeostasis was confirmed by a significant decrease in intracellular ATP levels, with approximately 43% reduction in MOLM-13 cells and 36% in MV4-11 cells compared to untreated controls in complete media. To test this synergy in vivo, we xenografted NSG mice with MV4-11 cells and assigned them to four Groups: (1) regular diet + vehicle, (2) regular diet + VEN, (3) serine/glycine-free (-SG) diet + vehicle, and (4) -SG diet + VEN. Mice on the -SG diet exhibited significantly delayed AML progression and reduced leukemic burden compared to those on the regular diet. Disease progression in Groups 2 and 3 was comparable, with average survival extended to 50 days versus 38 days in Group 1. Remarkably, all mice in Group 4 (-SG diet + VEN) remained alive at day 60, the time of the abstract submission. In summary, our findings identify serine metabolism as a critical metabolic vulnerability in AML. Targeting serine availability synergizes with venetoclax to induce profound ATP stress and leukemic cell death. This strategy offers a novel therapeutic paradigm in which dietary interventions, rather than additional chemotherapeutics, can be leveraged to enhance therapeutic efficacy while minimizing chemo-mediated systemic toxicity.