Metabolic clogging of mannose triggers dNTP loss and genomic instability in human cancer cells
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eLife assessment
The authors present valuable findings regarding the mechanism of high mannose induced cellular toxicity in cancer cells. The evidence supporting genomic instability as the anti-cancer activity of mannose is convincing with multiple orthogonal approaches showing consistent results, but the conclusions related to metabolic remodeling could be further strengthened by additional metabolomics data. While the findings are limited to genetically modified cancer cell lines cultured in vitro, this work will be of interest to cell biologists working on cancer metabolism.
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Abstract
Mannose has anticancer activity that inhibits cell proliferation and enhances the efficacy of chemotherapy. How mannose exerts its anticancer activity, however, remains poorly understood. Here, using genetically engineered human cancer cells that permit the precise control of mannose metabolic flux, we demonstrate that the large influx of mannose exceeding its metabolic capacity induced metabolic remodeling, leading to the generation of slow-cycling cells with limited deoxyribonucleoside triphosphates (dNTPs). This metabolic remodeling impaired dormant origin firing required to rescue stalled forks by cisplatin, thus exacerbating replication stress. Importantly, pharmacological inhibition of de novo dNTP biosynthesis was sufficient to retard cell cycle progression, sensitize cells to cisplatin, and inhibit dormant origin firing, suggesting dNTP loss-induced genomic instability as a central mechanism for the anticancer activity of mannose.
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eLife assessment
The authors present valuable findings regarding the mechanism of high mannose induced cellular toxicity in cancer cells. The evidence supporting genomic instability as the anti-cancer activity of mannose is convincing with multiple orthogonal approaches showing consistent results, but the conclusions related to metabolic remodeling could be further strengthened by additional metabolomics data. While the findings are limited to genetically modified cancer cell lines cultured in vitro, this work will be of interest to cell biologists working on cancer metabolism.
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Reviewer #1 (Public Review):
In this manuscript, Harada et al. build upon prior studies in honeybees and mammalian cells that high levels of mannose impair proliferation, glucose entry into glycolysis. Here, they find that an inability to adequately metabolize mannose results in dNTP depletion and impaired DNA synthesis at replication forks, which sensitizes to chemotherapy. They provide solid evidence that dNTP depletion is sufficient to impair proliferation and increase chemosensitivity, although causality in the context of an inability to metabolize mannose is not established.
Strengths:
This is a very rigorous, well-designed study and the findings are valuable and broadly interesting for the metabolism and cancer communities. The methods are comprehensive and the experimental details in the legends are complete.Weaknesses:
When …Reviewer #1 (Public Review):
In this manuscript, Harada et al. build upon prior studies in honeybees and mammalian cells that high levels of mannose impair proliferation, glucose entry into glycolysis. Here, they find that an inability to adequately metabolize mannose results in dNTP depletion and impaired DNA synthesis at replication forks, which sensitizes to chemotherapy. They provide solid evidence that dNTP depletion is sufficient to impair proliferation and increase chemosensitivity, although causality in the context of an inability to metabolize mannose is not established.
Strengths:
This is a very rigorous, well-designed study and the findings are valuable and broadly interesting for the metabolism and cancer communities. The methods are comprehensive and the experimental details in the legends are complete.Weaknesses:
When giving context to their work, the authors focus heavily on what is known about mannose metabolism in honeybees and do not discuss thoroughly what is known in cancer cells, including prior work that performed very in-depth metabolic phenotyping of mannose phosphate isomerase low and high cells. The claim that the activity of the pentose phosphate pathway is not affected by mannose is not completely justified by the data presented, as pathway flux is not examined. Moreover, the mechanistic connection between mannose and dNTP depletion is not established. Finally, causality for dNTP depletion in cell cycle perturbation and chemosensitivity is not established. -
Reviewer #2 (Public Review):
Harada et al. investigated the mechanism by which high mannose levels inhibit cellular proliferation and enhance chemotherapy. The authors used CRISPR-Cas9 to delete mannose phosphate isomerase (MPI), a key enzyme for metabolizing mannose, in human cancer cells. They found that MPI knockout leads to decreased proliferation of cancer cells when challenged with supraphysiologic concentrations of mannose. Mannose challenge increased sensitivity to both cisplatin and doxorubicin chemotherapy. It also induced slow cell-cycling with impaired entry into the S phase and progression to mitotic phase. Proteomic analysis revealed down-regulation of cell-cycle related proteins following mannose challenge. Specifically, MCM2-7 proteins are decreased, indicating a failure of replication fork progression. The authors show …
Reviewer #2 (Public Review):
Harada et al. investigated the mechanism by which high mannose levels inhibit cellular proliferation and enhance chemotherapy. The authors used CRISPR-Cas9 to delete mannose phosphate isomerase (MPI), a key enzyme for metabolizing mannose, in human cancer cells. They found that MPI knockout leads to decreased proliferation of cancer cells when challenged with supraphysiologic concentrations of mannose. Mannose challenge increased sensitivity to both cisplatin and doxorubicin chemotherapy. It also induced slow cell-cycling with impaired entry into the S phase and progression to mitotic phase. Proteomic analysis revealed down-regulation of cell-cycle related proteins following mannose challenge. Specifically, MCM2-7 proteins are decreased, indicating a failure of replication fork progression. The authors show that high mannose conditions disengage dormant origin sites from DNA synthesis during replication stress induced by cisplatin, confirming relevance to induced chemotherapy sensitivity. Metabolic analysis revealed decreased glycolytic activity, increased oxidative phosphorylation, and depleted nucleotides. Finally, pharmacologic inhibition of de novo dNTP biosynthesis using hydroxyurea treatment produced similar effects on cell-cycle progression, chemotherapy sensitivity, and inhibition of DNA synthesis from dormant origins, indicating that high mannose induced depletion of dNTP pools may be the major mechanism behind the anti-cancer effects of mannose.
Strengths: Overall, the authors used a robust approach with several techniques showing consistent results. The use of multiple clones and cell lines increases confidence in the reported findings. Additionally, the re-expression of MPI in MPI-KO cells eliminated the sensitivity to high mannose conditions, increasing confidence that the findings are not due to off-target effects. The authors are thorough in characterizing the defects in cell-cycle progression and have robust molecular evidence to support the failure of DNA synthesis from dormant origins during chemotherapy-induced replication stress. The use of both proteomics and metabolomic techniques generates a robust picture of molecular effects of mannose challenge. Lastly, the demonstration of similar mechanistic effects by pharmacologic inhibition of de novo dNTP synthesis provides support that depletion of dNTPs is a major cause for the anti-cancer effects of high mannose.
Weaknesses: While the conclusions of this paper are supported by strong and consistent evidence, there are limitations in the relevance of the models used. The study was conducted using cancer cells genetically engineered to not express MPI. However, cancer cells ubiquitously express MPI. Drawing conclusions about metabolic remodeling based on metabolite pool sizes alone is not recommended, as pool sizes can increase or decrease due to changes in production or consumption. Isotope labeling studies would reconcile the reasons for accumulation or depletion of metabolite pool sizes. Lastly, in Figure 3, the authors show down regulation of cell cycle progression genes in response to mannose challenge. However, there is also upregulation of proteins related to various cell death mechanisms including ferroptosis and necrosis, suggesting there may be additional mechanisms to explain the effects of mannose challenge. It is unclear why the cell-cycle explanation was pursued without addressing other possibilities.
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Reviewer #3 (Public Review):
The manuscript approaches an important problem associated with mannose challenge and subsequent changes in metabolism and DNA replication. The researchers employed MPI-KO human cancer cells to explore the key mechanism behind the anti-cancer activity of mannose, and demonstrated that the large influx of mannose exceeding the capacity to metabolize it, that is, the onset of 'honeybee syndrome', induced dramatic metabolic remodeling that led to dNTP loss.
• They established MPI-KO human cancer cells using the CRISPR-Cas9 system, and exploited the mannose auxotrophy and sensitivity observed in MPI-KO mouse embryonic fibroblasts (MPI- KO MEFs) (DeRossi et al., 2006). The addition of a physiological concentration of mannose (50 μM, unchallenged) to culture medium supported the proliferation of MPI-KO MEFs. …
Reviewer #3 (Public Review):
The manuscript approaches an important problem associated with mannose challenge and subsequent changes in metabolism and DNA replication. The researchers employed MPI-KO human cancer cells to explore the key mechanism behind the anti-cancer activity of mannose, and demonstrated that the large influx of mannose exceeding the capacity to metabolize it, that is, the onset of 'honeybee syndrome', induced dramatic metabolic remodeling that led to dNTP loss.
• They established MPI-KO human cancer cells using the CRISPR-Cas9 system, and exploited the mannose auxotrophy and sensitivity observed in MPI-KO mouse embryonic fibroblasts (MPI- KO MEFs) (DeRossi et al., 2006). The addition of a physiological concentration of mannose (50 μM, unchallenged) to culture medium supported the proliferation of MPI-KO MEFs. However, mannose challenge increased the sensitivity of MPI-KO HT1080 cells to DNA replication inhibitors (i.e., cisplatin and doxorubicin) when the cells had been preconditioned with excess 5 mannose prior to the drug treatment.
• Thus, induction of honeybee syndrome suppresses cell proliferation and increases chemosensitivity in MPI-KO human cancer cell models.
• These results suggest that mannose challenge severely impairs the entry of the cells into S phase and its progression to mitotic phase. Strikingly, however, switching of the mannose-challenge medium to the mannose-unchallenged medium after long-term mannose challenge (6 days) resulted in robust cell proliferation.
• The researchers observed downregulation of proteins related to the cell cycle and DNA replication in mannose-challenged cells (Fig. 3A), which were enriched with the mini-chromosome maintenance 2-7 (MCM2-7) complex.
• Together, these results indicate that mannose challenge disengages dormant origins from DNA synthesis during replication stress, thus exacerbating DNA damage.
• Our finding that DNA synthesis from dormant origins during replication stress is highly sensitive to the dNTP pool size is in good agreement with the therapeutic advantages of RNR inhibition in enhancing the efficacy of radiochemotherapy (Kunos and Ivy, 2018).
The work is of potentially great importance in understanding the action of mannose on cancer cells and the resulting sensitization to anti-cancer agents. -
