Metabolism of fatty acids and ketone bodies for glioblastoma growth: Implications for Ketogenic Diet Therapy

Background

The utilization of different nutrient sources by cancer cells is key for the understanding of cancer cell metabolism, which supports cell growth and cancer progression. Indeed, many genes related to cancer (oncogenes, which promote tumor growth, and tumor suppressors) are related to cellular metabolism. The functioning of the metabolic machinery of cancer cells needs to be fully known to find effective therapies against cancer.

Glioblastoma is one of the most aggressive forms of cancer and the most prevalent primary malignant brain tumor. The most common form in humans is also the most aggressive one, glioblastoma grade IV, also known as glioblastoma multiforme (GBM). Glioblastoma is caused by the uncontrolled growth of glial cells and their precursors in the brain. Glial cells support neurons in the central nervous system. GBM is usually resistant to the standard of care for this type of cancer, which consists of surgical resection, radiotherapy and chemotherapy. Thus, there is a clear and urgent need for effective therapies (Davis, 2016).

Sperry et al. studied the metabolism of glioblastoma cells in vitro and in vivo, with a focus on the capability of GBM cells to utilize fatty acids and ketone bodies (Sperry et al., 2019). Recently, ketogenic diets have been proposed to be beneficial for GBM. Ketogenic diets are low-carbohydrate, high-fat diets, which induce fatty acid oxidation and thus the production of ketone bodies. The reason behind these beneficial effects is based on the dependence of GBM cells on glucose. Ketogenic diets would reduce glucose availability for the brain cells, limiting GBM growth selectively, since healthy brain cells grow well by oxidizing ketone bodies. However, the ability of GBM to oxidize fatty acids and ketone bodies is not well known.

 

The findings

The researchers studied how GBM cells oxidize fatty acids and ketone bodies, and its implication for tumor growth. They used U87 glioma cell line and primary human GBM gliomasphere cell lines. When these cells were treated with palmitic acid (C16:0, a saturated long-chain fatty acid) and 3-hydroxybutyrate (3-OHB, a ketone body), these compounds enhanced cell growth. Fatty acids were used to obtain energy in these cells since the increase in cell growth was dependent on fatty acid oxidation. Knock-down of CPT1A, a protein needed for the entry of fatty acids into the mitochondria for beta-oxidation, or treatment with etomoxir, a CPT1 inhibitor, prevented the increased cell growth.

They translated their findings in vivo, by injecting control and sh-CPT1A U87 cells into NSG mice (immunodeficient mice in which tumors can grow). In control mice, tumor growth was promoted under a ketogenic diet. They confirmed that enhanced tumor growth was dependent on fatty acid oxidation since mice injected with sh-CPT1a U87 cells presented reduced tumor growth and an improvement in the survival rate.

In an in vitro tracer-based metabolomics study, by adding fully-labeled 13C-palmitate to U87 cells, they found that palmitic acid carbons were incorporated into acetyl-CoA molecules and TCA intermediates in control U87 cells. Fatty acid oxidation inhibition with etomoxir reduced the incorporation of carbons into acetyl-CoA and TCA intermediates, confirming that fatty acids were used to generate energy. However, the dose of etomoxir used in this study is elevated (100 µM) and has been reported to have off-target effects (Divakaruni et al., 2018).

A recent report reached similar conclusions regarding the reliance of GBM cells on fatty acid oxidation (Duman et al., 2019). In summary, the implications of the results found in this study are relevant for GBM. Ketogenic diets have been recently proposed as adjuvant therapy for GBM. The findings of this study, even though not definitive, demand a word of caution on the use of ketogenic diets in GBM patients.

 

Comments and questions for the authors

In general, I think the study is well done and answers most of the research questions proposed at the beginning. The obtention of similar results in gliomaspheres obtained from GBM patients and in U87 glioma cells is a strong point of this study. The facts that orthotopic tumor growth is reduced and mice survival is improved when fatty acid oxidation is inhibited (sh-CPT1A cells) are also relevant for the translatability of these results. However, the use of Etomoxir as a specific fatty acid oxidation inhibitor has raised some concerns in the scientific community (Divakaruni et al., 2018). The dose used here (100 µM) has been demonstrated to present off-target effects which may confound the final results obtained here.

 

My questions are:

 

Have the authors tried lower concentrations of Etomoxir (like 3 µM) in the same set of experiments reported here? The authors could also consider the utilization of other fatty acid oxidation inhibitors, such as L-aminocarnitine, which have been proved to be more specific. If either of those works, the authors could consider to treat mice receiving orthotopic tumors and check tumor growth and survival.

 

In Figure 6E, the growth of gliomaspheres when supplemented with palmitic acid (50 mM?, again maybe is 50 µM) or 3-OHB (1.25 mM) does not differ from the control cells. However, in Figure 5E-F, HK157 cell growth was promoted by fatty acids (≧50 µM) or ketone bodies supplementation (≧2.5 mM) under physiological conditions. How do the authors explain these differences in similar experiments? It is also interesting that at higher doses of ketone bodies (≧5 mM), the increased growth of HK157 gliomaspheres was blunted (also in U87 control cells supplemented with 10 mM of 3-OHB). What do the authors think about this effect?

 

References:

 

Davis, M. E. (2016). Glioblastoma: Overview of Disease and Treatment. Clin J Oncol Nurs 20, S2–S8. https://doi.org/10.1188/16.CJON.S1.2-8

Divakaruni, A. S., Hsieh, W. Y., Minarrieta, L., Duong, T. N., Kim, K. K. O., Desousa, B. R., Andreyev, A. Y., Bowman, C. E., Caradonna, K., Dranka, B. P., et al. (2018). Etomoxir Inhibits Macrophage Polarization by Disrupting CoA Homeostasis. Cell Metab. 28, 490-503.e7. https://doi.org/10.1016/j.cmet.2018.06.001

Duman, C., Yaqubi, K., Hoffmann, A., Acikgöz, A. A., Korshunov, A., Bendszus, M., Herold-Mende, C., Liu, H.-K. and Alfonso, J. (2019). Acyl-CoA-Binding Protein Drives Glioblastoma Tumorigenesis by Sustaining Fatty Acid Oxidation. Cell Metabolism 30, 274-289.e5. https://doi.org/10.1016/j.cmet.2019.04.004

Sperry, J., Belle, J. E. L., Condro, M. C., Guo, L., Braas, D., Vanderveer-Harris, N., Kim, K. K. O., Pope, W. B., Divakaruni, A. S., Lai, A., et al. (2019). Metabolism of fatty acids and ketone bodies for glioblastoma growth: Implications for Ketogenic Diet Therapy. bioRxiv 659474. https://doi.org/10.1101/659474

Tags: cancer, glioblastoma, ketogenic diet, lipids, metabolism

Posted on: 19 August 2019

doi: https://doi.org/10.1242/prelights.13450

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