When cancer takes hold, the cellular metabolism of malignant tissues reaches new heights as larger amounts of energy are needed to generate lipids for membrane biosynthesis and tumour signal transduction. It is therefore hoped that research into lipid metabolism pathways in cancer may yield specific therapeutic targets. For the most part, cancer cells rely on de novo fatty acid synthesis, making enzymes in this pathway particularly promising targets. In a study in BMC Cancer, John Laterra from the Kennedy Krieger Institute, USA, and colleagues investigate the role of a key enzyme in the activation step of fatty acid synthesis, namely the Acyl-CoA synthetase ACSVL3. Probing its role in the malignant brain tumour, glioblastoma multiforme (GBM), they specifically look to GBM stem cells that are thought to be especially important for tumour progression. Here Laterra discusses the growing interest in cancer metabolism and the potential of ACSVL3 as a therapeutic target.
What led to your interest in cancer stem cell metabolism, and particularly your investigation of lipid metabolism pathways?
The Laterra lab has an interest in brain cancer stem-like cells, particularly those associated with glioblastoma because they are thought to contribute disproportionately to tumour growth, recurrence, and resistance to treatment (i.e. tumour propagating cells). The lab of Paul Watkins (also at the Kennedy Krieger Institute, USA) had been investigating enzymes of lipid metabolism in inherited metabolic diseases and found that one enzyme – ACSVL3 – was abundant in several human malignancies. It initially seemed logical to assess ACSVL3 levels in glioblastoma cells given the fundamental importance of lipids in cell signaling pathways known to be aberrantly upregulated in glioblastoma and in other cancers. Our initial collaborative study (Cancer Res, 2009, 69:9175-9182) found that inhibiting endogenous ACSVL3 expression in glioblastoma cells inhibited their tumourigenicity. This finding led to the evaluation of ACSVL3 in glioblastoma tumour-propagating stem-like cells that, based on the cancer stem cell hypothesis, determine tumourigenicity.
Your study looks specifically at ACSVL3. What was previously known about this enzyme and its role in cancer and tumourigenesis?
Surprisingly little is known about ACSVL3. Little or no ACSVL3 is found in glial cells of adult brain, yet the enzyme is abundant in glioblastoma. Knocking down ACSVL3 by RNA interference decreased the malignant growth properties of glioblastoma cells, and decreased their tumourigenicity in mice. Reducing the ACSVL3 level in glioblastoma cells decreased signaling through oncogenic receptor tyrosine kinases known to drive glioblastoma malignancy (Cancer Res, 2009, 69:9175-9182). This showed for the first time that ACSVL3 supports glioblastoma malignancy and further suggested a role for this enzyme in regulating PI3-kinase signalling and Akt activation by oncogenic receptor tyrosine kinases.
What were your key findings? Were you surprised by any of them?
One key finding was that ACSVL3 expression was significantly higher in glioblastoma stem-like cells than in the general population of glioblastoma cells, and that expression of ACSVL3 coincided with stem cell markers such as CD133, ALDH, Sox-2, Nestin, and Musashi-1. Another key finding was that forcing the differentiation of stem-like cells resulted in decreased ACSVL3 expression and conversely, knockdown of ACSVL3 by RNA interference induced differentiation of stem-like cells. As with glioblastoma multiforme (GBM) cells, lowering of ACSVL3 expression in stem-like cells decreased growth rate and receptor tyrosine kinase-mediated signalling. The less surprising conclusion from these new results is that ACSVL3 affects cell proliferation and receptor tyrosine kinase signalling similarly in glioblastoma stem-like cells and bulk-population cells. The more surprising result is that ACSVL3 plays a role in cell phenotype regulation (i.e. cell stemness) that can explain why inhibiting ACSVL3 expression inhibits tumourigenicity, a property that correlates with tumour cell stemness.
Your study shows that knock down of ACSVL3 prevents glioblastoma propagation. What further research is needed to determine how your findings could be translated to the clinic?
Our studies used RNA interference to inhibit ACSVL3 expression since specific inhibitors of ACSVL3 enzymatic activity don’t currently exist. While it is likely that the effects of knockdown of ACSVL3 expression are attributed to diminished enzymatic activity, we can’t absolutely rule out the possibility that enzymatic-independent effects are responsible for the observed anti-tumour effects. Resolving this question and developing specific ACSVL3 enzyme inhibitors are critical to the clinical translation of our findings. We are currently developing high-throughput strategies for screening drugs and chemical libraries to identify candidate small molecule ACSVL3 inhibitors.
Do you think that ACSVL3 may play a similar role in cancer stem cells in other tissue types?
We have not yet looked at stem cells from other types of cancer. However, we do know that ACSVL3 is abundant in the majority of lung and prostate cancers that we have examined, and we suspect that it will be overexpressed in stem cells derived from these malignancies.
Numerous fatty acid metabolism enzymes exist, and are differentially expressed in various tissues. Do you think some of these enzymes may also have a role in cancer stem cells?
Yes, that is probably true. There have been a few reports where lipogenic enzymes such as acetyl-CoA carboxylase and fatty acid synthase are upregulated in cancer stem cells and fatty acid synthase inhibitors have been found to have anti-cancer effects in laboratory studies.
How do you think an understanding of cellular metabolism in cancer stem cells will aid progress in cancer research?
Current concepts point to a critically important role for the stem-like phenotype in the formation of solid tumours and their resistance to therapy. Effective therapies will need to target the stem-like cells in addition to the bulk population of tumour cells. The stem-like cell populations are known to differ biochemically from bulk non-stem populations though the extent to which this is true for metabolic pathways remains poorly defined. Understanding this will ensure that future therapies directed at tumour metabolism do not spare the stem-like cell subsets.
The link between sugar metabolism and cancer is well established, and lipid metabolism is now being explored. To what extent can cellular insights help us understand how diet and lifestyle affect cancer risk?
Elucidating the cellular mechanisms of carbohydrate and lipid metabolism that drive malignancy may help us understand dietary and behavioural influences on cancer risk. However, showing that specific aspects of carbohydrate and/or lipid metabolism at the cancer cell level drives malignancy does not mean that behavioural or dietary changes will be sufficient to overcome the oncogenic effects. Research efforts in this direction are likely more relevant to developing novel pharmacologic therapies.
What’s next for your research?
We want to explore in more depth the molecular mechanisms by which ACSVL3 supports cancer cell stemness and malignancy. In doing so we may identify additional novel therapeutic targets for clinical translation. We are actively in search of small molecule inhibitors of ACSVL3 for their promising therapeutic value.