The complexity of cancer is not overlooked. The ability of cancer cells to surpass biochemical processes is extraordinary, especially in times of high stress due to limited oxygen supply, a condition known as hypoxia. Tumors under hypoxic conditions are often categorized as more aggressive, meaning highly proliferative and invasive into surrounding tissues, and cause a decrease in chemotherapy sensitivity1. Consequently, tumors under hypoxic conditions tend to correlate with a poorer prognosis and decreased disease-free survival1. Although there are protocols to overcome hypoxic conditions, the mechanism for which cancer cells are able to survive under such conditions is not well known.
Very recently, studies have indicated that cancer cells show an increase in acetate intake under hypoxic conditions as an alternative carbon source to produce acetyl-CoA, which is typically derived from glucose, glutamine, and fatty acids. Acetyl-CoA plays an active role in various biochemical reactions involving macromolecule biosynthesis and energy production to support cell growth and proliferation. Additionally, acetyl-CoA molecules have the ability to donate an acetyl group to another molecule, a process known as acetylation, which has epigenetic implications to control gene expression and therefore, protein function. For example, histone acetylation in yeast from acetyl-coA has been uncovered to be involved in the synthesis of lipids and to be involved with cell cycle progression. However, under hypoxia, there is a decrease in acetyl-coA availability, and therefore, a decrease in histone acetylation rate.
So, how can cancer cells survive under hypoxic conditions if the cells are acetyl-CoA deficient and are unable to synthetize macromolecules? The answer may lie in the hands of acetate. Unfortunately, the function of acetate has been overlooked due to its relatively low physiological concentration, and therefore, the mechanism to how cancer cells are able to utilize acetyl-coA under hypoxia is unclear. To shed some light on yet another role of acetyl-coA, Goa et al.2 investigates the involvement of acetate in cancer cell metabolic adaption.
Before investigating how cancer cells utilize acetate, the authors must first confirm an increase in acetate intake under hypoxic conditions. It was observed that hypoxic liver cancer cells absorbed more than 80% of acetate from the culture medium, whereas cancer cells in a normal oxygen condition only absorbed about 20% of the available acetate. In addition, quantification of acetate consumption was determined with another liver cancer cell line, in which the cells absorbed significant, if not all, of the available acetate concentration from the surrounding tissue. This confirmed some role of acetate under hypoxic conditions.
Next, knowing that acetate has some role in histone acetylation, the authors investigated the specific histones that were being activated in hypoxic conditions with the addition of exogenous acetate. The authors treated the liver cancer cells with acetate under hypoxia and observed that the addition of acetate counteracted the decrease in histone acetylation that is normally observed under hypoxic conditions. The authors were able to track the acetylation source by carbon labeling exogenous acetate, and observing the corresponding acetyl-coA groups activate various histones. In particular, acetate addition created a significant increase in H3K9, (histone 3, lysine 9), H3K27, and H3K56 acetylation levels, but not H3K14, H3K18, H3K23, and H3K36 acetylation levels, indicating that acetate induced histone acetylation was specific to particular histones. The authors also proved that histone acetylation was time and dose-dependent on the addition of acetate.
Next, with the knowledge that acetate is causing specific histone acetylation, the authors investigated the biological effect on cancer metabolism induced by acetylation. In other words, the authors were interested in which genes were being activated and transcribed after histone acetylation with the presence of acetate. To accomplish this, mRNA expression levels of metabolic genes were detected under normal and hypoxic conditions. Amplification of mRNA encoding proteins encompassing varying metabolic pathways including glycolysis, TCA (tricarboxylic acid) cycle, fatty acid synthesis, cholesterol metabolism, and more were performed to determine which metabolic pathways may be affected by acetate-induced histone acetylation under normal and hypoxic conditions. As a result, mRNA levels of two genes were identified, FASN and ACACA, to have been activated by more than two-fold with the addition of acetate under hypoxia, while no activation effect was observed under normal conditions. FASN and ACSS2 are promoter regions known to have roles in lipid synthesis, suggesting a role of acetate in lipid synthesis by activating these two genes. In addition ACSS1 and ACSS2 mRNA levels, which code for the two main enzymes involved in acetyl-CoA production from acetate, were replenished back to normal levels with the addition of acetate, suggesting that these two genes play an important role in cancer cells adapting to hypoxia.
To debrief, at this point, the authors have uncovered that exogenous acetate allows for specific histone acetylation, causing for the expression of FASN and ACACA genes that are involved in lipid synthesis which are normally suppressed under hypoxic conditions without acetate. ACSS1 and ACSS2 mRNA levels also were elevated with the addition of acetate under hypoxic conditions. This leads to the next question of how these two genes are involved in lipid synthesis. It is already known that ACSS1 and ACSS2 are the two main enzymes involved in acetyl-CoA production from acetate. Thus, the authors sought to determine the involvement of the two enzymes with the promoter regions FASN and ACACA that are involved in lipid metabolism. To investigate this relationship, one or both of the ACSS1/2 enzymes were suppressed and FASN and ACACA mRNA levels were measured. When the ACSS1/2 were doubly knocked out, the acetate-increased histone acetylation levels at FASN and ACACA promoter regions were blocked! What this revealed was that enzymes ACSS1/2 catalyzed exogenous acetate into acetyl-coA, which induces histone-acetylation at FASN and ACACA promoter regions that when activated, code for proteins involved with lipid synthesis (Figure 1). The ability for cancer cells to survive under hypoxic stress with the availability of exogenous acetate suggests that lipogenic gene expression was vital for cell survival.
Through this study, the authors uncovered the role of acetate in epigenetic regulation to enhance lipid synthesis and promote tumor survival under hypoxic conditions. Undoubtedly, this can have clinical implications by providing a new target for cancer therapies. If there is a way to target either ACSS1/2 or FASN and ACACA promoter regions specific to cancer cells, then this stands as a potential therapeutic tool for preventing cancer growth survival under hypoxic conditions. The implications of this paper shed some light on the vast uncertainty that still remains in the biochemistry of cancer.
- Vaupel, P. Mayer, A. Hypoxia in cancer: significance and impact on clinical outcome. Cancer Metastasis Review 26, 225 (2007).
- Gao, X. et al. Acetate functions as an epigenetic metabolite to promote lipid synthesis under hypoxia. Nature Communications 7 11960 (2016).
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