Abstract
Methionine is an essential amino acid used, beyond protein synthesis, for polyamine formation and DNA/RNA/protein methylation. Cancer cells require particularly high methionine supply for their homeostasis. A successful approach for decreasing methionine concentration is based on the systemic delivery of methionine γ-lyase (MGL), with in vitro and in vivo studies demonstrating its efficacy in cancer therapy. However, the mechanisms explaining how cancer cells suffer from the absence of methionine more significantly than non-malignant cells are still unclear. We analyzed the outcome of the human colorectal adenocarcinoma cancer cell line HT29 to the exposure of MGL for up to 72 h by monitoring cell viability, proteome expression, histone post-translational modifications, and presence of spurious transcription. The rationale of this study was to verify whether reduced methionine supply would affect chromatin decondensation by changing the levels of histone methylation and therefore increasing genomic instability. MGL treatment showed a time-dependent cytotoxic effect on HT29 cancer cells, with an IC50 of 30 µg/ml, while Hs27 normal cells were less affected, with an IC50 of >460 µg/ml. Although the levels of total histone methylation were not altered, a loss of the silencing histone mark H3K9me2 was observed, as well as a decrease in H4K20me3. Since H3K9me2/3 decorate repetitive DNA elements, we proved by qRT-PCR that MGL treatment leads to an increased expression of major satellite units. Our data indicate that selected histone methylation marks may play major roles in the mechanism of methionine starvation in cancer cells, proving that MGL treatment directly impacts chromatin homeostasis.
Highlights
A common feature of cancer cell metabolism is the ability to adapt to a frequently nutrient-poor environment to acquire the necessary supplements to both sustain high rates of cellular proliferation and build new biomass
The proliferation of methionine γ-lyase (MGL)-treated HT29 cells is strongly inhibited at 72 h, with the number of viable cells lowered to 25% of the initial value
Hs27 normal human fibroblast cell line was much less affected by MGL treatment (Figure 2B), showing an IC50 > 0.46 mg/ml. These findings indicate that HT29 cells, as many other cancer cell lines according to the literature, are highly susceptible to the effects of MGL, that is, to the depletion of methionine, more than non-malignant cells
Summary
A common feature of cancer cell metabolism is the ability to adapt to a frequently nutrient-poor environment to acquire the necessary supplements to both sustain high rates of cellular proliferation and build new biomass. Deregulation in the uptake or consumption of glucose and amino acids is part of the known cancer-associated metabolic changes. The former is the Warburg effect, that is, the aerobic energy dependence on glycolysis (Warburg, 1925; Liberti and Locasale, 2016). Cancer cells are unable to proliferate in methioninedepleted media because they are universally methionine addicted; that is, cancer cells over-use methionine for transmethylation and proliferation (Chello and Bertino, 1973; Hoffman and Erbe, 1976; Coalson et al, 1982; Stern et al, 1983; Stern et al, 1984; Hoffman, 2015) This effect has been named the “Hoffman effect” (Hoffman, 2015). Methionine is 1) a building block amino acid for protein synthesis; 2) the precursor of S-adenosylmethionine (SAM), which plays a key role in most of the processes that involve a methyl transfer, such as DNA, RNA, and histone methylation and metabolite transformation; 3) involved in polyamine formation, in the transsulfuration pathway for cysteine and creatine biosynthesis; and 4) involved in onecarbon metabolism (Figure 1)
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