Methionine Restriction, Not Cysteine Restriction, Is a Cancer-specific Vulnerability

Recently, there have been numerous publications on the induction of ferroptosis by cysteine restriction in cancer cells. The present report aimed to determine whether cysteine restriction (CR) is a cancer-specific vulnerability in comparison with methionine restriction (MR), which is a known cancer-specific vulnerability.

Human cancer cell lines (HCT116 colon cancer, 143B osteosarcoma or HT1080 fibrosarcoma) and normal human fibroblasts (Hs27) were cultured in Dulbecco's modified Eagle's medium (DMEM) with dialyzed fetal bovine serum from which methionine or cysteine or both or neither had been depleted. Cancer and normal cells were co-cultured in 12-well plates under the above conditions. HCT116 cells expressing green fluorescent protein, and 143B and HT1080 cells expressing red fluorescent protein, were visualized by fluorescence microscopy. Normal fibroblasts and cancer cells were visualized by phase-contrast microscopy as well.

In co-culture, of either 143B, HCT116 or HT1080 with Hs27 human fibrosarcoma, CR was toxic to Hs27 normal fibroblasts as well as to all three cancer cell lines. In contrast, MR was toxic only to the cancer cells but not normal fibroblasts. Dual CR and MR was toxic to normal and cancer cells.

For all three cancer cell lines, HCT116 colon cancer, HT1080 fibrosarcoma and 143B osteosarcoma, both MR and CR were highly inhibitory in the co-cultures with Hs27 normal fibroblasts. In all cases MR had only a slight effect on normal fibroblasts, but CR was highly toxic to normal fibroblasts. Thus, MR is a cancer-specific vulnerability in contrast to CR which is toxic to both normal and cancer cells and is not a cancer-specificity vulnerability. Therefore, attempting to induce ferroptosis of cancer cells by CR does not appear to have potential as an effective cancer therapy.