Tumor-Specific S/G₂-Phase Cell Cycle Arrest of Cancer Cells by Methionine Restriction

Cancer cells require elevated amounts of methionine (MET) and arrest their growth under conditions of MET restriction (MR). This phenomenon is termed MET dependence. Fluorescence-activated cell sorting (FACS) first indicated that the MET-dependent SV40-transformed cancer cells were arrested in the S and G₂ phases of the cell cycle when under MR. This is in contrast to a G₁-phase accumulation of cells, which occurs only in MET-supplemented medium at very high cell densities and which is similar to the G₁ cell-cycle block which occurs in cultures of normal fibroblasts at high density. When the human PC-3 prostate carcinoma cell line was cultured in MET-free, homocysteine-containing (MET^-HCY⁺) medium, there was an extreme increment in DNA content without cell division indicating that the cells were blocked in S phase. Recombinant methioninase (rMETase) treatment of cancer cells also selectively trapped cancer cells in S/G₂: The cell cycle phase of the cancer cells was visualized with the fluorescence ubiquitination cell cycle indicator (FUCCI). At the time of rMETase-induced S/G₂-phase trap, identified by the cancer cells' green fluorescence by FUCCI imaging, the cancer cells were administered S-phase-dependent chemotherapy drugs, which interact with DNA or block DNA synthesis such as doxorubicin, cisplatin, or 5-fluorouracil (5-FU) and which were highly effective in killing the cancer cells. In contrast, treatment of cancer cells with drugs in the presence of MET, only led to the majority of the cancer cell population being blocked in G₀/G₁ phase, identified by the cancer cells becoming red fluorescent in the FUCCI system. The G₀/G₁ blocked cells were resistant to the chemotherapy. MR has the potential for highly effective cell-cycle-based treatment strategy for cancer in the clinic.