The Conditions of Non-rescuability of Methioninase-treated Cancer Cells by Methionine Replenishment: The Point of No Return

Abstract

Background/aim: The aim of the present study was to determine the extent of methioninase treatment beyond which methionine-addicted cancer cells could no longer be rescued by methionine repletion.

Materials and methods: Methionine-addicted HCT-116 human colon-cancer cells were treated in vitro with 1, 3, and 6 unit/ml of recombinant methioninase (rMETase) in methionine-containing growth media in 96-well culture dishes for eight days. rMETase was removed from six culture wells at various time points, and the cells were subsequently given normal growth medium containing 100 μM methionine. Cell number was determined every other day using the WST-8 cell-counting kit.

Results: When HCT-116 human colon-cancer cells were treated with 1 unit rMETase/ml, the cells could be fully rescued on days 2, 4, and 6 by removing rMETase and adding normal methionine-containing growth medium. When treated with 3 units of rMETase/ml, the cells could be fully rescued by methionine replenishment after two days of treatment. However, after four and six days of treatment, the cells were partially rescued; the cells did not die, but stopped proliferating. In contrast, treatment with 6 units of rMETase/ml led to proliferation arrest when rescue was attempted on days 2 and 4, and no rescue was possible on day 6, as the cells continued to die.

Conclusion: Three distinct responses to methionine replenishment following rMETase treatment were observed in HCT-116 cells. First, complete recovery and resumption of proliferation occurred with low-dose rMETase at all time points and with short-term treatment at intermediate doses of r-METase. Second, prolonged intermediate-dose treatment or short-term high-dose treatment, resulted in continued proliferation arrest without further cell death. Third, high-dose rMETase treatment for six days led to cell death.

Keywords: HCT-116 cells; Hoffman effect; Methionine rescue; colon cancer; methioninase; methionine addiction; methionine restriction.