INTRACELLULAR METABOLISM OF 5-METHYLTETRAHYDROFOLATE AND 5-FORMYLTETRAHYDROFOLATE IN A HUMAN BREAST-CANCER CELL-LINE

This report describes the intracellular metabolism of 5-methyltetrahyd rofolate (5-methyl-H(4)PteGlu) and 5-formyltetrahydrofolate (5-formyl- H(4)PteGlu) to the various folate forms and their respective polygluta mated states in the MCF-7 human breast-cancer cell lice. The intracell ular folate distribution observed in MCF-7 cells treated with 5-methyl -H(4)PteGlu was similar to that seen in cells treated with 5-formyl-H( 4)PteGlu. In cells exposed to 5-formyl-H(4)PteGlu for 24 h, the folate pool consisted of 103 +/- 10 pmol/mg 10-formyl-H(4)PteGlu, 120 +/- 18 pmol/mg H(4)PteGlu, and 71 +/- 18 pmol/mg 5-methyl-H(4)PteGlu versus 88 +/- 5, 54 +/- 20 and 87 +/- 10 pmol/mg, respectively, for cells exp osed to 5-methyl-H(4)PteGlu. Only the difference seen in H(4)PteGlu le vels between cells exposed to either 5-methyl-H(4)PteGlu or 5-formyl-H (4)PteGlu reached statistical significance (P <0.05). In the absence o f vitamin B12, exposure to 5-methyl-H(4)PteGlu resulted in 154 +/- 17 pmol/mg 5-methyl-H(4)PteGlu along with only 8 +/- 5 pmol/mg 10-formyl- H(4)PteGlu and 4 +/- 2 pmol/mg H(4)PteGlu, thus demonstrating the mark ed dependence on vitamin B12 for the metabolism of 5-methyl-H(4)PteGlu to the other intracellular folates. 5-10-Methylene- H(4)PteGlu (2 +/- 1.3 pmol/mg) was detected only in cells exposed to 5-formyl-H(4)PteGl u for 24 h, not in cells treated with 5-methyl-H(4)PteGlu. The profile of polyglutamates detected in cells treated with either 5-formyl-H(4) PteGlu or 5-methyl-H(4)PteGlu for 24 h was not significantly different , although cells treated with 5-methyl-H(4)PteGlu tended to have less conversion to the higher polyglutamates (Glu3-Glu5) as compared with t hose treated with 5-formyl-H(4)PteGlu. In 5-methyl-H(4)PteGlu-treated cells grown in the absence of vitamin B12, the pentaglutamate was the only polyglutamate form detected, accounting for only 11% of the total folate pool. Since there does not appear to be a greater formation of the optimal reduced-folate forms necessary to achieve enhanced thymid ylate synthase (TS) inhibition through ternary-complex formation in ce lls exposed to 5-methyl-H(4)PteGlu versus 5-formyl-H(4)PteGlu, these s tudies suggest that the use of 5-methyl-H(4)PteGlu would not be advant ageous over that of 5-formyl-H(4)PteGlu in combination regimens with t he fluoropyrimidines.