Basal levels of metallothionein I and II expression in mouse embryo fibroblasts enhance growth in low folate through a cell cycle mediated pathway.

The impact of basal (non-induced) expression levels of metallothionein I and II on the growth of mouse embryo fibroblasts in standard DMEM/F-12 containing 8.8 microm folic acid, and in DMEM/F12 without hypoxanthine, thymidine or folic acid, containing 15 nm or 15 pm[6S]-folinic acid, was assessed by comparing wild-type MT (+/+) and homozygous null MT (-/-) cell lines. No difference in growth rate was observed between the two in DMEM/F12, although MT (-/-) cells displayed a 6-fold decrease in p27(Kip1), a two fold increase in p53 and a slight increase in p21(Waf1). After 6 days in culture, the growth rate for MT (-/-) cells in 15 nm or 15 pm[6S]-folinic acid was half that of MT (+/+). After an additional 6 days in 15 n m folate, both MT (+/+) and (-/-) cells maintained their respective growth rates, while those in 15 pm had ceased to grow. During the initial 6 days in 15 nm folate, neither cell population displayed an increase in apoptosis or a change in cell cycle distribution, even though MT (-/-) cells sustained an additional 4-fold increase in p21(Waf1)and a 6-fold decrease in cyclin E expression. At day 12, however, the MT (-/-) population, but not MT (+/+), underwent a 7-fold increase in apoptosis coupled with a 3 fold increase in S phase cells. Hence, the basal level of MT I and II constitutively expressed in MT (+/+) cells enhances growth in 15nM [6S]-folinic acid by preventing S phase arrest and apoptosis. Copyright 2001 Academic Press.