Severe folate restriction results in depletion of and alteration in the composition of the intracellular folate pool, moderate sensitization to methotrexate and trimetrexate, upregulation of endogenous DHFR activity, and overexpression of metallothionein II and folate receptor alpha that, upon folate repletion, confer drug resistance to CHL cells.

DC-3F/FA3 cells (FA3) were derived from antifolate-sensitive CHL cells by selection for growth in folate-free media containing 15 pM [6S]-5CHOFH4. These cells undergo a 30-fold decrease in intracellular folates, overexpress folate receptor alpha (FR alpha) and metallothionein II, and display increased sensitivity to the dihydrofolate reductase (DHFR) targeted anti-folates methotrexate (MTX) and trimetrexate (TMTX), which can be attributed primarily to the folate pool status. Upon folate repletion by growth in 15 nM [6S]-5CHOFH4, they display a 5- and 10-fold increase in resistance to both drugs, respectively, even though folate pools are restored by only 43%. Enforced overexpression of FR alpha in transfectants cultured in nanomolar folate did not confer resistance to MTX but did support a modest 2-fold increase in resistance to TMTX. Enforced overexpression of MTII had a similar effect, but when both were overexpressed together no increase in resistance beyond that conferred by each one separately was noted, suggesting that both confer resistance to TMTX through a common downstream mechanism. Analysis of three independent low folate selected clones, FA3, FA7, and FA14, showed that each had a 5- to 6-fold increase in DHFR activity accompanied by a similar increase in DHFR protein level. However, no differences were detected in the DHFR gene copy number or in the steady-state amount of DHFR mRNA, suggesting that a posttranscriptional mechanism was responsible for the increase in DHFR expression.