Intrinsic resistance of cervical squamous cell carcinoma cell lines to methotrexate (MTX) as a result of decreased accumulation of intracellular MTX polyglutamates.

The causes of intrinsic of intrinsic MTX resistance in four human cervical squamous cell carcinoma cell lines (SiHa, C-33A, ME-180, C-4I) with different sensitivities to methotrexate (MTX) were determined. The in situ or whole-cell assay for thymidylate synthase (TS) was used to screen for known causes of MTX resistance, including increased dihydrofolate reductase (DHFR), altered DHFR, impaired MTX uptake, and decreased formation of MTX polyglutamates. While all four cell lines displayed initial sensitivity to MTX as demonstrated by a TS activity of < 20% following a 3-hr incubation with MTX, TS activity recovered in three of the four cell lines following a 4-hr incubation in drug-free media. Determination of MTX-polyglutamate accumulation in the four cell lines following a 24-hr incubation with 10 microM [3H]MTX revealed that a higher total intracellular level of MTX polyglutamates was achieved in the sensitive cell line (SiHa), with 54% occurring as long-chain tri-, tetra-, or pentaglutamates. The three resistant cell lines were found to contain less total MTX polyglutamates, with only 38, 14, and 13% occurring as long-chain MTX polyglutamates. Intrinsic MTX resistance in some cervical squamous cell carcinoma cell lines appears to be associated with a diminished accumulation of long-chain MTX polyglutamates, which are preferentially retained intracellularly.