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DNA replication in methotrexate-treated human lymphoblasts.

A Fridland, T P Brent

European Journal of Biochemistry

|September 15, 1975

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Methotrexate, a dihydrofolate reductase inhibitor, rapidly halts DNA synthesis in lymphoblasts. However, DNA replication continues semiconservatively, suggesting initiation inhibition rather than chain elongation impairment.

Area of Science:

Molecular Biology
Cell Biology
Biochemistry

Background:

Methotrexate is a chemotherapy drug that inhibits dihydrofolate reductase, an enzyme crucial for DNA synthesis.
Understanding the precise mechanism of methotrexate's effect on DNA replication is vital for cancer treatment strategies.

Purpose of the Study:

To investigate the detailed effects of methotrexate on DNA synthesis and replication in human lymphoblasts.
To determine whether methotrexate inhibits DNA chain initiation or elongation.

Main Methods:

Exposure of human lymphoblast cultures to methotrexate.
Measurement of DNA synthesis rates using 5-bromodeoxyuridine incorporation.
Analysis of DNA replication mode via alkaline CsCl density gradients.
Assessment of DNA chain growth rates using alkaline sucrose gradient sedimentation analysis.

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Main Results:

Methotrexate caused an >80% decrease in DNA synthesis within 30 minutes.
DNA synthesis continued for at least 6 hours post-methotrexate exposure, proceeding semiconservatively.
Sedimentation analysis revealed accumulation of 80S DNA fragments, indicating inhibited initiation.
Synthesized DNA remained stable and elongated to full size upon removal of methotrexate.

Conclusions:

Methotrexate inhibits DNA replication initiation in human lymphoblasts.
Chain elongation is not directly inhibited by methotrexate.
The findings elucidate a key mechanism of methotrexate's cytotoxic effect, independent of direct DNA chain elongation inhibition.