The ability of hypoxia to modify the gene expression of thymidylate synthase in tumour cells in vivo

Purpose: Hypoxic cells in tumours are resistant to 5-fluorouracil (5-FU). This in vivo study investigated the ability of hypoxia to regulate the gene expression of thymidylate synthase (TS), the target enzyme of 5-FU. Materials and methods: C3H mammary carcinomas, grown in the feet of female CDF1 mice, were used for all experiments. Mice were placed in a 10% oxygen environment for various time periods and the tumour oxygen status was determined with an Eppendorf oxygen electrode. The animals were then injected with BrdU (100 mg/kg, i.p.). Tumours were excised and immediately frozen (-80 degrees C) until isolation of total RNA. The mRNA was reversibly transcribed to complementary DNA and the resulting cDNA amplified in a multiplex PCR reaction, with beta-actin as the internal reference gene. Results: One hour of low oxygen breathing made tumours significantly more hypoxic. This increase was maintained for a maximum incubation period of 48 h. In the same tumours, no change in TS gene expression was seen with up to 3 h of low oxygen breathing. At longer times it decreased, reaching significance at 12-24 h and remaining at this lower level for up to 48 h. BrdU labelling was significantly reduced after breathing low O-2 for 24 h (p = 0.001). Conclusion: Hypoxia-induced down-regulation of TS gene expression was observed. This would be expected to make hypoxic tumour cells more sensitive to 5-FU. Other mechanisms must be responsible for the previously reported resistance to this drug.