E. Ehrnrooth et al., The ability of hypoxia to modify the gene expression of thymidylate synthase in tumour cells in vivo, INT J RAD B, 75(7), 1999, pp. 885-891
Purpose: Hypoxic cells in tumours are resistant to 5-fluorouracil (5-FU). T
his 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 deter
mined with an Eppendorf oxygen electrode. The animals were then injected wi
th BrdU (100 mg/kg, i.p.). Tumours were excised and immediately frozen (-80
degrees C) until isolation of total RNA. The mRNA was reversibly transcrib
ed 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 h
ypoxic. 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 signif
icance at 12-24 h and remaining at this lower level for up to 48 h. BrdU la
belling was significantly reduced after breathing low O-2 for 24 h (p = 0.0
01).
Conclusion: Hypoxia-induced down-regulation of TS gene expression was obser
ved. This would be expected to make hypoxic tumour cells more sensitive to
5-FU. Other mechanisms must be responsible for the previously reported resi
stance to this drug.