Differential regulation of c-fos expression in estrogen-induced hamster renal tumors compared with kidney not due to creation of an estrogen-response element by point mutation in the gene's flanking sequence
✍ Scribed by Stephen F. Sarabia; Joachim G. Liehr
- Publisher
- John Wiley and Sons
- Year
- 1999
- Tongue
- English
- Weight
- 150 KB
- Volume
- 24
- Category
- Article
- ISSN
- 0899-1987
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✦ Synopsis
The conversion of a palindromic sequence, GGTCTnnnAGACC, in the 5′-flanking region of the murine c-fos proto-oncogene into a functional estrogen-response element by a single base change into GGTC A / G nnnAGACC has previously been postulated [Nawaz et al., 1993] as a possible mechanism of the induction of tumors by estrogens. This attractive hypothesis has been investigated in estradiol-induced Syrian hamster kidney tumors, in H-301 kidney tumor cells (a cell line derived from the Syrian hamster tumor), and in normal kidney tissue. The c-fos gene is differentially regulated by a classical estrogen receptor-mediated process in tumors, whereas in the acutely treated kidney, estradiol induces c-fos expression independent of estrogen-receptor function. In this study, we identified in the 5′-flanking region of the hamster kidney c-fos gene the sequence AGTCCnnnAGACC, which closely resembled but did not appear to function as an estrogen-response element. No mutations were detected in this sequence or in the 5′-flanking region of c-fos genes from three different primary tumors and from H-301 tumor cells. To rule out the possibility of a low copy number of mutant alleles in a tumor sample, polymerase chain reaction-based single-strand conformation polymorphism analysis was performed on 372 base pairs of the 5′-flank of the c-fos gene (-367 to +5 base pairs relative to the transcription start point). Nine different kidney tumor DNA samples and five normal kidney tissue samples (controls) produced an identical pattern of DNA bands, suggesting a lack of natural polymorphisms and mutations in this region of the c-fos gene. Acute treatment of hamsters with 17β-estradiol for 6 h significantly induced renal c-fos mRNA expression, whereas control levels of c-fos were restored by co-treatment with estradiol and either N-acetyl-L-cysteine or α-naphthoflavone. We concluded that the previously observed change in regulatory control of c-fos expression in kidney versus estradiol-induced tumors does not involve the creation of a functional estrogen-response element by single point mutation in the 5′-flanking region of the gene. Additionally, c-fos expression in estradiol-treated hamster kidneys appears to be mediated by free radicals generated by the catechol metabolites of estradiol and not by the activation of any estrogen receptor.