Transcriptional regulation of rat cyclin D1 gene by CpG methylation status in promoter region.

Cyclin D1, a G(1)/S cell cycle-regulating oncogene, is known to be transcriptionally regulated by numerous growth factors. We cloned and characterized the rat cyclin D1 gene 5'-flanking region and, by species- and subspecies-matched transient transfection studies, found that a basic promoter structure with a cAMP response element and two continuous Sp1-binding sites was crucial for the steady-state expression of the cyclin D1 gene. Furthermore, the methylation status especially around two continuous Sp1-binding sites was found to be an important epigenetical mechanism determining the steady-state expression level in rat leukemic cell lines K4D, K4DT, and K4D16. Whether or not epigenetic control of the cyclin D1 gene existed among normal rat tissues was further examined by high sensitivity mapping of the methylated cytosine. In normal rat tissues, the methylated cytosines at non-CpG loci within two continuous Sp1-binding sites were observed in uterine stromal cells of the basal layer and found to be demethylated in the functioning layer, possibly by a passive demethylation mechanism through cell division. Since in the passive demethylation process Sp1-binding sites remain methylated in a part of the cell population, methylated cytosines at Sp1-binding sites may be essential for keeping a number of the stromal cells in the basal layer live against estrogen-induced proliferation that leads to either apoptosis or compaction.