Reduced CpG methylation is associated with transcriptional activation of the bone-specific rat osteocalcin gene in osteoblasts.
Chromatin remodeling of the bone-specific rat osteocalcin (OC) gene accompanies the onset and increase in OC expression during osteoblast differentiation. In osseous cells expressing OC, the promoter region contains two nuclease hypersensitive sites that encompass the elements that regulate basal tissue-specific and vitamin D-enhanced OC transcription. Multiple lines of evidence indicate that DNA methylation is involved in maintaining a stable and condensed chromatin organization that represses eukaryotic transcription. Here we report that DNA methylation at the OC gene locus is associated with the condensed chromatin structure found in cells not expressing OC. In addition, we find that reduced CpG methylation of the OC gene accompanies active transcription in ROS 17/2.8 rat osteosarcoma cells. Interestingly, during differentiation of primary diploid rat osteoblasts in culture, as the OC gene becomes increasingly expressed, CpG methylation of the OC promoter is significantly reduced. Inhibition of OC transcription does not occur by a direct mechanism because in vitro methylated OC promoter DNA is still recognized by the key regulators Runx/Cbfa and the vitamin D receptor complex. Furthermore, CpG methylation affects neither basal nor vitamin D-enhanced OC promoter activity in transient expression experiments. Together, our results indicate that DNA methylation may contribute indirectly to OC transcriptional control in osteoblasts by maintaining a highly condensed and repressed chromatin structure.
Villagra
Alejandro
A
Departamento de Biologia Molecular, Facultad de Ciencias Biologicas, Universidad de Concepcion, Casilla 160-C, Concepcion, Chile.
Gutiérrez
José
J
Paredes
Roberto
R
Sierra
José
J
Puchi
Marcia
M
Imschenetzky
Maria
M
Wijnen Av
Andre van
Av
Lian
Jane
J
Stein
Gary
G
Stein
Janet
J
Montecino
Martin
M
D000818:Animals; D001665:Binding Sites; D002417:Cattle; D002454:Cell Differentiation / Q000502:physiology; D018899:CpG Islands / Q000502:physiology*; D019175:DNA Methylation; D005786:Gene Expression Regulation; D010006:Osteoblasts / Q000166:cytology / Q000502:physiology*; D015675:Osteocalcin / Q000096:biosynthesis / Q000235:genetics*; D011401:Promoter Regions, Genetic / Q000187:drug effects / Q000235:genetics*; D051381:Rats; D014157:Transcription Factors / Q000502:physiology; D015533:Transcriptional Activation; D014407:Tumor Cells, Cultured; D014807:Vitamin D / Q000494:pharmacology
D014157:Transcription Factors; D015675:Osteocalcin; D014807:Vitamin D