Epigenetic regulation of alkaline phosphatase in human cells of the osteoblastic lineage

Epigenetic mechanisms play an important role in the tissue-specific regulation of gene expression. This study analyzed the relationship between tissue non-specific alkaline phosphatase (ALPL) gene expression and the methylation of a CpG island located in its proximal region. Gene expression was analyzed by real time RT-qPCR in primary human osteoblasts (hOBs), the osteoblastic cell line MG-63, the mammary cell line MCF-7, and bone tissue. DNA methylation was analyzed by qMSP in those cells and also in lining osteoblasts and in osteocytes obtained from human bone samples by laser-assisted capture. hOBs expressed much more ALPL mRNA than MG-63 cells (7.3±3.2 vs. 0.2±0.1 arbitrary units, respectively). hOBs showed a very weak DNA methylation (<10%), whereas MG-63 had a higher degree of methylation (58±6%). Likewise, MCF-7 cells, which scarcely expressed ALPL, had a hypermethylated CpG island. Thus, the degree of methylation in the CpG island was inversely associated with the transcriptional levels of ALPL in the studied cells. Furthermore, treatment with the DNA demethylating agent AzadC induced a 30-fold increase in ALPL expression, in MG-63 cells, accompanied by a parallel increase in alkaline phosphatase activity. However, AzadC did not affect ALPL levels in the already hypomethylated hOBs. In addition, in microdissected osteocytes, which do not express alkaline phosphatase, the CpG island was highly methylated (>90%), whereas lining osteoblasts showed an intermediate degree of methylation (58±13%). These results suggest an important role of DNA methylation in the regulation of ALPL expression through the osteoblast-osteocyte transition.