EPIGENETIC CONTROL OF PROGRAMMED CELL-DEATH - INHIBITION BY 5-AZACYTIDINE OF 1,25-DIHYDROXYVITAMIN D3-INDUCED PROGRAMMED CELL-DEATH IN C6.9GLIOMA-CELLS

In mammalian DNA cytosine methylation occurs specifically at CpG dinuc leotide. Although the full array of function of DNA methylation is yet to be elucidated, it is well established that DNA methylation is an i mportant mechanism involved in gene expression, DNA replication and ca ncer. Rat glioma C6.9 cells undergo programmed cell death (PCD) after treatment with 1,25-dihydroxyvitamin D3 (1,25-D3). Hence, these cells were used to study whether DNA methylation was involved in the control of PCD. We found that 1,25-D3-mediated PCD of C6.9 cells was suppress ed by exposure of the cells to the DNA demethylating agents 5-azacytid ine (5-AzaC) and 5-aza-2'-deoxycytidine. This effect remains detectabl e several cell divisions following removal of 5-AzaC and, therefore, i nvolves DNA methylation as an epigenetic regulatory mechanism of PCD. Accordingly, internucleosomal fragmentation, a feature of apoptosis th at is detected in 1,25-D3-treated cells, is no longer observable after treatment of these cells with 5-AzaC. However, 5-AzaC does not totall y suppress the responsiveness of C6.9 cells to 1,25-D3 since the induc tion of the c-myc gene remains unaffected. These results suggest that a change in DNA methylation pattern could suppress 1,25-D3-mediated PC D through the expression of previously hypermethylated genes such as p roto-oncogenes with death-repressor activity, endogenous virus sequenc es or even genes inducing change in the differentiated state of these cells. (C) 1998 Elsevier Science Ireland Ltd.