DNA METHYLATION ACCOUNTS FOR THE INHIBITION OF COLLAGEN-VI EXPRESSIONIN TRANSFORMED FIBROBLASTS

The expression of collagen VI, an adhesive glycoprotein of the extrace llular matrix, is completely inhibited in virally transformed fibrobla sts and in many cell lines derived from spontaneous mesenchymal tumors . Here we present evidence that DNA methylation plays an important rol e in this inhibition: (a) The mRNA level for DNA methyltransferase is highly increased in simian virus 40 (SV40)-transformed fibroblasts com pared with normal cells and this increase correlates with the decrease of the mRNA level for collagen VI. (b) Methylation of the alpha 2(VI) collagen promoter in vitro abolishes promoter activity in a transient transfection assay. (c) Genonmc sequencing reveals extensive methylat ion of the promoter region in SV40-transformed cells, but virtually no methylation of the corresponding region in normal cells. Increased me thylation is also observed in a rhabdomyosarcoma cell line. (d) Two Of the cis-acting elements of the a2(VI) collagen promoter lose their af finity for transcription factor AP2 when methylated in vitro as demons trated by gel retardation experiments. DNA methylation is therefore in volved in the silencing of the a2(VI) collagen gene. It seems likely t hat the same mechanism is also responsible for the repression of other transformation-sensitive proteins.