Suppression of prolactin gene expression in GH cells correlates with site-specific DNA methylation

Site-specific methylation of the rat prolactin and growth hormone promoters correlates with gene expression

The methylation patterns of the rat prolactin (rPRL) (positions -440 to -20) and growth hormone (rGH) (positions -360 to -110) promoters were analyzed by bisulfite genomic sequencing. Two normal tissues, the anterior pituitary and the liver, and three rat pituitary GH3 cell lines that differ considerably in their abilities to express both genes were tested. High levels of rPRL gene expression were correlated with hypomethylation of the CpG dinucleotides located at positions -277 and -97, near or within positive cis-acting regulatory elements. For the nine CpG sites analyzed in the rGH promoter, an overall hypomethylation-expression coupling was also observed for the anterior pituitary, the liver, and two of the cell lines. The effect of DNA methylation was tested by measuring the transient expression of the chloramphenicol acetyltransferase reporter gene driven by a regionally methylated rPRL promoter. CpG methylation resulted in a decrease in the activity of the rPRL promoter which was proportional to the number of modified CpG sites. The extent of the inhibition was also found to be dependent on the position of methylated sites. Taken together, these data suggest that site-specific methylation may modulate the action of transcription factors that dictate the tissue-specific expression of the rPRL and rGH genes in vivo.

CpG methylation represses the activity of the rat prolactin promoter in rat GH3 pituitary cell lines

In the present report, we have investigated the role of DNA methylation on the binding and trans-acting properties of transcription factors involved in the regulation of the rat prolactin (rPRL) gene, specifically Pit-1. To this aim we took advantage of a model system composed of three GH3 rat pituitary tumor cell lines that greatly differed in the extent of rPRL gene methylation and in the level of rPRL gene expression. Northern blot analyses indicated that identical species of Pit-1 mRNA were present to similar extent in the three GH3 cell lines. Electrophoretic mobility shift assays further demonstrated that Pit-1 was present in nuclear extracts and displayed equal affinities to bind the 1P responsive element encompassing the -65 to -38 region of the rPRL promoter, whatever the GH3 cell line tested. These data suggested that differential expression of the rPRL gene among cell lines did not result from variable amounts of Pit-1. By combining in vitro methylation and transient transfection experiments with a rPRL promoter-driven CAT construct, we showed that extensive methylation at CpG sites abolished the expression of the reporter gene. Furthermore, in vivo competition assays demonstrated that CpG methylation inhibited gene expression by preventing the binding of transcription factors We propose that related mechanisms linked to DNA methylation might alter the activity of the endogenous PRL gene in the low expressing cell line.

Regulation of folate-binding protein gene expression by DNA methylation in methotrexate-resistant KB cells

Folate-binding protein (FBP) is responsible for the cellular transport of folate and methotrexate (MTX) in human KB (nasopharyngeal epidermoid carcinoma) cells. The levels of membrane-associated FBP and FBP mRNA are decreased 70-80% in an MTX-resistant KB subline (KB1BT) (Hsueh C-T and Dolnick BJ, Oncol Res 4: 497-505, 1992). Southern blot analysis did not reveal any differences in FBP gene organization or copy number between KB1BT and KB cells. However, there was a 70% decrease in the FBP gene transcription rate and no change in FBP mRNA stability in KB1BT cells. Assessing genomic DNA methylation by MspI and HpaII restriction analysis suggested that the FBP gene in KB1BT cells was more methylated than in KB cells. These alterations in the expression, transcription rate and DNA methylation state of the FBP gene did not change when KB1BT cells were grown in the absence of MTX for 8 months (MTX-free KB1BT). When MTX-free KB1BT cells were exposed to 2.5 microM 5-aza-2'-deoxycytidine for 72 hr, the FBP gene became hypomethylated and the levels of membrane-associated FBP and FBP mRNA increased by 2- to 3-fold. These data indicate that decreased FBP gene expression in KB1BT cells results from increased DNA methylation.

Hypomethylation of the decorin proteoglycan gene in human colon cancer

We have previously reported that the connective tissue stroma of human colon carcinoma contains elevated amounts of decorin, a small proteoglycan involved in the regulation of matrix formation and cell proliferation. These biochemical changes were correlated with increased mRNA levels and general hypomethylation of the decorin gene in human colon cancer DNA. In this report we use a quantitative polymerase chain reaction method coupled with digestion of the DNA template by methylation-sensitive restriction endonucleases to investigate in detail the location of hypomethylated sites in decorin gene. We demonstrate that a specific site in the 3' region of the gene, encompassing codons 360-361, is specifically hypomethylated in both colon carcinoma and benign polyp. In contrast, three HpaII sites, clustered in the 5' untranslated region, show full methylation in normal and neoplastic DNA. The lack of such changes in ulcerative colitis DNA suggests that chronic inflammation alone is not sufficient to alter cytosine methylation in the decorin gene. These results suggest the possibility that the 3' region of the decorin-coding sequence may be involved in the control of decorin gene expression.

Prolactin-deficient GH3B3 cells are defective in the utilization of the endogenous prolactin promoter yet are fully competent to initiate transcription from a transfected prolactin promoter

Transcription of the prolactin (PRL) gene has been analyzed in wild-type D6, PRL-deficient B3, and revertant r16 GH3 cells. Levels of processed nuclear transcripts from the PRL gene were substantially reduced in the deficient line compared to wild-type cells and returned to greater than wild-type levels in the revertant line. Rare PRL transcripts in the deficient line contained the same 5' end found on transcripts in wild-type and revertant cells as judged by primer extension and S1 nuclease protection assays, implying that the cells are deficient in utilization of the normal wild-type promoter. Deficient cells also contained wild-type levels of the PRL- and growth hormone-specific transcription factor pit-1/GHF-1, and no difference was found in the ability of extracts from wild-type and deficient cells to retard various restriction fragments from both the proximal and the distal PRL promoter regions. The deficient and wild-type cells were equally competent in initiating transcription from a transfected rat PRL promoter containing both the distal and proximal promoter elements. These observations imply that PRL-deficient cells are not defective in a trans-activating factor functioning on these PRL promoter fragments (trans model). Rather, inefficient use of the PRL promoter in the variant cells may reflect an increased methylation state of the PRL gene itself (cis model).

Hypomethylation of the rat tyrosine hydroxylase gene correlates with its expression in several cell types

Changes in the methylation patterns of genes have been implicated in controlling tissue-specific gene expression. Since the mechanism which causes the developmental appearance of tyrosine hydroxylase (TH) is unknown, we examined the extent of DNA methylation in the rat TH gene is PC12, F4 and liver cells, which differ in their expression of TH. Using methylation-sensitive restriction endonucleases, we found that hypomethylation of the TH gene correlated with its expression in these cells.