Transcriptional silencing of the inhibin-α gene in human gastric carcinoma cells

Abstract

Although inhibin was first identified as a hormone regulating pituitary FSH secretion, it was later recognized to act as a tumor suppressor in the gonad and adrenal glands. Recently, the alpha subunit of this dimeric hormone (inhibin-α) was reported to be involved in prostate tumorigenesis. To identify additional roles outside the reproductive axis, we investigated inhibin-α gene activity and subsequent cell fate in human gastric cancer cells. The results were as follows: all the gastric cancer cells had at least one of a set of abnormalities including hypermethylation of the promoter, mutation of the 5′-UTR or allelic imbalance including LOH in the inhibin-α gene. Hypermethylation of the promoter and mutation of the 5′-UTR in inhibin-α were observed in SNU-1, SNU-5 and SNU-484 cells. LOH was observed in AGS, KATO III, SNU-5, SNU-484 and SNU-668 cells. Treatment with 5-AzaC, a demethylating agent, induced demethylation of the inhibin-α promoter in the SNU-1, SNU-5 and SNU-484 cell lines, with the CpG5 site being strongly influenced by 5-AzaC. In addition, inhibin-αmRNA and protein were maintained at low levels in most of the gastric cancer cell lines. These low levels of mRNA and protein expression could be increased in most lines by treatment with 5-AzaC. These increased inhibin-α expression levels seemed to be closely associated with apoptosis and suppression of cell growth. Taken together, our results reveal that the inhibin-α gene is transcriptionally silenced in human gastric cancer cells, and that reactivation of the gene suppresses their growth characteristics. This suggests that inhibin-α may have a more general tumor suppressor activity outside the reproductive axis.