Repression of glycoprotein hormone alpha-subunit gene expression and secretion by activin in alpha T3-1 cells.

Abstract

The alpha T3-1 cell line, a GnRH-responsive gonadotroph cell line developed by targeted oncogenesis in transgenic mice, was used to study regulation of the glycoprotein hormone alpha-subunit by activin. Transient transfection assays established that activin suppressed transcription of both the human and mouse alpha-subunit genes. Initial studies demonstrated that activin decreased transcription of -846 and -180 human alpha-subunit-luciferase constructs by about 30%, but that inhibin and follistatin were without effect. Subsequent studies to localize sequences mediating responses to activin were carried out using a series of 5'-deletions (-507 to -133) of the mouse alpha-subunit promoter fused to luciferase. The luciferase activity of the -507-base pair construct was decreased by 60-70% in the presence of activin, and follistatin prevented this decrease. There were significant stepwise losses of activin responsiveness when sequences between -507 and -424, -424, and -288, and -288 and -205 base pairs were eliminated. Clustered point mutations of the mouse alpha-subunit gene, shown previously to reduce basal expression and GnRH responsiveness, were tested to further identify sequences mediating activin repression. Constructs containing a mutated (-337 to -330) pituitary glycoprotein hormone basal element (PGBE) showed significant loss of activin responsiveness in the context of both the native promoter (-507 to +46) and a minimal promoter downstream of the -507 to -205 region of the mouse alpha-subunit gene, whereas mutation of sequences (-406 to -399) in the GnRH-response element had no effect. Multimers of the PGBE element (-344 to -300) were insufficient to mediate a full activin response when linked to a minimal promoter. When added together with GnRH to transfected cells, activin abolished the stimulatory effect of GnRH on alpha-transcription. Secretion of free alpha-subunit by alpha T3-1 cells decreased 10-50% after exposure to activin for approximately 20 h, and steady state levels of alpha-subunit messenger RNA (mRNA) decreased by about 20-25% after 24-72 h. As changes in activin sensitivity could modulate its action, activin receptor II mRNA levels were measured by Northern blot hybridization at various times after activin (or inhibin) treatment. The three species of ActRII mRNA present in alpha T3-1 cells (approximately 6, 3, and 0.5 kilobases) were unaffected up to 72 h by these treatments. These observations provide the first demonstration that activin regulates a gonadotropin subunit gene at the level of transcription. Suppression of transcription of the mouse alpha-subunit gene by activin appears to involve several segments of the alpha-promoter, one of which is in the region of the PGBE. Thus, alpha T3-1 cells may provide a favorable system to further identify the DNA sequences and nuclear factors through which activin acts to alter transcription.