Constitutive somatostatin receptor subtype 2 activity attenuates GH synthesis

Somatostatin signals predominantly through somatostatin receptor (SSTR) subtype 2 to attenuate GH release. However, the independent role of the receptor in regulating GH synthesis is unclear. Because we had previously demonstrated constitutive SSTR2 activity in mouse corticotrophs, we now analyzed GH regulation in rat pituitary somatotroph (GC) tumor cells, which express SSTR2 exclusively and are devoid of endogenous somatostatin ligand. We demonstrate that moderately stable SSTR2 overexpression (GpSSTR2(WT) cells) was associated with decreased GH promoter activity, GH mRNA, and hormone levels compared with those of control transfectants (GpCon cells). In contrast, levels of GH mRNA and peptide and GH promoter activity were unchanged in GpSSTR2(DRY) stable transfectants moderately expressing DRY motif mutated SSTR2 (R140A). GpSSTR(2DRY) did not exhibit an enhanced octreotide response as did GpSSTR2(WT) cells; however, both SSTR2(WT)-enhanced yellow fluorescent protein (eYFP) and SSTR2(DRY)-eYFP internalized on octreotide treatment. Suberoylanilide hydroxamic acid (SAHA), a histone deacetylase inhibitor, increased GH synthesis in wild-type GC cells and primary pituitary cultures. GpSSTR2(WT) cells induced GH synthesis more strongly on SAHA treatment, evident by both higher GH peptide and mRNA levels compared with the moderate but similar GH increase observed in GpCon and GpSSTR2(DRY) cells. In vivo SAHA also increased GH release from GpSSTR2(WT) but not from control xenografts. Endogenous rat GH promoter chromatin immunoprecipitation showed decreased baseline acetylation of the GH promoter with exacerbated acetylation after SAHA treatment in GpSSTR2(WT) compared with that of either GpSSTR(2DRY) or control cells, the latter 2 transfectants exhibiting similar GH promoter acetylation levels. In conclusion, modestly increased SSTR2 expression constitutively decreases GH synthesis, an effect partially mediated by GH promoter histone deacetylation.