Identification of transcriptional regulatory elements in the human somatostatin receptor sst2 promoter and regions including estrogen response element half-site for estrogen activation.

The somatostatin receptor subtype 2 (sst2) mediates inhibition of hormone secretion and cell proliferation, and modulates neurotransmission. Its expression is widespread in various normal tissues and many malignant cells, and is up-regulated by estrogen in breast cancer cells. This study was undertaken to investigate molecular mechanism of transcriptional regulation of the human sst2 gene, for which an additional exon (exon 1) in the 5'-untranslated region was recently found. Transient transfection and mutational analysis showed that the immediate 5'-upstream region containing two Sp1 (-54/-45 and -88/-79) and an ATF/CRE (-69/-62) sites provided full promoter activity. An EMSA together with transfection analysis in Sp1-deficient Drosophila Schneider line (SL2) cells showed that Sp1 acted on the proximal Sp1 site, whereas Sp3, Sp1, and Sp2 did on the distal Sp1 site. Activating transcription factor-2 (ATF)-2, c-Jun, and cyclic AMP response element-binding protein (CREB) interacted with the ATF/CRE site. Transcriptional activation by estrogen occurred through two different regions; one included these proximal elements and the other existed in the upstream region containing estrogen response element (ERE) half-site (-348/-344) and GC-rich sequence (-447/-414). This upstream estrogen responsiveness was observed in a human breast cancer T47D cell, but not in GH(3) or estrogen receptor alpha (ERalpha) -expressing HeLa cells, and was potentiated by overexpression of ERalpha or ERbeta, whose binding to the ERE half-site was verified by EMSAs. A chromatin immunoprecipitation assay suggested that ERalpha was recruited to the ERE half-site after estrogen treatment in T47D cells. The present results should provide a molecular basis for transcriptional regulation in a variety of physiological and pathological contexts of sst2-expressing tissues.