Transcriptional activation of mouse sst2 somatostatin receptor promoter bytransforming growth factor-beta - Involvement of Smad4

The sst2 somatostatin receptor is an inhibitory G protein-coupled receptor, which exhibits anti-tumor properties. Expression of sst2 is lost in most h uman pancreatic cancers. We have cloned 2090 base pairs corresponding to th e genomic DNA region upstream of the mouse sst2 (msst2) translation initiat ion codon (ATG). Deletion reporter analyses in mouse pituitary AtT-20 and h uman pancreatic cancer PANC-1, BxPC-3, and Capan-1 cells identify a region from nucleotide -260 to the ATC: codon (325 base pairs) showing maximal act ivity, and a region between nucleotides -2025 and -260 likely to comprise s ilencer or transcriptional suppressor elements. In PANC-1 and AtT-20 cells, transforming growth factor (TGF)-beta up-regulates msst2 transcription. Tr ansactivation is mediated by Smad4 and Smad3, The cis-acting region respons ible for such regulation is comprised between nucleotides -1115 and -972 an d includes Spl and CAGA-box sequences. Expression of Smad4 in Smad l-defici ent Capan-1 and BxPC-3 cells restores TGF-beta -dependent and -independent msst2 transactivation. Expression of Smad4 in BxPC-3 cells reestablishes bo th endogenous sst2 expression and somatostatin-mediated inhibition of cell growth. These findings demonstrate that msst2 is a new target gene for TGF- beta transcription regulation and underlie the possibility that loss of Sma d4 contributes to the lack of sst2 expression in human pancreatic cancer, w hich in turn may contribute to a stimulation of tumor growth.