EXPRESSION OF MULTIPLE SOMATOSTATIN RECEPTOR GENES IN ATT-20 CELLS - EVIDENCE FOR A NOVEL SOMATOSTATIN-28 SELECTIVE RECEPTOR SUBTYPE

The pattern of expression of somatostatin receptor (SSTR) genes and ge ne products in AtT-20 cells was characterized in an attempt to explain the SST-28 binding selectivity that typifies these cells. AtT-20 cell s expressed multiple SSTR mRNAs. Paradoxically, this included mRNA for three of the four SST-14 selective receptors: SSTR2 (++++), SSTR1 (+) , SSTR4 (+). The SST-28 selective SSTR5 was expressed as a 3.8-kilobas e (kb) transcript of reLatively Low abundance (+) in contrast to norma l mouse pituitary which dispLayed high Levels (+++) of a 2.4-kb SSTR5 MRNA. Immunoblot analysis of solubilized membranes with an antipeptide SSTR2 antibody revealed a single SSTR2 protein of 72 +/- 2 kDa. Prein cubation of AtT-20 cell membranes with SSTR2 antibody reduced I-125-[L eu8,D-Trp22,Tyr25]SST-28 binding sites by 38%. Residual binding sites exhibited a 4.9-fold increase in affinity for SST-28, a 2.6-fold decre ase in affinity for SST-14, and an SST-28:SST-14 potency ratio of 40:1 compared with a potency ratio of 3.5:1 in control membranes. These re sults demonstrate the expression of four SSTR genes in AtT-20 cells of which SSTR2 predominates. Blockade of SSTR2 with antibody exposes hig h affinity SST-28 selective sites with comparable binding characterist ics to those reported for cloned SSTR5. These SST-28 binding sites may arise from a SSTR5 variant encoded by a high molecular weight 3.8-kb transcript or more likely from another as yet undiscovered member of t he SST-28 selective SSTR subfamily.