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Silencing of Secretin Receptor Function by Dimerization with a Misspliced Variant Secretin Receptor in Ductal Pancreatic Adenocarcinoma¹

Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic, Rochester, Minnesota 55905

Secretin receptors that are key for regulation of healthy pancreatic ductal epithelial cells have been reported to be functionally absent on ductal pancreatic adenocarcinomas. Here, we examine the possible presence and function of molecular forms of the secretin receptor in pancreatic cancer cell lines and in primary tumors. Surprisingly, reverse transcription-PCR and sequencing demonstrated wild-type secretin receptor mRNA in each of four cell lines and three primary tumors. Lack of biological response to nanomolar concentrations of secretin was best explained by the demonstrated coexpression of a second and predominant transcript in each of the cell lines and tumors. This represented a variant of the secretin receptor in which the third exon was spliced out to eliminate residues 44–79 from the NH₂-terminal tail. This spliceoform has only recently been recognized in a rare gastrinoma, where it was incapable of binding secretin or signaling, and possessed dominant-negative activity to suppress hormone action at the wild-type secretin receptor (1) . Overexpression of wild-type secretin receptor in Panc-1 cells driven by transfection of fully processed cDNA resulted in normal responsiveness to low concentrations of secretin, establishing the ability of these cells to produce a receptor capable of normal biosynthesis, trafficking, and signaling. Bioluminescence resonance energy transfer demonstrated that the variant receptor could form a heterodimer with wild-type receptor, providing a molecular mechanism for its dominant-negative activity. This suggests that missplicing is responsible for expression of a secretin receptor variant having the ability to suppress the function of wild-type receptor by a direct interaction. In 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays in receptor-bearing Chinese hamster ovary cells, the secretin receptor was shown to have growth-inhibitory effects. Suppression of this activity in pancreatic carcinoma might, therefore, facilitate tumor growth and progression of this aggressive neoplasm.

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