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 NH(2)-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.