GPR93 Activation by Protein Hydrolysate Induces CCK Transcription and Secretion in STC-1 Cells By

In the intestinal lumen, protein hydrolysate increases the transcription and release of cholecystokinin (CCK) from enteroendocrine cells of the duodenal-jejunal mucosa. Our recent discovery that a G protein coupled receptor (GPCR), GPR93, is activated by dietary protein hydrolysate causing induced intracellular calcium mediated signaling events in intestinal epithelial cells, raises a possibility that GPR93 might be involved in the protein hydrolysate induction of CCK expression and/ or secretion. Using the enteroendocrine STC-1 cells as a model, the present study demonstrates that increasing expression of GPR93 amplifies the peptone induction of endogenous CCK mRNA levels. A similar increase in CCK transcription, indicated by the luciferase reporter activity driven by an 820 bp CCK promoter, is also observed in response to peptone at a dose as little as 6.25 mg/ ml, but not to lysophosphatidic acid (LPA), an agonist of GPR93. We discovered that the up-regulation of CCK transcription involves ERK1/2, PKA, and calmodulin dependent protein kinase (CaMK) mediated pathways. Additionally, GPR93 activation by peptone induces a response in CCK release at 15 minutes, which continues over a 2-hour period. The cAMP level in STC-1 cells overexpressing GPR93 is induced at a greater extent by peptone than by LPA suggesting a possible explanation of the different effects of peptone and LPA on CCK transcription and secretion. Our data indicate that GPR93 can contribute to the observed induction of CCK expression and secretion by peptone, and provide evidence that GPCRs can transduce dietary luminal signals. Page 2 of 40