The intestine is a multi-functional dynamic organ which must respond instantaneously to diverse environmental cues for normal body maintenance. Highly integrated regulatory systems are necessary for proper intestinal adaptation to the luminal environment. My study is part of a larger project that aims to broaden the understanding of how luminal nutrients modulate intestinal functions. I hypothesize that guanine nucleotide-binding protein-coupled receptors (GPCR) are part of the intestinal regulatory system that directly responds to luminal macronutrients.

Through this dissertation, I, for the first time, present evidence showing that GPCR (GPR93) directly responds to luminal protein digests (i.e. enzymatic hydrolysate of dietary protein, peptone) and, as a consequence, modulates the intestinal physiology, exemplified by the regulation of the synthesis and secretion of enterohormone and the change in immediate early gene response. As shown in Chapter 1, I establish GPR93 as an authentic protein hydrolysate responsive GPCR and present the intracellular changes (intracellular calcium and ERK1/2 phosphorylation) caused by the activation of this receptor in the intestinal cell culture model, hBRIE380i cells. The synergistic enhancement of activation when GPR93 is stimulated by its known agonist, lysophosphatidic acid, together with peptide fragments (peptone, cefaclor, or enzymatic digests of fatty acid free bovine serum albumin) clearly demonstrates that activation of GPR93 by protein hydrolysate results in a distinct signaling pathway, compared to activation of GPR93 by lysophosphatidic acid. Protein hydrolysate specific activation of GPR93 is further supported by the modulation of cholecystokinin synthesis and secretion in enteroendocrine STC-1 cells (Chapter 2) and the transcriptional regulation of cfos gene in enterocyte hBRIE380i cells (Chapter 3), all of which are not observed when the same receptor is stimulated by lysophosphatidic acid.

The existence of a protein hydrolysate responsive GPCR that plays a unique regulatory role in intestinal functions provides a solid argument for identifying intestinal mucosal GPCRs as direct luminal macronutrient induced signal transducers.