Deciphering the effects of microbial products on Peptide YY expression and secretion in human enteroendocrine L-cells

The human gut exerts major functions, mainly due to its localization and by forming an active barrier between a complex environment made of the gut microbiota, digested food products and secreted elements by the host. The main functions of the gut are digestion and absorption of nutrients and it is the first pool of immune cells and a barrier against pathogens, but the gut is also a main endocrine organ secreting more than twenty different hormones. These hormones regulate a wide range physiological functions including food intake, energy metabolism or digestion. Enteroendocrine cells, a sparse family of intestinal epithelial cells, produce and secrete these hormones in response to the activation of a variety of receptors that sense luminal content. Among them, L-cells secrete GLP-1 and Peptide YY (PYY) that are implicated in the regulation of insulin secretion, food intake and intestinal motility. They are mainly found in the distal ileum and in the colon where the microbiota is the densest. Gut microbiota ferments fibers into short chain fatty acids (SCFAs), produces vitamins, participates in regulation of host immune system and is a barrier against pathogens. The cross talk between microbiota and intestinal epithelium is important to maintain the local homeostasis, and is mediated by host receptors recognizing microbial products. Among them, Toll-like receptors (TLRs) recognize conserved microbial associate molecular patterns (MAMPs) and participate to the host innate immunity. Some microbial products also have important functions for the host such has SCFAs that are an important energy substrate for colonocytes and can also activate different signaling pathways. It was shown that fiber-rich diets, increasing production of SCFAs, as well as direct administration of SCFAs in the colon in humans or mice increased PYY plasma levels through mechanisms still undeciphered. Taking advantage of human cell lines as L-cell models, we assessed the different effects of SCFAs and TLR stimulation on PYY expression and secretion and calcium signaling in these cells. We showed that TLRs are functionally expressed in these cells at the exception of TLR4 and TLR8, and that butyrate, one of the three main SCFAs produced by the microbiota increases cell sensitivity to TLR stimulation by increasing their expression. Moreover, TLR stimulation increases Pyy expression by a fold of two but has little effect on secretion. SCFAs differently regulate Pyy expression. Propionate and butyrate highly increase Pyy expression by a fold of 40 and 120 respectively, and their effects are mainly mediated by inhibition of lysine/histone deacetylases whereas acetate increases expression of Pyy by a fold of 1.8 through stimulation of FFAR2 and FFAR3. SCFAs also induce a strong FFAR2-dependent oscillatory response monitoring PYY secretion whereas butyrate via FFAR3 and GPR109a decreases cytosolic calcium concentration and consequently reduces secretory responses. Thus, SCFAs differently increase PYY production and secretion depending of their chain length. Altogether, these results highlight the role human L-cells in microbiota-host crosstalk by sensing microbial products through expression of TLRs and their responses to SCFAs modulating PYY production and secretion. Furthermore, we deciphered some of the mechanisms implicated in beneficial host response to enriched fiber diets and increased production of SCFAs.