Table 3.

The characteristics of SCFAs and lactic acid receptors [33,59,60,61].

Receptor	Ligand	Protein G	Exspression	Physiological Function
FFAR2—Free fatty acid receptor 2 (GPR43)	Acetate, propionate, butyrate	Gi/o, Gq11	Small intestinal epithelium, colonic, colonic LP cells, leukocytes in small intestinal LP, adipocytes, polymorphonuclear cells, skeltal muscle, spleen and heart etc.	Apetite control, anti-lipolysis, increased insulin sensitivity, preadipocyte differentiation, expansion and differentiation of Tregs, protection against IBD, apoptosis of human colon cancer cel line etc.
FFAR3—Free fatty acid receptor 3 (GPR41)	Acetate, propionate, butyrate	Gi/o	Small intestinal epithelium, colonic, colonic LP cells (mast cells), peripheral nervous system, peripheral mononuclear cells, bone marrow spleen, adipocytes, lymph nodes, etc.	Leptin expression, oxygen consumption rate, increased energy expenditure, decreased food intake, hematopoiesis of DCs from bone marrow, increased DC precursors alleviating asthma and Treg cells etc.
HCA1—Hydroxycarboxylic acid receptor 1 (GPR81)	lactate	(Gi)	Predominantly in adipose tissue, minor in kidney, skeletal muscle, liver, intestinal tissue, rat and human brain, mouse primary cortical neuronal cells, macrophages, etc.	Modulation of cortical neuron activity, and enterocyte turnover in response to starvation-refeeding, anti-lipolysis, anti-inflammatory on macrophages, reduced symptom of cancer and IBD in mouse models of hepatitis and pancreatitis, etc.
HCA2—Hydroxycarboxylic acid receptor 2 (GPR109A)	Niacin, ketone bodies, β-hydroxybutyric acids, butyrate	Gi/o, Gβγ	Apical membrane of colonic and small intestinal epithelium, monocytes, adipocytes, macrophages, DCs, neutrophils, retinal pigment epithelium, etc.	Improved epithelial barrier function, anti-lipolysis, decrease of triglyceride, protection against CRC and colitis, increase of Treg generation and IL-10 producing T cells, etc.
Olfr78 (murine) OR51E2 (human)	Acetate, propionate	NR	Neurons, epithelial enteroendocrine cells of colon, enteroendocrine cells, renal afferent arteriole, smooth muscle cells, etc.	Regulation of hormone secretion (GLP-1, PYY) and blood pressure, etc.
PPARγ (Peroxisome proliferator-activated receptor gamma)	Propionate, butyrate	NR	Large intestine adenocarcinoma cells, etc.	Regulation of lipid metabolism, a joining factor between the gut microflora composition and accumulation of the adipose tissue, etc.

Abbreviations: CRC—colorectal cancer; DC—dendritic cell; GLP-1, glucagon-like peptide; GPR—G-protein coupled receptor; IBD—inflammatory bowel disease; IL-10 (interleukin-10); LP—lamina propria; NR—not reported; Olfr—olfactory receptor; PYY—peptide YY; Treg—regulatory T cell.