Member, Cancer Immunology, Inflammation and Tolerance Program
Assistant Professor Biochemistry and Molecular Biology
Assistant Professor of Graduate Studies
Georgia Cancer Center
1410 Laney Walker Blvd., CN-1162
Research projects in Singh Laboratory are aimed at studying how interactions between diet, gut microbiota, genes, and immune cells regulate intestinal inflammation, carcinogenesis, immunity, and tolerance in the gastrointestinal tract. Current specific areas are outlined below.
Gpr43 and Gpr109a as critical link between dietary fiber and intestinal health
The human gut is colonized with trillions of bacteria, collectively termed as “gut microbiota,” which is comprised of ~1000 different species with ~3 million genes collectively. In the colon, dietary fiber is fermented by gut commensal bacteria into short chain fatty acids (SCFAs), butyrate, acetate, and propionate. SCFAs interact with G-protein coupled receptors Gpr41, Gpr43, and Gpr109a. Among SCFAs, butyrate has received the most attention for its anti-inflammatory effects in colon. Butyrate enemas are known to decrease colonic inflammation during active colitis. Gpr109a is a cell surface receptor for butyrate and niacin (vitamin B3). Niacin is also produced by gut microbiota. Suboptimal consumption of dietary fiber is associated with enhanced risk of colonic inflammation. Niacin deficiency leads to spontaneous development of inflammation in the intestine. The Singh laboratory has shown that butyrate and niacin regulate induction of colonic Treg cells, and Gpr109a plays an important role in this process. Acting via Gpr109a, niacin suppresses colonic inflammation and carcinogenesis in animals fed diets deficient in dietary fiber or in animals depleted of gut-microbiota. These findings demonstrate that Gpr109a acts as a critical link that mediates anti-inflammatory and anti-carcinogenic effects of dietary fiber/gut microbiota in the intestine. Gpr109a is highly expressed by innate immune cells; macrophage, dendritic cells and neutrophils, whereas its expression on T and B cells is undetectable. Gpr109a signaling induces expression of anti-inflammatory molecules aldehyde dehydrogenase 1a (Aldh1a) and IL-10 and suppression of pro-inflammatory cytokine such as IL-6.
The Singh laboratory is also evaluating the role of Gpr43 in intestinal inflammation and carcinogenesis. Their data shows that Gpr43 regulate the composition of gut microbiota and suppresses inflammation and carcinogenesis in the intestine. Preliminary studies indicate that Gpr109a and Gpr43 suppress intestinal inflammation and carcinogenesis by distinct and non-redundant mechanisms. They are using various immunological, cellular, molecular and genomics approaches for our studies. Some of the important active projects are:
1) How dietary fiber and SCFA receptors, Gpr109a, Gpr43, Gpr41 affect the composition of gut microbiota, as well as cellular, molecular and secreted byproducts (metabolites) that affect intestinal inflammation and carcinogenesis.
2) Roles of dietary fiber, Gpr109a and Gpr43 in immunity and tolerance to normal gut microbiota and intestinal pathogens.
3) Signaling events downstream of Gpr109a relevant to induction of Aldh1a and IL-10 and other anti-inflammatory molecules in macrophages, dendritic cells (DCs) leading to induction of Treg cells in colon and suppression of inflammation and carcinogenesis.
4) Gpr43-induced signaling in innate immune cells and gut epithelium leading to suppression of intestinal inflammation and carcinogenesis
5) Relative contributions of different mechanisms induced by SCFA and dietary fiber in promotion of intestinal health under steady state and conditions of infection.
Gurav A, Sivaprakasam S, Bhutia YD, Singh N, and Ganapathy V. (2015) Slc5a8, a Na+-coupled high-affinity transporter for short-chain fatty acids, is a conditional tumor suppressor in colon that protects against colitis and colon cancer under low-fiber dietary conditions (Under Revision in Biochem Journal).
Singh N, Gurav A, Sivaprakasam S, Brady E, Padia R, Shi H, Thangaraju M, Prasad PD, Manicassamy S, Munn DH, Lee JR, Offermanns S, Ganapathy V*. (2014) Activation of Gpr109a, Receptor for Niacin and the Commensal Metabolite Butyrate, Suppresses Colonic Inflammation and Carcinogenesis Immunity 40:128-39.
Ganapathy V, Thangaraju M, Prasad PD, Martin PM, Singh N. (2013) Transporters and receptors for short-chain fatty acids as the molecular link between colonic bacteria and the host. Curr Opin Pharmacol. 13:869-74.
Singh N, Thangaraju M, Prasad PD, Martin PM, Lambert NA, Boettger T, Offermanns S, Ganapathy V.(2010)Blockade of dendritic cell development by bacterial fermentation products butyrate and propionate through a transporter (Slc5a8)-dependent inhibition of histone deacetylases.J Biol Chem. 285:27601-8.
Singh N, Yamamoto M, Takami M, Seki Y, Takezaki M, Mellor AL, Iwashima M. (2010) CD4+CD25+ regulatory T cells resist a novel form of CD28- and Fas-dependent p53-induced T cell apoptosis. J Immunol. 184:94-104.
Singh N*, Huang L, Qin H. (2010) Defective TCR induced apoptosis of T cells and rejection of transplanted immunogenic tumors in P53-/- mice. Eur J Immunol. 40:559-68. (* corresponding author)
Pacholczyk R, Kern J, Singh N, Iwashima M, Kraj P, Ignatowicz L. (2007) Nonself-Antigens Are the Cognate Specificities of Foxp3(+) Regulatory T Cells. Immunity27:493-504.
Singh N, Chandler PR, Seki Y, Baban B, Takezaki M, Kahler DJ, Munn DH, Larsen CP, Mellor AL, Iwashima M. (2007) Role of CD28 in fatal autoimmune disorder in scurfy mice. Blood 110:1199-206.
Sharma MD, Baban B, Chandler P, Hou DY, Singh N, Yagita H, Azuma M, Blazar BR, Mellor AL, Munn DH. (2007) Plasmacytoid dendritic cells from mouse tumor-draining lymph nodes directly activate mature Tregs via indoleamine 2,3-dioxygenase. J Clin Invest. 117:2570-82.
Singh N, Seki Y, Takami M, Seki Y, Baban B, Chandler P, Khosravi D, Zheng X, Takezaki M, Lee JR, Mellor AL, Bollag W, Iwashima M. (2006) Enrichment of regulatory CD4+CD25+ T cells by inhibition of phospholipase D signaling. Nature Methods. 3:629-36.