Role of HCA2 in Regulating Intestinal Homeostasis and Suppressing Colon Carcinogenesis

Abstract

Hydroxycarboxylic acid receptor 2 (HCA₂) is vital for sensing intermediates of metabolism, including β-hydroxybutyrate and butyrate. It also regulates profound anti-inflammatory effects in various tissues, indicating that HCA₂ may serve as an essential therapeutic target for mediating inflammation-associated diseases. Butyrate and niacin, endogenous and exogenous ligands of HCA₂, have been reported to play an essential role in maintaining intestinal homeostasis. HCA₂, predominantly expressed in diverse immune cells, is also present in intestinal epithelial cells (IECs), where it regulates the intricate communication network between diet, microbiota, and immune cells. This review summarizes the physiological role of HCA₂ in intestinal homeostasis and its pathological role in intestinal inflammation and cancer.

Keywords: HCA2; anti-inflammatory; colon cancer; intestinal homeostasis; intestinal inflammation; microbiota; mucosal immunity.

Figure 1

HCA₂ triggers different downstream signaling pathway in different cell types. (A) In adipocytes, activation of HCA₂ triggers a Gαi-mediated inhibition of adenylate cyclase activity, which leads to lower intracellular cAMP levels, reduced protein kinase A (PKA) activity, and further reduces the activity of hormone sensitive lipase (HSL), an important lipolytic enzyme. This inhibition of lipolysis results in a decreased release of free fatty acids into the circulation. (B) In hepatocytes, activation of HCA₂ mediates the protein kinase C (PKC)-extracellular signal-regulated kinase (ERK) signaling pathway, leading to phosphorylation of AMP-activated protein kinase (AMPK) and inhibition of acetyl CoA carboxylase (ACC). This results in an inhibition of hepatic de novo lipogenesis and a remarkable decrease of lipid accumulation in liver. (C) In colonocytes, ligand binding to HCA₂ suppresses NF-κB activation and activates NLRP3 inflammasome, which recruits caspase-1 and promotes the maturation of IL-18 for secretion. (D) In retinal pigment epithelium, HCA₂ exerts either an anti-inflammatory response through the Gαi/cAMP/NF-κB pathway, or apoptosis through the Gβγ/Ca²⁺/NOX4/ROS/JNK pathway. (E) Within Langerhans cells, HCA₂-mediated Gα_i activation primarily results in the Gβγ-complex released from activated Gα_i, thereby increasing intracellular calcium concentration by mobilizing Ca²⁺ release from the endoplasmic reticulum and ultimately driving the formation of PGD₂ and PGE₂, which are released to the dermal layer and cause cutaneous flushing reaction. (F) In macrophages, activation of HCA₂ involves inhibition of NF-κB, thereby exerting an anti-inflammatory effec. HCA₂ activation in macrophages also represses F-actin and blocks Gβγ signaling to inhibit chemokine-induced migration of macrophages. HCA₂ also shows anti-inflammatory effects in Parkinson's disease models by inhibiting the phosphorylation of the NF-κB p65 signaling pathway in microglia. HCA₂ suppresses LPS-induced NF-κB activation in human monocytes, resulting in decreased transcription of IL-6, TNFα, and MCP-1. In DCs, HCA₂ activation decreases IL-6 level and increases IL-10 level, also upregulates expression of RALDH1, which synthesizes RA from retinol. RA and IL-10 promote Treg cell proliferation. In neutrophils, niacin-mediated HCA₂ activation inhibits PKA activity and subsequently increase BAD levels, which drives apoptosis of neutrophils.

Figure 2

HCA₂ regulates gut immune homeostasis. Butyrate produced by gut microbiota fermentation activates HCA₂ expressed on IECs, macrophages, and dendritic cells. In IECs, HCA₂ stimulation is associated with increased inflammasome activation, which processes pro-IL-18 into mature IL-18, which is critical in regulating mucosal immunity and epithelial integrity. IL-18 and ligand induced anti-inflammatory IL-10 (from intestinal macrophages and dendritic cells) promote naïve T cells differentiation and proliferation into immunomodulator Treg cells, which protect intestine from inflammation and colitis associated cancer. IL-18 and IL-10 also decrease the proinflammatory Th1 and Th17 cell number. In addition, HCA₂ decreases proinflammatory cytokine IL-6 and TNF-α expression in intestinal macrophages and inhibits dendritic cell-induced IL-23 production to suppress ILC3-associated colonic inflammation.

Figure 3

HCA₂ plays a critical role in the suppression of colonic cancer. (A) Expression of HCA₂ is silenced in colon cancer cells. This tumor-associated silencing of HCA₂ involves DNMT1-mediated DNA methylation. (B) IFN-γ reverses DNA methylation-mediated HCA₂ silencing without altering the methylation status of the HCA₂ promoter. STAT1 is rapidly activated by IFN-γ and binds to the p300 promoter to activate p300 transcription, resulting in a permissive chromatin conformation at the HCA₂ promoter to allow HCA₂ transcription in colon carcinoma cells. Reexpression of HCA₂ in cancer cell lines induces apoptosis by inhibiting of Bcl-2, Bcl-xL, cyclin D1, and NF-κB activity and upregulating the death receptor pathway in a ligand-dependent manner. (C) In mouse models of inflammation-associated colon cancer caused by AOM and DSS, the intestinal epithelium of HCA₂^−/− mice display upregulated expression of colon cancer-promoting genes and decreased genes that inhibit colitis and colon carcinogenesis. HCA₂^−/− mice also exhibit a severe impairment of IL-10 and IL-18 production, which leads to a decrease in Treg cell number.