HCAR2 and lung carcinoma: Also, the expression of GPR109A is significantly higher in acute promyelocytic leukemia cells and lung cancer cells.136 Niacin and butyric acid as GPR109A agonists contribute to the ectopic expression of GPR109A in colon cancer cells, inducing cell apoptosis.112 Colonic epithelial cells use butyrate produced by gut microbiota to synthesize β-hydroxybutyrate.