RNA sequencing in human HepG2 hepatocytes reveals PPAR-alpha mediates transcriptome responsiveness of bilirubin


Bilirubin is a potent antioxidant that reduces inflammation and the accumulation of fat. There have been reports of gene responses to bilirubin, which was mostly attributed to its antioxidant function. Using RNA sequencing, we found that biliverdin, which is rapidly reduced to bilirubin, induced transcriptome responses in human HepG2 hepatocytes in a peroxisome proliferator-activated receptor (PPAR)-alpha-dependent fashion (398 genes with >2-fold change; false discovery rate P < 0.05). For comparison, a much narrower set of genes demonstrated differential expression when PPAR-alpha was suppressed via lentiviral shRNA knockdown (23 genes). Gene set enrichment analysis revealed the bilirubin-PPAR-alpha transcriptome mediates pathways for oxidation-reduction processes, mitochondrial function, response to nutrients, fatty acid oxidation, and lipid homeostasis. Together, these findings suggest that transcriptome responses from the generation of bilirubin are mostly PPAR-alpha dependent, and its antioxidant function regulates a smaller set of genes.