Exploring Bilirubin's Therapeutic Effects on Consequences of Fetal Hyperglycemia Through PPARa Activation

Gestational Diabetes Mellitus (GDM) is one of the most common medical conditions in pregnancy and is defined by elevations in maternal glucose levels due to inadequate insulin signaling leading to hyperglycemia which can reach the fetus. The pathophysiology of fetal hyperglycemia (FH) is linked to the risk of developing obesity and type II diabetes later in life. In addition to hyperglycemia, other metabolites, hormones, growth factors and cytokines are also associated with GDM and may contribute to the net effects on the fetus. To solely focus on phenotypes due to FH, we have previously characterized a zebrafish model of FH in which direct glucose exposure to the developing embryos (72-96 hours post fertilization, hpf) results in elevated free glucose concentration, induced de novo lipogenesis, and development of insulin resistance which persist into adulthood. RNA seq of whole FH embryos reveal biliverdin reductase a ( blvra ) and b ( blvrb ) as two of the most up-regulated genes. These enzymes convert biliverdin to bilirubin, an antioxidant recently proposed to act as a metabolic hormone in modulating signaling pathways through protein phosphorylation and activating nuclear receptors such as peroxisome proliferator-activated receptor alpha (PPAR-α). We hypothesize that activation of the blvr pathway to produce bilirubin has a protective effect against the adverse phenotypes induced by FH. To test this hypothesis, we pharmacologically activated the blvr pathway by treating 72 hpf control and FH zebrafish embryos to either biliverdin (9μM) or bilirubin (1μM) for 24h. Both biliverdin and bilirubin treatment significantly increased circulating bilirubin levels which resulted in decreased de novo lipogenesis in FH embryos as seen by Oil Red O staining, qPCR, and TAG concentration (0.50 vs 1.45 mg/dL) at 96 hpf. Increased circulating bilirubin levels also rescued insulin signaling in FH embryos as seen by phosphorylated AKT/AKT Westerns (fold change of 1.38 vs 0.21). By blocking the PPAR-α signaling pathway (GW6471), we were able to prevent the bilirubin rescue of de novo lipogenesis. Additionally, RNA seq of FH embryos revealed 14 genes associated with PPAR signaling pathway downregulated while bilirubin treatment upregulated these same genes. RNA seq also detected 72 differentially expressed genes involved in insulin signaling in bilirubin treated embryos compared to FH embryos. Overall, these proof-of-concept data indicate bilirubin as a therapeutic potential for FH induced obesity and insulin resistance and confirm bilirubin’s function as a metabolic hormone through PPARα. Future studies will utilize our novel blvra and a blvrb loss of function zebrafish mutants and metabolomics to evaluate the association of metabolites with FH. By utilizing these mutants along with our FH model, we aim to further understand the protective role that the biliverdin reductase pathway plays in lipogenesis and insulin resistance induced by FH. This work was supported by the National Institutes of Health T32HL105324 (M.L). This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.