The Effects of Bilirubin and Lumirubin on Metabolic and Oxidative Stress Markers

Aleš Dvořák,Nikola Vrzáčková,Kateřina Žížalová,Lucie Muchová,Kateřina Pospíšilová,Libor Vítek,Marek Vecka,Jana Křížová,Jaroslav Zelenka,Nikola Capková

doi:10.3389/fphar.2021.567001

Aleš Dvořák, Nikola Vrzáčková + Show 8 more

Open Access

https://doi.org/10.3389/fphar.2021.567001

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Abstract

For severe unconjugated hyperbilirubinemia the gold standard treatment is phototherapy with blue-green light, producing more polar photo-oxidation products, believed to be non-toxic. The aim of the present study was to compare the effects of bilirubin (BR) and lumirubin (LR), the major BR photo-oxidation product, on metabolic and oxidative stress markers. The biological activities of these pigments were investigated on several human and murine cell lines, with the focus on mitochondrial respiration, substrate metabolism, reactive oxygen species production, and the overall effects on cell viability. Compared to BR, LR was found to be much less toxic, while still maintaining a similar antioxidant capacity in the serum as well as suppressing activity leading to mitochondrial superoxide production. Nevertheless, due to its lower lipophilicity, LR was less efficient in preventing lipoperoxidation. The cytotoxicity of BR was affected by the cellular glycolytic reserve, most compromised in human hepatoblastoma HepG2 cells. The observed effects were correlated with changes in the production of tricarboxylic acid cycle metabolites. Both BR and LR modulated expression of PPARα downstream effectors involved in lipid and glucose metabolism. Proinflammatory effects of BR, evidenced by increased expression of TNFα upon exposure to bacterial lipopolysaccharide, were observed in murine macrophage-like RAW 264.7 cells. Collectively, these data point to the biological effects of BR and its photo-oxidation products, which might have clinical relevance in phototherapy-treated hyperbilirubinemic neonates and adult patients.

Highlights

Bilirubin (BR) (Figure 1A), the major product of the heme catabolic pathway in the intravascular compartment, has been identified as a molecule of unique biological significance
Due to the impact of BR on mitochondrial metabolism, we investigated the possible effects of LR and BR on the production of intracellular metabolites of the TCA cycle, known to affect energy balance and to modulate multiple cellular functions (Martinez-Reyes and Chandel, 2020)
The still scarce data obtained until now suggests some biological activity of these products (Jasprová et al, 2018), which may account for the reported clinical observations (Raghavan et al, 2005; Xiong et al, 2011; Mcnamee et al, 2012; Arnold et al, 2014; Khan et al, 2016; Auger et al, 2019; Safar and Elsary, 2020)

Summary

Introduction

Bilirubin (BR) (Figure 1A), the major product of the heme catabolic pathway in the intravascular compartment, has been identified as a molecule of unique biological significance. Long-term, mildly elevated BR concentrations protect mitochondria and the respiratory chain, with a concomitant decrease of reactive oxygen species (ROS) and pro-inflammatory cytokine production (Zelenka et al, 2016); these observations are consistent with our previous in vitro and in vivo data, further demonstrating the anti-inflammatory effects of BR (Valaskova et al, 2019) This data are in line with recent observations on beneficial effects of BR on metabolic pathways implicated in pathogenesis of diabetes, metabolic syndrome and obesity (Stec et al, 2016; Hinds and Stec, 2019) proposing BR as a signaling molecule with “real” endocrine activities (Hinds and Stec, 2018; Vitek, 2020). These pathways include those activated by PPARα (Stec et al, 2016; Hinds and Stec, 2018; Hinds and Stec, 2019), signaling pathways activated by other nuclear as well as cytoplasmic receptors are likely to contribute as well (Vitek, 2020)

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