A new, bifunctional recombinant protein was expressed as the fusion product of human elastin‐like polypeptide (HELP) and the bilirubin‐binding protein UnaG. The engineered product displays both the HELP‐specific property of forming a functional hydrogel matrix and the UnaG‐specific capacity of emitting green fluorescence upon ligand binding. The new fusion protein has been proven to be effective at detecting bilirubin in complex environments with high background noise. A cell culture model of the stress response, consisting of bilirubin released in the cell culture medium, was set up to assess the bilirubin‐sensing properties of the functional matrix obtained by cross‐linking the HELP moiety. Our engineered protein allowed us to monitor cell induction by the release of bilirubin in the culture medium on a nanomolar scale. This study shows that elastin‐like protein fusion represents a versatile platform for the development of novel and commercially viable analytical and biosensing devices.

We present a protocol for the study of bilirubin uptake in the isolated and perfused rat liver. The liver is perfused with an albumin-free saline buffered solution supplemented with glucose, pyruvate and lactate, in the absence of oxygen, at a physiologically low flow rate. Fractions of the venous effluent are collected and analyzed for bilirubin, bilirubin glucuronide and biomarkers of liver integrity. The liver preparation is viable and intact for 1 h after isolation from the general circulation, with constant levels of both bilirubin and bilirubin glucuronide (< 2 nM) in the effluent. Up to 12 boli of 10 nmol bilirubin can be sequentially injected into the portal vein without and with molecules that target sinusoidal membrane transporters of organic anions. Selective inhibition of bilirubin or bilirubin glucuronide uptake is detected as transient peaks in the effluent (Cmax up 6 to 60 nM). This protocol allows collecting repeated observations in the same liver, thus reducing the animal number by a factor of 10.

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Background & Aims: One of the organ-specific functions of the liver is the excretion of bilirubin into the bile. Membrane transport of bilirubin from the blood to the liver is not only an orphan function, as there is no link to the protein/gene entities that carry it out, but also a poorly characterised function. The aim of this study was to investigate the pharmacology of bilirubin uptake in the liver of the female Wistar rat to improve basic knowledge in this neglected area of liver physiology.Methods: We treated isolated, perfused rat livers with repeated single-pass, albumin-free bilirubin boli. We monitored both bilirubin and bilirubin glucuronide in perfusion effluent with a biofluorometric assay. We tested the ability of nine molecules known to be substrates or inhibitors of sinusoidal membrane transporters to inhibit the hepatic uptake of bilirubin.Results: We found that cyanidin 3-glucoside and malvidin 3-glucoside are the only molecules that inhibit bilirubin uptake. These dietary anthocyanins resemble bromosulfophthalein (BSP), a substrate of several sinusoidal membrane transporters. The SLCO-specific substrates estradiol-17 beta-glucuronide, pravastatin, and taurocholate inhibited only bilirubin glucuronide uptake. Cyanidin 3-glucoside and taurocholate acted at physiological concentrations. The SLC22-specific substrates indomethacin and ketoprofen were inactive. We demonstrated the existence of a bilirubin glucuronide transporter that is inhibited by bilirubin, a fact reported only once in the literature.Conclusions: Data indicate that bilirubin and bilirubin glucuronide are transported into the liver via pharmacologically distinct membrane transport pathways. Some dietary anthocyanins may physiologically modulate the uptake of bilirubin into the liver.