Hepatic uptake of bilirubin: the effect of cyanidin 3-glucoside and pravastatin
The initial tests were performed with two different inhibitors (molecular targets and kinetics in Suppl. Table 1). One was C3G (chemical structure in Suppl. Fig. 3), which inhibits the transport activity of bilitranslocase (Ki = 5.8 µM). The other was pravastatin, a specific substrate of human OATP1B1, Oatp2 (Slco1a4) and Oat (Slc22a7) from rat liver, but not of bilitranslocase.
Fig. 1a shows that when BR was injected alone or with pravastatin, some BR (10.9 pmol; 0.11%) was recovered in the effluent, whereas when it was co-injected with C3G, a sharp and high BR peak occurred, indicating inhibition of the uptake of BR. This effect was transient, as the next BR bolus was again almost completely absorbed.
In this study, inhibition of uptake of BR by C3G was performed at least once in all experiments testing pravastatin and other inhibitors as a functional control of liver preparations. No differences were observed between the BR peaks elicited by C3G that might depend on the order of administration (Suppl. Fig. 3). Therefore, we performed a comprehensive analysis of all C3G-induced peaks compared to all controls. Figure 1b shows peak analysis of BR boli alone or with either C3G or pravastatin. In contrast to pravastatin, which was inactive, C3G caused a 3-fold increase in AUC. The effect of C3G on Cmax was even more pronounced, increasing 6-fold. We found that the tmax of the BR peak was decreased by C3G and increased by pravastatin (peak parameters in Suppl. Fig. 4).
Fig. 1C shows the frequency of C3G effect size from case to case. The data show that the most common observation (21%) was a 2-fold increase in AUC, although 61% of observations were above this value and notable effects (> 5-fold increase) were obtained in 16% of cases. Negative effects (AUC ratio = 1) were in the minority (7.5 %) and occurred randomly (and never serially) with some preparations.
The dose-response relationship was analysed at either constant (Suppl. Fig. 5A) or variable BR (Suppl. Fig. 5B) dose. It was found that small changes in bolus concentrations around the value of 0.05 mM resulted in large changes in the peak areas of BR, which might explain the variable effect size of C3G, shown above in Fig. 1C.
Another anthocyanin, such as malvidin 3-glucoside but not peonidin 3-glucoside (chemical structures in Suppl. Fig. 3), inhibited the uptake of BR similarly to C3G (Suppl. Fig. 6).
Hepatic uptake of cyanidin 3-glucoside: the effect of bilirubin
If C3G inhibits a BR transporter in the liver, one might think that the opposite should be true. Therefore, we repeatedly injected C3G in increasing concentrations, alone or with 0.05 mM BR (Fig. 2). C3G in the effluent peaked with Cmax in the µM range. AUC analysis showed no net uptake of C3G into the liver. However, P3G in effluent increased linearly and was not affected by bolus administration. The recovery of P3G in the 200 fractions analyzed (10 boli) was 31.12 nmol, or approximately 9% of the total C3G injected (344 nmol). If approximately 1% of each C3G bolus is retained during its passage through the sinusoids, this tiny fraction cannot be estimated by the AUC calculation. Accordingly, inhibition of uptake by BR would not be detectable because an overwhelming proportion of C3G (approximately 99%) passed through the sinusoids. These data are consistent with the results of in vivo distribution, where only 0.64 % of the injected dose was recovered in the liver [20].
Hepatic uptake of bilirubin and bilirubin glucuronide: the effects of cyanidin 3-glucoside, pravastatin and estradiol-17 beta-glucuronide
To investigate the reason for the ineffectiveness of pravastatin in relation to the uptake of BR, we improved the analysis of liver effluent by measuring both BR and bilirubin glucuronide (BRG), the presence of which in the effluent of perfused rat liver was reported long ago [21]. The efflux of BRG through sinusoidal Mrp3 and its downstream reuptake by sinusoidal Oatp is referred to as BRG hopping [22]. Assuming that hopping should also be observed in the intact and viable isolated perfused liver, we analysed BRG in perfusion effluent and tested whether its reuptake could be inhibited by Oatp-specific substrates (Suppl. Table 1).
In this series of experiments, we first tested the effect of the high-affinity Oatp substrate ß-estradiol-17 beta-glucuronide (E17G). Fig. 3A shows one of these experiments, which began with two consecutive BR boli to induce intrahepatic BRG synthesis. The boli sequence continued with pairs of boli containing BR and inhibitors (C3G or E17G), interspersed with pairs of BR-only boli. Both BR and BRG could already be detected in the first fractions, suggesting constitutive BRG efflux from the intracellular stores. Intra-portal injection of BR did not alter the apparent steady-state level of sinusoidal BRG. In contrast, when BR was co-injected with C3G or E17G, sharp spikes of BRG occurred in the effluent. However, there was a difference between C3G and E17G, as only C3G elicited simultaneous spikes of BR and BRG (boli 3-4), whereas E17G elicited only BRG spikes (boli 7-8), even when injected without BR (boli 11-12). The different pattern of C3G and E17G on sinusoidal uptake of BR and BRG was observed several times (Fig. 3B-C-D; Suppl. Fig. 7A). Pravastatin acted like E17G in that it inhibited only the uptake of BRG (Suppl. Fig. 7B).
From these data, we conclude that BR was transported to the liver (99.9% of the cumulative dose, in this experiment), conjugated by UDP-glucuronosyltransferase (UGT1A1) (to an undetermined extent), and a minute amount of BRG (0.13% of the estimated absorbed BR fraction, in this experiment) was transported back to the sinusoid. It should be noted that under all circumstances, i.e. both before and after multiple administrations of BR, either alone or with inhibitors, the baseline BRG was constant.
Understanding the inhibition of bilirubin glucuronide uptake by cyanidin 3-glucoside
The dual action of C3G required further investigation to evaluate its ability to inhibit sinusoidal uptake of either BR or BRG. If C3G is an inhibitor of Oatp, it might be expected to inhibit reuptake of BRG even when injected in the absence of BR, similar to what has been observed with E17G and pravastatin.
In the experiment shown in Fig. 4A, pairs of BR boli were administered to induce intrahepatic BRG synthesis. When BR was co-injected with C3G, two overlapping peaks of BR and BRG (boli 3-4) appeared in the effluent, but when it was administered without BR (boli 7-8), no peak of BR or BRG appeared. Further boli of E17G, with or without BR, elicited only large peaks of BRG ( boli 11-12-13). Thus, while E17G acted independently of BR on a BRG transporter (presumably Oatp), C3G acted only in its presence, suggesting that BR is the actual and necessary inhibitor of another sinusoidal BRG transporter. However, we note that C3G elicited spikes of BR only when it was co-injected with BR, whereas it had no effect on basal levels of BR. This is generally the case when a transporter has allosteric properties, such as bilitranslocase [23]. In these cases, the inhibitors are inactive at very low substrate concentrations, at the base of the sigmoid curve.
The mutual independence of these transport pathways was further challenged in an experiment in which we tested the simultaneous injection of C3G and E17G instead of C3G with BR on the sinusoidal reuptake of BRG. We found that the resulting BRG peak was never higher than that elicited by E17G alone (Fig. 4B, boli 6 vs 7 and 11 vs 12).
To complete the characterization of the differential effects of C3G and E17G, we tested their effects on basal levels of BR and BRG detectable in perfusion effluent from livers never exposed to BR boli. As shown in Fig. 4C, basal levels of BR or BRG were not affected by C3G. Only E17G triggered spikes of BRG. Simultaneous administration of C3G and E17G elicited peak values that were not different from those elicited by E17G alone. The same result was obtained with pravastatin (Suppl. Fig. 8).
Overall, these data indicate that C3G does not directly inhibit BRG uptake. In contrast, the C3G-induced BR concentration peak was the agent that inhibited an unknown BRG transporter that was not directly targeted by either C3G or E17G.
Hepatic uptake of bilirubin and bilirubin glucuronide: the effect of non-steroidal anti-inflammatory drugs
Early functional studies had demonstrated the existence of a sinusoidal BRG membrane transporter that was competitively inhibited by BR and indocyanine green (ICG) but not by glycocholate [18][24]. We speculated that a possible candidate for this function might be rOat2 (Slc22a7), since ICG is a good inhibitor of its transport activity, unlike taurocholate and pravastatin [25]. We found that the Oat2 substrates ketoprofen or indomethacin (Suppl. Table 1B) did not affect the uptake of BR and BRG (Suppl. Fig. 9).
Hepatic uptake of bilirubin and bilirubin glucuronide: the effect of taurocholate
Finally, we studied the effect of taurocholate (TC) on the hepatic uptake of BR and BRG.
Taurocholate is transported in the liver by at least three different sinusoidal transporters (Suppl. Table 1). By using taurocholate, we aimed primarily to inhibit Ntcp, which is not targeted by E17G or pravastatin (Suppl. Table 1) but transports indocyanine green [26]. In human subjects carrying mutant forms of Ntcp [27], hypercholanemia may be associated with elevated blood levels of indirect bilirubin (BR) [28] and direct bilirubin (BRG) [29][30].
We observed that serial boli of increasing concentrations of TC triggered significant BRG spikes only when the concentration of TC was 2 mM. Remarkably, they had no effect on the uptake of BR (Fig. 5). We observed a concentration-dependent inhibition of BRG uptake by TC (Suppl Fig. 10). Overall, these data are consistent with previous reports that TC does not inhibit hepatic uptake of BR in isolated perfused rat liver [31].
The effect of cyanidin 3-glucoside and taurocholate on hepatic uptake of bilirubin and bilirubin glucuronide at physiological concentrations
To assess the effect of C3G and TC on the uptake of BR and BRG at physiological portal concentrations, we injected boli of 0.01 mM C3G (2 nmol) alone or with either 0.05 mM BR (10 nmol) or 2 mM TC (400 nmol). The effluent was analyzed for all relevant compounds, i.e. BR and BRG, C3G and P3G, and TC, to evaluate their concentrations in the effluent (data and statistics of basal concentrations and peak parameters in Suppl. Fig. 11).
Under these conditions, C3G reached 0.44 µM, which corresponds to a physiological value in rat portal blood after intestinal absorption [32] and in human systemic circulation [33]. It elicited peak levels up to 10 nM BR, which were higher than the basal levels BR of 0.9 nM (10-fold increase). TC , which was injected together with C3G, reached 24 µM, which is within the physiological range observed in fasting humans [34]. It triggered BRG peaks that reached 8.1 nM, which was above the basal value of 1.1 nM (7-fold increase), with no effect on C3G peaks.