We thank Dr. Ostrow and Dr. Tiribelli for their comments and we appreciate the opportunity to answer some aspects of the discussion on the regulation of transport proteins in diseased human livers.
Ostrow and Tiribelli may be correct that variation in age may affect the expression of transporters in liver, however, this is a hypothetical comment which cannot be tested because of ethical reasons which do not allow to perform liver biopsies in healthy children.
As already pointed out in our paper1 administration of ursodeoxycholic acid (UDCA) might contribute to changes in transporter expression (abstracts by Marschall et al.2 and by Trauner et al.3).In the paper by Marschall et al.4, which appeared after publication of our manuscript, protein levels of transporters were determined before and after UDCA treatment of apparently normal adults. An increase of MRP4 protein of only 1.6-fold was reported. This increase in response to UDCA is too low to explain the 5- to 9-fold increase of MRP4 in UDCA-treated PFIC children.1 Therefore, factors other than UDCA are likely to cause the upregulation of MRP4 in PFIC.
On the basis of some of our data, Ostrow and Tiribelli speculate on potential mechanisms of jaundice in PFIC. However, we did not observe any significant correlation between bilirubin levels and protein expression of single transporters (R2 values were below 0.10 for MRP2, OATP1B1 and OATP1B3 and high bilirubin levels were associated with low expression of MRP4). Besides the concerted regulation of multiple transporters, other mechanisms, such as substrate competition, should be considered as potential causes of jaundice in PFIC.
Ostrow and Tiribelli refer to the small increase in MRP1 mRNA expression in our study and mention that MRP1 exports unconjugated bilirubin from cells. We would like to stress that mRNA levels in liver tissue do not necessarily indicate the presence of MRP1 protein in hepatocytes. Indeed, we have not been able to detect MRP1 protein in human hepatocytes.5
Ostrow and Tiribelli point out that Mrp3/MRP3 upregulation occurs in Gunn rats and in patients with the Dubin–Johnson Syndrome (DJ), with PBC or with obstructive cholestasis. It is not surprising, that MRP3 was not elevated in PFIC because of the completely different genetic and pathophysiological background. The early event in PFIC is a defect in bile salt secretion while in Gunn rats, DJ, PBC or obstructive cholestasis an increase in unconjugated or conjugated bilirubin might represent the major effector of transporter regulation.