Role of multidrug resistance-associated protein 1 expression in the in vitro susceptibility of rat nerve cell to unconjugated bilirubin.

Nerve cell injury by unconjugated bilirubin (UCB) has been implicated in brain damage during neonatal hyperbilirubinemia, particularly in the preterm newborn. Recently, it was shown that UCB is a substrate for the multidrug resistance-associated protein 1 (Mrp1), an ATP-dependent efflux pump, which may decrease UCB intracellular levels. To obtain a further insight into the role of Mrp1 in the increased vulnerability of immature cells to UCB, we evaluated the mRNA and the protein levels of Mrp1 throughout differentiation in primary cultures of rat neurons and astrocytes. Furthermore, in order to provide supportive evidence for the role of Mrp1 in the protection of nerve cells from UCB-induced effects, we evaluated cell susceptibility to UCB when Mrp1 was inhibited with MK571 ((E)-3-[[[3-[2-(7-chloro-2-quinolinyl) ethenyl]phenyl]-[[3-dimethylamino)-3-oxopropyl]thio]methyl]thio]-propanoic acid). The results are the first to demonstrate that Mrp1 is expressed in neurons and that both mRNA and protein levels of Mrp1 increase with cell differentiation. Additionally, inhibition of Mrp1 was associated with an increase in UCB toxic effects, namely cell death, cell dysfunction, and secretion of interleukin (IL)-1beta, tumor necrosis factor (TNF)-alpha, as well as of glutamate. These results point to a novel role of Mrp1 in the susceptibility of premature babies to UCB encephalopathy, and provide a startup point for the development of a new therapeutic strategy.