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 (ILICB) 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 Mrpl in the increased vulnerability of immature cells to UCB, we evaluated the mRNA and the protein levels of Mrpl throughout differentiation in primary cultures of rat neurons and astrocytes. Furthermore, in order to provide supportive evidence for the role of Mrpl in the protection of nerve cells from ILICB-induced effects, we evaluated cell susceptibility to UCB when Mrpl was inhibited with MK571 ((E)-3-[[[3-[2-(7-chloro-2-quinolinyl) ethe nyl] phenyl]-[[3-dimethylamino)-3-oxop ro pyllthiolmethyl]thio]-propanoic acid). The results are the first to demonstrate that Mrpl is expressed in neurons and that both mRNA and protein levels of Mrpl increase with cell differentiation. Additionally, inhibition of Mrpl was associated with an increase in UCB toxic effects, namely cell death, cell dysfunction, and secretion of interleukin (IL)-1 beta, tumor necrosis factor (TNF)-alpha, as well as of glutamate. These results point to a novel role of Mrpl in the susceptibility of premature babies to UCB encephalopathy, and provide a startup point for the development of a new therapeutic strategy. (c) 2006 IBRO. Published by Elsevier Ltd. All rights reserved.