| The Multidrug resistance-associated proteins (Mrps) are ATP-dependent transporters involved in efflux of numerous endogenous and exogenous chemicals in a variety of tissues. Furthermore, Mrp expression is an important component in the disposition of chemicals in both naive and pathological conditions. Thus the purpose of this dissertation was to explore transcriptional mechanisms that can lead to alterations in Mrp expression. Study 1 examined basal and gender-specific expression in various tissues. Highest expression of the Mrp family members is as follows: Mrp1, 7, and 9 in testes; Mrp2 and 3 in intestine; Mrp4 in kidney; Mrp5 in brain; and Mrp6 in liver. Gender differences in Mrp expression were observed in kidney for Mrp3 and 4. Study 2 demonstrated that Mrp3 expression is female-predominant due to upregulation by estrogens, whereas Mrp4 is female-predominant due to repression by both androgen and male-pattern growth hormone secretion. In study 3, Mrp expression was examined after exposure to chemicals that specifically activate the aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), pregnane-X-receptor (PXR), peroxisome proliferator-activated receptor alpha (PPARalpha), and nuclear factor-E2-related factor 2 (Nrf2). The induction of hepatic Mrps by classical activators of AhR, CAR, and Nrf2 was observed, implicating these receptors in Mrp regulation. Utilizing Nrf2-null mice, Study 4 demonstrated that Nrf2 was necessary for increased Mrp3 and 4 expression after administration of butylated hydroxyanisole or oltipraz. Two putative Nrf2 response elements (AREs/EpREs) for Mrp3, and six ARE/EpREs for Mrp4 exist in the 5' flanking regions of these genes. Utilizing gel-shift assays, putative elements located at 9.9 and 10.6 kilobases (kb) upstream of the Mrp3 translational start site bound the Nrf2/MafK heterodimer, whereas for Mrp4, strong binding was observed with elements 3.8 and 30.8 kb upstream. Furthermore, Nrf2 translocation corresponded to Mrp1-4 up-regulation after glutathione depletion in gamma-glutamylcysteine-ligase-null mice. Study 5 demonstrated that chemical induction with perfluorodecanoic acid, an industrial surfactant, markedly increases Mrp2, 3, and 4 expression. Maximal induction of Mrp3 and 4 occurs via PPARalpha and Nrf2 activation, whereas Mrp2 induction is independent of these factors. In conclusion, these findings demonstrate that Mrp expression can be altered after hormonal and chemical administration. Furthermore, Nrf2 activation can cause increased Mrp expression, and stimulation of this pathway may explain the coordinate induction of Mrp efflux transporters and phase-I/II metabolism that is observed after chemical exposure, or in certain pathological conditions. Thus, dis-regulation of this normally beneficial pathway may be an important component of multidrug resistance. |
