Regulation and functional significance of the ABC transporter P-glycoprotein (ABCB1) in fish

生物体调节内源和外来化学物质，以减少它们的积累和潜在的毒性影响。化学动态平衡的机制包括：1)代谢产物的生物转化，2)代谢产物的膜外排运输，3)未代谢母体化合物的膜外排运输(如第三相转运：P-糖蛋白[P-gp，ABCB家族])。**跨膜蛋白P-gp，是三磷酸腺苷结合盒(ABC)运输蛋白超家族(ABCB1/MDR1)中的一种化学外排转运体。P-gp提供了一种对鱼类中污染物的多异种生物耐药机制(MXR)，类似于哺乳动物的多药耐药机制(MDR)。MXR是一种对生活在污染环境中的鱼类具有巨大适应价值的现象，它通过增强P-gp的表达来限制异物摄取。在鱼类中，P-gp主要在具有排泄(例如肝脏)、吸收(例如肠道)或屏障(例如血脑屏障[BBB])功能的器官中表达。**尽管近年来在哺乳动物中产生了大量关于P-gp的信息，但我们仍然不知道P-gp是如何作为外排泵工作的，也不知道它在生理上的作用是什么，尽管它被认为是唯一的ABC转运体，其唯一功能是保护细胞免受广泛化学物质的伤害。在哺乳动物中，P-gp运输多种化合物，包括内源性分子、药物和环境化学物质。在鱼类中，P-gp作用的已知底物的范围是有限的，似乎与哺乳动物不同。为了开始了解P-gp在化学毒代动力学中的作用，有必要为这两种底物定义一个共同的药效库。当鱼同时接触P-gp底物和化学混合物中的抑制剂时，这一点尤其重要；毒物动力学改变可能导致化学品积累增加和对毒性影响的敏感性。**关于鱼类暴露于异种生物后P-gp的构成或诱导水平的信息很少。同样有限的是关于P-gp调控或参与其诱导的信号通路的信息。有限的证据表明，对P-gp上调的控制可以发生在许多信号的反应中，包括特定的外源物质、内源性分子和应激。一个新兴的研究领域涉及核因子(如孕烷X受体[PXR])在P-gp表达中的作用。外源代谢酶可能通过PXR等途径存在共同和协调的信号机制。**建立良好的体外鱼类细胞模型系统和整个动物将被用来研究上述建议/描述的底物基础、P-gp水平、诱导、调节和信号通路。以血脑屏障(BBB)为模型组织，研究P-gp功能在整个生物体水平的生态环境意义。血脑屏障是脊椎动物外周循环和中枢神经系统之间的主要界面。BBB选择性的分子基础包括P-gp，它阻止一些外源物质进入中枢神经系统。关于BBB在鱼类中的重要性的信息很少。P-gp在其功能中的作用是一个正在研究的硬骨生理学领域。**本研究有几个目的：1)评估P-gp在硬骨鱼中的生理和毒理学作用，2)阐明P-gp表达的调控机制，3)增加对硬骨P-gp底物碱基的了解，4)确定P-gp与其他化学防御系统之间是否存在关系，以及5)确定鱼中血脑屏障的功能意义以及P-gp在其中的作用。

Organisms regulate endogenous and xenobiotic chemicals to reduce their accumulation and the potential for toxic effects. The mechanisms for chemical homeostasis include: 1) biotransformation to metabolites , 2) membrane efflux transport of metabolites , and 3) membrane efflux transport of unmetabolized parent compound (e.g. Phase III transport: P-glycoprotein [P-gp, ABCB family]).**The transmembrane protein P-gp, is a chemical efflux transporter in the ATP-binding cassette (ABC) transport protein superfamily (ABCB1/MDR1). P-gp provides a multixenobiotic resistance mechanism (MXR) to contaminants in fish that is analogous to the multidrug resistance (MDR) mechanism found in mammals . MXR is a phenomenon of tremendous adaptational value for fish living in polluted environments that acts through an enhanced expression of P-gp which restricts xenobiotic uptake. In fish, P-gp is predominantly expressed in organs that have an excretion (e.g. liver), absorption (e.g. intestine), or barrier (e.g. blood brain barrier [BBB]) function.**Although considerable information on P-gp in mammals has been generated in recent years, we still do not know how P-gp works as an efflux pump or what its roles are in physiology, although it is believed to be the only ABC transporter whose sole function is to protect cells against a wide range of chemicals. In mammals, P-gp transports a broad variety of compounds that include endogenous molecules, drugs, and environmental chemicals. In fish, the range of known substrates on which P-gp act is limited and appears to be different from mammals. Defining a common pharmacore for both substrates is necessary to begin to understand P-gp's role in chemical toxicokinetics. This is particularly important when fish are exposed to both P-gp substrates and inhibitors in chemical mixtures; altered toxicokinetics may lead to increased accumulation of chemicals and the susceptibility to toxic effects.**Very little information exists on the constitutive or inducible levels of P-gp following xenobiotic exposure in fish. Equally limited, is information regarding P-gp regulation or the signaling pathways involved in its induction. Limited evidence suggests that controls for the up-regulation of P-gp can occur in response to a number of signals, including specific xenobiotics, endogenous molecules, and stress. A newly emerging field of study concerns the roles of nuclear factors (e.g. Pregnane X receptor [PXR]) on P-gp expression. Common and coordinated signaling mechanisms may exist for xenobiotic metabolizing enzymes through pathways such as PXR. **Well-established in vitro fish cell model systems and whole animals will be used to investigate the substrate base, P-gp levels, induction, regulation, and signaling pathways proposed/described above. The ecological and environmental significance of P-gp function at the whole organism level will investigated using the blood brain barrier (BBB) as a model tissue. The BBB is the primary interface between the peripheral circulation and the CNS of vertebrates. The molecular basis of the selective nature of the BBB includes P-gp that prevents some xenobiotics from entering the CNS. Very little information exists regarding the significance of the BBB in fish. The role P-gp plays in its function is an area of teleost physiology that is under researched.**This research has several aims: 1) to assess the physiological and toxicological roles of P-gp in teleosts, 2) to elucidate the regulatory mechanisms underlying P-gp expression, 3) to increase knowledge regarding the teleost P-gp substrate base, 4) to determine if a relationship exists between P-gp and other chemical defense systems, and 5) to determine the functional significance of the BBB in fish and P-gp's role in it.