Fish may be exposed to an array of pharmaceuticals that are discharged into the aquatic environment, paralleling advances in medical knowledge, research and technology. Pharmaceuticals by their nature are designed to target specific receptors, transporters, or enzymes. Nuclear receptors (NRs) are often a key component of the therapeutic mechanism at play, and many of these are conserved among vertebrates. Consequently, fish may be affected by environmental pharmaceutical exposure, however there has been relatively little characterisation of NRs in fish compared with in mammals. In this thesis common carp (C. carpio) were exposed to selected pharmaceuticals in vitro and in vivo to investigate effects centred on the pregnane X receptor (PXR) and peroxisome proliferator-activated receptor alpha (PPARα), two key NRs involved in organism responses to pharmaceutical exposure. The PXR acts as a xenosensor, modulating expression of a number of xenobiotic metabolising enzymes (XMEs) in mammals. In a primary carp hepatocyte model it was shown that expression of a number of XMEs was altered on exposure to rifampicin (RIF), as occurs in mammals. This response was repressed by addition of ketoconaozle (KET; PXR-antagonist), indicating possible PXR involvement. The genes analysed showed up-regulation on exposure to ibuprofen (IBU) and clofibric acid (CFA), but not clotrimazole (CTZ) or propranolol (PRP). The lack of response to mammalian PXR-agonist CTZ was unexpected. In contrast, the same XME genes were found to be up-regulated in vivo after 10 days of exposure of carp to CTZ, although this response occurred only for a relatively high exposure concentration. CTZ was found to concentrate in the plasma (with levels up to 40 times higher than the water). Development and application of a reporter gene assay to measure PXR activation in carp (cPXR) and human PXR showed CTZ activation of cPXR, supporting data from the in vivo studies. Furthermore, activation was seen at concentrations as low as 0.01 μM. Interestingly RIF did not induce a response in the cPXR reporter gene assay, contrasting with the hepatocyte culture work. Taken together, the data presented here suggests divergence in the PXR pathway between mammals and fish in terms of ligand activation and downstream gene targets. PPARα was investigated in carp in vivo using CFA as a mammalian PPARα-agonist. Overall the resulting data suggested a broadly similar role for this NR in lipid homeostasis in fish as for mammals, with a number of PPARα-associated genes and acyl-coA oxidase (ACOX1) activity up-regulated in response to CFA exposure. A number of XMEs were also up-regulated by CFA (in vivo and in vitro), potentially extending the role of PPARα in fish (carp) to regulation of xenobiotic metabolism. The work presented has provided further characterisation of PXR and PPARα in fish. Elucidation of these pathways is vital to provide meaningful data in terms of establishing toxicity and mechanism-of-action data for pharmaceuticals and other compounds in fish, to allow validation of read-across approaches and ultimately aid in their environmental risk assessment. In vitro approaches are attractive ethically, financially and can provide useful mechanistic characterisation of compounds and the primary hepatocyte model and reporter gene assays used here show potential for the screening of pharmaceutical compounds in fish. However, further understanding of the metabolism of drugs and chemicals in fish is required to establish the true value of these methods for informing on possible effects in fish, in vivo.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:580044 |
| Date | January 2013 |
| Creators | Corcoran, Jenna Frances |
| Contributors | Tyler, Charles R.; Winter, Matthew J. |
| Publisher | University of Exeter |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/10871/13341 |
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