Studies on the role of peroxisome proliferators: in liver growth and neurodegenerative disorders

This thesis is divided into two main chapters. The first chapter relates to studies undertaken to gain insights into the mechanism of action on liver growth by the peroxisome proliferator (PP) ciprofibrate and the chemical cyproterone acetate (CPA) in rodents. Peroxisome proliferators are a class of chemicals that have diverse effects in rats and mice including increased DNA synthesis and peroxisome proliferation. Peroxisome proliferators include herbicides, plasticisers, hypolipidemic drugs and synthetic fatty acids. These chemicals act through ligand activation of nuclear membrane receptors termed ‘peroxisome-proliferator-activated receptors’ (PPARs), which ultimately activate nuclear transcription. PPs induce a cellular process in liver characterised by dramatic increases in the size and number of peroxisomes, correlated with both hepatocyte hypertrophy (i.e. an increase in the size of liver cells) and hyperplasia (i.e. an increase in the number of liver cells during replicative DNA synthesis and cell division). However, the mechanism of action of increased hepatocyte growth is not currently understood. Understanding the mechanism by which increased liver growth is induced by PPs in rodents will hopefully provide insights into how natural liver growth occurs and might have medical benefits for human health if the mechanism of PP toxicity can be overcome. Knowledge gained from the mechanism of PP activation might also then be applied to other chemical carcinogens. Therefore, firstly the mode of action of the peroxisome proliferator ciprofibrate was investigated. Previous work had indicated that two successive doses of ciprofibrate treatment separated by 24hr led to two rounds of liver cell replication, but it was not clear whether the same or different hepatocytes were involved in this growth response. To study this phenomenon, histochemical experimental work was undertaken to assess whether the same or different hepatocyte cells were stained during the two rounds of cell division following ciprofibrate treatment. The two histochemical stains used were EdU and BrdU, which are both base-pair analogues that stain nuclei undergoing DNA replication. It was hypothesized that if EdU was used to stain cells at 24 hr and then BrdU at 48 hr, that if the same cells were responding to ciprofibrate treatment then cells would be co-stained by both dyes, whereas if different cells were responding then there would be little or no double staining of hepatocyte cells. It was found that different cells were stained by the two dyes, indicating that ciprofibrate treatment was targeting different cells. Secondly, the mode of action of the carcinogen cyproterone acetate (CPA) on hepatocyte growth was investigated. Previous work had investigated the effects of CPA on hepatocyte growth in male and female rats and had suggested differences in response between the sexes. In the present study female rats were treated with CPA, to assess whether differences in labelling indices were present compared to previous male results. Female F-344/NHsd rats, aged 14-15 weeks, were treated with CPA and then injected with BrdU at 22 hr, and rats were killed 2 hr later. Results confirmed that the female rats had a considerably higher labelling index (50%) compared to male rats (6%). This suggested that upregulation of gene expression in female rats was much higher, which might provide an exciting opportunity to identify sets of genes involved in carcinogenic responses. To investigate whether there was any overlap between genes induced by ciprofibrate and CPA treatment a preliminary study was designed where female rats were gavaged with CPA and then killed 3 hr later. Real-time PCR analysis of a small number of target genes showed no consistent changes in expression between the present CPA and previous ciprofibrate treatment results, suggesting largely different modes of action of these chemicals. The second chapter of this thesis relates to studies undertaken to gain insights into neurodegenerative disorders. Neurodegeneration is a gradual loss of structure or function of neurons, which may lead to neuronal death. Neurodegenerative diseases including Parkinson’s, Alzheimer’s, and Huntington’s occur as a result of neurodegenerative changes. Several studies have suggested that PPARs have critical roles in reducing the brain inflammation which in turn might have a significant effect on reducing the fundamental processes involved. Work was performed using a mouse model of dementia with lewy body disease (Psmc1fl/fl; CaMKIIα-Cre) to represent neurodegenerative disorder, and involved parallel, in vivo and in vitro investigations to determine whether the development of neurodegenerative diseases occurs at the same rate in vitro and in vivo i.e. a comparison of rapidity of pathogenicity progression was made. Astrocytes were used to track the development of disease, given that these play a key role in neurological disorders, using an immunohistochemistry approach. A PPAR-γ analogue was used to investigate the role of PPARs in reducing astrocytes proliferation. To optimise the validity of the results, four controls were used including an antagonist T0070907 which abolished the effect of rosiglitazone treatment alone. The results on the effect of PPAR-γ agonist and rosiglitazone, after a week of treatment, showed that the PPAR-γ agonist inhibited astrocytes activation in both the cortex and hippocampus of the mutant mice organotypic slice culture. The number of GFAP +ve astrocytes was significantly decreased in mutant mice with 100 µM rosiglitazone in both areas, whereas 50 µM rosiglitazone showed a decrease in the number of astrocytes in the cortex, but the effect was less in the region of the hippocampus. This finding suggests that PPs such as rosiglitazone may have potential uses as therapeutic drugs to inhibit neurodegeneration