Abstract
Interferons and interleukins are induced during the acute phase response to viral infections and suppress drug metabolism by down-regulation of cytochrome P450 isoenzymes. Although numerous investigators have attempted to explain this phenomenon, the precise interferon-mediated molecular mechanism(s) which causes the suppression has yet to be identified. Previous workers have examined the suppression of clofibrate-induced CYP4A1 mRNA by the interferon inducer, poly IC, and indicated that the interferon-mediated inhibitory mechanism acts at a pretranslational stage of CYP4A1 expression (Renton et al, 1987, Renton and Knickle, 1990, Knickle et al, 1992). The results I present in this thesis have extended these investigations. 1. In preliminary studies I have confirmed the findings of Renton and coworkers and also shown that the constitutive (uninduced) expression of CYP4A1 protein, enzyme activity and mRNA is not down-regulated by poly IC. 2. I have used a cDNA probe for the peroxisome proliferator activated receptor (PPAR) to investigate the effects of clofibrate, poly IC and human interferon a on rat PPAR mRNA expression. Down-regulation of CYP4A1 mRNA is not preceded by down-regulation of PPAR mRNA synthesis. 3. Neither poly IC nor human interferon a affected the induction of CYP2E1 protein by isoniazid. 4. The species specificity of the interferon-mediated suppression of rat hepatic CYP4A1 expression has been examined using human interferon a. 5. Using nuclear run-on experiments, I have shown that the interferon-mediated suppression of CYP4A1 induction is due to the down-regulation of clofibrate-induced synthesis of CYP4A1 pre-mRNA. I have concluded from these investigations that the suppression of clofibrate-induced CYP4A1 by type I interferons is caused by an interferon-mediated molecular mechanism(s) which interferes with the clofibrate-responsive mechanisms governing the transcriptional induction of the CYP4A1 gene. In contrast, the induction of CYP2E1 by isoniazid, which occurs through protein stabilisation, is not affected by interferons. I have used these results to develop the proposal that the down-regulation of other cytochrome P450s in response to interferons or interleukins is most likely to be also at the level of pre-mRNA synthesis. Interferons are often used in conjunction with other drugs in the chemotherapy of a variety of diseases. Since many of these drugs have a narrow therapeutic index, their reduced metabolism could result in accumulation leading to enhanced toxicity. Clearly, an improvement in our understanding of the mechanisms by which interferons affect drug metabolism will help to predict, and therefore restrict, any toxicity caused by combination therapy with interferons.