Alleviating Cancer Drug Toxicity Through Targeted Inhibition of a Bacterial Enzyme and Exploring the Structural and Functional Properties of the Xenobiotic Nuclear Receptor PXR Public Deposited

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  • March 21, 2019
  • Wallace, Bret David
    • Affiliation: College of Arts and Sciences, Department of Chemistry
  • Although symbiotic GI bacteria are essential to human health, in some cases these organisms contribute to undesirable side effects during the treatment of disease. For example, the anticancer drug CPT-11 is employed against a range of human malignancies; however, its efficacy is severely limited by diarrhea generated by the reactivation of a glucuronidated drug metabolite in the gastrointestinal tract. The enzymes responsible for this dose-limiting side effect are bacterial beta-glucuronidases in our symbiotic intestinal microflora. We sought to selectively eliminate this GI-specific drug reactivation. We identified potent and novel bacterial beta-glucuronidase inhibitors through high-throughput screening. Hits exhibit nM-level in vitro inhibition against E. coli beta-glucuronidase and no effect on a mammalian beta-glucuronidase. We further elucidated the crystal structure of E. coli beta-glucuronidase both alone and in complexes with novel, potent inhibitors. Crystal structures established that selectivity was conferred through a loop unique to bacterial beta-glucuronidases. Oral administration of novel inhibitors in mice proved successful in alleviating CPT-11 induced toxicity. As such, drugs may be utilized to disrupt deleterious bacterial enzymatic activity in the human GI. The nuclear receptor PXR plays a key role in human biology and xenobiotic metabolism. This ligand-regulated transcription factor responds to a variety of chemically unique ligands, including many endogenous compounds and clinical drugs. In addition, PXR regulates the expression of a critical set of protective gene products involved in xenobiotic and endobiotic metabolism. The structural basis of the PXR ligand binding domain's remarkable and unique promiscuity has only recently come to light. An understanding of the PXR/RXR alpha complex through structural and functional studies is ever more important with the recently elucidated structure of the PPAR gamma /RXR alpha full-length complex and protein-DNA interactions. We show here the crystal structure of the PXR/RXR alpha LBD complex, revealing a heterotetrameric complex, unique to this particular nuclear receptor complex. In addition, we present, to our knowledge, the first of experimental methods for the expression, purification, and complex formation for the full-length PXR.1 and RXR alpha complex.
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  • partial fulfillment of the requirements for the degree Doctor of Philosophy in the Department of Chemistry.
  • Redinbo, Matthew R.
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  • University of North Carolina at Chapel Hill

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