Discovery Blocks Cancer Drug's Toxic Side Effect
Findings published in Science from the Redinbo Group, in collaboration with UNC School of Medicine, the Albert Einstein College of Medicine, and North Carolina Central University, may lead to the elimination of a debilitating side effect of CPT-11, a widely used but harshly potent treatment for colon cancer. The team of researchers, led by chemistry professor Matthew Redinbo from the University of North Carolina at Chapel Hill, has discovered that it is possible to target and block the enzyme, beta glucuronidase, which is thought to play a major role in causing the drug's side effects. "In a manner of speaking, we cured the bacteria's sweet tooth without damaging the microbes or intestines and, in the process, the drug's toxic side effect was alleviated," said Redinbo.
Study co-author, Sridhar Mani, professor of medicine and genetics at Einstein, said the severe diarrhea caused by CPT-11 can sharply limit the dosage that cancer patients can receive. "Our tests showed conclusively that the inhibitor identified by our UNC colleagues prevented diarrhea in mice that were also receiving CPT-11. We are hopeful that clinical trials will show that administering this inhibitor when patients start taking CPT-11 allows for improvement in the drug's anti-tumor effect in patients with cancer."
Mitochondrial Ribosome-Protein Interactions
Published in The Journal of Biological Chemistry, a collaboration between the Spremulli Group and the Fecko Group elaborates on how the mammalian mitochondrial inner membrane protein Oxa1L is involved in the insertion of a number of mitochondrial translation products into the inner membrane. During this process, the C-terminal tail of Oxa1L (Oxa1L-CTT) binds mitochondrial ribosomes and is believed to coordinate the synthesis and membrane insertion of the nascent chains into the membrane. The C-terminal tail of Oxa1L does not contain any Cys residues. Four variants of this protein with a specifically placed Cys residue at position 4, 39, 67, or 94 of Oxa1L-CTT were prepared. These Cys residues have been derivatized with a fluorescent probe, tetramethylrhodamine-5-maleimide, for biophysical studies.
Oxa1L-CTT forms oligomers cooperatively with a binding constant in the submicromolar range. Fluorescence anisotropy and fluorescence lifetime measurements indicate that contacts near a long helix close to position 39 of Oxa1L-CTT occur during oligomer formation. Fluorescence correlation spectroscopy measurements demonstrate that all of the Oxa1L-CTT derivatives bind to mammalian mitochondrial ribosomes. Steady-state fluorescence quenching and fluorescence lifetime data indicate that there are extensive contacts between Oxa1L-CTT and the ribosome-encompassing regions around positions 39, 67, and 94. The results of this study suggest that Oxa1L-CTT undergoes conformational changes and induced oligomer formation when it binds to the ribosome.
Deciphering Viral 'Zip-Codes'
All retroviruses contain a structure in their RNA genomes that functions like a cellular 'zip code' and directs the genome into nascent virus particles. Although the phenomena has been noted for nearly 30 years, the molecular mechanism was unknown.
As reported in PNAS scientists at UNC and at the National Cancer Institute lead by Professor Weeks have used SHAPE technology, invented in the Weeks laboratory, to map the virus packaging signal, the RNA 'zip code', at nucleotide resolution. Remarkably, mutating only 4 nucleotides in the newly identified structure abolishes specific RNA packaging and virus infectivity.
The results have broad implications for RNA recognition and illustrate how precise chemical tools make it possible to decipher complex biological interactions, including inside infectious viruses.