Researchers led by Penn State University have turned a deadly fungus into an effective anti-cancer agent. After isolating a new class of molecules from Aspergillus flavus, a toxic grain fungus linked to deaths in ancient tomb excavations, the researchers modified the chemicals and tested them on leukemia cells. The result? A promising anti-cancer agent that can compete with FDA-approved drugs, opening new avenues for the discovery of other fungal drugs.
A. Flavus as an Unexpected Source of a Promising New Cancer Therapy
“Mushrooms gave us penicillin,” says Sherry Gao, Presidential Penn Compact Associate Professor of Chemical and Biomolecular Engineering (CBE) and Bioengineering (BE) and lead author of a new study in Nature Chemical Biology on the findings. “These results show that many more drugs can be discovered from natural products.”
Aspergillus flavus, named after its yellow spores, has long been a microbial culprit. After archaeologists opened King Tutankhamun’s tomb in the 1920s, there were a number of premature deaths among excavation workers, fueling rumors that the pharaoh was cursed. Decades later, doctors theorized that fungal spores that had been dormant for thousands of years may have played a role. In the 1970s, a dozen scientists entered the tomb of Casimir IV in Poland. Ten of them died within a few weeks. Later investigations revealed that the tomb contained A. flavus, whose toxins can lead to lung infections, especially in people with weakened immune systems. Today, the same fungus is the unexpected source of a promising new cancer therapy.
Innovative Approach
The therapy in question is a class of ribosomally synthesized and post-translationally modified peptides, known as RiPPs, which are pronounced like the “rip” in a piece of cloth. The name refers to the way the compound is made – by the ribosome, a tiny cellular structure that makes proteins – and the fact that it is later modified, in this case to enhance its cancer-killing properties.
“Purifying these chemicals is difficult,” explains Qiuyue Nie, a postdoctoral researcher at the CBE and first author of the study. While thousands of RiPPs have been identified in bacteria, only a few have been found in fungi. This is partly because previous researchers had misidentified fungal RiPPs as non-ribosomal peptides and knew little about how fungi make these molecules. According to the researchers, the synthesis of these compounds is complicated. To find more RiPPs in fungi, they first examined a dozen strains of Aspergillus, which previous research suggested might contain more of these chemicals. By comparing the chemicals produced by these strains with known RiPP building blocks, they identified A. flavus as a promising candidate for further investigation.
Genetic analysis pointed to a specific protein in A. flavus as a source of fungal RiPPs. When the researchers switched off the genes that produce this protein, the chemical markers indicating the presence of RiPPs also disappeared. This novel approach – combining metabolic and genetic information – not only identified the source of fungal RiPPs in A. flavus, but could also be used to find other fungal RiPPs in the future.
Strong Effect on Leukemia
After purifying four different RiPPs, the scientists discovered that the molecules have a unique structure of interlocking rings. They named these molecules, which had not yet been described, after the fungus in which they were found: Asperigimycins. Even without modification, asperigimycins showed medical potential in combination with human cancer cells: two of the four variants had a strong effect against leukemia cells.
Another variant, to which the researchers added a lipid (fat molecule) that is also contained in the royal jelly that bees feed on during the development phase, showed an effect that was as good as cytarabine and daunorubicin, two FDA-approved drugs that have been used to treat leukemia for decades.
Instrument for the Development of Drugs
In order to understand why lipids increase the effectiveness of asperigimycins, the researchers selectively switched certain genes in the leukemia cells on and off. One gene, SLC46A3, proved to be crucial in ensuring that asperigimycins were able to penetrate the leukemia cells in sufficient quantities. This gene helps to excrete substances from the lysosomes, the tiny vesicles that collect foreign substances entering human cells.
Like asperigimycins, these chemicals have medicinal properties – nearly two dozen cyclic peptides have been approved since 2000 for the treatment of diseases as diverse as cancer and lupus – but many of them need to be modified to enter cells in sufficient quantities. “Now that we know that lipids can influence the transport of chemicals through this gene into cells, we have another tool for drug development,” says Nie.
Possibly One Day Clinical Trials on Humans
Through further experiments, the researchers found that asperigimycins probably interfere with the process of cell division. Remarkably, the compounds had little or no effect on breast, liver or lung cancer cells – or a range of bacteria and fungi – which, according to the experts, suggests that the disruptive effects of asperigimycins are specific to certain cell types, which is crucial for future drugs.
In addition to demonstrating the medicinal potential of asperigimycins, the researchers identified similar gene clusters in other fungi, suggesting that even more RiPPS could be discovered in fungi. The next step is to test asperigimycins in animal models in the hope of one day moving on to human clinical trials.