Years of results from the lab have revealed how marine bacteria make potent anti-cancer molecules.
The anti-cancer molecule Salinosporamide A, also known as Marizomb, is in Phase III clinical trials for the treatment of glioblastoma, a type of brain cancer. Scientists now understand for the first time the enzyme-driven process that activates the molecule.
Researchers at the Scripps Institution of Oceanography at the University of California, San Diego, have discovered that an enzyme called SalC assembles what the team calls a salinosporamide "warhead" against cancer. Scripps graduate student Katherine Bauman is the lead author of a paper explaining the assembly process in the March 21 issue of Nature Chemical Biology.
This work solves a nearly 20-year mystery about how marine bacteria make warheads unique to the salinomycin molecule and opens the door to future biotechnology for the manufacture of novel anticancer agents.
"Now that scientists understand how this enzyme makes salinosporamide warheads, this discovery could be used in the future to use the enzyme to produce other types of salinosporamides that attack not only cancer but also immune system diseases and infections caused by parasites," said co-author Bradley Moore, a Scripps College of Oceanography and Skaggs School of Pharmacy and Pharmaceutical Sciences Distinguished professor.
Scripps Institution of Oceanography PhD student Kate Bauman works with lab director Bradley Moore to further study new Salinispora cultures in a biosafety cabinet. These bacterial cultures produce Salinispora A, a potent anti-cancer agent currently in phase III clinical trials for glioblastoma.
Salisporamide has a long history at Scripps and the University of California, San Diego. Microbiologist Paul Jensen and marine chemist Bill Fenical of Scripps Oceanography discovered Salinosporamide A and the marine organisms that produce the molecule in 1990 after collecting microbes from tropical Atlantic sediments. Some of the clinical trials during drug development were conducted at the Moores Cancer Center at the University of California, San Diego Health.
"It was a very challenging 10-year project," said Moore, Bowman's advisor." Kate was able to bring together 10 years of early work to get us across the finish line.
A big problem for Bauman was to find out how many enzymes were responsible for folding the molecule into its active shape. Does it involve multiple enzymes or just one?
"I would put my money on more than one. In the end, it was just SalC. which was surprising," she said.
Moore said the salivosporangium molecule has the special ability to cross the blood-brain barrier, which explains its progress in clinical trials for glioblastoma. The molecule has a small but complex ring structure. It starts as a linear molecule that folds into a more complex circle.
"Nature makes it very simple. As chemists, we can't do what nature does to make such molecules, but nature does it with an enzyme," he said.
The enzyme involved is common in biology; it is an enzyme involved in the production of antibiotics such as erythromycin in human fatty acids and microorganisms.
Bauman, Percival Yang-Ting Chen of Morphic Therapeutics in Waltham, Massachusetts, and Daniella Trivella of the National Center for Energy and Materials Research in Brazil determined the molecular structure of SalC. To do so, they used the Advanced Light Source, a powerful particle gas pedal that produces X-rays, at the U.S. Department of Energy's Lawrence Berkeley National Laboratory.
"The SalC enzyme performs a very different reaction than normal ketone synthase," Bauman said. Normal ketosynthase is an enzyme that helps molecules form linear chains. In contrast, SalC makes salaminosulfonamide by forming two complex reactive ring structures.
An enzyme can form two ring structures that are difficult for synthetic chemists to make in the laboratory. With this information, scientists can now mutate the enzyme until they find forms that hold promise for inhibiting various types of diseases.
The marine bacteria involved are called Salinispora tropica and make salinospore amines to avoid being eaten by their predators. But scientists have discovered that salinispora tropica A can also treat cancer. They have isolated other salinispora tropica, but salinispora tropica A has characteristics that other salinispora tropica lack - including the biological activity that makes it harmful to cancer cells.
"Inhibiting this proteasome makes it a good anti-cancer agent," Bauman said, talking about protein complexes that degrade useless or damaged proteins. But there's another type of proteasome in immune cells. What if scientists were able to design a slightly different type of salinomycin than salinomycin A? One that inhibits the cancer-prone proteasome but excels at inhibiting the immune proteasome? This salivosporamide could be a highly selective treatment for autoimmune diseases that cause the immune system to turn on the body it is supposed to protect.
"That was the idea behind generating some of the other salicylic amides. Getting this enzyme SalC with a complex ring structure installed opens the door for the future," Baumann said.
Bradley Moore, Enzymatic assembly of the salinosporamide γ-lactam-β-lactone anticancer warhead, Nature Chemical Biology (2022). DOI: 10.1038/s41589-022-00993-w
