Scientists in Singapore have developed engineered probiotic gut bacteria that can transform a substance found naturally in broccoli and Brussels sprouts into a powerful cancer-preventing chemical. The National University of Singapore (NUS) team, led by Chun-Loong Ho, Ph.D., and associate professor Matthew Chang, Ph.D., engineered the harmless gut bacterium to attach specifically to the surface of colorectal cancer (CRC) cells and secrete an enzyme that converts the glucosinolate compounds in cruciferous vegetables into sulforaphane, an organic small molecule that has anticancer properties.
When tested in vitro, a combination of the engineered bacteria and broccoli extract almost completely prevented the growth of mouse and human CRC cells, without affecting any other type of cell. And when fed to a mouse model of CRC, the probiotic–vegetable combination reduced tumor numbers by 75% and stunted the growth of tumors that did develop.
“Mothers are right after all, eating vegetables is important,” Dr. Ho says. The authors report on their studies in Nature Biomedical Engineering, in a paper entitled “Engineered Commensal Microbes for Diet-Mediated Colorectal-Cancer Chemoprevention.”
CRC is one of the most common cancers worldwide, and while early diagnosis and surgery can result in complete remission, the prognosis isn’t so good for the 60% or so of patients who are initially diagnosed with advanced colorectal carcinomas. In an ideal world, we would have effective chemopreventive agents that help to stop tumor initiation, but commonly used strategies, including nonsteroidal anti-inflammatory drugs, folic acid, calcium, vitamin D, and antioxidants, aren’t effective for everyone, the authors note.
Eating foods that are high in natural metabolites such as isothiocyanates, indoles, quinones, alkaloids, and phenolics is an alternative approach to boosting chemoprevention. However, the authors acknowledge, “these naturally occurring chemopreventive metabolites often have low bioavailability and poor host absorption and therefore require high consumption of a particular food class to elicit effective chemoprevention.”
Another problem is that many potentially cancer-inhibiting metabolites are inactive until converted by enzymes into a bioactive form. One of these is glucosinolate, an isothiocyanate found in cruciferous vegetables—including broccoli, Brussels sprouts, bok choy, and salad rocket. Inactive glucosinolate in the vegetable is converted into bioactive sulforaphane by the plant 's own enzyme, myrosinase. “The conversion of glucosinolate to sulforaphane requires hydrolysis by the myrosinase enzyme (EC22.214.171.124), which is involved in the plant’s defense against herbivores,” the authors explain. Sulforaphane has been found to inhibit cancer cells by blocking the cell cycle, promoting the expression of proapoptotic factors, and downregulating antiapoptotic factors.
Myrosinase is produced in plants, but not in mammals, and the NUS team hypothesized whether a diet rich in cruciferous vegetables could have an anticancer effect if the required myrosinase enzyme was supplied and localized to target cancer cells in the gastrointestinal tract. They engineered the commensal Escherichia coli Nissle strain to target and bind to a cell-surface molecule found specifically on CRC cells, and also to produce and secrete myrosinase. The idea was that the enzyme produced by the bacteria would convert glucosinolate in dietary vegetables to sulforaphane, which would then arrest cancer cell growth and trigger apoptosis. Then, when the dead cancer cells are cleared, the engineered bacteria detach from the colorectal tissue and are passed out with feces.
The team first confirmed that the engineered E. coli bacteria specifically bound to CRC cells in vitro and secreted myrosinase. More than 95% of the cancer cells were cleared when exposed to a combination of the engineered bacteria and glucosinolate, whereas the treatment had no effect on breast, stomach, or primary cell lines. “Sulforaphane does not affect primary stem cells or other non-cancer cell lines and mammalian cells do not have the capacity to convert these compounds, which makes this myrosinase–glucosinolate combination a potential tool for targeting cancer cells,” the authors write.
Encouragingly, feeding the probiotic bacteria and broccoli extract to CRC-bearing mice resulted in more than 75% tumor reduction.
The researchers foresee that such probiotics could be used both to help prevent cancer but also to mop up any cancer cells that remain after surgery. It may also be feasible to develop probiotics that people can take as a dietary supplement to prevent CRC from developing, or to reduce recurrence after surgery. “One exciting aspect of our strategy is that it just capitalizes on our lifestyle, potentially transforming our normal diet into a sustainable, low-cost therapeutic regimen,” Dr. Chang states. “We hope that our strategy can be a useful complement to current cancer therapies.”