Combating Drug Resistance

The research of structural biologist Dr. Min Lu could help solve one of the world's most serious threats to health: antibiotic-resistant superbugs. The second round of NIH funding for his study of the mate transporter comes from an influx of newly targeted federal research dollars.

The human body has a myriad of mechanisms in place to protect it from all manner of assaults. But when the security system becomes the enemy, microbes can gain the upper hand. Antibiotics lose their power to heal. Stubborn infections kill.

Min Lu, PhD, associate professor in the Department of Biochemistry and Molecular Biology, is working to understand why patients develop resistance to medications. His investigation focuses on the MATE (multidrug and toxin extrusion) transporter, which works in bacterial cells to remove antimicrobial compounds, thus contributing to the global crisis of antibiotic resistance among pathogenic bacteria. Found primarily in the liver and kidneys, MATE proteins act as detoxifying machines in human cells.

“It’s a security system that keeps us alive,” Dr. Lu said. “But it can also prevent life-saving drugs, including treatments for cancer and diabetes, from doing their job.”

Growing antimicrobial resistance, which encompasses resistance to drugs that treat infectious diseases including HIV, tuberculosis and malaria, is resulting in protracted illness, higher treatment costs and increased risk of death around the globe, according to the World Health Organization. In the United States, 2 million people each year contract infections that are resistant to antibiotics; 23,000 die as a result.

A 2014 report by the President’s Council of Advisors on Science and Technology gauged the annual economic impact of antibiotic resistance at $55–$70 billion and recommended a coordinated course of action. The NIH this year received $100 million in federal funds to support basic science research aimed at combating the problem.

Dr. Lu received National Institutes of Health funding on his first-ever grant proposal for the MATE study, and the agency recently awarded him $1.3 million to continue his investigation through August 2019.

He and his team are investigating MATE transporters in bacteria, with the goal of studying them in humans. They have characterized two of the three MATE transporter subfamilies in  bacteria, which have similar protein sequences but very different mechanisms and slightly different structures, Dr. Lu said.

Knowing the sequences and structures of proteins can provide insight into their specific functions. To determine the structures, Dr. Lu’s team first purified the proteins, which enabled the proteins to form three-dimensional protein crystals. Next, the researchers collected X-ray diffraction data from the crystals to capture their crystalline shapes. Based on these images, the team generated hypotheses about which amino acids on the proteins were responsible for various activities.

“We changed amino acids in the transporters to see if we could make them less active,” Dr. Lu said. “If the protein still worked, then the amino acid we changed was not important. But if the cells no longer expressed the protein or if they expressed proteins that were not active, then the cell died and we knew the amino acid we changed was important. Understanding how these proteins work and how they remove drugs from cells can help us to figure out how to inhibit them or to design compounds or drugs that they cannot remove easily.” 

With a better understanding of how MATE transporters work, Dr. Lu and his colleagues have begun to test the effects of the proteins on various compounds, including the compound verapamil, used to treat arrhythmia, severe angina and high blood pressure.

It’s a security system that keeps us alive, but it can also prevent life-saving drugs, including treatments for cancer and diabetes, from doing their job.” -Min Lu, PhD

“We found that verapamil binds to the MATE transporter and inhibits the binding between it and its drug substrates,” Dr. Lu said. “It turns out that verapamil could be a potentially useful probe to uncover the therapeutically vulnerable sites on MATE transporters. This may open a new avenue of thought on battling drug resistance.”

The team hopes to soon go beyond bacterial models to apply their findings in humans. In particular, they are interested in investigating ways of circumventing the MATE transporter activity on the diabetes drug metformin.

“Human proteins are quite different from bacterial proteins, so understanding how bacterial transporters work doesn’t mean we understand how human proteins work,” Dr. Lu said. “There is still a lot more work to be done.”

This story first appeared in the Summer 2016 issue of Helix Magazine.