Silencing cells to combat multidrug resistance

Multidrug resistance is a growing health problem. Bacteria seem to have the last say despite the bombardment of drugs and antibiotics. Scientists discovered that one of the "culprits" is a molecular 'pump' known as P-glycoprotein. In healthy people, this protein plays a good role. It is part of the mechanism of the so-called "multidrug efflux pumps" of living cells -- with the ability to prevent the entry of toxic molecules by "pumping" them out to be discharged by the body. However, this protein when over expressed by cancer patients, are able to make cancer cells resistant to a wide range of chemotherapy drugs.

Bacteria too possess efflux pumps -- "transport" proteins which expel toxic substances which can kill them. Bacterial genomes which have been studied contain several different efflux pumps. One example is the E coli. Colonies of the bacteria can inhabit our intestines -- and may even be good for us as they trap and pump out toxic bile salt. A mutated form of E coli however, causes food poisoning -- and because of its effective "pumps" -- is highly resistant to drugs.

Researchers have attempted to tweak the molecular structure of drugs, to make them incompatible with the pump's binding sites so that they will not be expelled and could stay put at target sites to do their good work. This however, proves to be extremely difficult because, on the other hand, antibiotics need to bind to target sites in order to be effective.


The answer to a more effective battle against bacteria, perhaps lie in combating these "pumps". A team led by Dr Adam Yuan at the NUS Department of Biological Sciences, in collaboration with Temasek Life Sciences Laboratory (affiliated to NUS) is examining the structure of these "pumps" to see how they can be blocked. One of their priorities is to combat multidrug resistance in cancer patients. Said Dr Yuan who is also Principal Investigator of the Structural Biology Group at the Temasek Life Sciences Laboratory: "Any process that limits the levels of these pumps will increase the concentrations of cancer drugs, and one of our approaches is to focus on the regulators of multidrug resistance."

The team has managed to solve more than half dozen of regulators. The regulator proteins are crystallised and when exposed to extremely bright X-rays, the molecular structure is revealed, hence enabling the team to study how they work to regulate the "pumps". They have also solved four "ATP-binding" domain structures of multidrug resistant pumps. (ATP or adenosine triphosphate is a major source of energy for cellular reactions). Their efforts have paid off. Said Dr Yuan: "We have already come out with a novel 'template', a model for testing the effectiveness of target drugs to inhibit the pumps."

Combating viral infection

RNA "silencing" is one of the tools used by the body to fight viruses by "silencing" viral infection. (RNA or ribonucleic acid determines protein synthesis and the transmission of genetic information.) At Dr Yuan's lab at NUS and the Temasek Life Sciences Lab, researchers are using a combination of structural, biochemical and biophysical approaches to study RNA-protein interaction, and small compound-membrane protein recognition at atomic resolution in vitro. They are also using a combination of molecular and cellular biological approaches to study these important life-related processes in vivo.

"One of our long term goals is to gain a detailed understanding of the structural, biophysical and biological properties of key super-complexes within RNA interference pathway, and to relate this understanding to its in vivo function," said Dr Yuan.

The team is also using "Virus-induced gene silencing" or VIGS -- an approach harnessing plants' natural ability fight plant viruses through a RNA-mediated defence mechanism. From a plant model, the team hopes to come out with a novel template as well, for testing RNA-based drug targets.