Cancer patients fear the possibility that one day their cells might start rendering many different chemotherapy regimens ineffective. This phenomenon, called multidrug resistance, is responsible for virtually all cancer deaths among individuals who have undergone treatment.
TSRI Associate Professor Qinghai Zhang (right), shown here with Research Associate Sung Chang Lee, was a senior author of both studies.
Now scientists at The Scripps Research Institute (TSRI) have published a pair of studies showing how the primary protein responsible for multidrug chemotherapy resistance changes shape and reacts to therapeutic drugs.
“This information will help us design better molecules to inhibit or evade multidrug resistance,” said TSRI Associate Professor Qinghai Zhang, a senior author of both studies.
The proteins at work in multidrug resistance are V-shaped proteins called ABC transporters. ABC transporters are found in all kingdoms of life – from bacteria to humans. In humans, an important ABC transporter is P-glycoprotein (P-gp), which catches harmful toxins in a “binding pocket” and expels them from cells.
The problem is that in cancer patients, P-gp sometimes begins recognizing chemotherapy drugs and expelling them, too. Over time, more and more cancer cells can develop multidrug resistance, eliminating all possible treatments. A better understanding of P-gp and how it binds to molecules will help scientists develop better cancer drugs.
For one of the new studies, researchers looked at P-gp under one of TSRI's powerful electron microscopes. They also looked at MsbA, a similar transporter protein found in bacteria.
The electron microscopy (EM) work – spearheaded by a postdoctoral researcher Arne Moeller, working in the laboratory of Bridget Carragher and former TSRI Professor Clint Potter – solved a major problem in transporter research.
Until recently, researchers could only compare images of crystal structures made from transporter proteins. These crystallography images showed single snapshots of the transporter but didn't show how the shape of the transporters could change. Using EM, however, P-pg and MsbA could be captured in action.
The new research was also enabled by the development of new chemical tools. Previous studies were hampered by the fact that, outside the cell membranes, these transporter proteins turned into an unstructured mash.
“They looked like tofu,” said Sung Chang Lee, a research associate in Dr. Zhang's lab at TSRI and co-first author of the study.
In the study, the researchers used a solution of lipids and peptides to mimic natural conditions in the cell membrane. A novel chemical called beta sheet peptide, developed by the Zhang lab, was used to stabilize the protein and provide a new perspective.
Together with EM, this technique enabled the research team to capture a series of images showing how transporter proteins change shape in response to drug and nucleotide binding. They found that transporter proteins have an open binding pocket that constantly switches to face different sides of membranes.
“The transporter goes through many steps – it's like a machine,” said Dr. Zhang.
In the second study, the researchers investigated the drug binding sites of P-gp using higher-resolution X-ray crystallography.
Their findings show how P-gp interacts with drug-like molecules called ligands. The researchers studied crystals of the transporter bound to four different ligands to see how the transporters reacted.
The researchers found that when certain ligands bind to P-gp, they trigger local conformational changes in the transporter. Binding also increased the rate of ATP hydrolysis, which provides mechanical energy and may be the first step in the binding pocket closing process.
The team also found that ligands could bind to different areas of the transporter, leaving nearby slots open for other molecules. This suggested that it may be difficult to completely halt the drug expulsion process.
Dr. Zhang said the next step in this research is to develop molecules that evade P-gp binding.