Often maligned for its role in diabetes, sugar is critical for cells to signal to one another and their environment. These sugar units are typically attached to proteins on the surface of cells via the activity of enzymes.
The locations, lengths and sequences of sugar units on a protein can affect how it is recognized by certain drugs. Hence, the ‘glycosylation pattern’ of proteins has implications for the efficacy of targeted therapy, especially in the context of cancer.
“Currently, antibody-based cancer therapeutics are focused on targeting the correctly glycosylated protein that is overexpressed in cancer cells compared to normal cells,” said Sir David Lane, Chief Scientist of A*STAR. But because these correctly glycosylated proteins may also be expressed in normal cells, side effects of treatment occur.
“If we are able to target the aberrantly glycosylated proteins that are only expressed on neoplastic cells but not the normal tissues, we would gain access to significant enhancements in drug specificities and drug concentrations, without the fear of off-target toxicities associated with the use of higher drug doses,” Lane explained.
In a study published in Oncogene, his team was able to generate an antibody—called 6E6—that distinguishes between the glycosylation patterns on the RON receptor, a protein often expressed by aggressive cancers.
Lane recalls the serendipitous nature of the discovery. His team was originally disappointed that 6E6 did not bind well to correctly glycosylated RON expressed by cancer cells grown in a petri dish.
However, when the researchers injected 6E6 into mice engrafted with human cancer cells, they observed much stronger binding. It turned out that the tumors in mice were expressing much higher levels of unglycosylated RON, which 6E6 was sensitive to.
Importantly, tumor growth in mice treated with 6E6 was inhibited by almost 80 percent compared to a control group, indicating that 6E6 has therapeutic potential. The researchers are currently investigating the mechanism of this therapeutic effect, and their preliminary data suggests that 6E6 recruits a subset of immune cells known as natural killer cells to cause cell death in the tumors.
The researchers have also mapped the binding interactions between 6E6 and unglycosylated RON. They found that 6E6 recognizes and binds to a sequence of three amino acids in the alpha chain of RON; these amino acids form a loop constrained by a chemical bond known as a sulfydryl link.
“Our research highlights the importance of looking into glycosylation changes in neoplastic cells compared to normal cells,” said Xin Yu Koh, a Postdoctoral Research Fellow in Lane’s lab and the first author of the study. Going forward, the team aims to test their antibodies in more advanced preclinical animal models and explore combinational therapies.