Tweaking cancer cell response to ultrasound treatment

Cells have surface receptors called integrins that bind to repetitive domains present on the extracellular matrix (ECM) surrounding the cells, allowing them to grow and spread. A new study from the Department of Bioengineering (BE), Indian Institute of Science (IISc) and collaborators shows that tweaking the spacing between these binding domains on the ECM can boost the efficiency of ultrasound treatment applied to kill cancer cells.

“In a normal tissue, the spacing on the ECM is around 50-70 nanometres (nm), but in the tumour microenvironment, severe choking occurs due to excessive ECM secretion, which may reduce the binding spacing to below 50 nm,” explains Ajay Tijore, Assistant Professor in BE and corresponding author of the study published in Nano Letters. “We found more cancer cells being killed when the binding spacing is increased to around 50-70 nm.” 

Low-frequency ultrasound waves (39 kHz) can disrupt the cell membrane and trigger cell death in cancer cells. It is a relatively low-cost and non-invasive approach. Unlike normal cells, cancer cells do not have repair mechanisms that help them withstand the mechanical forces exerted by ultrasound waves. “Our work focused on fundamental understanding of ultrasound-mediated cancer cell killing and the effect of the ECM binding spacing on it,” explains S Manasa Veena, PhD student in BE and first author.

To mimic the integrin-ECM binding, the team constructed an array of gold nanodots separated by different distances (35, 50 and 70 nm) and allowed highly invasive cancer cells to attach to them. Then, they applied pulsed ultrasound waves.

When ultrasound was applied to cancer cells grown on the 50 nm and 70 nm platforms, their cell membranes were found to stretch due to forces exerted by a filament protein called myosin. More extracellular calcium is then pumped into the cytoplasm, which damages mitochondria and promotes cell death. Reduced spacing (35 nm), however, prevented the cells from binding more efficiently to the nanodots, and generating sufficient myosin forces to trigger cell death. “Just like how we touch a surface to feel the texture, cancer cells need to ‘pinch’ the ECM by applying myosin forces to sense the ECM architecture,” Tijore explains. 

The team realised that if there was a way to increase this spacing or mimic its effect, cancer cells might respond better to other treatments as well. 

“While doing these experiments, we stumbled upon literature on a drug called Cilengitide,” explains Veena. The drug works by blocking integrin-ECM binding. “It was one of the most widely studied drugs and went to Phase III clinical trials, but it failed." 

Using the same setup, the team decided to test what happens when a very low dose of Cilengitide – about 1,000 times lower than used in the trials – is given along with ultrasound treatment. “When the dose is very low, there are not enough drug molecules to bind all the integrin receptors. The cancer cell now starts thinking that the 35 nm spacing has increased,” Tijore explains. “We are fooling the cancer cell into thinking that the spacing has changed. Now the cell starts developing myosin forces, pumps more calcium inside, and this triggers cancer cell killing.”

Tijore’s team has collaborated with clinicians to test this combination approach on oral cancer tissue samples. “Oral cancer is a major problem in the Indian subcontinent. There is a lot of ECM deposition, leading to swelling and inflammation … an extreme choking of the tumour microenvironment,” he says. “This is what we are currently working on.”