The mechanical response of a tumour to high-frequency oscillation conveys information about its metastatic potential and response to therapy, researchers at the University of Lyon have shown. Jérémie Margueritat and colleagues probed the viscoelastic properties of cultured cancer cells by measuring how spontaneous acoustic waves affected the scattering of incident laser light (Phys. Rev. Lett. 10.1103/PhysRevLett.122.018101).
The researchers exploited a phenomenon called Brillouin light scattering (BLS), in which naturally occurring pressure waves change the medium’s local refractive index. As the size of the effect is determined by the material’s viscoelastic modulus, small-scale variations in the frequency of backscattered light give a high-resolution image of the sample’s mechanical properties.
In this case, Margueritat and colleagues shone a monochromatic laser onto spheroids (3D cell cultures) derived from colorectal carcinoma lines. Focusing the beam to a spot 10 μm across, the researchers measured the behaviour of poroelastic units of just a few cells.
A tumour’s mechanical properties arise from two components, which each react differently to stress at high and low frequencies.
“A tumour is composed of a rigid frame formed from the connected cytoskeleton. This is invaded by biological fluids, pretty much like a sponge,” explains co-author Thomas Dehoux. “If you compress this sponge slowly (low frequency) the fluids can escape freely, and the resistance to deformation is only borne by the frame (cytoskeleton). If you compress the sponge very fast, the fluids have no time to escape, and the resistance to deformation comes from the frame and the compressibility of the fluids that are trapped within the tumour.”
Investigations in the slow-compression regime have already suggested the importance of the elastic framework to tumour growth and treatment response, leading to therapies that aim to disrupt proteins governing intercellular adhesion. The fluid contribution at GHz frequencies, in contrast, has been relatively unstudied until now because of instrumental limitations.
“The interferometers used to analyse changes in the wavelength of the light that exits the sample were slow and required a lot of adjustments,” says Dehoux. “BLS thus remained used mostly in solid-state physics. Thanks to recent improvements in acquisition time, such techniques are now increasingly used in biophysics.”
This new approach revealed a marked heterogeneity across the cell spheroids in response to high-frequency deformation. The researchers found that spheroids consistently exhibited a distinct feature at their edges, defined by a step-like change in the storage and loss moduli — quantities that describe how the material stores and dissipates energy.
This structure is important for what it says about the likelihood that a tumour will proliferate, says Dehoux: “We have shown in a previous paper that this peripheral layer contains live cells that can potentially form tumours elsewhere. In less metastatic tissues, this layer is reduced, suggesting that the presence of the layer is linked to metastatic potential. We are now exploring the possibility to quantify metastatic potential from the probing with BLS of this process.”
Response monitoring
The technique also offers insight into processes that occur during chemotherapy. When the researchers bathed the spheroids in a solution of fluorouracil — a commonly used cancer drug — disaggregation of the cells was reflected in a rapid lowering of the storage modulus at the edge of the structure. Meanwhile, the storage modulus at the centre of the tumour remained unchanged, indicating a greater resistance to treatment.
Accessing such information would let clinicians see quickly and in detail whether a cancer treatment was effective, allowing them to modify the course if necessary. Until now, only a crude measure of treatment progress has been available, based on overall tumour volume.
“We are working to design an in vivo endoscopic probe that would allow measurement of the mechanical signature of a tumour in situ,” says Dehoux. Tumours start to change as soon as they are removed, so this is an important consideration. “Another main point is that our technique is label-free: it does not require fluorescent tags that would potentially alter the normal tissue physiology and response to drugs.”
