Accumulation of mechanical forces in tumors is related to hyaluronan content and tissue stiffness

<div><p>Hyaluronan is abundant in the extracellular matrix of many desmoplastic tumors and determines in large part the tumor biochemical and mechanical microenvironment. Additionally, it has been identified as one of the major physiological barriers to the effective delivery of drugs to solid tumors and its targeting with the use of pharmaceutical agents has shown to decompress tumor blood vessels, and thus improve tumor perfusion and efficacy of cytotoxic drugs. In this study, we investigated the contribution of hyaluronan to the accumulation of mechanical forces in tumors. Using experimental data from two orthotopic breast tumor models and treating tumors with two clinically approved anti-fibrotic drugs (tranilast and pirfenidone), we found that accumulation of growth-induced, residual forces in tumors are associated with hyaluronan content. Furthermore, mechanical characterization of the tumors revealed a good correlation of the accumulated forces with the elastic modulus of the tissue. Our results provide important insights on the mechano-pathology of solid tumors and can be used for the design of therapeutic strategies that target hyaluronan.</p></div

Swelling stress agrees well with growth-induced stress.

<p>The good correlation of the two types of stress is given by the good fit along the y = x dash line. Error bars represent the range of estimated growth-induced stress.</p

Schematic of tumor opening experiment and calculations.

<p><b>A</b>: Typical experimental procedure showing the tumor before and after the cut has been made. The measured tumor opening appears in the figure. <b>B</b>: Representative computational results in the beginning and at the end of the simulation. In the model, the tumor consists of two domains, the tumor and a peripheral layer with thickness 5% of the tumor diameter. The simulations were used for the calculation of the growth-induced stress from the measured displacement/opening of the tumor.</p

Effect of ECM composition and mechanical properties on growth-induced stress.

<p>(<b>A</b>) Collagen or (<b>B</b>) hyaluronan (HA) area fraction is not associated with growth-induced stress, whereas a relation seems to exist when (<b>C</b>) the ratio of HA/collagen area fraction is employed. (<b>D</b>) Growth-induced stress does not depend on tumor opening but there is a good correlation between growth-induced stress and tumor elastic modulus (<b>E</b>) as well as with the product of tumor opening and elastic modulus (<b>F</b>). Error bars represent the range of estimated growth-induced stress.</p

Schematic of growth-induced stress and swelling stress.

<p><b>A:</b> Growth-induced stress, <b>σ</b>^g, is equal to the stress required to close the tumor after the tumor relaxes and the stress is released. <b>B:</b> Swelling stress, <b>σ</b>^<b>c</b>, is the stress required to compress tumor to initial radius from swelled tissue condition.</p

Effect of ECM composition on tumor opening.

<p>(<b>A</b>) Collagen area fraction does not correlate with opening, whereas there is a correlation of tumor opening with hyaluronan (HA) area fraction (<b>B</b>) and the ratio of HA/collagen area fraction (<b>C</b>). Five tumor specimens (n = 5) from each tumor type were used.</p

Role of Constitutive Behavior and Tumor-Host Mechanical Interactions in the State of Stress and Growth of Solid Tumors

<div><p>Mechanical forces play a crucial role in tumor patho-physiology. Compression of cancer cells inhibits their proliferation rate, induces apoptosis and enhances their invasive and metastatic potential. Additionally, compression of intratumor blood vessels reduces the supply of oxygen, nutrients and drugs, affecting tumor progression and treatment. Despite the great importance of the mechanical microenvironment to the pathology of cancer, there are limited studies for the constitutive modeling and the mechanical properties of tumors and on how these parameters affect tumor growth. Also, the contribution of the host tissue to the growth and state of stress of the tumor remains unclear. To this end, we performed unconfined compression experiments in two tumor types and found that the experimental stress-strain response is better fitted to an exponential constitutive equation compared to the widely used neo-Hookean and Blatz-Ko models. Subsequently, we incorporated the constitutive equations along with the corresponding values of the mechanical properties - calculated by the fit - to a biomechanical model of tumor growth. Interestingly, we found that the evolution of stress and the growth rate of the tumor are independent from the selection of the constitutive equation, but depend strongly on the mechanical interactions with the surrounding host tissue. Particularly, model predictions - in agreement with experimental studies - suggest that the stiffness of solid tumors should exceed a critical value compared with that of the surrounding tissue in order to be able to displace the tissue and grow in size. With the use of the model, we estimated this critical value to be on the order of 1.5. Our results suggest that the direct effect of solid stress on tumor growth involves not only the inhibitory effect of stress on cancer cell proliferation and the induction of apoptosis, but also the resistance of the surrounding tissue to tumor expansion.</p></div

Experimental measurements for the elastic modulus and tan(<i>δ</i>) of the two tumor types.

<p>Experimental measurements for the elastic modulus and tan(<i>δ</i>) of the two tumor types.</p

Values of the mechanical properties of the two tumor types derived by fitting the model to the experimental stress-strain curves.

<p>Standard errors are shown in parenthesis.</p>a<p>The Poisson’s ratio was taken to be 0.2 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0104717#pone.0104717-Roose1" target="_blank">[22]</a>.</p

Effect of tumor-host mechanical interactions on tumor state of stress and growth.

<p>Dependence of A) state of stress and B) growth rate of tumors on the mechanical properties of the host tissue. The host tissue was modeled as a compressible neo-Hookean material with Poisson’s ratio of 0.2 and three values of the shear modulus were used, <i>µ</i> = 10, 15 and 30 kPa. The stiffer the host tissue is, the higher the stress in the tumor and the lower its growth rate becomes.</p