Solid stress inhibits the growth of multicellular tumor spheroids

Introduction

The growth, morphogenesis, and homeostasis of tissue are tightly regulated by the stress field; examples can be found in the bone (external load) and blood vessel (systemic pressure). Vice versa, stress can be induced by cellular processes; examples are traction forces and residual stresses in tissues.

Researchers hypothesized that the stress may affect

• Tumor growth rate
• Growth pattern in vitro/in vivo
• Tumor physiology and blood flow - vessel wall collapse, chronic vascular/lymphatic occlusion
• Metastasis

However, the stress generated was difficult to quantify and we did not know how the stress affects proliferation/apoptosis/cellular density. The tumor growth in vivo also involves a gradual displacement of the surrounding matrix.

In this study, the authors addressed these concerns by seeding tumor cells in an agarose gels (the cells formed spheroids) - an in vitro study. The effect of the external stress was simulated by deformation of the gel. Also, when the spheroids grow, they generated stress, which can be calculated from the size of the spheroids and properties of the gel.

Techniques used:

• TUNEL (Terminal deoxynucleotidyl transferase dUTP Nick End Labeling) - apoptosis
• PCNA (Proliferating Cell Nuclear Antigen) marker - proliferation
• PI (Propidium iodide) staining - cell density

Results

Solid stress > spheroid growth?

Increasing agarose concentration increases initial stiffness of the matrix.

• Low concentrations (<0.8%): Growth rates and final sizes were shown to not change much when the concentration changed, but were significantly lower than free-suspension controls.
• High concentrations (>0.9%): Growth was inhibited, lower clonal efficiency (?)
  • Threshold for significant growth inhibition at 0.9-1% conc. Sizes not decrease progressively.

So, increase in initial matrix stiffness does accelerate response to growth-induced stress, as shown by spheroid growth kinetics and clonal efficiency.