Tissue Stiffness Drives Tumor Formation

(Philadelphia, PA) - The relationship betweentissue rigidity and tumor formation is fairly well established;however, what is not so well understood is what happens on a molecularlevel that contributes to such stiffness. Now, for the first time,researchers at the University of Pennsylvania School of Medicine haveshown that tumor formation is generated by a complex interaction ofboth mechanical as well as chemical signals, and the resulting tissuestiffening induces molecular signals that promote the cancerousbehavior of cells. Penn's interdisciplinary research team-drawn fromthe fields of Biomedical Engineering and Cell and DevelopmentalBiology-has demonstrated clearly that force, growth, and tumor behaviorare inextricably linked and this enhanced understanding of thenecessary fusion of these factors may lead to the development of newtumor therapies or targets.

"This study identifies the connection between oncogenes and themechanics of the cell and its microenvironment in animal and cultureexperimental models," explains senior author Valerie Weaver, PhD,Assistant Professor of Pathology and Laboratory Medicine."Specifically, we have defined the vitality of mechanical force as anintegral factor in tumor development." Weaver and colleagues publishedtheir findings as the cover-story in the September issue of CancerCell.

Weaver and first authors, Matthew J. Pascek and NastaranZahir-both graduate students in Bioengineering-used a three-dimensionalgel on which they grew breast cancer cells and could precisely controland measure the stiffness of the surrounding microenvironment. "Wefound that tissue rigidity enhances cell growth and destroys tissueorganization to promote tumor-like behavior in normal cells," saysPascek. "This happens because the stiffness helps to activate keygrowth signaling pathways and increase cell tension."

Cells use surface receptors called integrins to communicatewith the outside tissue environment, which consists primarily ofconnective tissue. Integrins regulate cell growth, death, and movement,as well as tissue organization. Integrins also play a role in celldivision and proliferation through ERK, an extracellularsignal-regulating molecule. Despite the fact that integrins werediscovered and their activity found to be aberrant in tumors decadesago, how integrins could become altered and their importance to cancerhas remained contentious among researchers.

Weaver and colleagues found that tissue stiffness inducestumor-like behavior in cells through ERK and Rho, another regulatorymolecule. Although researchers have long appreciated that oncogenessuch as Ras and Erb2 drive cell growth via the ERK pathway, this studyshowed how high levels of ERK also prime a cell to contract more viaintegrins.

Integrin activity also regulates the Rho molecular pathway,which in tumors regulates the stiffness of the cytoskeleton, acollection of protein filaments within a cell that give shape and thecapacity for directed movement. When the researchers increased thestiffness of the gel in which experimental cells were grown, Rhoactivity increased, as well as the number and size of focal adhesions,clusters of integrins that create a connection between integrins andthe cytoskeleton.

Overall, the researchers found that a self-perpetuating programof tissue destruction is set up-through changed integrin signaling-tocreate a double-pronged drive toward aberrant cell behavior. Both thestiffness of connective tissue surrounding developing tumors and theincreased activity or expression of oncogenes can promote cells tobecome cancerous. For example, the researchers found that as stiffnessincreased in connective tissue, the cells of a normal breast ductstarted to grow atypically, causing the structure of the duct todegrade, as the uncontrolled cell growth of duct-lining cells invadedthe duct tube.

The researchers also discovered that when cell tension becomesgreat enough, it overrides normal tissue behavior, but is reversible."We showed that some breast tumors with elevated signaling for thegrowth factor ERK also have high tension and that their behavior wouldreturn to normal by inhibiting cell tension," says Zahir. With thisknowledge, Weaver's group is now looking to see whether drugs thatinhibit cell contractility could help prevent early metastasis. Theyare also fine-tuning how different cell types react to different levelsof stiffness and how this is important for normal cell behavior, aswell as aberrant activity and structure.

This research was funded by the Department of Defense and theNational Institutes of Health. Co-authors are Kandice R. Johnson,Johnathon N. Lakins, Gabriela I. Rozenberg, Amit Gefen, Cynthia A,Reinhart-King, Susan S. Margulies, David Boettinger, and Daniel A.Hammer, all from Penn; as well as Mica Dembo from Boston University.