Integrins sense extracellular-matrix rigidity to regulate mammary-epithelial tissue phenotype

Tumors are stiffer than normal tissue, but whether this biophysical alteration actively contributes to malignant behavior has not been assessed. Cellular integrins physically interact with the tissue environment by tugging on the extracellular matrix (ECM). Because integrins are mechanotransducers that regulate cell fate, we asked if tissue stiffness could regulate mammary epithelial tissue behavior by modulating integrin adhesion assembly. We found that compliant matrices equivalent in stiffness to the normal mammary gland induce the assembly of focal complexes that supported normal morphogenesis. Rigid matrices, on the other hand, drove focal adhesion maturation to disrupt tissue morphogenesis and enhance epithelial growth. We determined that the focal complex to focal adhesion transition is triggered by integrin clustering. To better understand how matrix stiffness influences integrin clustering, we built a stochastic model of integrin-ligand interactions within a deformable cell-ECM interface. With this model, we showed that integrins spontaneously cluster on rigid matrices through a glycocalyx-mediated mechanism that does not require cytoskeletal adaptors or cell contractility. Increasing matrix compliance in the model, however, diminished integrin clustering. Experimentally, we determined that clustering ultimately was linked to focal adhesion maturation by a tension-dependent feedback circuit. Specifically, integrin clustering stimulated ERK and Rho activation, which in turn increased cytoskeletal contractility to drive focal adhesion assembly and additional integrin clustering. Matrix rigidity was thus sensed, at least in part, by a myosin-independent, integrin-clustering mechanism and a myosin-dependent, cell-signaling circuit. Perturbation of any aspect of this mechano-sensory process compromised tissue phenotype. Indeed, enhancing matrix rigidity, inducing integrin clustering, or increasing ERK- or Rho-mediated tension disrupted epithelial morphogenesis in 3D culture models. Remarkably, we found that highly-contractile, EGF-transformed epithelia with high ERK and Rho activity could be phenotypically reverted to tissues lacking focal adhesions if Rho-contractility or EGF was decreased. These studies suggest new possibilities for cancer intervention based on normalizing the mechano-biology of the tumor.