Cancer is a major cause of death that threatens human health worldwide1. It has been estimated that there are 19.3 million new cancer cases worldwide in 2020, which will rise to 30.2 million by 2040.2 Amount various factors that cause cancer death, metastasis has been found to be responsible for approximately 90% of cancer deaths3. Studies have indicated that the tumor microenvironment (TME) plays a key role in tumor metastasis4. TME is composed of stromal cells, extracellular matrix (ECM) and biochemical factors, that differs from normal tissue microenvironment in composition, microstructure and physical properties5. Cancer-associated fibroblasts (CAFs) originated from activated fibroblasts are the major stromal cells in TME6, that boost tumor growth and progression7. CAFs promote tumor metastasis via secreting of soluble cytokines producing chemotactic gradients facilitating tumor cell migration.8, 9 In addition, CAFs are responsible for secretion of matrix proteins that stiffen the matrix10, 11 or for matrix remodeling by generation of tracks within the matrix that enhance tumor cell migration.12, 13 In a typical case, upon forming of cell–cell adhesion, CAFs have been demonstrated to promote invasion and metastasis of carcinoma cells through applying a physical force to drag tumor cells14, 15. Therefore, CAFs, as the abettor of tumor metastasis,16 has become an important target for cancer therapy.

Normally, only activated fibroblasts--CAFs can perform various cancer-promoting functions while normal fibroblasts (NFs) do not17, 18, 19. It has been demonstrated that ECM generated by NFs is softer than that secreted by CAFs18. For instance, in pancreatic tumor stroma, upon activation with transforming growth factor-β (TGF-β), pancreatic stellate cells (PSCs) are activated and then differentiate into α-SMA-expressing myofibroblast-like CAFs and produce abundant ECM20. As pancreatic cancer progresses, the matrix stiffness of the tissue becomes more rigid thus facilitated invasion and metastasis21. Therefore, attempt has been made to inhibit the cancer-promoting functions of CAFs by direct elimination of CAFs in the TME22. Unfortunately, rather than inhabitation of their cancer-promoting functions, the depletion of CAFs leads to more aggressive tumors in mice and a significant decrease in the survival rate22. The failure in direct elimination of CAFs has rose the trial by reversing of activated fibroblasts to the quiescent state using gold base nanoparticles23. This induced quiescence of PSCs has significantly enhanced the anti-tumor efficacy upon therapeutics.24 Therefore, we envision that the inhibitory effect on tumor progression can be achieved by reversing the activated CAFs to NFs, a process named CAF reprogramming.

Cell reprogramming refers to a process of resetting cell identity while keeping the gene sequence unchanged.25 Since the concept proposed at 1950s, various methods have been established to reprogram cells, including biochemical (e.g. nuclear transplantation26, lentiviral or retroviral transduction27, 28, growth factors29, small molecules30) and mechanical methods (e.g. topography31, substrate stiffness32, and nucleus compression33). Compared to biochemical methods, mechanical methods have advantages for no genetic integration and instantaneous transfection. These mechanical methods take effects based mainly on the principle of tuning the cellular mechanotransduction including modulation of force-bearing transmembrane receptors. For example, integrin (a force sensitive receptor that senses ECM stiffness) based mechanotransduction has been applied for cell reprogramming by engineering receptor adhesive biomaterials. Exposure of tail-tip fibroblasts on soft three-dimensioned polyethylene glycol (PEG) hydrogels resulted in iPSCs generation34. Polyacrylamide substrate presenting RGD peptide (an integrin binding ligand) increased the efficiency by two folds to reprogram fibroblasts to induced cardiomyocyte-like cells.35 In conclusion, it is possible to mechanically reprogram CAFs into their quiescent state by modulating of force-bearing transmembrane receptors.

Design biomaterials presenting ligands for integrin and cadherin (transmembrane receptor that mediates cell–cell interaction) ligations enable reversing of already differentiated stem cells.36, 37 Therefore, we hypothesized that modulating these force-sensitive transmembrane receptors by manipulating of ligands can be a promising strategy for cell reprogramming. We first confirmed that N-cadherin mediates intercellular interaction between activated PSCs. We then constructed a PEG hydrogel system with tunable substrate stiffness and adhesion ligands (with integrin/RGD ligation mimicking cell-matrix interaction and N-cadherin/HAVDI ligation mimicking cell–cell interaction). Next, we explored whether mechanical modulation of N-cadherin/HAVDI ligation enables the reprogramming of activated PSCs into their quiescent state and the mechanical dosing dependent efficiency of reprogramming. Finally, we discussed the differentiation potential of the reprogrammed cells. This study sheds light on the new ways of CAF reprogramming by modulation of their force-sensitive transmembrane receptors and thus provides a promising guidance for clinical treatment of cancers.