Substrate stiffness modulates integrin α5 expression and ECM-associated gene expression in fibroblasts

Biomaterials, like polydimethylsiloxane (PDMS), are soft, biocompatible, and tuneable, which makes them useful to delineate specific substrate factors that regulate the complex landscape of cell-substrate interactions. We used a commercial formulation of PDMS to fabricate substrates with moduli 40 kPa, 300 kPa, and 1.5 MPa, and cultured HMF3S fibroblasts on them. Gene expression analysis was performed by RT-PCR and Western blotting. Cellular and nuclear morphologies were analyzed using confocal imaging, and MMP-2 and MMP-9 activities were determined with gelatin zymography. Results, comparing mechanotransduction on PDMS substrates with control petridishes, show that substrate stiffness modulates cell morphologies and proliferations. Cell nuclei were rounded on compliant substrates and correlated with increased tubulin expression. Proliferations were higher on stiffer substrates with cell cycle arrest on softer substrates. Integrin alpha5 expression decreased on lower stiffness substrates, and correlated with inefficient TGF-beta; activation. Changes to the activated state of the fibroblast on higher stiffness substrates were linked to altered TGF-beta; secretion. Collagen I, collagen III, and MMP-2 expression levels were lower on compliant PDMS substrates as compared to stiffer ones; there was little MMP-9 activity on substrates. These results demonstrate critical feedback mechanisms between substrate stiffness and ECM regulation by fibroblasts which is highly relevant in diseases like tissue fibrosis.