FN was isolated from human plasma, FRET-labeled, and characterized according to previously published protocols (24 (link)). In short, FN-DA was double-labeled with Alexa Fluor 546 C-5 maleimide (A10258, Thermo Fisher Scientific) as acceptor on free cysteines, and with Alexa Fluor 488 succinimidyl ester (A20100, Thermo Fisher Scientific) as donor on lysines, and stored in PBS (0.5 mg/ml) at −80°C. The labeling ratio was determined by UV absorption measurements as 3.9 acceptors and 8.8 donors per molecule, respectively. The FN-FRET probe was calibrated as reported previously (24 (link), 25 (link)) by measuring the FRET intensity ratios upon denaturation by different concentrations of GdnHCl in solution. For FN-FRET experiments, FN-DA was mixed with unlabeled FN at a ratio of 1:9 to prevent intermolecular energy transfer and added to the culture medium at the specified time point at a concentration of 50 μg/ml.