Lung Cell Mechanoresponse to Force Directionality Figure 2. Local cell stiffness depends on the directionality and integrin-specificity of the applied force in lung cells but not in PAA2+ gels. (A) Average resistance to compression (EC) and extension (ET) obtained in PAA+ gels with negatively-charged FE-AFM tips. (B) Corresponding EC/ ET data. The dashed line indicates the perfectly symmetric mechanoresponse (EC = ET). (C, E) EC and ET data probed with either bare or coated FE-AFM tips on (C) CCD-19Lu fibroblasts or (E) A549 parenchymal cells. (D, F) EC/ET data corresponding to (C) and (E), respectively. The legend of the bar filling patterns shown at the bottom apply to plots A–F. *P,0.05, **P,0.01 and ***P,0.005 were determined by paired Student’s t-test with respect to (w.r.t.) compression. +P,0.05, ++P,0.01 and +++P,0.005 were determined by unpaired Student’s t-test w.r.t. bare. doi:10.1371/journal.pone.0032261.g002 observed after few minutes in culture [6,27,28], we controlled the force during step II to held RGD-coated tips in contact with CCD19-Lu fibroblasts for 5 min. A representative force recording versus time during step III and IV after applying force control in step II is shown in Figure S4. Using this protocol we observed an increase in the asymmetric ratio (EC/ET = 51617) more than five- fold compared to the average EC/ET value observed after 30 s of tip-cell adhesion (Figure 2D). This raise was due to the marked increase in EC (1966 kPa), whereas ET (0.5360.04 kPa) and the adhesion strength force F* (0.7960.04 nN) exhibited little or no PLoS ONE | www.plosone.org 5 February 2012 | Volume 7 | Issue 2 | e32261