Fig. S1. Adhesion of cells either deposited on glass surface (glass) or on E0, E50, E200, E500, 0/2, 0/5 and 0/12. Cell adhesion was monitored after 4h of cell culture by acid phosphatase activity (AP) measured using the pNPP assay. Value of 100% corresponds to AP activity of cells cultured on glass. Error bars represent the s.e.m. derived from two independent experiments (triplicate well determinations, 4 measurements per well): *statistically significant difference (P<0.001, ANOVA, Tukey Test).

Fig. S2. Vinculin and actin on stiff substrates. (A) 4h of culture on glass surface (a, b), (0/2) (c, d), (0/5) (e, f) and (0/12) (g, h). Cells were labelled with phalloidin (a, c, e, g) and immunolabelled with anti-vinculin (b, d, f, h). Scale bar: 20 m. (B) Cells 4h cultured on (E200) (a) and (E500) (b) were immunolabelled with anti- phospho-FAK-Y397 (P-FAK-Y397) (a, b). Representative data from two independent experiments. Scale bar: 20 m. (C) Western blots of p44/p42 MAPK, phospho- p44/p42 MAPK (P-p44/p42 MAPK), FAK (FAK) and phospho-FAK-Y397 (P-FAK-Y397) protein contents in cells seeded 4h on (0/2), (0/5) and (0/12) films. Representative results from at least three independent experiments.

Fig. S3. Replication on substrate of 500 kPa (A) BrdU incorporation was visualized by indirect immunofluorescence in cells seeded 4h on (0/2) (a), (0/5) (b), (0/12) (c) and glass (d). Scale bar: 20 m. Representative data from three independent experiments. (B) cells seeded on (E500) films were fixed 1h (a, h), 2h (b, i), 3h (c, j), 4h (d, k), 5h (e, l), 6h (f, m) and 7h (g, n) after synchronization. Cells with anti-vinculin (a-g) with anti-BrdU (h-n). Scale bar: 20 m.

Fig. S4. Rac 1 with respect to elastic modulus. (A) cells were seeded on glass (a, b, c) and on (E500) (d, e, f). Cells were immunolabelled with anti-Rac1 (a-f). Cells were fixed 3h (a, d), 4h (b, e), and 7h (c, f) after synchronization. Scale bar: 20 m.

Fig. S5. (A) Transcription on stiff substrates 4h of culture on (0/2) (a), (0/5) (b), (0/12) (c) and glass (d). Cells were immunolabelled with anti-BrdU. Representative results from three independent experiments are shown. Scale bar: 20 m. (B) Cells were immunolabelled with anti-caspase-3, 4h of culture on (E0) (a, b), (E50) (c, d), (E200) (e, f) and E500 (g, h). Representative results from two independent experiments. Scale bar: 20 m. (C) Fluorescence intensity for caspase-3 signal (arbitrary units, A.U. in %) using imageJ, based on Ca, c, e, g: 300 cells were analysed per condition. Results are presented as s.e.m. for two independent experiments.