To determine if the appearance of the cells in the dormant clones was due to cortically rearranged F-actin, a characteristic of non-transformed mammary epithelial cells [33], we stained them with phallacidin. Figures 1 a and b demonstrate Temsirolimus nmr that 74.1 + 7.8% of these very large, quiescent cells have parallel bundles of cortical actin as compared with 33.0 + 11.5% of cells in growing colonies. This difference is significant at p < 0.01 (Student’s t test). Analogously,

MCF-10A non-transformed, immortalized mammary epithelial cells incubated on fibronectin also have a cortical actin distribution. To test the hypothesis that the re-differentiated state depends on outside-in signaling through re-expressed integrin α5β1, we incubated the cells growing on fibronectin with blocking antibodies to this integrin. Figure 2 a demonstrates that cortical rearrangement of F-actin requires binding of integrin α5β1 by fibronectin, while blocking antibody to integrin α2β1, also upregulated in these dormant cells [3], has no effect and acts as a negative control. The increase in the percent cells with cortical actin from 28.6 + 0.9%

X-396 in vivo of growing cells to 67.9 + 6.6% of dormant cells and the decrease back to 21.6 + 8.5% due to blocking of integrin α5β1 binding are statistically significant (p < 0.005, p < 0.001, respectively, Fig. 2b). Antibody to integrin α2β1 had no effect with 66.0 + 13.2% of the cells having cortical actin (p > 0.05). Other characteristics of these dormant cells, including increased nuclear size (Fig. 2c) and increased cytoplasm to nucleus ratios (Fig. 2d) were also partially reversed by blocking

antibody to integrin α5β1. The mean longitudinal nuclear axis increased from 15.4 + 2.0 μm in growing cells to 26.7 + 3.7 μm in dormant cells (p < 0.001) and was reversed to 19.8 + 4.0 with blocking antibody to integrin α5β1 (p < 0.001). Blocking antibody to integrin α2β1 did not have an affect. Similarly, the mean square of the cytoplasm to nucleus ratios increased from 4.6 + 1.9 in growing cells to 17.2 + 10.7 in dormant cells and was reversed to 9.48 + 5.6 with blocking Tau-protein kinase antibody to integrin α5β1 (p < 0.001). Blocking antibody to integrin α2β1 did not have an affect on this characteristic either. Fig. 1 Cortical actin stabilization in dormant breast cancer cells. a MCF-7 cells incubated with or without FGF-2 10 ng/ml on fibronectin-coated cover slips for 6 days at clonogenic density were stained with BODIPY-Phallacidin (green actin staining) and DAPI (blue nuclear staining) and photographed at 400 x magnification (see Materials and Methods). A 20 μM size bar is included in all fluorescence photographs in all figures. MCF-10A cells were incubated on fibronectin-coated slides and stained in a similar manner as controls and demonstrate morphological similarity with dormant MCF-7 cells. Arrows indicate prominent places of cortical actin redistribution around the perimeter of the cytoplasm.