Figure 1.

The miR-106b∼25 cluster is upregulated in drug-resistant cells. (a) Doxorubicin-resistant cells (MD60) bypass drug-induced senescence. SA-β-galactosidase staining after treatment with 60 ng/ml doxorubicin for 24 h followed by 5 days drug-free culture. Percentage of senescent cells is indicated after monitoring at least six fields of view (typical variation approximately 10%). Bar represents 30 μm. (b) Cell cycle profiles by flow cytometry after propidium iodide staining. Both MTMEC and MD60 cells showed an initial G2 arrest after exposure to doxorubicin for 24 h (middle panels), but MTMEC cells did not proliferate any further, with some cells showing a tetraploid DNA content (upper right panel), whereas drug-resistant MD60 cells did resume proliferation and showed a cell cycle profile (lower right panel) similar to that of untreated cells (left panels). X axis represents DNA content and y axis cell number. (c and d) Expression of miRs 106b, 93 and 25 was determined by reverse transcription and real-time PCR using TaqMan probes and was normalized to the expression of U6 RNA. Expression was determined in previously described models from pre-tumorigenic ESFs⁵ (c) and (d) drug-resistant derivatives of MTMEC cells (d). Data represent the average±S.D. of three different experiments (*P<0.05). In all cases, expression data show fold upregulation in drug-resistant cells with respect to drug-sensitive cells