miR-155 limits the TGF-β1–mediated induction of P15 and P21 and cell cycle arrest in DLBCL. (A) Ectopic expression of miR-155 in 3 DLBCL cell lines decreased the expression of its target SMAD5. (B) DLBCL cell lines stably expressing an empty vector (MSCV) or miR-155 were exposed to 5 ng/mL of TGF-β1 or vehicle control (-) for 1 hour. Quantitative real-time RT-PCR measurements of P15 (upper panel) and P21 (bottom panel) showed a significantly impaired induction of these genes in miR-155–expressing cells than in their isogeneic controls (P < .05, Student t test). Data shown are mean ± standard deviation of cells exposed to TGF-β1 normalized by vehicle-treated cells of a representative assay performed in triplicate; 2 to 4 biological replicates of this experiment were completed. (C) Expression of miR-155 rendered 3 DLBCL cell lines significantly less sensitive to TGF-β1–induced cell cycle arrest than their isogenic controls (P < .01, Student t test). Data shown are the percentage of cells that were arrested in G0/G1 following exposure to TGF-β1 for 48 hours in a representative assay performed in triplicate. Specifically, the display shows the percentage of TGF-β1–exposed minus vehicle-exposed cells in G0/G1. The complete cell cycle profile is shown in supplemental Figure 3. The results of this assay were confirmed in at least 3 biological replicates.