miR-155 controls TGF-β1–mediated regulation of RB and cell cycle progression in normal mature B lymphocytes. (A) Western blot analysis in mature B lymphocytes isolated from 4 pairs of WT/miR-155 KO mice shows a consistently higher SMAD5 expression in miR-155 null cells. (B) Western blot–based examination of the phosphorylation levels of RB’s Ser780 following TGF-β1 (4 pairs of mice) or BMP4 (2 pairs of mice) exposure depicts a more pronounced decrease in this residue’s phosphorylation, and consequently RB activation, in miR-155 null cells. β-actin immunoblotting was used as loading control. Densitometric quantification of the SMAD5 (A) or pRB Ser780 (B) is shown under each panel. (C) Homozygous deletion of miR-155 rendered mature B lymphocytes significantly more sensitive to TGF-β1–induced cell cycle arrest (left panel), with consequent decrease in cells in the S phase (right panel) (P < .05, Student t test). On the left, data shown are the mean ± standard deviation of the percentage of B lymphocytes that were arrested in G0/G1 following exposure to TGF-β1 for 48 hours, in 4 pairs of WT vs miR-155 KO mice. On the right, the percentage of cells in S phase after TGF-β1 exposure is shown. The data displayed in this figure represent 4 biological replicates, that is, the comparison between 8 animals, 4 miR-155 WT and 4 KO mice. The complete cell cycle profile is shown in supplemental Figure 8.