p15 and p21 inhibition recapitulates miR-155 suppression of TGF-β1 signals in DLBCL. (A) Real-time RT-PCR quantification of P15 and P21 in 3 DLBCL cell lines shows a dominant expression of P15 in Ly7 and P21 in Ly18 and DHL5 (P < .01, Student t test). (B) Western blot analysis of hypo-pRB was performed in the DLBCL cell line Ly7 transiently expressing an siRNA control or 2 sequences specific for p15. Upon TGF-β1 exposure (5 ng/mL), the abundance of the hypo-pRB (active RB) isoform increases in the control siRNA Ly7 cells, but not in their p15 KD isogenic counterparts. (C-D) Western blot analyses of hypo-pRB were performed in the DLBCL cell lines Ly18 and DHL5 transiently expressing an siRNA control or 2 sequences specific for p21. Upon TGF-β1 exposure (5 ng/mL), the abundance of the hypo-pRB (active RB) isoform increases in the control siRNA cells, but not, or to a lesser extent, in p21 KD cells. The effectiveness of the siRNA-mediated p15 and p21 suppression is shown in supplemental Figure 7. β-actin imunoblotting was used to confirm proper loading. The data shown in this figure were confirmed in 1 to 3 biological replicates.