MicroRNA-155 controls RB phosphorylation in normal and malignant B lymphocytes via the noncanonical TGF-β1/SMAD5 signaling module

MicroRNA-155 (miR-155) plays pleiotropic roles in the biology of normal and malignant B lymphocytes, including the modulation of the transforming growth factor β (TGF-β) pathway via the targeting of SMAD5. However, the extent of the miR-155-mediated disruption of the TGF-β1/SMAD5 axis remainsto be elucidated. To address this issue, we used the miR-155 knockout (KO) mouse and diffuse large B-cell lymphoma (DLBCL) cell lines ectopically expressing miR-155. In the DLBCL models, expression of miR-155 blocked TGF-β1-mediated activation of the retinoblastoma protein (RB), decreasing the abundance of the inhibitory pRB-E2F1 complex and limiting G0/G1 arrest. Genetic knockdown of SMAD5, p15, or p21 recapitulated these effects, establishing a circuitry whereby the targeting of SMAD5 by miR-155 blunts the TGF-β1-induced transcription of p15 and p21, thus sustaining RB phosphorylation and inactivity. Next, we demonstrated that SMAD5 levels are elevated in mature Blymphocytes from the miR-155KO mice, which displaya heightened sensitivity to TGF-b1 characterized by suppression of RB phosphorylation and more pronounced G0/G1 cell cycle arrest. Our findings suggest thatamiR-155-mediated perturbation of the RB/E2F axis may play a role in DLBCL pathogenesis, and contribute to the reduced number of germinal center B cells and impaired T cell-dependent antibody response found in the miR-155 KO mice.