Distinctive Mechanism for Sustained TGF-b Signaling and Growth Inhibition: MEK1 Activation-Dependent Stabilization of Type II TGF-b Receptors

There are multiple mechanisms by which cells evade TGF-b–mediated growth inhibitory effects. In this report, we describe a novel mechanism by which cells become resistant to TGF-b–mediated growth suppression. Although having all the components of the TGF-b signaling pathway, different cell lines, RL, HaCaT, and BJAB, have different sensitivities toward TGF-b–induced growth suppression. The TGF-b resistance of RL, a B-cell lymphoma cell line, was due to ligand-induced downregulation of TGF-b receptor II (TbRII) and only transient TGF-b induced nuclear translocation of Smad2 and Smad3. With low-dose phorbol 12-myristate 13-acetate (PMA) or anti-IgM treatment, TGF-b sensitivity was restored by stabilizing TbRII expression and sustaining TGF-b signaling. The MEK inhibitor, U0126, blocked both PMAand anti-IgM–induced upregulation of TbRII. In HaCaT and BJAB, two TGF-b–sensitive cell lines, which had higher basal levels of phospho-MEK and TbRII compared with RL, U0126 induced downregulation of TbRII and blocked subsequent TGF-b signaling. Similar results were also obtained with normal B cells, where MEK1 inhibitor downregulated TbRII and subsequent TGF-b signaling. Constitutively active MEK1, but not constitutively active ERK2, induced upregulation of TbRII. Furthermore, TbRII physically interacted with the constitutively active MEK1, but not with wild-type MEK1, indicating involvement of active MEK1 in stabilizing TbRII. Collectively, our data suggest a novel mechanism for MEK1 in regulating the sensitivity to TGF-b signaling by stabilizing TbRII. Mol Cancer Res; 9(1); 78–89. 2010 AACR.