RhoB, Not RhoA, Represses the Transcription of the Transforming Growth Factor β Type II Receptor by a Mechanism Involving Activator Protein 1

Abstract

The transforming growth factor-␤ (TGF-␤) type I (T␤R-I) and type II (T␤R-II) receptors are responsible for transducing TGF-␤ signals. We have previously shown that inhibition of farnesyltransferase activity results in an increase in T␤R-II expression, leading to enhanced TGF-␤ binding, signaling, and inhibition of tumor cell growth, suggesting that a farnesylated protein(s) exerts a repressive effect on T␤R-II expression. Likely candidates are farnesylated proteins such as Ras and RhoB, which are both farnesylated and involved in cell growth control. Neither a dominant negative HaRas, constitutively activated Ha-Ras, or a pharmacological inhibitor of MEK1 affected T␤R-II transcription. However, ectopic expression of RhoB, but not the closely related family member RhoA, resulted in a 5-fold decrease of T␤R-II promoter activity. Furthermore, ectopic expression of RhoB, but not RhoA, resulted in a significant decrease of T␤R-II protein expression and resistance of tumor cells to TGF-␤-mediated cell growth inhibition. Deletion analysis of the T␤R-II promoter identified a RhoB-responsive region, and mutational analysis of this region revealed that a site for the transcription factor activator protein 1 (AP1) is critical for RhoB-mediated repression of T␤R-II transcription. Electrophoretic mobility shift assays clearly showed that the binding of AP1 to its DNA-binding site is strongly inhibited by RhoB. Consequently, transcription assays using an AP1 reporter showed that AP1-mediated transcription is down-regulated by RhoB. Altogether, these results identify a mechanism by which RhoB antagonizes TGF-␤ action through transcriptional down-regulation of AP1 in T␤R-II promoter.

Interleukin‐1β impairment of transforming growth factor β1 signaling by DOWN‐REGULATION of transforming growth factor β receptor type II and up‐regulation of Smad7 in human articular chondrocytes

“…In our studies, using the more specific MEK inhibitor U0126, no effect of ERK-1/2 phosphorylation blockade was observed on the levels of TGF␤RII mRNA or protein, which suggests that another pathway is involved. Similar results have previously been observed in studies using the human pancreatic carcinomaderived cell line Panc1 (38). Moreover, SB203580, an inhibitor of the p38 pathway, decreased the expression of TGF␤RII to levels similar to those following IL-1␤ treatment.…”

Androgen Control of Cell Proliferation and Cytoskeletal Reorganization in Human Fibrosarcoma Cells

“…Interestingly, RhoGDI does not bind to RhoB, which is consistent with the finding that RhoB is always in association with membranes (57). Last, only RhoB has been shown to inhibit NFB signaling through stabilization of the IB complex (58), and to repress the transcription of TGF␤ type II receptors by a mechanism involving AP-1 (59). The RhoB signaling pathways in HT-AR1 cells are not known, but they could involve any number of Rho effector proteins that have been identified in other cells.…”

E-cadherin transcriptional downregulation by promoter methylation but not mutation is related to epithelial-to-mesenchymal transition in breast cancer cell lines

“…As an AP1 site has been identified in the TGFBR2 promoter, upregulation of FOSL1 and FOSB may stimulate TGFBR2 transcription. Interestingly, RHOB binds to the promoter of TGFBR2 and in this way prevents AP1-dependent transcription creating a negative feedback loop that regulates TGFb signal transduction (Adnane et al, 2002). RHOB was significantly downregulated in the 'Fibroblastic' cluster.…”