Transforming growth factor beta 1 (TGF-1) induces growth suppression in a variety of cell types. The signaling molecules that are responsible for transmitting TGF-b signal include Smad2, 3, and 4. Upon TGF-b engagement, Smad2 and 3 become phosphorylated and form heteromeric complexes with Smad4 and translocate to nucleus, where in association with transcriptional co-activators or repressors, binds to the promoters of TGF-b-responsive genes. In our laboratory, we are studying a diffuse large B-cell lymphoma cell line, DB that lacks TGF-b responsiveness with respect to growth suppression. Our goal is to identify the deficit(s) in TGF-b signaling pathway in DB cells. Preliminary data indicate that the TGF-b-mediated phosphorylation of Smad2 and Smad3 were absent in DB cells, whereas TGF-b-induced phosphorylation of both Smad3 and Smad2 were observed in a TGF-b-responsive B-cell lymphoma cell line RL in presence of low dose of PMA. Examination of the status of the TGF-b receptors reveals that both RL and DB cells have TGF-b receptors I on their cell surface, whereas TGF-b receptors II were present only on the cell surface of RL cells. We have demonstrated that transfection of wild-type, but not a C-terminal truncated form of receptor II rendered the DB cells responsive to TGF-b mediated growth suppression. Analysis of the TGF-b receptor II gene revealed the absence of the receptor II message, which was reversed upon treatment with demethylating agent, indicating that the promoter methylation might be the cause of gene silencing. Promoter analysis revealed CpG methylations at -25 and -140 that correlated with the gene silencing. We have shown that the promoter methylation was also involved in silencing TGF-b receptor II gene in another B-cell lymphoma cell line, Akata. We are currently investigating the role of PKC in stabilization of TGF-b receptor II on the cell surface of B-cell lymphoma cell lines.
| Chen, Gang; Ghosh, Paritosh; Osawa, Hiroshi et al. (2007) Resistance to TGF-beta 1 correlates with aberrant expression of TGF-beta receptor II in human B-cell lymphoma cell lines. Blood 109:5301-7 |
