Resistance to TGF-β1 correlates with aberrant expression of TGF-β receptor II in human B-cell lymphoma cell lines

Resistance to transforming growth factor (TGF)–β1–mediated growth suppression in tumor cells is often associated with the functional loss of TGF-β receptors. Here we describe two B-cell lymphoma cell lines (DB and RL) that differ in their sensitivity to TGF-β1–mediated growth suppression. The TGF-β1–resistant cell line DB lacked functional TGF-β receptor II (TβRII) in contrast to the TGF-β–responsive cell line RL, whereas both cell lines had comparable levels of receptor I (TβRI). Lack of functional TβRII was correlated with the lack of TGF-β1–induced nuclear translocation of phospho-Smad3 and phospho-Smad2, the lack of nuclear expression of p21Cip1/WAF1, and the down-regulation of c-Myc in DB cells. Transfection of wild-type, but not a C-terminal–truncated, form of TβRII rendered the DB cell line responsive to TGF-β1–mediated growth suppression. Analysis of the TβRII gene in DB cells revealed the absence of TβRII message, which was reversed upon 5′-azacytidine treatment, 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. These data suggest that promoter methylation plays an important role in TβRII gene silencing and subsequent development of a TGF-β1–resistant phenotype by some B-cell lymphoma cells.