Mutations of tumor necrosis factor α-responsive serine residues within the C-terminal transactivation domain of human transcription factor REL enhance its in vitro transforming ability

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Abstract
The human c-rel gene (REL), encoding an NF-B transcription factor, is amplified or mutated in several human B-cell lymphomas and can transform chicken lymphoid cells in vitro. We have previously shown that certain deletions of C-terminal transactivation sequences enhance REL's transforming ability in chicken spleen cells. In this report, we have analysed the effect of single amino-acid changes at select serine residues in the C-terminal transactivation domain on REL's transforming ability. Mutation of either of two TNF-inducible serine residues (Ser460 and Ser471) to nonphosphorylatable residues (alanine, asparagine, phenylalanine) made REL more efficient at transforming chicken spleen cells in vitro. In contrast, mutation of Ser471 to a phosphorylation mimetic aspartate residue impaired REL's transforming ability, even though it increased REL's inherent transactivation ability as a GAL4-fusion protein. Alanine mutations of several other serine residues within the transactivation domain did not substantially affect REL's transforming ability. Transactivation by GAL4-REL fusion proteins containing either transformation enhancing or nonenhancing mutations at serine residues was generally similar to wild-type GAL4-REL. However, more transforming mutants with mutations at either Ser460 or Ser471 differed from wild-type REL in their ability to transactivate certain B-site reporter genes. In particular, the SOD2 promoter, encoding manganese superoxide dismutase, was activated less strongly by the more transforming REL mutant REL-S471N in transient assays, but REL-S471N-transformed chicken spleen cells had increased levels of MnSOD protein as compared to wild-type REL-transformed cells. Taken together, our results show that mutations of certain serine residues can enhance REL's transforming ability in vitro and suggest that these mutations increase REL-mediated transformation by altering REL's ability to modulate the expression of select target genes. Furthermore, phosphorylation of Ser471 may be involved in REL-mediated modulation of transformation-specific target gene expression. Lastly, these results suggest that similar mutations in the REL transactivation domain contribute to the development of certain human B-cell lymphomas.