We have shown previously that Smad3 and Smad4 mediate TGF-beta1-induced IgA expression. In the present study, we examined the involvement of Runx3 in this process. Overexpression of Runx3 in mice increased germ-line alpha (GLalpha) transcription, and transcription was further augmented when B lymphoma and LPS-activated murine spleen cells were co-transfected with Smad3/4. Overexpression of Runx3 and Smad3/4 increased IgA secretion by both cell types in response to TGF-beta1. p300, which has histone acetyltransferase activity, further augmented TGF-beta1-induced GLalpha transcription promoted by Smad3/4 and Runx3. These observations were confirmed by examining the influence of Smad3/4, Runx3 and p300 on the expression of endogenous GLalpha and post-switch alpha transcripts.E1A, an inhibitor of p300, blocked both GLalpha promoter activity and the enhancement of endogenous GLalpha transcription by Smad3/4 and Runx3. We conclude that p300 cooperates with Smad3/4 and Runx3 in stimulating TGF-beta1-induced GLalpha transcription and subsequent IgA isotype expression, while E1A inhibits these cooperative effects.