Retinoic acid induces expression of Ig germ line α transcript, an IgA isotype switching indicative, through retinoic acid receptor

Retinoic acid (RA) is considered to possess an activity of IgA isotype switching. Thus far, TGF-beta 1 is known to be the most powerful IgA isotype switch factor. To elucidate the molecular mechanisms underlying the Ig germ line (GL) alpha transcriptional regulation by RA, we constructed three different sizes of mouse GL alpha promoter reporters; short-GL alpha(-130/+14), middle-GL alpha(-448/+72) and long-GL alpha(-3028/+72). Based on luciferase assay, RA increased the activity of all three GL alpha promoter reporters by approximately 2-fold and the effect was further enhanced by TGF-beta 1. Overexpression of Smad3/4 increased TGF-beta 1-induced GL alpha promoter activities but had no effect on RA-induced GL alpha promoter activities. In order to analyze the characteristics of the RA-inducible GL alpha promoter region, we also constructed two mutant reporters: Smad3 binding elements (SBEs)-substituted short-GL alpha (short-GL alpha mSBE) and Runx3 binding elements (RBEs)-substituted short-GL alpha (short-GL alpha mRBE) promoter reporters. Promoter activities of the two mutant reporters to RA were comparable to that of wild type reporter, while those of the two mutant reporters to TGF-beta 1 were markedly diminished as compared to that of WT short-GL alpha. Finally, RA-induced GL alpha transcription was virtually disappeared by LE540, an antagonist of RA receptor (RAR). Taken together, these results suggest that RA induces GL alpha transcription mainly through RAR pathway, where neither Smad3/4 nor Runx3 is involved.