Identification of tocopherol-associated protein as an activin/TGF-β-inducible gene in mast cells

Previous studies have demonstrated that treatment with activin A and TGF-β₁, members of the TGF-β family, stimulated maturation of mouse bone marrow-derived cultured mast cells (BMMC), which was characterized by morphology and gene expression of mouse mast cell proteases (mmcps). In order to gain a better understanding of activin A- and TGF-β₁-induced maturation in mast cells, we investigated the genes that were up-regulated in response to treatment with these two members of the TGF-β family. The cDNA microarray analyses indicated that in BMMC, five genes were induced by treatment with 4 nM activin A for 2 h. Tocopherol-associated protein (Tap) was one of the induced genes, and the Tap induction in response to activin A treatment was confirmed by real-time RT-PCR analyses. Treatment with TGF-β₁ at 200 pM but not BMP-2 at 4 nM also increased Tap gene transcript in BMMC. Activin A-induced Tap expression was detected in BMMC but not in RAW264 macrophage-like cells, B16 melanoma cells or P19 embryonic carcinoma cells. Treatment with > 1 μM SB431542, an inhibitor of activin and TGF-β type I receptors ALK4/5, reduced responsiveness of Tap expression to TGF-β₁, whereas < 0.5 μM SB431542 effectively reduced TGF-β₁-induced expression of mmcp-1 and mmcp-7. These results suggest that inhibitory effects of SB431542 are different between TGF-β-induced genes. Reporter assays indicated that Tap expression enhances transcription mediated by the activin/TGF-β pathway. Thus, the present results suggest that Tap induction in response to activin/TGF-β occurs predominantly in mast cells and serves as a positive regulator in activin/TGF-β signaling.