Transcriptional Mediator Subunit MED1/TRAP220 in Stromal Cells Is Involved in Hematopoietic Stem/Progenitor Cell Support through Osteopontin Expression

MED1/TRAP220, a subunit of the transcriptional Mediator/TRAP complex, is crucial for various biological events through its interaction with distinct activators such as nuclear receptors and GATA family activators. In hematopoiesis, MED1 plays a pivotal role in optimal nuclear receptor-mediated myelomonopoiesis and GATA-1-induced erythropoiesis. In this study, we present evidence that MED1 in stromal cells is involved in supporting hematopoietic stem and/or progenitor cells (HSPCs) through osteopontin (OPN) expression. We found that the proliferation of bone marrow (BM) cells cocultured with MED1 knockout (Med1-/-) mouse embryonic fibroblasts (MEFs) was significantly suppressed when compared to the control. Furthermore, the number of long-term culture-initiating cells (LTC-ICs) was attenuated for BM cells cocultured with Med1-/- MEFs. The vitamin D receptor (VDR)- and Runx2-mediated expression of OPN, as well as Mediator recruitment to the Opn promoter, was specifically attenuated in the Med1-/- MEFs. Addition of OPN to these MEFs restored the growth of cocultured BM cells and the number of LTC-ICs, both of which were attenuated by the addition of the anti-OPN antibody to Med1+/+ MEFs and to BM stromal cells. Consequently, MED1 in niche appears to play an important role in supporting HSPCs, by upregulating VDR- and Runx2-mediated transcription on the Opn promoter.

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Sumitomo A, Ishino R, Urahama N, Inoue K, Yonezawa K, Hasegawa N, Horie O, Matsuoka H, Kondo T, Roeder RG, Ito M