Abstract
The c-myb protooncogene encodes a transcription factor, c-Myb, which is highly expressed in immature hematopoietic cells. c-Myb is required for many critical aspects of blood cell development including lineage fate selection, proliferation, and at multiple time points during early myeloid, and B and T lymphoid cell development. GATA-3, which belongs to a family of zinc finger transcription factors, is also required at several steps in early T cell development, and specifically in regard to this communication, for the development of T helper type 2 (Th2) cells. A recent study by Maurice et al (EMBO2007, 26:3629–3640) reported that c-myb regulates T helper cell lineage commitment in developing mouse thymocytes via regulation of GATA-3 expression. As we were unaware of any studies that have addressed the role of c-Myb and GATA-3 in normal human peripheral blood lymphocytes (PBL), we explored the potential regulatory relationship between these transcription factors in cells of this type. Proceeding from the murine studies, we performed a chromatin immunoprecipitation assay (ChIP) which showed that c-Myb bound the GATA-3 downstream promoter in naïve CD4+ T cells under conditions designed to promote Th2 growth. Such binding was not observed in cells stimulated under Th1 promoting conditions. The interaction of c-Myb and GATA-3 proteins was also detected in cell lysates under Th2 cell promoting conditions by immunoprecipitation with both anti-c-Myb, and anti-GATA-3 polyclonal antibodies. Of note, immunoprecipitation with these same antibodies did not show binding of either protein to STAT6. Additional studies revealed that c-Myb activated a GATA-3 minimal promoter by direct binding to a conserved c-Myb binding site in peripheral blood T cells. Of even greater interest, in 293T cells, GATA-3 activated its own promoter ~6 fold when c-Myb was co-expressed in 293T cells. In the absence of c-Myb, GATA-3 did not significantly activate its own promoter in these cells. We have recently shown that c-Myb binds to MLL via menin. A ChIP assay also showed that MLL and Menin bound to the GATA-3 promoter suggesting that c-Myb and GATA-3 form a co-activator complex on the GATA-3 promoter with MLL. Finally, to explore the role of c-myb expression in human peripheral blood naive CD4+ T cells, we employed c-Myb targeted, and control, short hairpin RNA (shRNA) expressed from a lentivirus vector. This strategy yielded a sequence specific 80–90% knockdown of c-Myb expression in our hands. Stimulation of naive peripheral blood CD4+ T cells expressing the c-Myb directed shRNA with cytokines promoting Th2 cell formation (IL-4, IL-2, and anti-IL-12 antibody) blocked the up-regulation of GATA-3 mRNA expression ~90% compared to cells in which a control shRNA had been expressed. Flow cytometric analysis revealed that intracellular IL-4 expression also was diminished. In contrast, silencing c-myb had no effect on T-bet mRNA expression, or intracellular interferon-expression in the cells induced to undergo Th1 cell formation with IL-12, IL-2 and anti-IL-4 antibody. We conclude from these studies that c-Myb regulates developmental programs specific for Th2, as opposed to Th1, cell development. We hypothesize that such control is exerted in peripheral blood T lymphocytes, at least in part, through direct control of GATA-3, whose expression is auto-regulated with the assistance of c-Myb, and perhaps MLL, acting as transcriptional co-factors.
c-Myb, Menin, GATA-3, and MLL form a dynamic transcription complex that plays a pivotal role in human T helper type 2 cell development