Stromal-cell derived factor (SDF)-1α/CXCL12 and its cognate receptor, CXCR4, play a crucial role in the trafficking of normal hematopoietic stem/progenitor cells (HSPC) and their homing/retention in bone marrow. Consequently, modulation of CXCR4 expression in HSPC could be applied therapeutically to improve the efficiency of HSPC transplantation. It is known that gene expression can be regulated by chromatin remodelling. Two groups of histone modifying enzymes, histone acetyltransferase (HAT) and histone deacetylase (HDAC) participate in the regulation of chromatin structure, and hence gene expression. Disruption of normal HAT or HDAC activities has been found in many human cancers. Recently, several structurally diverse and highly specific HDAC inhibitors (HDI) have been reported. They act as strong modulators of growth, differentiation and apoptosis in several types of cancer, particularly acute myeloid leukemia (AML). However, very little is known regarding the effects of HDI on HSPC. We have previously shown that a specific short-chain fatty acid HDI, valproic acid (VPA), enhances CXCR4 expression and function in normal HSPC (

). In order to determine whether other structurally diverse classes of HDI are able to influence CXCR4 expression in HSPC through chromatin remodelling, we investigated the effect of potent hydroxamic acid HDI Trichostatin A (TSA) on CXCR4 in normal HSPC. We examined the effect of TSA on CXCR4 expression (by FACS and real-time RT-PCR), modulation of CXCR4 transcription (chromatin immunoprecipitation (X-ChIP) analysis) and on functional response towards an SDF-1α gradient (by chemotaxis assay) of HSPC (CD34+ cells from cord blood (CB) and the models of immature hematopoietic cells expressing CD34 antigen, namely AML cell lines KG-1a and KG-1). Cells were incubated for 24 h in IMDM supplemented with 20% FCS in the presence of TSA (0.1 μM). We found that TSA increases the percentage of CXCR4-expressing CB CD34+, KG-1a, KG-1 cells (2.5-, 8- and 3-fold, respectively). This effect was also confirmed at the mRNA level in CB CD34+, KG-1a and KG-1 cells (by about 2.5-, 5- and 2.5-fold up-regulation, respectively). Moreover, X-ChIP analysis showed a significant increase in association of acetyl-histone H4 binding to the CXCR4 promoter in CB CD34+ and KG-1 cells (2- and 1.7-fold, respectively). TSA was also shown to significantly increase the chemotaxis of KG-1a cells towards SDF-1α (20 ng/mL), which was inhibited by AMD3100, a potent antagonist of CXCR4. We conclude that other HDI such as TSA regulate CXCR4 expression in HSPC by chromatin remodelling and we suggest that priming of HSPC with HDI may improve their homing and engraftment into bone marrow, especially in CB transplantation.

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