The TAL1 (or SCL) gene encodes a basic helix-loop-helix transcription factor that can activate or repress gene expression, depending on its interaction partners. TAL1 proteins are expressed in the vascular and hematopoietic systems and in erythroid, megakaryocytic, and mast cell precursors. TAL1 is essential for hematopoietic commitment and vascular remodeling during embryogenesis and is also important to the differentiation of erythroid and megakaryocytic progenitors postnatally. Although a mouse Tal1 cDNA was cloned from a bone marrow (BM) macrophage cDNA library and we reported expression of Tal1 protein in mouse bone marrow mononuclear cells (
), no studies have examined the function of this transcription factor in the monocyte-macrophage lineage. To address this issue, we characterized Tal1 expression in primary mouse BM monocytes and M1 monocytic leukemia cells induced to differentiate in vitro. We isolated BM cells from wild-type C57BL6/J mice and purified a mononuclear cell population by pronase treatment and density gradient centrifugation on horse serum. Monocytes were then induced to differentiate in culture, initially in the presence of both interleukin-3 (IL-3) and macrophage colony-stimulating factor (M-CSF) and subsequently in the presence of L-cell conditioned medium. Tal1 expression was maintained in these cultures, declining by only 14% over the full culture period, while calcitonin receptor expression, a marker of osteoclastic differentiation, was extinguished. In parallel, interleukin-6 receptor (IL-6R) and macrophage colony-stimulating factor receptor (M-CSFR) mRNA increased 3.5- and 114-fold, respectively. In M1 cells induced to differentiate with recombinant murine IL-6, Tal1 protein abundance decreased similarly with differentiation. To assess the function of Tal1 in this cell lineage, a full-length Tal1 cDNA was introduced into both primary BM monocytes and M1 cells via retroviral gene transfer. Puurified monocytes were transduced 24 hr after isolation with wild-type Tal1 or a DNA-binding-defective Tal1 mutant, T192P, in the MSCV-GFP vector. GFP-expressing cells were then sorted and induced to differentiate. Gene expression analysis using semi-quantitative PCR carried out after 8 days of culture showed increased expression of Tal1 mRNA in cells transduced with the wild-type Tal1 cDNA compared to the vector control. This was accompanied by upregulation of IL-6R and M-CSFR expression relative to both parental vector- and mutant-transduced cells. Northern blot analysis of M1 cells transduced with the Tal1 cDNA showed similar upregulation of M-CSFR mRNA relative to vector control cells. In summary, these studies show that the TAL1 transcription factor is expressed by murine monocytes during differentiation to macrophages and suggest a role for TAL1 in both gene expression and differentiation of this lineage. Further, they suggest a requirement for direct TAL1 DNA binding.
Disclosure: No relevant conflicts of interest to declare.
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