Mechanisms of SHP-1 P2 promoter regulation in hematopoietic cells and its silencing in HTLV-1-transformed T cells

Abstract

The Src homology‐2‐containing protein‐tyrosine phosphatase 1 (SHP‐1), is a negative regulator of cell signaling. It is also considered a tumor suppressor gene because of its ability to antagonize the action of tyrosine kinases. Although SHP‐1 is expressed strongly in hematopoietic cells, decreased expression has been observed in various hematological malignancies, which suggests a central involvement of SHP‐1 in leukemogenesis. We have shown previously that human T cell lymphotropic virus type‐1 (HTLV‐1) Tax‐induced promoter silencing (TIPS) is an early event causing down‐regulation of SHP‐1 expression, which is dependent on NF‐κB. In this study, DNase I footprinting and EMSA also revealed binding of transcription factors, specificity protein 1 (Sp1) and octamer‐binding transcription factor 1 (Oct‐1) to the P2 promoter, and site‐directed mutagenesis confirmed that these factors contribute to the basal P2 promoter activity. Chromatin immunoprecipitation (CHIP) assays showed that Sp1, Oct‐1, NF‐κB, CREB‐1, and RNA polymerase II interacted with the core SHP‐1 P2 promoter in CD4⁺ T cells and Jurkat cells but not in HTLV‐1‐transformed MT‐2 and HUT102 cells when HTLV‐1 Tax is present. Furthermore, bisulfite sequencing of the SHP‐1 P2 core region revealed heavy CpG methylation in HTLV‐1‐transformed cells compared with freshly isolated CD4⁺ T cells and HTLV‐1‐noninfected T cell lines. A significant inverse correlation between degree of CpG methylation and expression of SHP‐1 mRNA or protein was observed. Taken together, our data support the notion that in HTLV‐1‐transformed CD4⁺ T cells, TIPS causes dissociation of transcription factors from the core SHP‐1 P2 promoter, which in turn leads to subsequent DNA methylation, an important early step for leukemogenesis.