The Zinc Finger Transcription Factor Egr-1 Activates Macrophage Differentiation in M1 Myeloblastic Leukemia Cells

Tissue inhibitor of matrix metalloproteinase-1 overexpression in M1 myeloblasts impairs IL-6-induced differentiation

The balance between matrix metalloproteinase (MMP) and tissue inhibitor of matrix metalloproteinase (TIMP) is important for extracellular matrix interactions of hematopoietic cells. MMP-independent growth modulating activity for TIMP-1 on B lymphocytes and erythroid progenitors has also been described, but a role for TIMP-1 in myelomonocytic differentiation has not been previously reported. In this study, we demonstrate that TIMP-1 overexpression impairs differentiation of the myeloblastic M1 cell line following interleukin (IL)-6 stimulation. We generated retroviral vectors coexpressing human TIMP-1 and the green fluorescent protein (GFP) and stably transduced murine M1 myeloid cells. TIMP-1 expressing cells showed a large reduction in IL-6-induced macrophage differentiation in vitro that was reversible with a specific monoclonal antibody. The differentiation delay in M1/TIMP-1 cells was also specifically reversible by pharmacologic phosphatidylinositol-3 kinase (PI3-K) inhibition. Additionally, overexpression of a TIMP-1/GFP fusion protein also impaired M1 differentiation and this protein was localized to the cell surface, consistent with an autocrine receptor-mediated mechanism. Surprisingly, TIMP-1 transduced cells had a selective advantage for growth in IL-6, indicating that functional effects on growth and differentiation of M1 cells were primarily through an autocrine mechanism. Intrinsic TIMP-1 expression in myeloid leukemia cells might thus impact upon survival or differentiation.

Reduced tumorigenicity of murine leukemia cells expressing protein-tyrosine phosphatase, PTPepsilon C

Recently, we reported that a cytosolic isoform of protein-tyrosine phosphatase epsilon (PTP epsilon C), when overexpressed, inhibits terminal differentiation and apoptosis of murine M1 myeloblastic leukemia cells induced by interleukin-6. To determine whether these observed effects in vitro correspond to a tumorigenicity of PTP epsilon C-expresser (M1- epsilon C) cells in vivo, parent M1 and M1- epsilon C cells were intravenously inoculated into scid or nude mice, and survival of mice receiving these cell lines was monitored. Unexpectedly, both scid and nude mice inoculated with M1- epsilon C cells showed significantly prolonged survival time than those receiving parent M1 cells. While parent M1 cells inoculated by intravenous injection formed metastatic tumors in the spleen, expression of PTP epsilon C suppressed tumor development in the spleen. The results suggest a suppressive role of PTP epsilon C in tumorigenesis.

Cdc25A stability is controlled by the ubiquitin-proteasome pathway during cell cycle progression and terminal differentiation

Members of the cdc25 family are protein phosphatases that play pivotal roles in cell cycle progression. Cdc25A has been shown to be a critical regulator of the G1/S transition of mammalian cells and to be a myc-target gene with oncongenic properties. We investigated the regulation of cdc25A during terminal differentiation using myeloblastic leukemia M1 cells, that can be induced to undergo differentiation into macrophages by interleukin-6 (IL-6) treatment. In this report it is shown that cdc25A protein is degraded by the ubiquitin-proteasome machinery in both terminally differentiating and cycling cells. Cdc25A was found to have two major peaks of accumulation during cell cycle progression, one in G1 and the other in S/G2. Evidence was obtained that degradation of cdc25A by the ubiquitin-proteasome machinery in terminally differentiating myeloid cells is accelerated compared to cycling cells. Moreover, deregulated expression of c-myc in M1 cells, which had been previously shown to block terminal differentiation, was also found to block IL-6 induced degradation of cdc25A.