The frequency of extramedullary infiltration (EMI) in acute myeloblastic leukemia (AML) is reported up to 40% and most prevalent in myelo-monoblastic and monoblastic subtypes of AML (M4 and M5 according to FAB classification). The majority of patients with EMI suffered poor prognosis. To explore mechanism underlying EMI, we analyzed SHI-1 cells, a highly invasive human acute monocytic leukemia cell line, and found their strong expression of matrix metalloproteinase 2 (MMP-2), membrane type 1 MMP (MT1-MMP) and tissue inhibitor of metalloproteinase 2 (TIMP-2). SHI-1 cells showed higher invasive ability to traverse reconstituted basement membranes (Matrigel) and stronger activation of proMMP-2 than other leukemia cell line such as NB4, K562, U937 and THP-1 cells. When co-cultured with bone marrow stromal cells (BMSCs), the invasive capacity and proMMP-2 activation of SHI-1 cells enhanced remarkably. Furthermore, the inhibition of MMP-2, MT1-MMP, or TIMP-2 by small interfering RNA (siRNA) substantially impaired SHI-1 cells invasion and decreased proMMP-2 activation. In the contrast, up-regulated expression of TIMP-2 for 2-3 folds level increased cell invasion and proMMP-2 activation. These results demonstrated that constitutively high expression of MMP-2, MT1-MMP and TIMP-2 in SHI-1 cells facilitated cell invasion by promoting proMMP-2 activation. Moreover, up-regulation of TIMP-2 exhibited not a repressive but an activating effect on SHI-1 cells invasion. Our study indicated that increasing TIMP-2 in AML patients with EMI may potentially cause adverse effects, particularly in patients containing high levels of MMP-2 and MT1-MMP.
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