Membrane type-matrix metalloproteinases (MT-MMPs) play a key role in tumor cell progression and metastasis but their role in hematological malignancy and leukemic cell dissemination is still unclear. To date, six MT-MMPs have been identified, of which four (MT1-, MT2-, MT3- and MT5-MMP) possess a trans-membrane domain that tethers the enzyme to the plasma membrane and two (MT4- and MT6-MMP) are anchored to the cell surface via a glycophosphatidyl inositol domain. MT-MMPs are known to activate pro-forms of MMPs. We examined the expression of all six MT-MMPs in 13 myeloid and lymphoid cell lines using RT-PCR, Western blotting and FACS. We found that

  1. MT1-MMP was expressed on most of the cell lines tested,

  2. MT2-MMP was expressed in myeloid cell lines such as THP-1, HEL, K562 and U937, and in T cell lines such as Jurkat and CEM, but not in any of the tested B cell lines,

  3. MT3-MMP was not expressed in any of the cell lines except for THP-1,

  4. MT4-MMP was strongly expressed in all myeloid but not lymphoid cell lines and

  5. MT5-MMP and MT6-MMP were expressed in most of the myeloid cell lines.

Next, we examined the expression of all six MT-MMPs in primary AML samples and found that MT1-MMP is expressed in 20 out of 24 AML patient samples as detected by RT-PCR and Western blotting, MT2-MMP and MT4-MMP were expressed in 21 and 22 out of 24 samples, respectively, and MT6-MMP was expressed in 17 out of the 17 samples tested. In contrast, MT3- and MT5-MMPs were not found in the primary AML samples. Zymographic analysis showed that the pro form of MMP-2 was secreted in media conditioned by AML blasts and became activated only when these AML blasts were co-cultured with BM stromal cells. Since TNF-α is endogenously secreted by AML blasts we next stimulated these cells with human recombinant TNF-α and found strong upregulation of MT1-MMP and MT6-MMP. Moreover, zymography demonstrated that TNF-α-stimulated AML blasts are more potent in activation of pro-MMP-2 than unstimulated cells, indicating that activation may occur via upregulation of MT-MMPs. Furthermore, we found that TNF-α stimulation led to stronger upregulation/induction of MT1- and MT2-MMPs in CD34+ cells than in CD34- cells isolated from AML patients. In conclusion, we report here for the first time that MT1-, 2-, 4- and 6-MMPs are expressed in AML blasts and suggest that these MT-MMPs may contribute to the invasive phenotype of this malignancy.

Author notes

Disclosure: No relevant conflicts of interest to declare.