Mtss1 Suppresses BCR-ABL Induced Cell Migration and Is Downregulated in CML Stem Cells.

Migration and adhesion properties of hematopoietic stem cells (HSC) are disrupted in chronic myeloid leukemia (CML). Egression of these cells from the bone marrow is associated with cytoskeletal changes including actin remodeling. In a microarray screen of differentially regulated genes in HSC from BCR-ABL positive transgenic mice, we found downregulation of multiple genes involved in actin-associated changes of cell structure, adhesion, and migration (i.e. intersectin-1, cortactin, Mtss1, synaptopodin, and Gem GTPase). Mtss1 was further studied since it has been described to be a binding partner of Rac, which is essential for BCR-ABL mediated transformation, and a potential tumor suppressor. Using quanitative RT-PCR, Mtss1 downregulation (6-fold) was confirmed in HSC and unfractionated bone marrow and spleen cells from SCLtTA/BCR-ABL transgenic mice after 3 weeks of BCR-ABL induction as well as in human BCR-ABL positive cell lines. Treatment of BCR-ABL positive (32D/BCR-ABL, K562, KYO-1) but not BCR-ABL negative (32D, U937) cell lines with 5μ M Imatinib led to upregulation of Mtss1 mRNA (5- to 10-fold) and protein, suggesting that downregulation of Mtss1 is dependent on BCR-ABL kinase activity. Retroviral transduction of Mtss1 into 32D/BCR-ABL cells almost completely inhibited BCR-ABL induced cell motility of individual cells seeded on murine bone marrow stromal cells in time-lapse video experiments over the course of two hours. Interestingly, we found that retroviral transduction of Mtss1 into 32D/BCR-ABL cells completely suppressed migration of these cells to extra-hematopoietic sites in vivo upon intravenous transplantation into syngeneic C3H mice. Moreover, when Mtss1-transduced cells were injected subcutaneously, the size of the tumors was significantly decreased as compared to empty vector-transduced 32D/BCR-ABL cells (p<0.05), confirming that Mtss1 may be a tumor suppressor.

These results demonstrate that Mtss1 antagonizes BCR-ABL induced cell migration and is downregulated by BCR-ABL in CML stem cells, suggesting that downregulation of Mtss1 and other cytoskeletal adaptor proteins may be required for egression of CML stem cells from the bone marrow niche. Since the same Mtss1 protein domain is responsible for both Rac and actin binding, Mtss1 may interfere with Rac function and thereby inhibit the effects of Rac on migration and cytoskeletal dynamics of HSC.