In solid cancers, invasion and metastasis account for more than 90% of mortality. However, in the current armory of anticancer therapies, a specific category of anti-invasion and antimetastatic drugs is missing. Here, we coin the term ‘migrastatics’ for drugs interfering with all modes of cancer cell invasion and metastasis, to distinguish this class from conventional cytostatic drugs, which are mainly directed against cell proliferation. We define actin polymerization and contractility as target mechanisms for migrastatics, and review candidate migrastatic drugs. Critical assessment of these antimetastatic agents is warranted, because they may define new options for the treatment of solid cancers.

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IntroductionDifferentiation of hematopoietic stem cells and progenitors into various lineages is controlled by a complex array of extrinsic and intrinsic factors. [1][2][3][4] Myeloid and erythroid blood cells develop from a common myeloid progenitor, which differentiates into either megakaryocytes and erythrocytes, or granulocytes and macrophages. Several experimental strategies including gene targeting, expression pattern analysis, antisense, and overexpression studies led to the identification of transcription factors that are required for formation, survival, and proliferation of multilineage progenitors and that direct the differentiation and maturation of individual lineages. Specifically, factors like SCL, Rbtn2, GATA-2, and c-Myb were found to be essential for multipotent cells. 5 In more mature cells, the expression of SCL, GATA-1, c-Myb, Rbtn2, FOG, and EKLF is a prerequisite for proper development of the erythroid lineage. On the other hand, factors of the C/EBP family, PU.1, Egr-1, c-Myb, and AML-1 play an important role in the myeloid lineage. [6][7][8][9] It has been suggested that in the common myeloid progenitor the concentrations of intracellular lineage-determining regulators are to be low and balanced. 10,11 During the commitment process this balance is disturbed either in a stochastic or instructed manner, resulting in the prevalence of a particular set of factors and the subsequent development of the respective lineage. Several factors were identified that instruct progenitor cells to develop along a specific lineage. Using the model of primary chicken myb-etstransformed multipotent progenitors (MEP), 12 it was demonstrated that PU.1 and C/EBP␤ forced differentiation of MEP progenitors into myeloid cells, and C/EBP␣ only into eosinophils. 13,14 In mammalian cells, Egr-1 was found to commit myeloid cells into the macrophage lineage with the concomitant block to differentiation into granulocytes. 15 The c-myb proto-oncogene is essential for early definitive myeloid and erythroid cells as documented by gene targeting experiments. 16 It is required for the expansion of immature cells of the myeloid, erythroid, and lymphoid lineages and is downregulated during their terminal differentiations. 17 The v-myb gene transduced by avian myeloblastosis virus (AMV), as well as its truncated homologue, transduced as the myb-ets fusion by E26 leukemia virus, are oncogenes that specifically affect the developmental programs of avian hematopoietic cells. AMV v-Myb interferes exclusively with the development of the macrophage lineage of both primitive and definitive hematopoietic cells by blocking terminal differentiation of macrophage precursors and activating their self-renewal capacities in vitro. It causes fatal acute monoblastic leukemia in chicks. E26 v-Myb-Ets fusion protein transforms multipotent progenitors of primitive hematopoietic cells (blastoderm), and committed erythroid and myeloid cells of bone marrow. It induces erythroid leukemia in infected chicks. [18][19][20][21][22] Thus, the E26 v-Myb-E...

Metastatic spreading of cancer cells is a highly complex process directed primarily by the interplay between tumor microenvironment, cell surface receptors, and actin cytoskeleton dynamics. To advance our understanding of metastatic cancer dissemination, we have developed a model system that is based on two v-src transformed chicken sarcoma cell lines-the highly metastatic parental PR9692 and a non-metastasizing but fully tumorigenic clonal derivative PR9692-E9. Oligonucleotide microarray analysis of both cell lines revealed that the gene encoding the transcription factor EGR1 was downregulated in the non-metastatic PR9692-E9 cells. Further investigation demonstrated that the introduction of exogenous EGR1 into PR9692-E9 cells restored their metastatic potential to a level indistinguishable from parental PR9692 cells. Microarray analysis of EGR1 reconstituted cells revealed the activation of genes that are crucial for actin cytoskeleton contractility (MYL9), filopodia formation (MYO10), the production of specific extracellular matrix components (HAS2, COL6A1-3) and other essential pro-metastatic abilities.

Satellite cells represent a heterogeneous population of stem and progenitor cells responsible for muscle growth, repair and regeneration. We investigated whether c-Myb could play a role in satellite cell biology because our previous results using satellite cell-derived mouse myoblast cell line C2C12 showed that c-Myb was expressed in growing cells and downregulated during differentiation. We detected c-Myb expression in activated satellite cells of regenerating muscle. c-Myb was also discovered in activated satellite cells associated with isolated viable myofiber and in descendants of activated satellite cells, proliferating myoblasts. However, no c-Myb expression was detected in multinucleated myotubes originated from fusing myoblasts. The constitutive expression of c-Myb lacking the 3′ untranslated region (3′ UTR) strongly inhibited the ability of myoblasts to fuse. The inhibition was dependent on intact c-Myb transactivation domain as myoblasts expressing mutated c-Myb in transactivation domain were able to fuse. The absence of 3′ UTR of c-Myb was also important because the expression of c-Myb coding region with its 3′ UTR did not inhibit myoblast fusion. The same results were repeated in C2C12 cells as well. Moreover, it was documented that 3′ UTR of c-Myb was responsible for downregulation of c-Myb protein levels in differentiating C2C12 cells. DNA microarray analysis of C2C12 cells revealed that the expression of several muscle-specific genes was downregulated during differentiation of c-Myb-expressing cells, namely: ACTN2, MYH8, TNNC2, MYOG, CKM and LRRN1. A detailed qRT-PCR analysis of MYOG, TNNC2 and LRRN1 is presented. Our findings thus indicate that c-Myb is involved in regulating the differentiation program of myogenic progenitor cells as its expression blocks myoblast fusion.

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CAS is a docking protein, which was shown to act as a mechanosensor in focal adhesions. The unique assembly of structural domains in CAS is important for its function as a mechanosensor. The tension within focal adhesions is transmitted to a stretchable substrate domain of CAS by focal adhesion-targeting of SH3 and CCH domain of CAS, which anchor the CAS protein in focal adhesions. Mechanistic models of the stretching biosensor propose equal roles for both anchoring domains. Using deletion mutants and domain replacements, we have analyzed the relative importance of the focal adhesion anchoring domains on CAS localization and dynamics in focal adhesions as well as on CAS-mediated mechanotransduction. We confirmed the predicted prerequisite of the focal adhesion targeting for CAS-dependent mechanosensing and unraveled the critical importance of CAS SH3 domain in mechanosensing. We further show that CAS localizes to the force transduction layer of focal adhesions and that mechanical stress stabilizes CAS in focal adhesions.

Comparing the gene expression profiles of metastatic and nonmetastatic cells has the power to reveal candidate metastasis-associated genes, whose involvement in metastasis can be experimentally tested. In this study, differentially expressed genes were explored in the v-src-transformed metastatic cell line PR9692 and its nonmetastatic subclone PR9692-E9. First, the contribution of homeodomain only protein X (HOPX) in metastasis formation and development was assessed. HOPX-specific knockdown decreased HOPX expression in the nonmetastatic subclone and displayed reduced cell motility in vitro. Critically, HOPX knockdown decreased the in vivo metastatic capacity in a syngeneic animal model system. Genomic analyses identified a cadre of genes affected by HOPX knockdown that intersected significantly with genes previously found to be differentially expressed in metastatic versus nonmetastatic cells. Furthermore, 232 genes were found in both screens with at least a two-fold change in gene expression, and a number of high-confidence targets were validated for differential expression. Importantly, significant changes were demonstrated in the protein expression level of three metastaticassociated genes (NCAM, FOXG1, and ITGA4), and knockdown of one of the identified HOPX-regulated metastatic genes, ITGA4, showed marked inhibition of cell motility and metastasis formation. These data demonstrate that HOPX is a metastasis-associated gene and that its knockdown decreases the metastatic activity of v-src-transformed cells through altered gene expression patterns.