Neuronal differentiation and axonal growth are controlled by a variety of factors including neurotrophic factors, extracellular matrix components, and cell adhesion molecules. Here we describe a novel and very efficient neuritogenic factor, the metastasis-related Mts1 protein, belonging to the S100 protein family. The oligomeric but not the dimeric form of Mts1 strongly induces differentiation of cultured hippocampal neurons. A mutant with a single Y75F amino acid substitution, which stabilizes the dimeric form of Mts1, is unable to promote neurite extension. Disulfide bonds do not play an essential role in the Mts1 neuritogenic activity. Mts1-stimulated neurite outgrowth involves activation of phospholipase C and protein kinase C, depends on the intracellular level of Ca 2؉ , and requires activation of the extracellular signal-regulated kinases (ERKs) 1 and 2.

Mts1 protein (S100A4 according to a new classification) has been implicated in the formation of the metastatic phenotype via regulation of cell motility and invasiveness. Previously we have demonstrated that Mts1 protein interacted with the heavy chain of nonmuscle myosin in a calcium-dependent manner. To elucidate the role of the Mts1-myosin interaction, we mapped the Mts1-binding region on the myosin heavy chain molecule. We prepared proteolytically digested platelet myosin and a series of overlapped myosin heavy chain protein fragments and used them in a blot overlay with Mts1 protein. Here we report that the Mts1-binding site is located within a 29-amino acid region, at the C-terminal end of the myosin heavy chain (between 1909 -1937 amino acids). Two-dimensional phosphopeptide analysis showed that Mts1 protein inhibits protein kinase C phosphorylation of the platelet myosin heavy chain at Ser-1917. We hypothesize that Mts1 protein regulates cytoskeletal dynamics of the metastatic cells through modulation of the myosin phosphorylation by protein kinase C in calcium-dependent fashion.

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S100A4 (also known as Mts1, metastasin, p9Ka, pEL98, CAPL, calvasculin, Fsp-1, placental calcium-binding protein) belongs to the family of EF-hand calcium-binding proteins, whose expression is elevated in a number of pathological conditions. Although it is well documented that S100A4 is expressed in cancer cells and contributes to tumor cell motility and metastatic progression, the exact underlying mechanisms remain elusive. An important characteristic feature of S100 proteins is their dual function, inside and outside the cell. In this review, we focus on the intracellular function of S100A4. The review contains structural analysis of S1004 in comparison with other members of S100 proteins. Possible modes of the interaction of S100 proteins with targets are described. Several examples of best-studied molecular interactions involving S100A4 with heavy chain of nonmuscle myosin IIA, LAR-interacting protein liprin beta1 and tumor suppressor protein p53 are provided. We suggest that the binding of S100A4 to these molecules is critical for the S100A4 function. Further studies of the implications of these interactions in different molecular pathways may shed additional light on the role of S100A4 protein in the control of tumor cell motility and migration. We discuss the approaches for down-regulation of S100A4 expression and their potential for application in the clinics.

ALG-2 was isolated in a screen for proteins involved in programmed cell death and is the first Ca(2+)-binding protein found to be directly involved in apoptosis. We have generated polyclonal antibodies that are suitable for detecting ALG-2 using different immunological methods. Three commercial antibodies against ALG-2 did neither detect mouse recombinant ALG-2 nor endogenous ALG-2 in Jurkat cell lysates, whereas our own affinity-purified antibody recognized recombinant as well as endogenous ALG-2. The specificity of the antibody was shown by preabsorbtion experiments and on ALG-2-deficient cells using Western blot analysis and immunohistochemistry. Western blot analysis of 15 different adult mouse tissues demonstrated that ALG-2 is ubiquitously expressed. We found that ALG-2 was more than threefold overexpressed in rat liver hepatoma compared to normal rat liver using Western blot analysis, a result confirmed by immunohistochemical analysis. Staining of four different lung cancer tissue microarrays including specimens of 263 patients showed that ALG-2 is mainly localized to epithelial cells and significantly up-regulated in small-cell lung cancers and in non-small-cell lung cancers. Our results lead to the conclusion that ALG-2 beside its known proapoptotic functions may be a player in survival pathways.