Pes of tumors [4] and respective proteins were therefore considered as targets for anti-metastatic treatment. Moreover, several 
reports have described prognostic power of CD44 gene products in different cancer types [5,6,7]. CD44 gene products are transmembrane proteoglycans, which act as cell-cell and cell-matrix adhesion molecules and principle receptors of hyaluronan (HA) [8]. They exist as multiple isoforms that are generated from a single primary gene transcript by alternative splicing combined with varying posttranslational glycosylation [9]. As a result, the extracellular domains of CD44 isoforms consist of variable regions, encoded by variant exons, and are glycosylated to different extents. The standard isoform CD44s, also described as CD44H (hematopoietic), is the smallest CD44 gene product and does not contain any of the variable extracellular regions encoded by the variant exons. The structural differences in the extracellular domains of the CD44 isoforms largely define a wide range of biological functions in development, inflammation, haematopoiesis, wound healing, immune response and cancer [4]. In tumor biology, evidence has accumulated that metastasis in different types of cancer is directly or inversely related to the expression ofCD44 Silencing Promotes Osteosarcoma MetastasisCD44 gene products, depending on the expression pattern of CD44 isoforms and on the organ of origin. Several studies have shown up-regulated expression of CD44 isoforms in many human tumors, including gastric cancer, pancreatic cancer, lung and renal cell cancer [6,10,11,12]. Some reports point to important functions of CD44s in tumor progression [13]. However, in other tumor types, such as neuroblastoma and prostate cancer, the absence of CD44 gene products correlated with poor prognosis [7,14]. In prostate carcinoma cells, CD44 was even considered as a metastasis-suppressor gene [4,15]. Thus, the functions of CD44 gene products in tumor get IQ1 biology are controversially discussed. Little is known on potential functions of CD44 gene products in OS pathophysiology and in OS metastasis in particular. Two immunohistochemical studies of OS tissue specimens revealed discrepant results. Kim et al. reported that overexpression of the CD44v5 isoform (CD44s with an additional amino acid sequence in the extracellular domain encoded by variant exon 5) in tumor tissue correlated significantly with metastatic disease and, consequently, lower survival rates of the patients [16]. In the tumor samples analyzed by Kuryu et al., on the other hand, a poor prognosis for OS patients correlated with overexpression of the CD44 isoforms that contain in the extracellular domain the amino acid sequence encoded by variant exon 6 (CD44v6) [17]. In both studies, immunostaining with CD44 antibodies detecting all expressed CD44 isoforms in tumor tissue together (pan CD44 antibodies) was not a prognostic indicator. As outlined before, HA is the principle ligand of CD44 gene products. In OS, a few studies with established OS cell lines revealed evidence for a tumor promoting effect of HA. Hosono et al. showed that in vitro and in vivo tumorigenic properties of the OS cell lines MG63 and LM8 were inhibited by HA oligosaccharides that perturbed the HA-rich LED 209 web pericellular matrix [18]. A study performed by Nishida et al. showed diminished retention of HA and inhibition of tumorigenicity of MG63 OS cells upon antisense inhibition of HA synthase (HAS)-2 [19]. HAS-3-derived HA enhanced the cel.Pes of tumors [4] and respective proteins were therefore considered as targets for anti-metastatic treatment. Moreover, several reports have described prognostic power of CD44 gene products in different cancer types [5,6,7]. CD44 gene products are transmembrane proteoglycans, which act as cell-cell and cell-matrix adhesion molecules and principle receptors of hyaluronan (HA) [8]. They exist as multiple isoforms that are generated from a single primary gene transcript by alternative splicing combined with varying posttranslational glycosylation [9]. As a result, the extracellular domains of CD44 isoforms consist of variable regions, encoded by variant exons, and are glycosylated to different extents. The standard isoform CD44s, also described as CD44H (hematopoietic), is the smallest CD44 gene product and does not contain any of the variable extracellular regions encoded by the variant exons. The structural differences in the extracellular domains of the CD44 isoforms largely define a wide range of biological functions in development, inflammation, haematopoiesis, wound healing, immune response and cancer [4]. In tumor biology, evidence has accumulated that metastasis in different types of cancer is directly or inversely related to the expression ofCD44 Silencing Promotes Osteosarcoma MetastasisCD44 gene products, depending on the expression pattern of CD44 isoforms and on the organ of origin. Several studies have shown up-regulated expression of CD44 isoforms in many human tumors, including gastric cancer, pancreatic cancer, lung and renal cell cancer [6,10,11,12]. Some reports point to important functions of CD44s in tumor progression [13]. However, in other tumor types, such as neuroblastoma and prostate cancer, the absence of CD44 gene products correlated with poor prognosis [7,14]. In prostate carcinoma cells, CD44 was even considered as a metastasis-suppressor gene [4,15]. Thus, the functions of CD44 gene products in tumor biology are controversially discussed. Little is known on potential functions of CD44 gene products in OS pathophysiology and in OS metastasis in particular. Two immunohistochemical studies of OS tissue specimens revealed discrepant results. Kim et al. reported that overexpression of the CD44v5 isoform (CD44s with an additional amino acid sequence in 
the extracellular domain encoded by variant exon 5) in tumor tissue correlated significantly with metastatic disease and, consequently, lower survival rates of the patients [16]. In the tumor samples analyzed by Kuryu et al., on the other hand, a poor prognosis for OS patients correlated with overexpression of the CD44 isoforms that contain in the extracellular domain the amino acid sequence encoded by variant exon 6 (CD44v6) [17]. In both studies, immunostaining with CD44 antibodies detecting all expressed CD44 isoforms in tumor tissue together (pan CD44 antibodies) was not a prognostic indicator. As outlined before, HA is the principle ligand of CD44 gene products. In OS, a few studies with established OS cell lines revealed evidence for a tumor promoting effect of HA. Hosono et al. showed that in vitro and in vivo tumorigenic properties of the OS cell lines MG63 and LM8 were inhibited by HA oligosaccharides that perturbed the HA-rich pericellular matrix [18]. A study performed by Nishida et al. showed diminished retention of HA and inhibition of tumorigenicity of MG63 OS cells upon antisense inhibition of HA synthase (HAS)-2 [19]. HAS-3-derived HA enhanced the cel.
