CCAR1 promotes DNA repair via alternative splicing.

DNA repair is directly performed by hundreds of core factors and indirectly regulated by thousands of others. We massively expanded a CRISPR inhibition and Cas9-editing screening system to discover factors indirectly modulating homology-directed repair (HDR) in the context of ∼18,000 individual gene knockdowns. We focused on CCAR1, a poorly understood gene that we found the depletion of reduced both HDR and interstrand crosslink repair, phenocopying the loss of the Fanconi anemia pathway. CCAR1 loss abrogated FANCA protein without substantial reduction in the level of its mRNA or that of other FA genes. We instead found that CCAR1 prevents inclusion of a poison exon in FANCA. Transcriptomic analysis revealed that the CCAR1 splicing modulatory activity is not limited to FANCA, and it instead regulates widespread changes in alternative splicing that would damage coding sequences in mouse and human cells. CCAR1 therefore has an unanticipated function as a splicing fidelity factor.

A Molecular Voyage: Multiomics Insights into Circulating Tumor Cells.

Circulating tumor cells (CTCs) play a pivotal role in metastasis, the leading cause of cancer-associated death. Recent improvements of CTC isolation tools, coupled with a steady development of multiomics technologies at single-cell resolution, have enabled an extensive exploration of CTC biology, unlocking insights into their molecular profiles. A detailed molecular portrait requires CTC interrogation across various levels encompassing genomic, epigenetic, transcriptomic, proteomic and metabolic features. Here, we review how state-of-the-art multiomics applied to CTCs are shedding light on how cancer spreads. Further, we highlight the potential implications of CTC profiling for clinical applications aimed at enhancing cancer diagnosis and treatment. SIGNIFICANCE: Exploring the complexity of cancer progression through cutting-edge multiomics studies holds the promise of uncovering novel aspects of cancer biology and identifying therapeutic vulnerabilities to suppress metastasis.

Metastasis Unleashed: Hyposialylation Empowers Chemo-Evasive Circulating Tumor Cell Clusters in Breast Cancer.

Therapy resistance is frequently observed in cancer patients with distant metastases and effective management of metastatic disease remains challenging. Unraveling the cellular mechanisms and molecular targets fueling metastatic spread is crucial for advancing cancer therapies. In a recent issue of Cancer Discovery, Dashzeveg and colleagues revealed that loss of terminal sialylation in glycoproteins within circulating tumor cell clusters is a dynamic process that contributes to cellular dormancy, facilitates evasion of chemotherapy, and enhances metastatic seeding. Furthermore, the study identifies the glycoprotein podocalyxin (PODXL) as a potential target for counteracting the metastasis of quiescent tumor cells associated with paclitaxel treatment in triple-negative breast cancer.

Travelling under pressure - hypoxia and shear stress in the metastatic journey.

Cancer cell invasion, intravasation and survival in the bloodstream are early steps of the metastatic process, pivotal to enabling the spread of cancer to distant tissues. Circulating tumor cells (CTCs) represent a highly selected subpopulation of cancer cells that tamed these critical steps, and a better understanding of their biology and driving molecular principles may facilitate the development of novel tools to prevent metastasis. Here, we describe key research advances in this field, aiming at describing early metastasis-related processes such as collective invasion, shedding, and survival of CTCs in the bloodstream, paying particular attention to microenvironmental factors like hypoxia and mechanical stress, considered as important influencers of the metastatic journey.

A new time dimension in the fight against metastasis.

Despite advances in uncovering vulnerabilities, identifying biomarkers, and developing more efficient treatments, cancer remains a threat because of its ability to progress while acquiring resistance to therapy. The circadian rhythm governs most of the cellular functions implicated in cancer progression, and its exploitation therefore opens new promising directions in the fight against metastasis. In this review we summarize the role of the circadian rhythm in tumor development and progression, with emphasis on the circadian rhythm-regulated elements that control the generation of circulating tumor cells (CTCs) and metastasis. We then present data on chronotherapy and discuss how circadian rhythm investigations may open new paths to more effective anticancer treatments.

Circulating tumour cells in gastrointestinal cancers: food for thought?

Gastrointestinal (GI) cancers account for 35% of cancer-related deaths, predominantly due to their ability to spread and generate drug-tolerant metastases. Arising from different locations in the GI system, the majority of metastatic GI malignancies colonise the liver and the lungs. In this context, circulating tumour cells (CTCs) are playing a critical role in the formation of new metastases, and their presence in the blood of patients has been correlated with a poor outcome. In addition to their prognostic utility, prospective targeting of CTCs may represent a novel, yet ambitious strategy in the fight against metastasis. A better understanding of CTC biology, mechanistic underpinnings and weaknesses may facilitate the development of previously underappreciated anti-metastasis approaches. Here, along with related clinical studies, we outline a selection of the literature describing biological features of CTCs with an impact on their metastasis forming ability in different GI cancers.

EMP1-positive cells found guilty of metastatic relapse in colorectal cancer.

Metastatic recurrence develops in 30%-40% of colorectal cancer (CRC) patients in the years that follow surgical removal of the primary tumor. In a recent issue of Nature, Cañellas-Socias et al. identify a distinct population of CRC cells, marked with epithelial membrane protein 1 (EMP1), accountable for metastatic relapse.

Inhibition of the activin receptor signaling pathway: A novel intervention against osteosarcoma.

Osteosarcoma is a cancer of pathological bone remodeling with high mortality and severe comorbidity. New therapies are urgently needed. Activin A, a member of the transforming growth factor ß (TGFß) superfamily, has been suggested to stimulate proliferation and invasion of osteosarcoma cells in vitro, thus representing a potential therapeutic target. In this study, inhibition of the activin receptor signaling pathway was explored as a therapy for osteosarcoma. In a murine intratibial osteosarcoma xenograft model, two types of inhibitors were tested: (a) a soluble activin type IIA decoy receptor (ActRIIA-mFc), or (b) a modified variant of follistatin (FSTΔHBS -hFc), either alone or in combination with a bisphosphonate. Both inhibitors reduced primary tumor development by nearly 50% compared to vehicle treatment. When ActRIIA-mFc was combined with bisphosphonate, the effect on tumor size became even more pronounced (78% reduction vs. vehicle). Moreover, FSTΔHBS -hFc increased body weight in the face of tumor progression (14% increase vs. vehicle), and ActRIIA-mFc reduced the number of lung metastases when combined with bisphosphonate. The present study demonstrates a novel approach to treating osteosarcoma and encourages further investigation of inhibition of the activin receptor signaling pathway as an intervention against the disease.

Exploring the Role of Osteosarcoma-Derived Extracellular Vesicles in Pre-Metastatic Niche Formation and Metastasis in the 143-B Xenograft Mouse Osteosarcoma Model.

The pre-metastatic niche (PMN) is a tumor-driven microenvironment in distant organs that can foster and support the survival and growth of disseminated tumor cells. This facilitates the establishment of secondary lesions that eventually form overt metastasis, the main cause of cancer-related death. In recent years, tumor-derived extracellular-vesicles (EVs) have emerged as potentially key drivers of the PMN. The role of the PMN in osteosarcoma metastasis is poorly understood and the potential contribution of osteosarcoma cell-derived EVs to PMN formation has not been investigated so far. Here, we characterize pulmonary PMN development using the spontaneously metastasizing 143-B xenograft osteosarcoma mouse model. We demonstrate the accumulation of CD11b+ myeloid cells in the pre-metastatic lungs of tumor-bearing mice. We also establish that highly metastatic 143-B and poorly metastatic SAOS-2 osteosarcoma cell-derived EV education in naïve mice can recapitulate the recruitment of myeloid cells to the lungs. Surprisingly, despite EV-induced myeloid cell infiltration in the pre-metastatic lungs, 143-B and SAOS-2 EVs do not contribute towards the 143-B metastatic burden in the context of both spontaneous as well as experimental metastasis in severe-combined immunodeficient (SCID) mice. Taken together, OS-derived EVs alone may not be able to form a functional PMN, and may perhaps require a combination of tumor-secreted factors along with EVs to do so. Additionally, our study gives a valuable insight into the PMN complexity by providing the transcriptomic signature of the premetastatic lungs in an osteosarcoma xenograft model for the first time. In conclusion, identification of regulators of cellular and molecular changes in the pre-metastatic lungs might lead to the development of a combination therapies in the future that interrupt PMN formation and combat osteosarcoma metastasis.

Osteosarcoma-Derived Extracellular Vesicles Induce Lung Fibroblast Reprogramming.

Tumor-secreted extracellular vesicles (EVs) have been identified as mediators of cancer-host intercellular communication and shown to support pre-metastatic niche formation by modulating stromal cells at future metastatic sites. While osteosarcoma, the most common primary malignant bone tumor in children and adolescents, has a high propensity for pulmonary metastases, the interaction of osteosarcoma cells with resident lung cells remains poorly understood. Here, we deliver foundational in vitro evidence that osteosarcoma cell-derived EVs drive myofibroblast/cancer-associated fibroblast differentiation. Human lung fibroblasts displayed increased invasive competence, in addition to increased α-smooth muscle actin expression and fibronectin production upon EV treatment. Furthermore, we demonstrate, through the use of transforming growth factor beta receptor 1 (TGFBR1) inhibitors and CRISPR-Cas9-mediated knockouts, that TGFß1 present in osteosarcoma cell-derived EVs is responsible for lung fibroblast differentiation. Overall, our study highlights osteosarcoma-derived EVs as novel regulators of lung fibroblast activation and provides mechanistic insight into how osteosarcoma cells can modulate distant cells to potentially support metastatic progression.

The miR-143/145 Cluster, a Novel Diagnostic Biomarker in Chondrosarcoma, Acts as a Tumor Suppressor and Directly Inhibits Fascin-1.

Chondrosarcoma is the second most frequent bone sarcoma. Due to the inherent chemotherapy and radiotherapy resistance and absence of known therapeutic targets, clinical management is limited to surgical resection. Consequently, patients with advanced disease face a poor prognosis. Hence, elucidating regulatory networks governing chondrosarcoma pathogenesis is vital for development of effective therapeutic strategies. Here, miRNA and mRNA next generation sequencing of different subtypes of human chondrogenic tumors in combination with in silico bioinformatics tools were performed with the aim to identify key molecular factors. We identified miR-143/145 cluster levels to inversely correlate with tumor grade. This deregulation was echoed in the miRNA plasma levels of patients and we provided the first evidence that circulating miR-145 is a potential noninvasive diagnostic biomarker and can be valuable as an indicator to improve the currently challenging diagnosis of cartilaginous bone tumors. Additionally, artificial upregulation of both miRNAs impelled a potent tumor suppressor effect in vitro and in vivo in an orthotopic xenograft mouse model. A combined in silico/sequencing approach revealed FSCN1 as a direct target of miR-143/145, and its depletion phenotypically resembled miR-143/145 upregulation in vitro. Last, FSCN1 is a malignancy-promoting factor associated with aggressive chondrosarcoma progression. Our findings underscore miR-143/145/FSCN1 as important players in chondrosarcoma and may potentially open new avenues for specific therapeutic intervention options. © 2020 American Society for Bone and Mineral Research.

A bispecific antibody targeting IGF-IR and EGFR has tumor and metastasis suppressive activity in an orthotopic xenograft osteosarcoma mouse model.

Osteosarcoma is a highly aggressive bone cancer and the second most frequent cause of cancer-associated death in childhood and adolescence. Pulmonary metastases account for the high mortality rate in osteosarcoma patients. Therefore, novel therapeutic approaches, efficiently restraining the metastatic disease, are mandatory for a significant improvement of the currently poor patients' survival. Although initial studies with antibodies targeting insulin-like growth factor receptor (IGF-IR) showed promising potential for the treatment of patients with bone and soft tissue sarcomas, phase II clinical trials revealed variable results, which implied activation of alternative signaling pathways leading to therapy resistance. Since a cross-talk between IGF-IR and the epidermal growth factor receptor (EGFR) has been demonstrated in several cancer types, co-targeting of these two receptors was considered in the present study as a valuable therapeutic strategy to overcome single-agent treatment resistance in osteosarcoma. The effects of IGF-IR and/or EGFR targeting by intraperitoneal administration of the monospecific IGF-IR antibody R1507 or the EGFR antibody Cetuximab or the bispecific IGF-IR/EGFR antibody XGFR* on primary tumor growth and pulmonary metastasis were investigated in an intratibial human xenograft osteosarcoma mouse model. In vitro functional assays demonstrated that targeting IGF-IR and EGFR didn't affect osteosarcoma cell viability, but inhibited ligand-activated intracellular signaling and cell migratory capacity. The blocking potential of ligand-induced signaling in vitro was similar for all antibodies, but, in vivo, only XGFR* treatment significantly inhibited intratibial primary tumor growth and pulmonary metastasis. The therapeutic response to XGFR* was associated with an infiltration of innate immune system effector cells into the tumor microenvironment. Taken together, our study highlights the bispecific anti-IGF-IR/EGFR antibody XGFR* as an innovative promising effective candidate for the treatment of metastatic osteosarcoma and provides the rationale for future clinical studies.

Targeting αvß3 and αvß5 integrins inhibits pulmonary metastasis in an intratibial xenograft osteosarcoma mouse model.

Osteosarcoma is an aggressive bone cancer that has a high propensity for metastasis to the lungs. Patients with metastatic disease face a very poor prognosis. Therefore, novel therapeutics, efficiently suppressing the metastatic process, are urgently needed. Integrins play a pivotal role in tumor cell adhesion, motility and metastasis. Here, we evaluated αvß3 and αvß5 integrin inhibition with cilengitide as a novel metastasis-suppressive therapeutic approach in osteosarcoma. Immunohistochemical analysis of αvß3 and αvß5 integrins expression in a tissue microarray of tumor specimens collected from osteosarcoma patients revealed that αvß5 integrin is mainly found on tumor cells, whereas αvß3 is predominantly expressed by stromal cells. In vitro functional assays demonstrated that cilengitide dose-dependently inhibited de novo adhesion, provoked detachment and inhibited migration of osteosarcoma cell lines. Cilengitide induced a decline in cell viability, blocked the cell cycle in the G1 phase and caused anoikis by activation of the Hippo pathway. In a xenograft orthotopic mouse model cilengitide minimally affected intratibial primary tumor growth but, importantly, suppressed pulmonary metastasis. The data demonstrate that targeting αvß3 and αvß5 integrins in osteosarcoma should be considered as a novel therapeutic option for patients with metastatic disease.

Altered CXCL12 expression reveals a dual role of CXCR4 in osteosarcoma primary tumor growth and metastasis.

PURPOSE: Better understanding of the molecular mechanisms of metastasis-the major cause of death in osteosarcoma (OS)-is a key for the development of more effective metastasis-suppressive therapy. Here, we investigated the biological relevance of the CXCL12/CXCR4 axis in OS. METHODS: We interfered with CXCL12/CXCR4 signaling in CXCR4-expressing human 143-B OS cells through stable expression of CXCL12, of its competitive antagonist P2G, or of CXCL12-KDEL, designed to retain CXCR4 within the cell. Intratibial OS xenograft mouse model metastasizing to the lung was used to assess tumorigenic and metastatic potential of the manipulated cell lines. RESULTS: Constitutive expression of native CXCL12 promoted lung metastasis without affecting tumor growth. Stable expression of P2G or CXCL12-KDEL significantly accelerated tumor growth but diminished lung metastasis. Tumors grown from P2G- or CXCL12-KDEL-expressing cells contained higher levels of CXCR4-encoding mRNA going along with a higher percentage of CXCR4-expressing tumor cells. Lung metastases of all groups were predominantly enriched with CXCR4-expressing tumor cells. CONCLUSION: Higher abundance of CXCR4 possibly contributed to increased local retention of tumor cells by bone marrow-derived CXCL12, reflected in the increased primary tumor growth and decreased number of lung metastases in P2G and CXCL12-KDEL groups. Higher percentage of CXCR4-expressing lung metastatic cells compared to the corresponding primary tumors point to important functions of the CXCL12/CXCR4 axis in late steps of metastasis. In conclusion, based on the here reported results, local treatment of lung metastases with novel CXCR4-targeting therapeutics might be considered and favored over anti-CXCR4 systemic therapy.

An in vitro osteosarcoma 3D microtissue model for drug development.

Osteosarcoma (OS) is the most common primary malignant bone tumour in children and adolescents. Therapy today includes surgical removal of the tumour and neoadjuvant and adjuvant chemotherapy. The 5-year survival rates for patients with localised disease are between 50 and 70%, but in patients with metastases the prognosis remains poor (∼ 20%). The aim of this study was the development of a biological relevant OS 3D microtissue model, which is suitable for drug development. Microtissues were formed by the hanging drop method with the established OS cell lines SaOS-2, HOS and MG-63, as well as with cells derived from osteoblastic and chondroblastic OS patient material. Histological characterisation of the microtissues with H/E- and Ki-67-(proliferation), as well as apoptosis staining (TUNEL) revealed the inherent histological heterogeneity of OS. Microtissues from SaOS-2 and HOS cell lines were exposed to doxorubicin, cisplatin, taurolidine, pemetrexed and taxol and the viability was assessed by the CellTiter-GLO(®) Luminescent Cell Viability Assay. The obtained IC50-values for 3D cultures were all higher (1.7 to >16,000-fold) when compared to corresponding cells grown in 2D monolayer culture, except for pemetrexed that was inactive in 2D and 3D cultures. Doxorubicin did not affect the viability of chondroblastic monolayer cultures whereas on 3D microtissues an IC50-value of 2.3 µM was obtained. The 3D microtissues reflect the tissue heterogeneity of OS and are potential suitable tools for drug development towards personalised medicine.

CXCR4 antibody treatment suppresses metastatic spread to the lung of intratibial human osteosarcoma xenografts in mice.

Current combined surgical and neo-adjuvant chemotherapy of primary metastatic osteosarcoma (OS) is ineffective, reflected by a 5-year survival rate of affected patients of less than 20 %. Studies in experimental OS metastasis models pointed to the CXCR4/CXCL12 homing axis as a novel target for OS metastasis-suppressive treatment. The present study investigated for the first time the CXCR4-blocking principle in a spontaneously metastasizing human 143B OS cell line-derived orthotopic xenograft mouse model. The highly metastatic 143B cells, unlike the parental non-metastatic HOS cells, express functional CXCR4 receptors at the cell surface, as revealed in this study by RT/PCR of gene transcripts, by FACS analysis with the monoclonal anti CXCR4 antibody 12G5 (mAb 12G5) and by CXCL12 time- and dose-dependent stimulation of AKT and ERK phosphorylation. A significantly (p < 0.05) higher CXCL12 dose-dependent chemotactic response of 143B compared to HOS cells in a Boyden chamber trans-well migration assay suggested a crucial role of the CXCL12/CXCR4 homing axis in 143B cell lung metastasis. Repetitive treatment of mice with 143B cell-derived intratibial tumors given intravenous bolus injections of mAb12G5 indeed inhibited significantly (p < 0.01) the number of X-gal-stainable lung micrometastases of lacZ-transduced 143B cells. Antibody treatment had also a mild inhibitory effect on primary tumor growth associated with remarkably less osteolysis, but it did not affect the number of developing lung macrometastases. In conclusion, these results demonstrate considerable potential of high-affinity CXCR4-blocking agents for OS tumor cell homing suppressive treatment in metastasizing OS complementary to current (neo)-adjuvant chemotherapy.

Expression of the chemokine receptor CXCR7 in CXCR4-expressing human 143B osteosarcoma cells enhances lung metastasis of intratibial xenografts in SCID mice.

More effective treatment of metastasizing osteosarcoma with a current mean 5-year survival rate of less than 20% requires more detailed knowledge on mechanisms and key regulatory molecules of the complex metastatic process. CXCR4, the receptor of the chemokine CXCL12, has been reported to promote tumor progression and metastasis in osteosarcoma. CXCR7 is a recently deorphanized CXCL12-scavenging receptor with so far not well-defined functions in tumor biology. The present study focused on a potential malignancy enhancing function of CXCR7 in interaction with CXCR4 in osteosarcoma, which was investigated in an intratibial osteosarcoma model in SCID mice, making use of the human 143B osteosarcoma cell line that spontaneously metastasizes to the lung and expresses endogenous CXCR4. 143B osteosarcoma cells stably expressing LacZ (143B-LacZ cells) were retrovirally transduced with a gene encoding HA-tagged CXCR7 (143B-LacZ-X7-HA cells). 143B-LacZ-X7-HA cells co-expressing CXCR7 and CXCR4 exhibited CXCL12 scavenging and enhanced adhesion to IL-1ß-activated HUVEC cells compared to 143B-LacZ cells expressing CXCR4 alone. SCID mice intratibially injected with 143B-LacZ-X7-HA cells had significantly (p<0.05) smaller primary tumors, but significantly (p<0.05) higher numbers of lung metastases than mice injected with 143B-LacZ cells. Unexpectedly, 143B-LacZ-X7-HA cells, unlike 143B-LacZ cells, also metastasized with high incidence to the auriculum cordis. In conclusion, expression of the CXCL12 scavenging receptor CXCR7 in the CXCR4-expressing human 143B osteosarcoma cell line enhances its metastatic activity in intratibial primary tumors in SCID mice that predominantly metastasize to the lung and thereby closely mimic the human disease. These findings point to CXCR7 as a target, complementary to previously proposed CXCR4, for more effective metastasis-suppressive treatment in osteosarcoma.

Characterization of different osteosarcoma phenotypes by PET imaging in preclinical animal models.

UNLABELLED: The aim of this study was to characterize the different phenotypes of osteosarcoma by PET, comparing the uptake of 3 tracers ((18)F-FDG, (18)F-fluoromisonidazole [(18)F-FMISO], and (18)F-fluoride) in preclinical mouse models that reflect the heterogeneity of the human disease. METHODS: Mouse LM8 osteosarcoma, human 143B, and Caprin-1 stably overexpressing SaOS-2 cells were injected intratibially in C3H and severe-combined immunodeficient mice. PET imaging with (18)F-FDG, (18)F-FMISO, and (18)F-fluoride was performed in these mouse models, and a ratio between the standardized uptake value of the primary tumor and a control area of bone was calculated and compared among the models. Histology and immunohistochemistry were performed to confirm the PET findings. RESULTS: The pattern of tracer uptake differed among the primary tumors of the 3 models in accordance with the histology and immunohistochemistry on primary tumor sections. The osteolytic tumors in the 143B model showed the highest uptake of (18)F-FDG, an indicator of glucose metabolism, which was significantly higher (P < 0.05) than in the SaOS-2/Caprin-1 model and correlated with the percentage of Ki67-positive cells in the primary tumors. Hypoxia, indicated by (18)F-FMISO accumulation, was higher in the SaOS-2/Caprin-1 and 143B cell line-derived tumors (P < 0.01). Finally (18)F-fluoride, a marker of bone remodeling, correlated with the osteoblastic phenotype. The SaOS-2/Caprin-1 cell-derived tumors showed a significantly higher uptake than the moderately osteoblastic LM8 (P < 0.05) and the osteolytic 143B (P < 0.01) cell line-derived tumors. CONCLUSION: Differential PET imaging with tracers indicating metabolic activity, hypoxia, or bone remodeling will be helpful for the characterization of different osteosarcoma phenotypes and subsequent evaluation of more specific treatment modalities targeting the processes that are predominant in each specific tumor type or subtype.

Silencing of CD44 gene expression in human 143-B osteosarcoma cells promotes metastasis of intratibial tumors in SCID mice.

Osteosarcoma (OS) is the most frequent primary malignant bone cancer in children and adolescents with a high propensity for lung metastasis. Therefore, it is of great importance to identify molecular markers leading to increased metastatic potential in order to devise more effective therapeutic strategies that suppress metastasis, the major cause of death in OS. CD44, the principal receptor for the extracellular matrix component hyaluronan (HA), is frequently found overexpressed in tumor cells and has been implicated in metastatic spread in various cancer types. Here, we investigated the effects of stable shRNA-mediated silencing of CD44 gene products on in vitro and in vivo metastatic properties of the highly metastatic human 143-B OS cell line. In vitro, CD44 knockdown resulted in a 73% decrease in the adhesion to HA, a 57% decrease in the migration rate in a trans-filter migration assay, and a 28% decrease in the cells' capacity for anchorage-independent growth in soft agar compared to the control cells, implicating that CD44 expression contributes to the metastatic activity of 143-B cells. However, making use of an orthotopic xenograft OS mouse model, we demonstrated that reduced CD44 expression facilitated primary tumor growth and formation of pulmonary metastases. The enhanced malignant phenotype was associated with decreased adhesion to HA and reduced expression of the tumor suppressor merlin in vivo. In conclusion, our study identified CD44 as a metastasis suppressor in this particular experimental OS model.

CD44 enhances tumor formation and lung metastasis in experimental osteosarcoma and is an additional predictor for poor patient outcome.

Formation of metastases in the lungs is the major cause of death in patients suffering from osteosarcoma (OS). Metastases at presentation and poor response to preoperative chemotherapy are strong predictors for poor patient outcome. The elucidation of molecular markers that promote metastasis formation and/or chemoresistance is therefore of importance. CD44 is a plasma membrane glycoprotein that binds to the extracellular matrix component hyaluronan (HA) and has been shown to be involved in metastasis formation in a variety of other tumors. Here we investigated the role of CD44 expression on OS tumor formation and metastasis. High CD44 expression, evaluated with a tissue microarray including samples from 53 OS patients and stained with a pan-CD44 antibody (Hermes3), showed a tendency (p < 0.08) to shortened overall survival. However, nonresponders and patients with lung metastases and high CD44 expression had significantly poorer prognosis than patients with low CD44 expression. Overexpression of the standard CD44 isoform (CD44s) and its HA-binding defective mutant R41A in osteoblastic SaOS-2 cells resulted in HA-independent higher migration rates and increased chemoresistance, partially dependent on HA. In an orthotopic mouse model of OS, overexpression of CD44s in SaOS-2 cells resulted in an HA-dependent increased primary tumor formation and increased numbers of micrometastases and macrometastases in the lungs. In conclusion, although CD44 failed to be an independent predictor for patient outcome in this limited cohort of OS patients, increased CD44 expression was associated with even worse survival in patients with chemoresistance and with lung metastases. CD44-associated chemoresistance was also observed in vitro, and increased formation of lung metastases was found in vivo in SCID mice.