Cluster of differentiation 44 promotes osteosarcoma progression in mice lacking the tumor suppressor Merlin

Merlin is a versatile tumor suppressor protein encoded by theNF2gene. Several lines of evidence suggest that Merlin exerts its tumor suppressor activity, at least in part, by forming an inhibitory complex with cluster of differentiation 44 (CD44). Consistently, numerousNF2mutations in cancer patients are predicted to perturb the interaction of Merlin with CD44. We hypothesized that disruption of the Merlin-CD44 complex through loss of Merlin, unleashes putative tumor- or metastasis-promoting functions of CD44. To evaluate the relevance of the Merlin-CD44 interaction in vivo, we compared tumor growth and progression inCd44-positive andCd44-negativeNf2-mutant mice. HeterozygousNf2-mutant mice were prone to developing highly metastatic osteosarcomas. Importantly, while the absence of theCd44gene had no effect on the frequency of primary osteosarcoma development, it strongly diminished osteosarcoma metastasis formation in theNf2-mutant mice. In vitro assays identified transendothelial migration as the most prominent cellular phenotype dependent on CD44. Adhesion to endothelial cells was blocked by interfering with integrin alpha 4 beta 1(very late antigen-4, VLA-4) on osteosarcoma cells and CD44 upregulated levels of integrin VLA-4 beta 1subunit. Among other putative functions of CD44, which may contribute to the metastatic behavior, the passage through the endothelial cells also appears to be critical in vivo, as CD44 significantly promoted formation of lung metastasis upon intravenous injection of osteosarcoma cells into immunocompromised mice. Altogether, our results strongly suggest that CD44 plays a metastasis-promoting role in the absence of Merlin.