Carcinogen-induced oral cavity squamous cell carcinoma (OSCC) incurs significant morbidity and mortality and constitutes a global health challenge. To gain further insight into this disease, we generated cell line models from DMBA-induced murine primary OSCC capable of tumor formation upon transplantation into immunocompetent wild-type mice. While several lines grew rapidly and were capable of metastasis, some grew slowly and did not metastasize. Aggressively growing lines displayed ERK1/2 activation, which stimulated expression of CD44, a marker associated with EMT and putative cancer stem cells. MEK inhibition upstream of ERK1/2 decreased CD44 expression and promoter activity and reduced cell migration and invasion. Conversely, MEK1 activation enhanced CD44 expression and promoter activity, whereas CD44 attenuation reduced in vitro migration and in vivo tumor formation. Extending these findings to freshly resected human OSCC, we confirmed a strict relationship between ERK1/2 phosphorylation and CD44 expression. In summary, our findings identify CD44 as a critical target of ERK1/2 in promoting tumor aggressiveness and offer a preclinical proof of concept to target this pathway as a strategy to treat head and neck cancer.

CD44 is a major cell-surface receptor for hyaluronic acid (HA), a glycosaminoglycan component of extracellular matrix. HA-CD44 interactions have been implicated in leukocyte extravasation into an inflammatory site. This study examined the role of CD44 in acute inflammatory responses during pneumonias induced by Escherichia coli and Streptococcus pneumoniae using CD44-deficient mice. In E. coli-induced pneumonia, neutrophil accumulation in the lungs and edema formation was increased by 84% and 88%, respectively, in CD44-deficient mice compared to wild-type mice. In contrast, no difference was observed between these genotypes in S. pneumoniaeinduced pneumonia, and the HA content in the lungs decreased after instillation of S. pneumoniae, but not E. coli, in both genotypes. Studies to determine the mechanisms for this enhanced response showed that: 1) neutrophil apoptosis was not different between these two genotypes in either type of pneumonia; 2) CD44 deficiency resulted in enhanced mRNA expression of several inflammatory genes; and 3) CD44-deficient neutrophils migrated through Matrigel in response to chemoattractants faster and in greater numbers than wild-type neutrophils in vitro and this increase was in part dependent on HA content in the Matrigel. These data demonstrate that CD44 deficiency results in enhanced inflammation in E. coli but not S. pneumoniae-induced pneumonia, suggesting a previously unrecognized role for CD44 in limiting the inflammatory response to E. coli. Neutrophil accumulation at inflammatory sites is an important feature of acute inflammatory responses. During pulmonary inflammation, circulating neutrophils become sequestered within pulmonary capillaries, a process that does not require rolling. Neutrophils then migrate across the endothelium and through the pulmonary interstitium, which contains fibroblasts and extracellular matrix proteins such as collagens and proteoglycans.1-4 Finally, neutrophils migrate between alveolar epithelial cells, often between a type II and a type I pneumocyte, into the airspace. 1,5 CD44 is a type I transmembrane glycoprotein that is expressed by most cell types, including leukocytes, and is the major cell surface receptor for hyaluronan (HA). HA is a nonsulfated glycosaminoglycan component of the extracellular matrix and plays a major role in maintenance of tissue integrity.6 -8 CD44 has 10 different splice variants and neutrophils express the standard CD44 isoform, CD44s.9 CD44 may modulate immunological and inflammatory responses through at least two mechanisms. First, CD44 may play important roles in modulating leukocyte extravasation. Interaction between CD44 and HA is implicated in lymphocyte rolling and extravasation, and optimal binding of these two molecules is modulated by proinflammatory cytokines such as tumor necrosis factor-␣.10 -14 The role of CD44 in mediating neutrophil emigration during acute inflammatory responses is less well understood. Second, CD44 is capable of inducing signal transduction pathways and cell activation. Ligation of C...