Epithelial-mesenchymal transition (EMT) is a tightly regulated process that is critical for embryogenesis but is abnormally activated during cancer metastasis and recurrence. Here we show that a switch in CD44 alternative splicing is required for EMT. Using both in vitro and in vivo systems, we have demonstrated a shift in CD44 expression from variant isoforms (CD44v) to the standard isoform (CD44s) during EMT. This isoform switch to CD44s was essential for cells to undergo EMT and was required for the formation of breast tumors that display EMT characteristics in mice. Mechanistically, the splicing factor epithelial splicing regulatory protein 1 (ESRP1) controlled the CD44 isoform switch and was critical for regulating the EMT phenotype. Additionally, the CD44s isoform activated Akt signaling, providing a mechanistic link to a key pathway that drives EMT. Finally, CD44s expression was upregulated in high-grade human breast tumors and was correlated with the level of the mesenchymal marker N-cadherin in these tumors. Together, our data suggest that regulation of CD44 alternative splicing causally contributes to EMT and breast cancer progression.

Kras-induced non-small-cell lung adenocarcinoma is the major subtype of lung cancers and is associated with poor prognosis. Using a lung cancer mouse model that expresses a cre-mediated KrasG12D mutant, we identified a critical role for the cell surface molecule CD44 in mediating cell proliferation downstream of oncogenic Kras signaling. The deletion of CD44 attenuates lung adenocarcinoma formation and prolongs the survival of these mice. Mechanistically, CD44 is required for the activation of Krasmediated signaling through the mitogen-activated protein kinase (MAPK) pathway and thus promotes tumor cell proliferation. Together, these results reveal an unrecognized role for CD44 in oncogenic Kras-induced lung adenocarcinoma and suggest that targeting CD44 could be an effective strategy for halting Kras-dependent carcinomas.

Epithelial-mesenchymal transition (EMT) is a tightly regulated process that is critical for embryogenesis. When abnormally activated, EMT promotes cancer metastasis and recurrence. Here we show that splice isoform switching constitutes a novel mechanism that controls EMT. We identified a shift in CD44 expression from variant isoforms (CD44v) to the standard isoform (CD44s) during EMT. This isoform switch to CD44s, which is regulated at the level of alternative splicing, is essential for cells to undergo EMT and is required for the formation of breast tumors that display EMT characteristics in mice. We demonstrate that the splicing factor ESRP1 controls the CD44 isoform switch and is critical for regulating the EMT phenotype. Furthermore, the CD44s isoform activates Akt signaling and inhibits cell death, providing a mechanistic link to a key pathway that drives EMT. Finally, CD44s expression is upregulated in high-grade human breast tumors and correlates with the level of the mesenchymal marker N-cadherin in these tumors. Together, these data suggest that regulation of CD44 alternative splicing causally contributes to EMT and breast cancer progression. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 325. doi:1538-7445.AM2012-325