Purpose: A subpopulation of pancreatic ductal adenocarcinoma (PDAC) cells is thought to be inherently resistant to chemotherapy or to give rise to tumor cells that become resistant during treatment. Here we determined the role of CD44 expression and its isoforms as a marker and potential target for tumor cells that give rise to invasive and gemcitabineresistant tumors. Experimental Design: RT-PCR, Western blotting, and DNA sequencing was used to determine CD44 isoform and expression levels. Flow cytometry was used to sort cells on the basis of their CD44 expression level. CD44 expression was knocked down using shRNA. Tumorigenic properties were determined by clonogenic and Matrigel assays, IHC, tumor growth in vivo using luciferase imaging and by tumor weight. Results: Weidentified an invasive cell population that gives rise to gemcitabine-resistant tumors. These cancer cells express a high level of CD44 standard isoform and have an EMT phenotype (CD44s/EMT). In vivo, CD44s/EMT engraft and expand rapidly and give rise to tumors that express high levels of CD44 isoforms that contain multiple exon variants. CD44low-expressing cells show continued sensitivity to gemcitabine in vivo and knockdown of CD44 in CD44s/EMT cells increases sensitivity to gemcitabine and decreases invasiveness. Conclusions: PDAC cells expressing high levels of CD44s with a mesenchymal-like phenotype were highly invasive and developed gemcitabine resistance in vivo. Thus, initial targeting CD44 or reversing the CD44high phenotype may improve therapeutic response.
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