CD44 Expressed on Cancer-Associated Fibroblasts Is a Functional Molecule Supporting the Stemness and Drug Resistance of Malignant Cancer Cells in the Tumor Microenvironment

Cells constituting the tumor microenvironment are attractive targets for developing new cancer therapies. Here we show that cancer-associated fibroblasts (CAFs) support tumor growth in vivo and maintain the stemness of cancer stem/initiating cells in an in vitro model using an established CAF cell line. We found that CD44 is abundantly expressed on CAFs. This molecule is a cancer stem cell marker in several tumors, but its role in tumorigenesis when expressed by CAFs has not been investigated. It is generally accepted that hypoxic and hyponutritional conditions are triggers of cancer malignancy. We found that CAFs strongly express CD44 in hypoxic and avascular areas in the tumor and that its expression on established CAFs is upregulated under hypoxic and hyponutritional conditions in vitro. In addition, CAF CD44-positivity in tumor tissues was increased after treatment with inhibitors of angiogenesis. Using cocultures and tumor sphere formation assays, CAFs from wild-type mice were found to sustain the stemness of cancer stem/initiating cells, while CD44-deficient CAFs did not. Furthermore, CD44 was involved in malignant cancer cell drug resistance mechanisms. In conclusion, our study suggests that CD44 on CAFs is a functional molecule contributing to the maintenance of cancer stem cell populations in the tumor microenvironment.