Cancer Res. 2020 Aug 14:canres.1079.2020. doi: 10.1158/0008-5472.CAN-20-1079. Online ahead of print.
Tumor-associated macrophages (TAM) in the tumor microenvironment (TME) cooperate with cancer stem cells (CSC) to maintain stemness. We recently identified CD44 as a surface marker defining head and neck squamous cell carcinoma (HNSCC) CSC. PI3K-4EBP1-SOX2 activation and signaling regulate CSC properties, yet the upstream molecular control of this pathway and the mechanisms underlying crosstalk between TAM and CSC in HNSCC remain largely unknown. Because CD44 is a molecular mediator in the TME,
we propose here that TAM-influenced CD44 signaling could mediate stemness via the PI3K-4EBP1-SOX2 pathway, possibly by modulating availability of hyaluronic acid (HA), the main CD44 ligand. HNSCC IHC was used to identify TAM/CSC relationships, and in vitro co-culture spheroid models and in vivo mouse models were used to identify the influence of TAM on CSC function via CD44. Patient HNSCC-derived TAM were positively and negatively associated with CSC marker expression at non-invasive and invasive edge regions, respectively. TAM increased availability of HA and increased cancer cell invasion. HA binding to CD44 increased PI3K-4EBP1-SOX2 signaling and the CSC fraction, whereas CD44-VCAM-1 binding promoted invasive signaling by Ezrin/PI3K. In vivo, targeting CD44 decreased PI3K-4EBP1-SOX2 signaling, tumor growth, and CSC. TAM depletion in syngeneic and humanized mouse models also diminished growth and CSC numbers. Lastly, a CD44 isoform switch regulated epithelial-to-mesenchymal plasticity as CD44s and CD44v8-10 determined invasive and tumorigenic phenotypes, respectively. We have established a mechanistic link between TAM and CSC in HNSCC that is mediated by CD44 intracellular signaling in response to extracellular signals.