Simple SummaryAn RNA aptamer targeting the extracellular sheddase domain of Adam8 was isolated and characterized both in vitro and in vivo. In co-cultures of human mesenchymal stem cells with human MDA-MB-231 breast cancer or Hep G2 cell liver cancer cells, the aptamer blocked extracellular Adam8 activities with associated reversal of the previously established cancer-derived osteopontin-induced myofibroblast cancer-associated fibroblast phenotype (myCAF). Our results suggest that the signal pathways that initiate the development of the myCAF phenotype may be distinct from those required for maintenance, that extracellular Adam8 sheddase activity is required for maintenance of the myCAF phenotype, and that this aptamer may serve as a vehicle for the further investigation of this new pathway.AbstractCancer progression depends on an accumulation of metastasis-supporting physiological changes, which are regulated by cell-signaling molecules. In this regard, a disintegrin and metalloproteinase 8 (Adam8) is a transmembrane glycoprotein that is selectively expressed and induced by a variety of inflammatory stimuli. In this study, we identified Adam8 as a sox2-dependent protein expressed in MDA-MB-231 breast cancer cells when cocultured with mesenchymal-stem-cell-derived myofibroblast-like cancer-associated fibroblasts (myCAF). We have previously found that myCAF-induced cancer stemness is required for the maintenance of the myCAF phenotype, suggesting that the initiation and maintenance of the myCAF phenotype require distinct cell-signaling crosstalk pathways between cancer cells and myCAF. Adam8 was identified as a candidate secreted protein induced by myCAF-mediated cancer stemness. Adam8 has a known sheddase function against which we developed an RNA aptamer, namely, Adam8-Apt1-26nt. The Adam8-Apt1-26nt-mediated blockade of the extracellular soluble Adam8 metalloproteinase domain abolishes the previously initiated myCAF phenotype, or, termed differently, blocks the maintenance of the myCAF phenotype. Consequently, cancer stemness is significantly decreased. Xenograft models show that Adam8-Apt-1-26nt administration is associated with decreased tumor growth and metastasis, while flow cytometric analyses demonstrate a significantly decreased fraction of myCAF after Adam8-Apt-1-26nt treatment. The role of soluble Adam8 in the maintenance of the myCAF phenotype has not been previously characterized. Our study suggests that the signal pathways for the induction or initiation of the myCAF phenotype may be distinct from those involved with the maintenance of the myCAF phenotype.