Spontaneous formation of tumor spheroid on a hydrophilic filter paper for cancer stem cell enrichment

Emerging evidence has demonstrated that cancer stem cells (CSCs) play critical roles in tumor invasion, metastasis and recurrence. The specific targeting capability on CSCs is of high importance for the development of effective anti-tumor therapeutics. However, isolation, enrichment and cultivation of these special and rare groups of tumor cells for in vitro analyses is a nontrivial job and requires particular culture medium and environmental control. Herein, we established a low-cost and efficient method for CSC enrichment by culturing prostate cancer cells on a hydrophilic filter paper. We found that tumor spheroids could form spontaneously on a pristine filter paper solely with regular cell culture medium. The paper-grown cells had elevated expression of putative CSC markers, indicating increased stemness of the cancer cells. Moreover, increased resistance of the chemotherapeutic drug doxorubicin was observed on the formed CSC spheroids compared to regular culture. The properties of the filter paper were characterized to investigate the underlying mechanism behind the promoted tumor spheroid formation. The obtained results suggested that the excellent hydrophilicity of the cellulose fibers retarded the hydrophobic interaction-mediated cell anchoring on the cellulose fibers, while the limited space/niche between fibers promoted the aggregation of cells. In addition, biocompatible paper-based materials are able to realize convenient assembly of tissue-like structures for developing in vitro disease models or organs-on-paper applications. Therefore, hydrophilic filter papers could be a low-cost material for construction of various assay platforms for isolating and enriching CSCs, screening anti-tumor drugs, and constructing tumor models in vitro.