Cell hiding in colorectal cancer: correlation with response to chemotherapy in vitro and in vivo

Resistance to chemotherapy remains the main challenge for cancer treatment. One of the mechanisms of tumor escape from cytotoxic agents could be the formation of cell-in-cell (CIC) structures, in which the outer cell protects the inner cell from unfavorable environment. Such structures have been found in many tumor types, however, their link to chemosensitivity is elusive. Here, we tested whether the CIC structures can promote resistance of colorectal cancer cells to chemotherapy. To identify CIC structures in cell cultures and in tumor xenografts, both transmission electron microscopy and confocal fluorescence microscopy of live and fixed cells as well as tissue slices and histopathology were used. Cytogenetic analysis was performed to detect chromosome instability associated with the drug resistance. It was found that in the five colorectal cancer cell lines intrinsic chemoresistance positively correlated with the ability of cells to spontaneously form CIC structures. Cultured cells treated with oxaliplatin and Irinotecan and tumor xenografts treated with FOLFOX or FOLFIRI regimens displayed an increased number of CICs after the treatment. The release of the inner cell from CIC structure was observed after removal of the drug. The number of CICs in the cell lines and tumors with acquired resistance to oxaliplatin was higher than in the drug-naive counterparts. The development of chemoresistance was also accompanied by the changes in the cell's ploidy. These preliminary data clearly demonstrate the associations of CIC structures with chemoresistance of colorectal cancer in cultured cells and tumor xenografts and show the prospect of further clinical validation of CICs as a potential prognostic marker for treatment efficiency.