*3.1. Radiation Increased the Population of Radioresistant Rather Than Radiosensitive CD44+ Colorectal Cancer Cells*

Experimental and clinical data show that CSCs play an important role in tumor recurrence and resistance to therapy [7–9]. To investigate the relationship between radiation resistance and population of CSCs, we first confirmed the radiation resistance in various types of colorectal cancer cells, including in previously reported radioresistant and radiosensitive cells [17]. Colorectal cancer cells such as HCT116, DLD1, and HT29 were relatively resistant to radiation by annexin V/PI staining and colony formation assay. (Figure 1A,B); however, the expression of colorectal CSC markers such as CD44, CD133, and ALDH was similar to that of radiosensitive colorectal cancer cells under untreated conditions (Figure 1C). Next, we examined the effect of radiation on the expression of colorectal CSC markers. Flow cytometry and immunocytochemistry analysis (Figure 1C,D) showed that radiation increased the expression of CD133 and ALDH in all colorectal cancer cells except LoVo cells with unchanged expression of CD133, whereas CD44 expression was selectively increased in radioresistant colorectal cancer cells such as HCT116, DLD1, and HT29 (Figure 1C). These results suggested that resistance to radiation in radioresistant colorectal cancer cells may be acquired by radiation-upregulated expression of CD44, which is one of the markers of colorectal CSCs.
