Cancer stem-like properties in colorectal cancer cells with low proteasome activity

Purpose: One of the main reasons for cancer treatment resistance is the existence of cancer stem-like cells (CSCs). Here, we elucidated the relationship between low proteasome activity cells (LPACs) and CSCs. Experimental Design: The human colorectal cancer cell lines HCT116, SW480, DLD1, and KM12SM were engineered to stably express a green fluorescent molecule fused to the degron of ornithine decarboxylase, resulting in an accumulation of the fluorescence in LPACs. LPACs were isolated by flow cytometry. Treatment resistance (radio- and chemotherapy) and the capacity of LPACs to act as CSCs were analyzed. Microarray analysis was performed to reveal genes related to treatment resistance. The prognostic impact of potent genes was examined in 190 patients with colorectal cancer. Results: LPACs had a significantly increased capacity for radioresistance and chemoresistance (5-fluorouracil and oxaliplatin), significantly lower reactive oxygen species activity, and significantly increased sphere formation capacity compared with non-LPACs. The number of cells in the G₀-G₁ phase was significantly higher among LPACs. Subcutaneous injection of as few as 20 LPACs led to tumor formation in immunologically incompetent mice. Microarray analysis revealed that the expression of EP300-interacting inhibitor of differentiation 3 (EID3) was significantly increased in LPACs. In vitro assay revealed that EID3 positively controlled cell proliferation and treatment resistance. The high expression of EID3 was an adverse prognostic indicator in patients with colorectal cancer (P = 0.0400). Conclusions: LPACs have characteristic treatment resistance and act as CSCs in colorectal cancer. In addition, EID3 is one of the potential regulators of treatment resistance in colorectal cancer and may be a potential therapeutic target.