NDRG1 and colorectal carcinoma: Figure 4A shows that the protein expression level of NDRG1 was relatively lower in CRC cells than in normal epithelial colon cells, indicating a negative correlation between NDRG1 expression and malignancy. To determine the effect of apoptolidin A on NDRG1 expression in RKO and HCT116 cells, the cells were treated with apoptolidin A for 24 h and examined via immunoblot and real-time polymerase chain reaction (PCR) analysis. In both cell lines, apoptolidin A increased NDRG1 protein and mRNA expression in a concentration-dependent manner (Figure 4B,C).