CQ potentiates the G0/G1 arrest induced by 5-FU. (A) The analysis of changes in cell cycle was quantified by flow-cytometry after PI staining in HT-29 cells treated without or with 5-FU for 24, 48 h after 12-h pretreatment without or with CQ. Values were given as mean ± SD. Treatment of HT-29 cells with 5-FU alone resulted in increased intra-S arrest, but the G1 arrest was potentiated by CQ-pretreatment. Furthermore, G2/M progression of HT-29 cells was blocked by the treatment with 5-FU alone, and it was potentiated by the 12-h pretreatment of CQ (*, p < 0.05 vs. control). (B) The colony formation rate was quantified in HT-29 cells treated without or with 5-FU for 48 h after 12-h pretreatment without or with CQ. Values were given as mean ± SD. Treatment of cells with 5-FU alone resulted in a significant delay in the colony-forming ability, but at day 11 of culture, approximately 90% of the cells have formed colonies, suggestive that these cells were in a dormant state. Pre-treatment of cells with CQ prior to exposure to 5-FU resulted in potentiation of the inhibitory effect on the colony forming ability, which was reduced to approximately 35% of control untreated cells at day 11. CQ alone partially inhibited the colony forming ability of HT-29, but it was almost completely recovered by day 11.