PiHL promotes 5-FU resistance in colorectal HCT116 xenograft tumors. (A) PiHL reduced p53-mediated apoptosis induced by 5-FU (500 μM) in p53+/+ and p53-/- HCT116 cells. The percentage of apoptotic cells was determined by PI and Annexin V staining. (B) HCT116 p53+/+-PiHL, HCT116 p53-/--PiHL and their control cells were employed for xenograft tumor formation in nude mice. When tumor volumes reached 200 mm3, mice were treated with 5-FU (30 mg/kg daily) or vehicle for 12 days. Relative tumor volumes are presented as mean±s.e.m, n=6. The fold reduction of tumor volumes by 5-FU for each group was calculated. *p < 0.05, ***p < 0.001 (C, D) 5-FU-induced apoptosis was determined by IHC staining of cleaved caspase 3 (C) and TUNEL assay (D) in xenograft tumors. Scale bar, 50 mm. (E) The levels of p53 protein were determined in HCT116 p53+/+ and HCT116 p53-/- tumors treated with and without 5-FU.