Functional mechanism of the enhancement of 5-fluorouracil sensitivity by TUSC4 in colon cancer cells.

5-Fluorouracil (5-FU) is the chemotherapeutic drug of choice for the treatment of metastatic colorectal cancer (CRC). Tumor suppressor candidate 4 (TUSC4), also referred to as nitrogen permease regulator-like 2 (NPRL2), is located at chromosome 3p21.3 and expressed in numerous normal tissues, including the heart, liver, skeletal muscle, kidney, and pancreas. The aim of the present study was to investigate the functional mechanism by which TUSC4 affects sensitivity to 5-FU and to determine its clinical significance in CRC. The results of the present study demonstrated that TUSC4 overexpression increases the sensitivity of HCT116 cells to 5-FU. The IC50 of 5-FU was reduced in cells transduced with TUSC4 compared with negative control (NC) cells, and the effect of TUSC4 on 5-FU sensitivity was time dependent. Following TUSC4 transduction in HCT116 cells, a proportion of the cells were arrested in the G1 phase of the cell cycle, and a reduction in the S phase population was observed. Flow cytometry analysis revealed that TUSC4 transduction and 5-FU treatment increased apoptosis compared with NC cells. The mechanism through which TUSC4 overexpression enhances 5-FU sensitivity involves the downregulation of the function of the PI3K/Akt/mTOR network. Furthermore, 5-FU upregulated caspase-3 and caspase-9, promoting apoptosis in TUSC4-overexpressing cells compared with cells that were transduced with TUSC4 or treated with 5-FU and NC cells. The findings of the present study indicate that TUSC4 has potential as a biomarker for the prediction of the response to 5-FU and prognosis in patients with colorectal cancer and other types of human cancer. TUSC4 may also act as a molecular therapeutic agent for enhancing the patient's response to 5-FU treatment.