Differential influence of TGFβ1 and TGFβ3 isoforms on cell cycle kinetics and postirradiation recovery of normal and malignant colorectal epithelial cells

Purpose: A clonogenic assay was used to determine the effects of the growth factors TGFβ1 and TGFβ3 on the radiation responses of a normal rat epithelial cell line (IEC6) and a human colonic carcinoma epithelial cell line (Widr). Methods and Materials: The radiation sensitivity and ability to recover from potentially lethal damage (PLD), of preconfluent monolayer cultures, was assessed in the presence of the growth factors for 24 h prior to, during, and after irradiation. Results: The surviving fractions of both cell lines assessed immediately following irradiation were unaffected by TGFβ1 or TGFβ3. However, TGFβ3 ( but not TGFβ1) significantly reduced the amount of PLD recovery in the Widr cells ( but not in the IEC6 cells). This was associated with a reduction in the shoulder region of the survival curve, rather than a change in slope. A comparative analysis of the effects of TGFβs 1 and 3 on cell cycle events in the two cell lines demonstrated significantly more Widr cells in the S phase, in the presence of TGFβ3 only, compared to the controls. This remained constant both before and immediately following irradiation. In the IEC6 cell line TGFβ3 produced an increase in the numbers of G1 phase cells, characteristic of a G1 arrest. Conclusion: It seems likely that TGFβ3-induced radiosensitisation in Widr cells, 6 h after a single dose of irradiation, is related to its effects on cell cycle events such that the failure of these cells to arrest in G1, either before or after irradiation, results in significantly reduced recovery from DNA damage. This, however, may not be the only mechanism by which this growth factor produces this effect. Indeed, it will also be necessary to investigate these effects in in vivo models and to determine the response to fractionated irradiation before the potential therapeutic benefit of both the differential effects observed between the two TGFβ isoforms and also between the malignant and normal cell lines can be fully assessed.