Effect of hypoxia-inducible factor 1α (HIF-1α) deficiency in tumor cells and murine tumor stroma on radiation sensitivity and growth of tumors

Abstract

Poor prognosis of many solid tumors is often associated with hypoxic regions and an increased level of hypoxia-inducible factor-1α (HIF-1α). Previous findings indicate that HIF-1α expression is relevant for radiation resistance. Furthermore, there are evidences that the tumor microenvironment is also involved in protecting tumor cells from irradiation (IR). To clarify the role of HIF-1α deficiency in apoptosis, DNA damage repair and radiosensitization in vitro, murine Lewis Lung Carcinoma (LLC) cells with a doxycycline-inducible knockdown for HIF-1α were used. The data provided strong evidence that depletion of HIF-1α increased IR-induced apoptosis and impairment of clonogenic potential of LLC cells and thus increased radiosensitivity of cells. Furthermore, HIF-1α deficient LLC cells demonstrated IR-induced increase of DNA damage measured by γH2AX foci and increased recruitment of RAD51, which suggests an alteration in homologous recombination repair. The level of 53BP1 IR-induced foci remained unchanged, as well as fast repair kinetics indicating that non-homologous end joining repair capacity was not affected. Next, this study investigated an effect of surrounding microenvironment on HIF-1α signaling in tumor cells and the effect of this interaction on the radiation sensitivity in vivo. The analysis of HIF-1α depletion in tumors in an allograft mouse model with wild-type C57BL/6 mice demonstrated unchanged tumor growth regardless of HIF-1α deficiency in tumor cells. Afterwards, a murine allograft model with HIF-1α conditional knockout was used for the investigation, whether HIF-1α elimination in surrounding stroma cells simultaneously with tumor cells affects the radiosensitivity of tumors. The tumor volume of the irradiated groups was not significantly changed regardless of HIF-1α deficiency in tumor cells and in the mice. However, the non-irradiated groups demonstrated that the volume of tumors with HIF-1α positive cells that grew in HIF-1α knockout mice was significantly decreased compared to tumors with HIF-1α depleted cells that grew in HIF-1α positive mice. In vitro results of present study indicate that inactivation of HIF 1α leads to an enhanced radiation sensitivity of tumor cells. In vivo results are more controversial, although the data indicated an influence of HIF-1α deficiency in stromal cells on the tumor growth. The demonstrated complexity of HIF-1α-related network in tumor cells and surrounding tissue during irradiation makes it an attractive future target for further investigation to enhance the outcome of radiation treatment.