Purpose: Cancer treatment by conventional radiotherapy is limited by normal tissue side-effects. Fibroblasts as "non-target" stromal cell type are considered as strong promoter of tumor growth and for developing a therapy resistant phenotype. Regarding application of novel molecular therapeutics combined with radiotherapy, evaluation of a specific targeted molecule in both tumor and normal cells is mandatory for efficacy and tolerability assessment. Previous work showed integrin-linked kinase (ILK), a mediator of beta-integrin signals and putative phosphorylator of AKT, as potent anti-survival regulator in human cancer cell lines.
Materials and methods: To evaluate the role of ILK in normal fibroblast survival, ILK-wild-type (ILK(fl/fl)), ILK(-/-) and ILK(N-terminal) and ILK(C-terminal) domain expressing fibroblasts were irradiated with X-rays on different substrata or in three-dimensional laminin-rich extracellular matrix (lrECM).
Results: On control substrata, ILK-deficient and ILK-mutant fibroblasts showed significant increase in radiation survival relative to ILK-wild-type cells. This effect was compensated by growth on ECM proteins and in 3D lrECM. ILK regulated AKT activity in a phosphatidylinositol-3 kinase (PI3K)-dependent manner. Upon PI3K inhibition, only ILK-wild-type fibroblasts showed significant radiosensitization.
Conclusions: These findings obtained in 3D cell cultures suggest ILK to be dispensable for the radiation survival response of normal fibroblasts. However, targeting the PI3K/AKT signaling axis pharmacologically might be critical for survival of normal fibroblasts exposed to ionizing radiation.