Nuclear epidermal growth factor receptor modulates cellular radio-sensitivity by regulation of chromatin access.

PURPOSE: Nuclear EGFR is involved in cellular stress management and regulation of cellular radio-sensitivity. The aim of this study was to elucidate the molecular mode of nuclear EGFR action.
METHODS: Radiation induced nuclear EGFR-shuttling and EGFR-foci formation was analyzed with immunohistochemistry and confocal microscopy. Composition of γH(2)AX-protein complexes was analyzed by western-blotting after immuno-precipitation. Functional relevance of nuclear EGFR was analyzed after siRNA mediated depletion of EGFR with respect to activation of ATM, histone H3 acetylation, residual DNA-damage and cell survival after irradiation.
RESULTS: Following radiation nuclear EGFR was localized in foci similar to γH(2)AX. EGFR co-localized in a sub-fraction of γH(2)AX-foci. Analysis of composition of γH(2)AX-complexes revealed presence of EGFR, ATM, promyelocytic leukemia protein (PML), histone H3 and hetero-chromatin binding protein (HP1) in response to radiation. Depletion of EGFR protein inhibited ATM activation due to inhibition of acetylase TIP60 activity following irradiation. Consequently, histone H3 acetylation and phosphorylation was blocked and chromatin could not be opened for repair. Thus, residual DNA-damage was increased 24 h after irradiation and cells were radio-sensitized. Comparable results were obtained when cells were treated with EGFR-NLS-peptide, which blocks EGFR nuclear shuttling specifically.
CONCLUSIONS: Nuclear EGFR is part of DNA-damage repair complex and is involved in regulation of TIP60-acetylase activity. TIP60 is essential for ATM activation and chromatin relaxation which is a prerequisite for DNA-repair in heterochromatic DNA. Thus interventional EGFR strategies during tumor treatment may also interact with DNA-repair by blocking access to damaged DNA.