Molecular interactions of ErbB1 (EGFR) and integrin- 1 in astrocytoma frozen sections predict clinical outcome and correlate with Akt-mediated in vitro radioresistance

Abstract

Treatment of astrocytoma is frequently hampered by radioresistance of the tumor. In addition to overexpression of ErbB1/EGFR, functional crosstalk between receptor tyrosine kinases and cell adhesion molecules may also contribute to therapy resistance. Acceptor photobleaching FRET was implemented on frozen sections of clinical astrocytoma to check the role of ErbB1-integrin-β1 interaction. U251 glioma subclones were obtained by introducing extra CHR7 material or the ErbB1 gene to test the relevance and mechanism of this interaction in vitro. Grade IV tumors showed higher ErbB1 and integrin-β1 expression and greater ErbB1-integrin-β1 heteroassociation than did grade II tumors. Of these, the extent of molecular association was a single determinant of tumor grade and prognosis in stepwise logistic regression. In vitro, integrin-β1 was upregulated, and radiosensitivity was diminished by ectopic ErbB1 expression. Great excess of ErbB1 provided colony forming advantage over medium excess but did not yield better radiation resistance or faster proliferation and decreased to medium level over time, whereas integrin-β1 levels remained elevated and defined the extent of radioresistance. Increased expression of ErbB1 and integrin-β1 was paralleled by decreasing ErbB1 homoassociation and increasing ErbB1-integrin-β1 heteroassociation. Microscopic two-sided FRET revealed that pixels with higher ErbB1-integrin-β1 heteroassociation exhibited lowed ErbB1 homoassociation, indicating competition for association partners among these molecules. Boosted Akt phosphorylation response to EGF accompanied this shift toward heteroassociation, and the consequentially increased radioresistance could be reverted by inhibiting PI3K. The clinically relevant ErbB1-integrin-β1 heteroassociation may be used as a target of both predictive diagnostics and molecular therapy.