Mechanism of lapatinib-mediated radiosensitization of breast cancer cells is primarily by inhibition of the Raf>MEK>ERK mitogen-activated protein kinase cascade and radiosensitization of lapatinib-resistant cells restored by direct inhibition of MEK.
OBJECTIVE
We recently showed that lapatinib, an EGFR/HER2 inhibitor, radiosensitized breast cancer cells of the basal and HER2+ subtypes. The purpose of this study was to identify the downstream signaling pathways responsible for lapatinib-mediated radiosensitization in breast cancer.


METHODS
Response of EGFR downstream signaling pathways was assessed by Western blot and clonogenic cell survival assays in breast tumor cells after irradiation (5Gy), lapatinib, CI-1040, or combined treatment.


RESULTS
In SUM102 cells, an EGFR+ basal breast cancer cell line, exposure to ionizing radiation elicited strong activation of ERK1/2 and JNK, which was blocked by lapatinib, and weak/no activation of p38, AKT or STAT3. Direct inhibition of MEK1 with CI-1040 resulted in 95% inhibition of surviving colonies when combined with radiation while inhibition of JNK with SP600125 had no effect. Lapatinib-mediated radiosensitization of SUM102 cells was completely abrogated with expression of constitutively active Raf. Treatment of lapatinib-resistant SUM185 cells with CI-1040 restored radiosensitization with 45% fewer surviving colonies when combined with radiation.


CONCLUSIONS
These data suggest that radiosensitization by lapatinib is mediated largely through inhibition of MEK/ERK and that direct inhibition of this pathway may provide an additional avenue of radiosensitization in EGFR+ or HER2+ breast cancers.
Sambade
Maria J
MJ
Department of Radiation Oncology, University of North Carolina at Chapel Hill, NC 27599, USA.
Camp
J Terese
JT
Kimple
Randall J
RJ
Sartor
Carolyn I
CI
Shields
Janiel M
JM
D000873:Anthracenes / Q000494:pharmacology; D001943:Breast Neoplasms / Q000378:metabolism* / Q000532:radiotherapy*; D045744:Cell Line, Tumor; D004789:Enzyme Activation / Q000187:drug effects; D066246:ErbB Receptors / Q000037:antagonists & inhibitors*; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D005260:Female; D006801:Humans; D000077341:Lapatinib; D048369:MAP Kinase Kinase 1 / Q000037:antagonists & inhibitors*; D020935:MAP Kinase Signaling System / Q000187:drug effects*; D048056:Mitogen-Activated Protein Kinase 9 / Q000037:antagonists & inhibitors; D020929:Mitogen-Activated Protein Kinase Kinases / Q000378:metabolism; D019908:Proto-Oncogene Proteins c-raf / Q000378:metabolism; D011799:Quinazolines / Q000494:pharmacology*; D011836:Radiation Tolerance / Q000187:drug effects*; D018719:Erb-b2 Receptor Tyrosine Kinases / Q000037:antagonists & inhibitors; D014407:Tumor Cells, Cultured
D000873:Anthracenes; D066246:ErbB Receptors; D048049:Extracellular Signal-Regulated MAP Kinases; D000077341:Lapatinib; D048369:MAP Kinase Kinase 1; D048056:Mitogen-Activated Protein Kinase 9; D020929:Mitogen-Activated Protein Kinase Kinases; D019908:Proto-Oncogene Proteins c-raf; D011799:Quinazolines; D018719:Erb-b2 Receptor Tyrosine Kinases; C432165:pyrazolanthrone