Background: Epidermal growth factor receptor (EGFR) inhibitor gefitinib (Iressa) is used for treating non-small cell lung cancer. Gefitinib also induces differentiation in acute myeloid leukemia (AML) cell lines and patient samples lacking EGFR by an unknown mechanism. Here we dissected the mechanism of gefitinib action responsible for its EGFR-independent effects.
Methods: Signaling events were analyzed by homogenous time-resolved fluorescence and immunoblotting. Cellular proliferation and differentiation were assessed by ATP measurement, trypan blue exclusion, 5-bromo-2'-deoxyuridine incorporation and flow-cytometry. Gefitinib and G protein-coupled receptor (GPCR) interactions were assessed by β-arrestin recruitment, luciferase and radioligand competition assays. Role of histamine receptors (HR) in gefitinib actions were assessed by HR knockdown or pharmacological modulation. EGFR and HR interaction was assessed by co-immunoprecipitation.
Results: Gefitinib reduced cyclic AMP content in both AML and EGFR-expressing cells and induced ERK phosphorylation in AML cells. Dibutyryl-cAMP or PD98059 suppressed gefitinib-induced AML cell cytostasis and differentiation. Gefitinib bound to and modulated HRs with subtype selectivity. Pharmacological or genetic modulations of H2 and H4 HRs (H2R and H4R) not only suppressed gefitinib-induced cytostasis and differentiation of AML cells but also blocked EGFR and ERK1/2 inhibition in MDA-MB-231 cells. Moreover, in MDA-MB-231 cells gefitinib enhanced EGFR interaction with H4R that was blocked by H4R agonist 4-methyl histamine (4MH).
Conclusion: HRs play critical roles in anti-cancer effects of gefitinib in both EGFR-deficient and EGFR-rich environments.
General significance: We furnish fresh insights into gefitinib functions which may provide new molecular clues to its efficacy and safety issues.
Keywords: Cancer; EGFR; GPCR; Gefitinib; Histamine receptor; Tyrosine kinase.
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