Treatment with gefitinib increased the XY1 portion of cells in GG phase by to in the two LKHER cell lines, with a corresponding decrease in the portion of cells in S and GM phases. Furthermore, no subG fraction, indicative of apoptosis, was observed following gefitinib treatment in any of the cell lines.In contrast, pWAFCIP expression was unchanged in all of the clones after gefitinib treatment.However, the underlying mechanism of why this difference in pkip expression levels by gefitinib appears remains to be further studied.Gefitinib inhibits constitutive association of HER with HER as well as pA, and basal HER phosphorylation in LKHER cells.LK cells Thujone expressed no detectable levels of EGFR, but sufficient levels of HER. Moreover, the constitutive association of HERHER is often observed in tumor cells overexpressing HER. We examined the effect of gefitinib on the association of HER with HER, and that of HER with pa.HER was coprecipitated with HER in LKHER and LK HER cells under basal growth conditions, indicating the presence of constitutive HERHER complexes.A, exponentially growing cells in serum medium were pretreated for hours with the indicated concentrations of gefitinib.SDSPAGE and probed with antibodies.The mean values for each phase of the cell cycle are shown on the graph and in the table.Treatment with gefitinib markedly abrogated HER phosphorylation as well as the association of HER with HER, and HER with pa.These nine cell lines expressed different levels of EGFR, HER, HER, and HER, but there appeared to be no correlation between EGFR andor HER expression and drug sensitivity. In this study, overexpression of HER in LK cells is shown to result in enhanced drug sensitivity to gefitinib. Thus, the expression of HER, but not EGFR, seems to modulate drug sensitivity to gefitinib in this NSCLC cell line.Therapeutic efficacy of gefitinib against NSCLC was found to be closely associated with somatic mutations in the EGFR kinase domain, and we found that one highly gefitinibsensitive NSCLC cell line, PC, also has a deletion mutation in exon of EGFR catalytic domain.Taken together, overexpression of HER seems to sensitize LK cells to gefitinib, plausibly coordinated with HER.This study reported that the effects of gefitinib are independent of EGFR expression levels, but are influenced by high HER expression. Moreover, recent studies have also reported that gefitinib has a good antitumor effect on tumors displaying higher HER levels. These studies, including our present study, consistently suggest that high HER expression confers increased sensitivity to the therapeutic effect of gefitinib.Gefitinib intrinsically shows very high affinity against EGFR, but about fold less affinity against HER than EGFR. HER phosphorylation under exponential growth conditions was moderately affected when gefitinib was used at high concentrations.Expression of HER is essential for the dimer formation of HER and HER, whereas exogenous addition of heregrin, a specific ligand for HER, could not further enhance dimer formation or HER phosphorylation, in LKHER transfectants. Treatment with gefitinib inhibited HER phosphorylation with release of pa in LKHER cells under basal growth conditions.Exponentially growing LKHER or LK mock in serum medium were treated with gefitinib for hours.

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