Amphiregulin promotes BAX inhibition and resistance to gefitinib in non-small-cell lung cancers

Abstract

Molecular resistance mechanisms affecting the efficiency of receptor tyrosine kinase inhibitors such as gefitinib in non-small-cell lung cancer (NSCLC) cells are not fully understood. Amphiregulin (Areg) overexpression has been proposed to predict NSCLC resistance to gefitinib and we have established that Areg-overexpressing H358 NSCLC cells resist apoptosis. Here, we demonstrate that Areg prevents gefitinib-induced apoptosis in NSCLC cells. We show that H358 cells are resistant to gefitinib in contrast to H322 cells, which do not overexpress Areg. Inhibition of Areg expression by small-interfering RNAs (siRNAs) restores gefitinib sensitivity in H358 cells, whereas addition of recombinant Areg confers resistance in H322 cells. Areg knockdown overcomes resistance to gefitinib and induced apoptosis in NSCLC H358 cells in vitro and in vivo. Under gefitinib treatment, Areg decreases the expression of the proapoptotic protein BAX, inhibits its conformational change and its mitochondrial translocation. Thus, in the presence of Areg, gefitinib-mediated apoptosis is reduced because BAX is sequestered in the cytoplasm. This suggests that treatments using epidermal growth factor receptor (EGFR) inhibitors may be poorly efficient in patients with elevated levels of Areg. These findings indicate the need for inhibition of Areg to enhance the efficiency of the EGFR inhibitors in patients suffering NSCLC.

Figure 1

Gefitinib effect in non-small-cell lung cancer (NSCLC) cells. (a) The MTT assay in H358 and H322 NSCLC cells treated with the indicated concentrations of gefitinib for 96 hours. (b,c) Effect of 0.5 or 1 µmol/l gefitinib on H358 and H322 cells. Apoptosis was analyzed after 96 hours by (b) detection of the active caspase-3 and flow cytometry or (c) after 24–96 hours after counting Hoechst-stained cells. Results are expressed as mean ± SD (n ≥ 3). *P < 0.05, **P < 0.01, ***P < 0.001, for comparison between H358 and H322 cells.

Figure 2

Areg inhibits gefitinib-induced apoptosis. (a) H322 cells were treated with 50 ng/ml recombinant human Areg and/or gefitinib as indicated. (b) H322 cells were treated with the indicated concentrations of Areg and 0.5 µmol/l gefitinib. (c) H358 cells were transfected with Areg or control siRNAs. The efficiency of Areg knockdown was assessed by enzyme-linked immunosorbent assay. Results are expressed as a rate of Areg released 48 hours after control siRNAs transfection and as mean ± SD (n = 3). (d) H358 cells were transfected with control or Areg siRNAs and treated with gefitinib and/or 50 ng/ml Areg. Apoptosis was scored after counting Hoechst-stained cells (a,b,d). Results are expressed as mean ± SD (n ≥ 3). *P < 0.05, **P < 0.01, for comparison between treated and control cells. Areg siRNAs, amphiregulin small-interfering RNAs.

Figure 3

Areg decreases the BAX protein level. H358 cells were transfected with control or Areg siRNAs. As indicated, 0.5 µmol/l gefitinib and/or 3 µmol/l MG132 were added. (a) Quantitative real-time RT-PCR for BAX mRNA on total RNA extracted from H358 cells. Relative gene expressions are expressed as a rate of BAX mRNA level after control siRNAs transfection. (b, c) Total cell lysates were subjected to western blotting using BAX antibody. Actin was used as protein level control. The values represent the relative intensity, quantified with ImageJ software, of BAX bands of treated samples to that of control cells, after being normalized to the respective actin and are represented by the mean ± SD (n = 4). *P ≤ 0.05 or **P ≤ 0.01 for comparison between control siRNAs in gefitinib-treated cells and the other treatments as indicated. Areg siRNAs, amphiregulin small-interfering RNAs.

Figure 4

Areg inactivates BAX in H358 cells. H358 cells were transfected with control or Areg siRNAs. As indicated, 0.5 µmol/l gefitinib and/or 10 µmol/l z-VAD-fmk were added. (a) Flow cytometry analysis of BAX immunostaining using activated-BAX antibody. Dotted histogram and IgG, irrelevant antibody; open histogram, control cells; filled histogram, treated cells as indicated. Percentages of activated-BAX-stained cells were expressed as mean ± SD (n ≥ 3). *P < 0.05, more significant than control. (b) H358 cells were fractionated into cytosolic and mitochondrial fractions. Both fractions extracts were subjected to western blotting using BAX antibody. Mitochondrial HSP70 was used for checking that cytosolic extracts were mitochondria-free and actin for loading control. (c) Apoptosis was scored after counting Hoechst-stained cells. Flow cytometry analysis of BAX immunostaining as described in a. Areg siRNAs, amphiregulin small-interfering RNAs.

Figure 5

Areg inactivates BAX in H322 cells. H322 cells were treated with 50 ng/ml Areg and/or 0.5 µmol/l gefitinib and/or 10 µmol/l z-VAD-fmk were added as indicated. (a) Flow cytometry analysis of BAX immunostaining using activated-BAX antibody. Dotted histogram and IgG, irrelevant antibody; open histogram, control cells; filled histogram, treated cells as indicated. Percentages of activated-BAX-stained cells were expressed as mean ± SD (n ≥ 3). **P < 0.01, more significant than control. (b) H322 cells were fractionated into cytosolic and mitochondrial fractions. Both fractions extracts were subjected to western blotting using BAX antibody. Mitochondrial HSP70 was used for checking that cytosolic extracts were mitochondria-free and actin for loading control. (c) Apoptosis was scored after counting Hoechst-stained cells. Flow cytometry analysis of BAX immunostaining as described in a.

Figure 6

Effects of combined treatment with gefitinib and Areg knockdown in vivo. Effects of combined treatment with gefitinib and Areg siRNAs on growth of H358 xenograft tumors in athymic nude mice. The mice were randomly assigned to one of four treatment groups. (a) Effect of gefitinib (10 mg/kg body weight) or vehicle, administered orally 5 days/week, and control or Areg siRNAs (16 µg/day, administered intraperitoneally 4 days/week) on tumor volume. Points, mean tumor volume (n = 8); bars, SE. *P < 0.05, **P < 0.01, ***P < 0.001 for comparisons between treated and control for each series of experiments. (b) Effect of gefitinib and Areg siRNAs on the expression of Areg and activated-caspase-3 in H358 xenograft tumors, assessed by western blotting. Actin was used as loading control. Areg siRNAs, amphiregulin small-interfering RNAs.