Heregulin induces Iressa-resistance of breast cancer cells through receptor heterodimerization.

Abstract

Abstract Abstract #2016 Background: Heregulins (HRGs) are known as soluble secreted growth factors that are involved via binding and activating HER3 and HER4 cell surface receptors in cell proliferation, metastasis, survival, and differentiation in normal and malignant tissues. We have previously shown that up-regulation of HRG is sufficient for development of mammary tumors independently of estrogen stimulation and HER2 overexpression. Moreover, inhibition of HRG expression suppresses the aggressive phenotype by decreasing HER activation and reducing matrix metalloproteinase-9 activity. Therefore, HRG appears to be a key modulator of tumorigenicity and metastasis in triple negative breast carcinomas. Resistance to anti-epidermal growth factor receptor (anti-EGFR) therapies such as gefitinib is an emerging clinical problem. In this regard we have used MDA231, HS578T and BT549 cells to study potential mechanisms of gefitinib resistance. These cell lines are EGF independent and express high levels of HRG and resistant to gefitinib. Our hypothesis is that HRG signaling through HER3 induces resistance to EGFR blockade. To demonstrate this, we knocked down HRG expression by RNA interference (RNAi) in MDA231, HS578T, and BT549 cells.
 Material and Methods: Down-regulation of HRG is observed in MDA231, HS578T, and BT549 cells transduced with HRG siRNA lentiviral vector pRNAt-U6.2. Sensitivity to gefitinib was determined by MTT assays and soft agar assays. Each experiment was repeated at least 3 times, all data were analyzed by ANOVA. The level of significance was set at p < 0.05.
 Results: Our results show that when comparing to their parental cells, all MDA231/HRGsiRNA, HS578T/HRGsiRNA and BT549/HRGsiRNA cells grew at a lower rate and become sensitive to gefitinib. Our results indicate that HRG can overcome the inhibitory effects of gefitinib on cell growth and HRG signaling allows at least in part, the acquisition of gefitinib-resistant phenotype.
 Discussion: Although the mechanism for the resistance is still under further investigation, our observation has implications for the effectiveness of anti-EGFR therapies in breast cancer where HRG induced receptor heterodimerization in EGFR-positive breast tumors. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 2016.