Mechanisms of acquired resistance to insulin-like growth factor 1 receptor inhibitor in MCF-7 breast cancer cell line

No studies have yet clarified the mechanism of acquired resistance to insulin-like growth factor-1 receptor (IGF-1R) tyrosine kinase inhibitor (TKI). Our previous study of 16 breast cancer cell lines found that only MCF-7 expressed high levels of insulin receptor substrate (IRS)-1 and was sensitive to the IGF-1R-TKI NVP-AEW541. We developed a model of acquired resistance to NVP-AEW541 by continuously exposing MCF-7 cells to NVP-AEW541, naming the model MCF-7-NR. Examination of the effects of NVP-AEW541 on cell growth and IGF-1R signaling in MCF-7 and MCF-7-NR cells showed much lower levels of IRS-1 in the latter than the former. While phosphorylation of Akt was completely inhibited by administration of NVP-AEW541 (3 µM) in both cell lines, phosphorylation of S6K remained high only in MCF-7-NR. The notion of Akt-independent S6K phosphorylation in MCF-7-NR was further supported by the fact that cell growth and phosphorylation of S6K was affected by administration of the Akt inhibitor perifosine to a lesser degree in MCF-7-NR than in MCF-7. Further, the mTOR inhibitor everolimus inhibited phosphorylation of S6K and cell growth equally in both lines. Screening of MCF-7 and MCF-7-NR lines for phosphorylation of 42 receptor tyrosine kinases with and without 3 µM NVP-AEW541 showed that Tyro3 phosphorylation remained high only in MCF-7-NR cells. Gene silencing of Tyro3 using siRNA resulted in reduced cell growth, decreased phosphorylation of phosphoinositide-dependent kinase-1 (PDK-1) and protein kinase C α/βII, reduced expression of cyclin D1 in the MCF-7-NR line, with minimal effects evident in the MCF-7 line. In summary, Akt-independent activation of mTOR/S6K appears to induce acquired resistance to NVP-AEW541, and an mTOR inhibitor may have therapeutic value in overcoming it. The Tyro3 upregulation and migration of control of cell growth and cyclin D1 expression from the IGF-1R- to Tyro3-dependent signal may also cause resistance to NVP-AEW541