Comparison of three approaches for inhibiting insulin-like growth factor I receptor and their effects on NSCLC cell lines in vitro

The insulin-like growth factor-1 receptor (IGF-1R) mitogenic signaling mediates malignant cell survival by many complex and redundant pathways. This study compared the effects of IGF-1R inhibition on viability and apoptosis of two NSCLC cell lines, using three different methods for the impairment of IGF-1R function: (IR3, an anti-IGF-1R antibody; tyrphostin AG1024, a tyrosine kinase inhibitor (TKI) and IGF-1R-small interfering RNA (siRNA). IGF-1R inhibition led to a decrease of cell survival and induced apoptosis in a manner depending on the approach used for the receptor inhibition. To find an explanation, we analyzed the effects of these treatments on three major antiapoptotic pathways evoked by IGF-1R signaling: IRS-1, Shc and 14.3.3-dependent mitochondrial translocation of Raf-1 kinase (mitRaf). (IR3 downregulated IRS-1 phosphorylation in A549 cells and Shc phosphorylation in U1810 cells. While in A549 cells AG1024 treatment decreased both IRS-1 and Shc phosphorylation, in U1810 cells the IRS-1 phosphorylation was only slightly affected and the Shc phosphorylation drastically downregulated. Neither (IR3 nor AG1024 had any effect on Raf-1 kinase translocation. Irrespective of the cell line, IGF-1R-siRNA treatment induced downregulation of both IRS-1 and Shc phosphorylation coupled with the abrogation of mitRaf. In addition, the IGF-1R-siRNA proved to be the most potent inducer of apoptosis suggesting that more than one antiapoptotic pathway in IGF-1R signaling should be inhibited to effectively induce apoptosis in lung cancer cells.