Altered splicing of ATG16-L1 mediates acquired resistance to tyrosine kinase inhibitors of EGFR by blocking autophagy in non-small cell lung cancer.

Despite the initial efficacy of using tyrosine kinase inhibitors of epidermal growth factor receptors (EGFR-TKIs) for treating patients with non-small cell lung cancer (NSCLC), resistance inevitably develops. Recent studies highlight a link between alternative splicing and cancer drug response. Therefore, we aimed to identify deregulated splicing events that play a role in resistance to EGFR-TKI. By using RNA sequencing, reverse-transcription PCR (RT-PCR), and RNA interference, we showed that overexpression of a splice variant of the autophagic gene ATG16-L1 that retains exon 8 and encodes the ß-isoform of autophagy-related protein 16-1 (ATG16-L1 ß) concurs acquired resistance to EGFR-TKI in NSCLC cells. Using matched biopsies, we found increased levels of ATG16-L1 ß at the time of progression in 3 of 11 NSCLC patients treated with EGFR-TKI. Mechanistically, gefitinib-induced autophagy was impaired in resistant cells that accumulated ATG16-L1 ß. Neutralization of ATG16-L1 ß restored autophagy in response to gefitinib, induced apoptosis, and inhibited the growth of in ovo tumor xenografts. Conversely, overexpression of ATG16-L1 ß in parental sensitive cells prevented gefitinib-induced autophagy and increased cell survival. These results support a role of defective autophagy in acquired resistance to EGFR-TKIs and identify splicing regulation of ATG16-L1 as a therapeutic vulnerability that could be explored for improving EGFR-targeted cancer therapy.

ARF promotes the degradation of the Epidermal Growth Factor Receptor by the lysosome.

Epidermal Growth Factor Receptor (EGFR) signaling regulates multiple cellular processes including proliferation, survival and apoptosis, and is attenuated by lysosomal receptor degradation. EGFR is a potent oncogene and activating mutations of EGFR are critical determinants of oncogenic transformation as well as therapeutic targets in non-small cell lung cancer. We previously demonstrated that wild type and mutant EGFRs repress the expression of the ARF tumor suppressor to promote the survival of lung tumor cells. In this study, using transient transfection systems in CHO EGFR-null cells as well as in various lung tumor cell lines carrying wild type or activated mutant EGFR, we show that ARF downregulates the expression of EGFR protein by reducing its half life. In wild type EGFR cells, ARF promotes canonical lysosomal degradation of the receptor through enhanced phosphorylation of EGFR-Y1045 and Cbl-Y731. In contrast, in mutant EGFR cells, ARF induces EGFR degradation by activating a non-canonical AKT-dependent lysosomal pathway. Taken together, these results uncover a feedback loop by which ARF may control EGFR turnover to restrain oncogenic signaling. They also highlight distinct degradation promoting pathways between wild type and mutant EGFRs in response to ARF.

Nuclear translocation of IGF1R by intracellular amphiregulin contributes to the resistance of lung tumour cells to EGFR-TKI.

Many Receptor Tyrosine Kinases translocate from the cell surface to the nucleus in normal and pathological conditions, including cancer. Here we report the nuclear expression of insulin-like growth factor-1 receptor (IGF1R) in primary human lung tumours. Using lung cancer cell lines and lung tumour xenografts, we demonstrate that the epidermal growth factor receptor-tyrosine kinase inhibitor (EGFR-TKI) gefitinib induces the nuclear accumulation of IGF1R in mucinous lung adenocarcinoma by a mechanism involving the intracellular re-localization of the growth factor amphiregulin. Amphiregulin allows the binding of IGF1R to importin-ß1 and promotes its nuclear transport. The nuclear accumulation of IGF1R by amphiregulin induces cell cycle arrest through p21WAF1/CIP1 upregulation, and prevents the induction of apoptosis in response to gefitinib. These results identify amphiregulin as the first nuclear localization signal-containing protein that interacts with IGF1R and allows its nuclear translocation. Furthermore they indicate that nuclear expression of IGF1R contributes to EGFR-TKI resistance in lung cancer.

Splice Variants of the RTK Family: Their Role in Tumour Progression and Response to Targeted Therapy.

Receptor tyrosine kinases (RTKs) belong to a family of transmembrane receptors that display tyrosine kinase activity and trigger the activation of downstream signalling pathways mainly involved in cell proliferation and survival. RTK amplification or somatic mutations leading to their constitutive activation and oncogenic properties have been reported in various tumour types. Numerous RTK-targeted therapies have been developed to counteract this hyperactivation. Alternative splicing of pre-mRNA has recently emerged as an important contributor to cancer development and tumour maintenance. Interestingly, RTKs are alternatively spliced. However, the biological functions of RTK splice variants, as well as the upstream signals that control their expression in tumours, remain to be understood. More importantly, it remains to be determined whether, and how, these splicing events may affect the response of tumour cells to RTK-targeted therapies, and inversely, whether these therapies may impact these splicing events. In this review, we will discuss the role of alternative splicing of RTKs in tumour progression and response to therapies, with a special focus on two major RTKs that control proliferation, survival, and angiogenesis, namely, epidermal growth factor receptor (EGFR) and vascular endothelial growth factor receptor-1 (VEGFR1).