Targeting microtubules sensitizes drug resistant lung cancer cells to lysosomal pathway inhibitors.

Oncogene-addicted cancers are predominantly driven by specific oncogenic pathways and display initial exquisite sensitivity to designer therapies, but eventually become refractory to treatments. Clear understanding of lung tumorigenic mechanisms is essential for improved therapies. Methods: Lysosomes were analyzed in EGFR-WT and mutant cells and corresponding patient samples using immunofluorescence or immunohistochemistry and immunoblotting. Microtubule organization and dynamics were studied using immunofluorescence analyses. Also, we have validated our findings in a transgenic mouse model that contain EGFR-TKI resistant mutations. Results: We herein describe a novel mechanism that a mutated kinase disrupts the microtubule organization and results in a defective endosomal/lysosomal pathway. This prevents the efficient degradation of phosphorylated proteins that become trapped within the endosomes and continue to signal, therefore amplifying downstream proliferative and survival pathways. Phenotypically, a distinctive subcellular appearance of LAMP1 secondary to microtubule dysfunction in cells expressing EGFR kinase mutants is seen, and this may have potential diagnostic applications for the detection of such mutants. We demonstrate that lysosomal-inhibitors re-sensitize resistant cells to EGFR tyrosine-kinase inhibitors (TKIs). Identifying the endosome-lysosome pathway and microtubule dysfunction as a mechanism of resistance allows to pharmacologically intervene on this pathway. Conclusions: We find that the combination of microtubule stabilizing agent and lysosome inhibitor could reduce the tumor progression in EGFR TKI resistant mouse models of lung cancer.

Phosphorylation of Mig6 negatively regulates the ubiquitination and degradation of EGFR mutants in lung adenocarcinoma cell lines.

Activating mutations in the kinase domain of epidermal growth factor receptor (EGFR) leads to the constitutively active kinase, improves the EGFR stability and promotes malignant transformation in lung adenocarcinoma. Despite the clinical significance, the mechanism by which the increased kinase activity stabilizes the receptor is not completely understood. Using SILAC phosphoproteomic approach, we identify that Mig6 is highly phosphorylated at S256 in EGFR mutants (19del and L858R). Loss of Mig6 contributes to the efficient degradation of EGFR wildtype and mutants in lung cancer cells. Mig6 regulates the recruitment of c-Cbl to EGFR as the ablation of Mig6 enables efficient ubiquitination of the EGFR mutants through elevated recruitment of c-Cbl. We show that the cells with activating mutants of EGFR inactivate Mig6 through phosphorylation at S256. Inactivated Mig6 causes inefficient ubiquitination of EGFR, leading to defective degradation of the receptor thus contributing to the increased stability of the receptor. Taken together, we show a novel function of Mig6 in regulating the ubiquitination of EGFR.

Cavin-2 regulates the activity and stability of endothelial nitric-oxide synthase (eNOS) in angiogenesis.

Angiogenesis is a highly regulated process for formation of new blood vessels from pre-existing ones. Angiogenesis is dysregulated in various pathologies, including age-related macular degeneration, arthritis, and cancer. Inhibiting pathological angiogenesis therefore represents a promising therapeutic strategy for treating these disorders, highlighting the need to study angiogenesis in more detail. To this end, identifying the genes essential for blood vessel formation and elucidating their function are crucial for a complete understanding of angiogenesis. Here, focusing on potential candidate genes for angiogenesis, we performed a morpholino-based genetic screen in zebrafish and identified Cavin-2, a membrane-bound phosphatidylserine-binding protein and critical organizer of caveolae (small microdomains in the plasma membrane), as a regulator of angiogenesis. Using endothelial cells, we show that Cavin-2 is required for in vitro angiogenesis and also for endothelial cell proliferation, migration, and invasion. We noted a high level of Cavin-2 expression in the neovascular tufts in the mouse model of oxygen-induced retinopathy, suggesting a role for Cavin-2 in pathogenic angiogenesis. Interestingly, we also found that Cavin-2 regulates the production of nitric oxide (NO) in endothelial cells by controlling the stability and activity of the endothelial nitric-oxide synthase (eNOS) and that Cavin-2 knockdown cells produce much less NO than WT cells. Also, mass spectrometry, flow cytometry, and electron microscopy analyses indicated that Cavin-2 is secreted in endothelial microparticles (EMPs) and is required for EMP biogenesis. Taken together, our results indicate that in addition to its function in caveolae biogenesis, Cavin-2 plays a critical role in endothelial cell maintenance and function by regulating eNOS activity.