Deciphering MET-dependent modulation of global cellular responses to DNA damage by quantitative phosphoproteomics.

Increasing evidence suggests that interference with growth factor receptor tyrosine kinase (RTK) signaling can affect DNA damage response (DDR) networks, with a consequent impact on cellular responses to DNA-damaging agents widely used in cancer treatment. In that respect, the MET RTK is deregulated in abundance and/or activity in a variety of human tumors. Using two proteomic techniques, we explored how disrupting MET signaling modulates global cellular phosphorylation response to ionizing radiation (IR). Following an immunoaffinity-based phosphoproteomic discovery survey, we selected candidate phosphorylation sites for extensive characterization by targeted proteomics focusing on phosphorylation sites in both signaling networks. Several substrates of the DDR were confirmed to be modulated by sequential MET inhibition and IR, or MET inhibition alone. Upon combined treatment, for two substrates, NUMA1 S395 and CHEK1 S345, the gain and loss of phosphorylation, respectively, were recapitulated using invivo tumor models by immunohistochemistry, with possible utility in future translational research. Overall, we have corroborated phosphorylation sites at the intersection between MET and the DDR signaling networks, and suggest that these represent a class of proteins at the interface between oncogene-driven proliferation and genomic stability.

Identification of a MET-eIF4G1 translational regulation axis that controls HIF-1α levels under hypoxia.

Poor oxygenation is a common hallmark of solid cancers that strongly associates with aggressive tumor progression and treatment resistance. While a hypoxia-inducible factor 1α (HIF-1α)-associated transcriptional overexpression of the hepatocyte growth factor (HGF) receptor tyrosine kinase (RTK) MET has been previously documented, any regulation of the HIF-1α system through MET downstream signaling in hypoxic tumors has not been yet described. By using MET-driven in vitro as well as ex vivo tumor organotypic fresh tissue models we report that MET targeting results in depletion of HIF-1α and its various downstream targets. Mechanistically, we provide evidence that MET regulates HIF-1α levels through a protein translation mechanism that relies on phosphorylation modulation of the eukaryotic initiation factor 4G1 (eIF4G1) on serine 1232 (Ser-1232). Targeted phosphoproteomics data demonstrate a significant drop in eIF4G1 Ser-1232 phosphorylation following MET targeting, which is linked to an increased affinity between eIF4G1 and eIF4E. Since phosphorylation of eIF4G1 on Ser-1232 is largely mediated through mitogen-activated protein kinase (MAPK), we show that expression of a constitutively active K-RAS variant is sufficient to abrogate the inhibitory effect of MET targeting on the HIF-1α pathway with subsequent resistance of tumor cells to MET targeting under hypoxic conditions. Analysis of The Cancer Genome Atlas data demonstrates frequent co-expression of MET, HIF-1α and eIF4G1 in various solid tumors and its impact on disease-free survival of non-small cell lung cancer patients. Clinical relevance of the MET-eIF4G1-HIF-1α pathway is further supported by a co-occurrence of their expression in common tumor regions of individual lung cancer patients.

Impact of MET targeting on tumor-associated angiogenesis and growth of MET mutations-driven models of liver cancer.

Deregulated expression of the MET receptor tyrosine kinase has been reported in up to 50% of patients with hepatocellular carcinoma, the most abundant form of liver cancers, and is associated with decreased survival. Consequently, MET is considered as a molecular target in this malignancy, whose progression is highly dependent on extensive angiogenesis. Here we studied the impact of MET small molecule inhibitors on angiogenesis-associated parameters and growth of xenograft liver models consisting of cells expressing MET-mutated variants M1268T and Y1248H, which exhibit constitutive kinase activity. We demonstrate that MET mutations expression is associated with significantly increased production of vascular endothelial growth factor, which is blocked by MET targeting only in cells expressing the M1268T inhibitor-sensitive but not in the Y1248H inhibitor-resistant variant. Decrease in vascular endothelial growth factor production is also associated with reduction of tyrosine phopshorylation of the vascular endothelial growth factor receptor 2 expressed on primary liver sinusoidal endothelial cells and with inhibition of vessel formation. Furthermore, MET inhibition demonstrated an efficient anti-tumor activity and considerable reduction in microvessel density only against the M1268T-derived intrahepatic tumors. Collectively, our data support the role of targeting MET-associated angiogenesis as a major biological determinant for liver tumor growth control.

Interplay between receptor tyrosine kinases and hypoxia signaling in cancer.

Deregulated signaling via receptor tyrosine kinase (RTK) pathways is prevalent in numerous types of human cancers and is commonly correlated with worst prognosis, resistance to various treatment modalities and increased mortality. Likewise, hypoxic tumors are often manifested by aggressive mode of growth and progression following an adaptive genetic reprogramming with consequent transcriptional activation of genes encoding proteins, which support tumor survival under low oxygen-related conditions. Consequently, both the hypoxia-inducible factor (HIF) system, which is the major mediator of hypoxia-related signaling, and numerous RTK systems are considered critical molecular targets in current cancer therapy. It is now evident that there is an intricate molecular crosstalk between RTKs and hypoxia-related signaling in the sense that hypoxia can activate expression of particular RTKs and/or their corresponding ligands, while some RTK systems have been shown to trigger activation of the HIF machinery. Moreover, signaling regulation of some RTK systems under hypoxic conditions has also been documented to take place in a HIF-independent manner. With this review we aim at overviewing the most current observations on that topic and highlight the importance of the potential co-drugging the HIF system along with particular relevant RTKs for better tumor growth control.

Targeting of the MET receptor tyrosine kinase by small molecule inhibitors leads to MET accumulation by impairing the receptor downregulation.

The MET receptor tyrosine kinase is deregulated primarily via overexpression or point mutations in various human cancers and different strategies for MET inhibition are currently evaluated in clinical trials. We observed by Western blot analysis and by Flow cytometry that MET inhibition by different MET small molecule inhibitors surprisingly increases in a dose-dependent manner total MET levels in treated cells. Mechanistically, this inhibition-related MET accumulation was associated with reduced Tyr1003 phosphorylation and MET physical association with the CBL ubiquitin ligase with concomitant decrease in MET ubiquitination. These data may suggest careful consideration for design of anti-MET clinical protocols.