Downstream signalling and specific inhibition of c-MET/HGF pathway in small cell lung cancer: implications for tumour invasion

The c- MET receptor can be overexpressed, amplified, or mutated in solid tumours including small cell lung cancer ( SCLC). In c- MET- overexpressing SCLC cell line NCI- H69, hepatocyte growth factor ( HGF) dramatically induced c- MET phosphorylation at phosphoepitopes pY1230/ 1234/ 1235 ( catalytic tyrosine kinase), pY1003 ( juxtamembrane), and also of paxillin at pY31 ( CRKLbinding site). We utilised a global proteomics phosphoantibody array approach to identify further c- MET/ HGF signal transduction intermediates in SCLC. Strong HGF induction of specific phosphorylation sites in phosphoproteins involved in c- MET/ HGF signal transduction was detected, namely adducin-alpha [ S724], adducin-gamma [ S662], CREB [ S133], ERK1 [ T185/ Y187], ERK1/ 2 [ T202/ Y204], ERK2 [ T185/ Y187], MAPKK ( MEK) 1/ 2 [ S221/ S225], MAPKK ( MEK) 3/ 6 [ S189/ S207], RB [ S612], RB1 [ S780], JNK [ T183/ Y185], STAT3 [ S727], focal adhesion kinase ( FAK) [ Y576/ S722/ S910], p38 alpha- MAPK [ T180/ Y182], and AKT1[ S473] and [ T308]. Conversely, inhibition of phosphorylation by HGF in protein kinase C ( PKC), protein kinase R ( PKR), and also CDK1 was identified. Phosphoantibody- based immunohistochemical analysis of SCLC tumour tissue and microarray established the role of c- MET in SCLC biology. This supports a role of c- MET activation in tumour invasive front in the tumour progression and invasion involving FAK and AKT downstream. The c- MET serves as an attractive therapeutic target in SCLC, as shown through small interfering RNA ( siRNA) and selective prototype c- MET inhibitor SU11274, inhibiting the phosphorylation of c- MET itself and its downstream molecules such as AKT, S6 kinase, and ERK1/ 2. Investigation of mechanisms of invasion and, ultimately, metastasis in SCLC would be very useful with these signal transduction molecules.