Non-canonical Raf-1/p70S6K signalling in non-small-cell lung cancer

Lung cancer is the leading cause of cancer-related death globally, with non-small-cell lung cancer (NSCLC) being the predominant subtype. Overall survival remains low for NSCLC patients, and novel targets are needed to improve outcome. Raf-1 is a key component of the Ras/Raf/MEK signalling pathway, but its role and downstream targets in NSCLC are not completely understood. Our previous study indicated a possible correlation between Raf-1 levels and ribosomal protein S6 kinase (p70S6K) function. In this study, we aimed to investigate whether p70S6K is a downstream target of Raf-1 in NSCLC. Raf-1 was silenced in NSCLC cell lines by using small hairpin RNA, and Raf-1 and p70S6K protein levels were measured via Western blot. p70S6K was then overexpressed following Raf-1 knock-down; then, cell proliferation, apoptosis and the cell cycle in NSCLC cell lines were examined. Tumour xenografts with NSCLC cells were then transplanted for in vivo study. Tumours were measured and weighed, and Raf-1 and p70S6K expression, cell proliferation and apoptosis were examined in tumour tissues by Western blot, Ki-67 staining and TUNEL staining, respectively. When Raf-1 was silenced, p70S6K protein levels were markedly decreased in the A549 and H1299 NSCLC cell lines. A significant decrease in NSCLC cell proliferation, a profound increase in apoptosis and cell cycle arrest were observed in vitro following Raf-1 knock-down. Overexpression of p70S6K after Raf-1 depletion effectively reversed these effects. Xenograft studies confirmed these results in vivo. In conclusion, Raf-1 targets p70S6K as its downstream effector to regulate NSCLC tumorigenicity, making Raf-1/p70S6K signalling a promising target for NSCLC treatment.