HRAS as a potential therapeutic target of salirasib RAS inhibitor in bladder cancer

The active form of the small GTPase RAS binds to downstream effectors to promote cell growth and proliferation. RAS signal enhancement contributes to tumorigenesis, invasion, and metastasis in various different cancers. HRAS proto-oncogene GTPase (HRAS), one of the RAS isoforms, was the first human oncogene for which mutations were reported in T24 bladder cancer (BC) cells in 1982, and HRAS mutation or upregulation has been reported in several cancers. According to data from The Cancer Genome Atlas, HRAS expression was significantly upregulated in clinical BC samples compared to healthy samples (P=0.0024). HRAS expression was also significantly upregulated in BC with HRAS mutation compared to patients without HRAS mutation (P<0.0001). The tumor suppressive effect of salirasib, a RAS inhibitor, has been reported in several cancer types, but only at relatively high concentrations. As such, RAS inhibitors have not been used for clinical applications. The aim of the current study was to investigate the therapeutic potential of targeting HRAS using salirasib and small interfering RNA (siRNA) and to characterize the mechanism by which HRAS functions using recently developed quantitative in vitro proteome-assisted multiple reaction monitoring for protein absolute quantification (iMPAQT), in BC cells. iMPAQT allows measurement of the absolute abundance of any human protein with the high quantitative accuracy. Salirasib and siRNA targeting of HRAS inhibited cell proliferation, migration and invasion in HRAS wild type and HRAS-mutated cell lines. Proteomic analyses revealed that several metabolic pathways, including the oxidative phosphorylation pathway and glycolysis, were significantly downregulated in salirasib-treated BC cells. However, the expression levels of hexokinase 2, phosphoglycerate kinase 1, pyruvate kinase, muscle (PKM)1, PKM2 and lactate dehydrogenase A, which are downstream of RAS and target genes of hypoxia inducible factor-1α, were not notably downregulated, which may explain the high concentration of salirasib required to inhibit cell viability. These findings provide insight into the mechanisms of salirasib, and suggest the need for novel therapeutic strategies to treat cancers such as BC.