Kinase suppressor of Ras 1 (KSR1) promotes liver carcinogenesis via activation of the RAS/RAF/MEK/ERK signaling pathway

Background & Aims: Mutations in the RAS and RAF genes are frequently observed in various human cancers, leading to persistent activation of the RAS/RAF/MEK/ERK signaling pathway and driving tumorigenesis. Intriguingly, hepatocellular carcinoma (HCC) reveals rare mutations in RAS and RAF, despite frequent activation of the RAS/RAF/MEK/ERK signaling. Here, we identify kinase suppressor of Ras 1 (KSR1) as a pivotal player in pathway activation and tumorigenesis in the liver. Methods: Gene expression data from human cancers were analyzed using publicly available databases. HCC cell lines stably expressing KSR1 were established to evaluate RAS/RAF/MEK/ERK signaling activity. Liver cancer was induced in C57BL/6 male mice via hydrodynamic tail vein injection. Tumor-bearing livers were formalin-fixed and processed for H&E staining and immunohistochemistry. Results: KSR1 expression was frequently upregulated in human HCC (n = 366, p <0.001) and strongly associated with activation of the RAS/RAF/MEK/ERK signaling pathway (n = 50, p <0.001). Ectopic expression of KSR1 led to increased phosphorylation of MEK1/2 and ERK1/2 in HCC cells and murine livers, confirming activation of the signaling pathway. Overexpression of KSR1 in murine livers in conjunction with P53 inactivation or c-Myc overexpression led to the development of HCC, which exhibited activated RAS/RAF/MEK/ERK signaling (n = 10 mice per group). Notably, the degrees of MEK phosphorylation and tumorigenesis in the liver induced by KSR1 overexpression were equivalent to those by an activated RAF, the bona fide kinase of MEK1/2. Intriguingly, liver tumors induced by activated RAS or RAF were efficiently suppressed by KSR1 knockdown or pharmacological inhibition of KSR1 (n = 10, p <0.01), highlighting the potential of KSR1 as a therapeutic target for cancers driven by pathway activation. Conclusions: Overexpression of KSR1 activates the RAS/RAF/MEK/ERK signaling pathway and drives hepatic tumorigenesis in the absence of mutations in RAS or RAF. Impact and implications: Our findings identify kinase suppressor of Ras 1 (KSR1) as an oncogenic driver in the liver and a key activator of the RAS/RAF/MEK/ERK signaling pathway. This study enhances our understanding of the role that scaffold proteins play in promoting oncogenic signaling cascades. The successful use of a pharmacological inhibitor of KSR to suppress in vivo tumor growth driven by RAS or RAF highlights the potential of KSR1 as a druggable target.