The tumor suppressor Rassf1a prevents mouse liver tumorigenesis and regulates levels of the oncogenic kinase Tbk1 and of beta tubulin gene expression in the liver

The tumor suppressor gene RASSF1A (Ras association domain family protein 1A) coding for a microtubule stabilizing protein is epigenetically silenced in most human cancers. As a binding partner of the kinases MST1 and MST2, the mammalian orthologues of the Drosophila Hippo kinase, RASSF1A is a potential regulator of the Hippo tumor suppressor pathway. RASSF1A shares these properties with the scaffold protein SAV1. The role of this pathway in human cancer has remained enigmatic because Hippo pathway components are rarely mutated. Rassf1a homozygous knockout mice developed liver tumors. However, heterozygous deletion of Sav1 or co-deletion of Rassf1a and Sav1 produced liver tumors with much higher efficiency than single deletion of Rassf1a. Analysis of RASSF1A binding partners by mass spectrometry identified the Hippo kinases MST1, MST2 and the oncogenic IkB kinase TBK1 as the most significantly enriched RASSF1A-interacting proteins. The transcriptome of Rassf1a-/- livers was more deregulated than that of Sav1+/- livers and the transcriptome of Rassf1a-/-, Sav1+/- livers was similar to that of Rassf1a-/- mice. We found that the levels of Tbk1 protein were substantially upregulated in livers lacking Rassf1a, and at the transcript level, factors regulating Tbk1 stability, including Usp2 and Dtx4, were also dysregulated. Furthermore, transcripts of several beta tubulin isoforms were increased in the Rassf1a-deficient liver genotypes presumably reflecting a role of Rassf1a as a tubulin-binding and microtubule-stabilizing protein. Our data suggest a multifactorial role of Rassf1a in suppression of liver carcinogenesis.

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Zhang X, Guo C, Wu X, Li AX, Liu L, Tsark W, Dammann R, Shen H, Vonderfecht SL, Pfeifer GP