MEK/ERK signaling downstream of mutant Kras drives biliary differentiation in murine cholangiocarcinoma

Introduction Activation of oncogenic RAS signaling in human liver cancer is mostly found in cholangiocarcinoma (CC), but rarely in hepatocellular carcinoma (HCC). In a genetic mouse model of RAS-activated cholangiocarcinoma (RPK; Rb^lox/lox;p53^lox/lox;Kras/lsl-Kras^G12D ), tumors show activation of PI3K/AKT and MEK/ERK signaling. However, the relevance of these RAS-dependent signaling pathways in tumor development is not completely understood. To investigate RAS dependent signaling pathways, animals with activation of oncogenic Kras and genetic inactivation of either PI3K-activated AKT (Pdk1^lox/lox ) or MEK/ERK signaling (Map2k1^lox/lox;Map2k2^-/- ) were generated.

Methods Adult RPK; Alb^CreER mice, RPK animals deficient for MEK proteins (RPK; Map2k1^lox/lox; Map2k2^-/-; Alb^CreER ) or deficient for PI3K-activated AKT signaling (RPK; Pdk1^lox/lox; Alb^CreER ) were injected with tamoxifen to induce tumor development. Liver samples were analyzed for cell proliferation, tumor development as well as for mRNA and protein expression.

Results Tumors in RPK animals showed activation of PI3K/AKT and MEK/ERK signaling by immunohistochemistry and western blot. 4 weeks after injection of tamoxifen, hepatocytes in RPK livers were actively proliferating, with significantly lower levels of proliferation detected in RPK; Pdk1^lox/lox; AlbCre^ER and RPK; Map2k1^lox/lox; Map2k2^-/-; AlbCre^ER knock-out models. In line with reduced proliferation at early time points, Mek1/2- as well as Pdk1-deleted RPK mice showed significantly delayed tumor development in comparison to RPK control mice. Strikingly, deficiency for downstream MEK/ERK signaling was associated with a change in tumor differentiation, leading to the development of HCC or mixed HCC/CC in 33% or 39% of tumors, respectively.

Conclusion In a preclinical model of RAS-dependent cholangiocarcinoma, both PI3K/AKT signaling and MEK/ERK-signaling drive early cell proliferation and tumor development. Impaired MEK signaling downstream of oncogenic Kras was associated with a striking change in tumor differentiation towards a HCC phenotype. This novel role of MEK/ERK signaling in differentiation towards the biliary lineage could provide an explanation for the low frequency of RAS mutations in HCC in comparison to CC.

Publication History

Article published online:
07 September 2021