FOXO1, an AKT downstream substrate, plays a role as tumor suppressor in HCC pathogenesis.

Abstract

e15627 Background: Hepatocellular carcinoma (HCC) represents the fifth most common tumor types in the world. However, the signaling pathways underlying hepatic pathogenesis are still poorly understood. Activation of PI3K/AKT signaling cascade has been implicated in HCC development. It has been well-established that the mammalian target of rapamycin complex 1 (mTORC1), is a major downstream effector of AKT. The reason why mTORC1 can be consistently activated is partly due to inactive mutations of a negative element regulating mTORC1, tuberous sclerosis 2 (TSC2), found in 10% human HCCs. Beyond TSC2, forkhead Box O transcription factor 1 (FOXO1), is another AKT substrate. It has been proved that FOXO1 plays a crucial role in the insulin signal pathway and liver regenerationrelying on AKT isoforms. However, the role of FOXO1 in HCC development has not been well established. Methods: FOXO1 expression was analyzed in human liver cancer from TCGA database and multiple HCC cell lines were used to verify the mechanism of how FOXO1 regulate HCC cell growth downstream of AKT activation and we further confirmed the role of FOXO1 in HCC development in vivo. Results: By using TCGA database the mRNA level of FOXO1 significantly decreases among the FOXO family, comparing tumor tissue to the surrounding tissue. The overall survival rate favors patients with high FOXO1 expression. The inhibition efficacies of MK2206, a pan-AKT inhibitor, in both TSC2 wide type cells and TSC2-null ones are not significantly different, indicating TSC2 is not the only target downstream of AKT. As FOXO1 is another major target, we knockdown endogenous FOXO1 in the same HCC cell lines, the activation of mTORC1 was no longer inhibited. Furthermore, we overexpress the activated FOXO1 (FOXO1 AAA) in TSC2-null HCC cell lines. Cell growth are significantly prohibited, implicating FOXO1 is a tumor suppressor in HCC cells. Overexpression of activated FOXO1 can also delay AKT depended HCC in met/β-catenin FVB mice model. Conclusions: AKT/mTORC1 can be a classic molecular pathway for HCC pathogenesis. Nevertheless, the parallel substrate of AKT, FOXO1, may also be an important factor in the TSC2 independent way.