The role of TAX1BP2 in hepatocellular carcinoma

Abstract

TAX1 Binding Protein 2 (TAX1BP2) has been found to be a centrosome duplication regulating protein. Previous findings have demonstrated that over-expression of TAX1BP2 suppresses centrosome over-duplication. Recently, our lab has revealed that TAX1BP2 is a novel tumor suppressor in hepatocellular carcinoma (HCC) regulated by cyclin-dependent protein kinase 2 (CDK2), nevertheless, the molecular mechanism of how TAX1BP2 regulates centrosome duplication and the link between its centrosome duplication regulatory ability and the tumor suppressing property remain elusive. With the aim to understand the roles of TAX1BP2 in HCC, the present study intended to investigate the link between centrosome duplication regulating ability and tumor suppressing property.

Polo-like kinase 4 (PLK4) is a special member of the Polo-like kinase family as its structure is diverged from other family members. Instead of having two Polo-boxes, it carries one Polo-box and one cryptic Polo-box. It has been shown that PLK4 is involved in the formation of centrioles, an important component of centrosome, and is a key regulator of centrosome duplication. Based on the functional similarity, it was hypothesized that PLK4 may function as a regulator of TAX1BP2. To define if PLK4 regulate TAX1BP2, the interaction between PLK4 and TAX1BP2, both in vivo and in vitro, was first confirmed using affinity pulldown and co-immunoprecipitation assays. To understand the significance of the physical interaction, in vitro and in vivo kinase assay were used to study the phosphorylation activity between PLK4 and TAX1BP2. It was demonstrated that TAX1BP2 is a potential substrate of PLK4. Centrosome duplication assay was also performed to investigate if over-expression of PLK4 abolished the centrosome over-duplication suppressing ability of TAX1BP2.

In order to delineate the signaling pathway of TAX1BP2, the interaction between TAX1BP2 and its cellular interacting partners was investigated in this study. Ten proteins were isolated as potential interacting partners of TAX1BP2 using Tandem affinity purification (TAP) coupled with Mass Spectrometry protein fingerprinting. Two of the ten proteins, the Ezrin and Mortalin, were confirmed to be binding partners of TAX1BP2 using affinity pull-down assay and TAP, respectively. The identification of the interacting partners suggested that TAX1BP2 may modulate centrosome duplication via alteration of the subcellular localization of Mortalin. These findings helped to delineate the signaling pathway of TAX1BP2 and enabled the better understanding of the roles of TAX1BP2 in tumor suppressor function of HCC.

In summary, we demonstrated that TAX1BP2 contains a centrosome duplication regulatory domain (CDRD) and its centrosome duplication regulating ability is critical for its tumor suppressing property. Moreover, three novel interacting partners of TAX1BP2, including Ezrin, PLK4 and Mortalin, are identified. Our findings provide a new insight into the roles of TAX1BP2 in centrosome duplication, hepatocarcinogenesis and metastasis.