Functional characterization of salt-inducible kinase 2 and its interaction with Tax1 binding protein 2 in hepatocellular carcinoma

Abstract

Salt-Inducible Kinase 2 (SIK2) is a member of the AMP-activated kinase family. Initially characterized for its roles in cellular metabolism, SIK2 has recently been shown to function as a negative regulator of apoptosis and a positive cell cycle regulator, possibly via its centrosome duplication-permissive phosphorylation of centrosomal Nek2-associated protein 1 (C-Nap1).Further, SIK2 overexpression has been reported in tumors of ovary, prostate and B cell origins. These suggest an oncogenic potential of SIK2.

Tax1 Binding Protein 2 (TAX1BP2) is a hepatocellular carcinoma (HCC) tumor suppressor highly homologous to C-Nap1. TAX1BP2 has been first shown to suppress centrosome overduplication and this function is coupled to its tumor suppressive role, which is later found modulated through phosphorylation by upstream kinases.

The multi-step process of hepatocarcinogenesis implies deregulation of cellular signaling pathways are common in HCC. So far, mechanistic involvement of SIK2 in HCC has not been identified. Common features between SIK2 and TAX1BP2 including involvement in centrosome duplication and being oncoprotein Tax-related, and an intuitive kinase-substrate relationship suggest a mechanistic link between them. The present study therefore intended to characterize the roles of SIK2 in HCC and identify the link between SIK2 and TAX1BP2.

Here, overexpression of SIK2 in human HCC samples was reported and a trend between SIK2 overexpression and increased tumor invasiveness was observed. An effector domain (ED) for SIK2 growth promoting ability was mapped to amino acid region 294 –338 where the ubiquitin-associated domain (UBA) resides. Immunofluorescence staining revealed the ED was also responsible for centrosome localization. Stable overexpression of SIK2 full length and kinase-defective (K49M) mutant significantly enhanced cellular migration while K49M promoted cell proliferation. The ED of SIK2 was also shown responsible for its co-localization with TAX1BP2 and the signal was intensified by SIK2 kinase activity. Further, physical interaction between SIK2 and TAX1BP2 was confirmed by endogenous immunoprecipitation and an in vitro kinase assay identified TAX1BP2 as a substrate of SIK2.

Taken together, the findings provide insights into the oncogenic potential of SIK2 in HCC and suggest a mechanistic linkage between SIK2 and TAX1BP2.