Salt-inducible kinase 2 : a potential metastasis suppressor in hepatocellular carcinoma

Abstract

Salt-inducible Kinase 2 (SIK2), a serine/threonine kinase under the AMP-activated protein kinase (AMPK) family, is reported to participate in vast numbers of molecular pathways that are vital to cell proliferation and mobility like the PI3K/AKT and LKB1/HDAC pathways. Aberrant level of SIK2 was observed in various malignancies including ovarian, colorectal, breast and gastric cancers. It has also been identified as a centrosome kinase playing a vital role in centrosome duplication. While emerging as an important protein in carcinogenesis, the role of SIK2 in hepatocellular carcinoma (HCC), a major contributor of cancer mortality due to frequent recurrence and metastasis, has not yet been well studied.

In this study, SIK2 expression was shown to be downregulated in HCC. Reduced SIK2 expression is correlated with poorer survival for HCC patients. Surprisingly, knockout of SIK2 resulted in slower growth rate in both in vitro cell assay and in vivo animal model. Cell cycle analysis suggested a potential delay in the G1-S transition. Also, loss of SIK2 sensitized HCC cells towards cisplatin treatment, while also made HCC cells more resistant to paclitaxel and doxorubicin treatments. Moreover, SIK2 depletion was found to drastically enhances the metastatic potential of HCC cells, including anchorage-independent growth, cell mobility and invasiveness, providing possible explanation on the clinical data. RNA-sequencing and mass spectrometry proteomic analysis revealed an upregulation on biological processes closely related to metastasis such as EMT and angiogenesis. While levels of epithelial marker like E-cadherin was downregulated in SIK2 knockout cells, expression of EMT and invasion promoter like ZEB1 and vimentin were upregulated. Phosphoproteomic analysis revealed potential downstream substrates of SIK2 that are related to HCC metastasis such as phospho-FLNA and phospho-DPYSL2. The above result unveiled a novel role of SIK2 in suppressing HCC metastasis.

To investigate the mechanism behind the metastasis suppression caused by SIK2, several SIK2 mutants were overexpressed in the SIK2 knockout cells. While both wild type and constitutively active form of SIK2 abolished the enhanced cell migration, kinase dead and centrosome targeting domain deletion mutant of SIK2 could not. These data suggested that SIK2 suppressed metastasis of HCC in both kinase- and centrosome-dependent manner.

In this study, I demonstrated an unexpected role of SIK2 in suppressing HCC metastasis, with the kinase and centrosome targeting activity seemed to be essential to the metastatic suppressive function of SIK2. Taken together, my data provides important insights on the development of SIK2 as a target for personalised HCC treatment in the future.