Loss of Sirtuin6 induces expression of the cancer-related transmembrane protein TMEM45A in HepG2 cells

Sirtuin 6 (SIRT6), a member of the sirtuin family of NAD+-dependent deacetylases is a key player in numerous cellular processes. The protein is crucial in aging-related pathways and cancer since it maintains genome stability and telomere integrity, promotes DNA repair and regulates metabolic homeostasis.

Loss or aberration of SIRT6 leads to epigenetic changes promoting a loosened chromatin state that ultimately results in major transcriptional deregulation. A cohort study with HCC patients confirmed the downregulation of SIRT6 in around 40% of HCC tissues and gene expression related to SIRT6-deficiency was associated with a worse disease outcome.

In order to better understand the role of SIRT6 in HCC we generated SIRT6-deficient HepG2 hepatoblastoma cells by using the CRISPR/CAS gene editing system. Cell clones lacking SIRT6 showed increased acetylation levels of Histone 3 at Lysine 9, 18 and 27. Even though no difference in proliferation was observed, the knockout cells showed an increased colony formation capacity. By analysis of SIRT6-dependent gene expression signatures using RNA-Sequencing, we identified several cancer-related genes. Among them was the hypoxia-inducible transmembrane protein 45 A (TMEM45A) which is upregulated in different types of cancer. TMEM45A is mainly located in the trans-golgi network and highly conserved in vertebrates. Although its biological function has not yet been elucidated, TMEM45A has been shown to be essential for chemoresistance of breast and liver cancer cells under hypoxic conditions.

We were able to confirm an elevated expression of TMEM45A in SIRT6-deficient HepG2 cells that was further increased under hypoxic conditions. Moreover we detected accumulation of acetylated HIF1α in SIRT6 knockout cells. Based on the direct interaction of SIRT6 and HIF1α we analyzed the molecular mechanism of TMEM45A expression and its functional consequences for liver cancer.