Panoply™ Human CTSK Knockdown Stable Cell Line

SIAH1 has been reported to be downregulated in hepatocellular carcinoma (HCC) and plays a crucial role in HCC progression. Here, researchers discovered that cathepsin K (CTSK), a protein that may interact with SIAH1, can inhibit SIAH1 protein levels. CTSK is highly expressed in HCC tissues. Inhibition or downregulation of CTSK suppresses HCC cell proliferation, while CTSK overexpression has the opposite effect. CTSK promotes HCC cell proliferation by regulating the SIAH1/protein kinase B (AKT) pathway, in which the AKT pathway promotes SIAH1 ubiquitination. Neural progenitor cell development downregulation 4 (NEDD4) was found to be a potential upstream ubiquitin ligase for SIAH1. Furthermore, CTSK mediates the ubiquitination and degradation of SIAH1 by increasing SIAH1 self-ubiquitination and recruiting NEDD4. Finally, the role of CTSK was confirmed in a xenograft mouse model. In summary, the oncogene CTSK is upregulated in human hepatocellular carcinoma tissues and promotes hepatocellular carcinoma cell proliferation by downregulating SIAH1.

Enrichment analysis using the Kyoto Encyclopedia of Genes and Genomes (KEGG) revealed that genes in the CTSK-overexpressing group were significantly enriched in the PI3K-AKT-mTOR pathway (Figure 1A). Previous reports have shown that SIAH1 partially inhibits tumor cell proliferation and invasion by downregulating AKT expression. Conversely, SIAH1 plays a crucial role in inhibiting HCC cell proliferation. Therefore, CTSK may promote HCC cell proliferation by regulating the SIAH1/AKT pathway. As expected, in CTSK-knockdown cells, SIAH1 protein levels were significantly increased, and AKT (S473) phosphorylation levels were decreased (Figure 1B). Conversely, CTSK overexpression produced the opposite effect (Figure 1C). More importantly, to confirm that the CTSK-induced changes in AKT phosphorylation were mediated by SIAH, the researchers conducted a rescue experiment by silencing SIAH1. In Huh7 and HepG2 cells, inhibition or knockdown of CTSK significantly upregulated SIAH1 expression, thereby reducing p-AKT (S473) levels, while silencing SIAH1 significantly blocked this regulation (Figure 1D). These results indicate that the downregulation of p-AKT (S473) induced by CTSK inhibition is mainly attributed to the upregulation of SIAH1 expression. The researchers performed CCK-8 assays to validate these results. Silencing SIAH1 significantly attenuated the cell proliferation inhibition induced by CTSK inhibition or knockdown (Figure 1E and F). In summary, these results suggest that the promoting effect of CTSK upregulation on HCC proliferation is mediated through the SIAH1/AKT pathway.

Figure 1. CTSK promotes the proliferation of HCC cells by regulating SIAH1/AKT pathway. (Zhang C, et al., 2023)