CTSL is expressed by cancerous tissues and encodes a lysosomal cysteine proteinase that regulates cancer progression and SARS-CoV-2 entry. Therefore, it is critical to predict the susceptibility of cancer patients for SARS-CoV-2 and evaluate the correlation between disease outcomes and the expression of CTSL in malignant cancer tissues. In the current study, we analyzed CTSL expression, mutation rate, survival and COVID-19 disease outcomes in cancer and normal tissues, using online databases. We also performed immunohistochemistry (IHC) to test CTSL expression and western blot to monitor its regulation by cordycepin (CD), and N6, N6-dimethyladenosine (m⁶₂A), respectively. We found that CTSL is conserved across different species, and highly expressed in both normal and cancer tissues from human, as compared to ACE2 or other proteinases/proteases. Additionally, the expression of CTSL protein was the highest in the lung tissue. We show that the mRNA expression of CTSL is 66.4-fold higher in normal lungs and 54.8-fold higher in cancer tissues, as compared to ACE2 mRNA expression in the respective tissues. Compared to other proteases/proteinases/convertases such as TMPRSS2 and FURIN, the expression of CTSL was higher in both normal lungs and lung cancer samples. All these data indicate that CTSL might play an important role in COVID-19 pathogenesis in normal and cancer tissues of the lungs. Additionally, the CTSL-002 isoform containing both the inhibitor_I29 and Peptidase_C1 domains was highly prevalent in all cancers, suggesting its potential role in tumor progression and SARS-CoV-2 entry in multiple types of cancers. Further analysis of the expression of CTSL mutant showed a correlation with FURIN and TMPRSS2, suggesting a potential role of CTSL mutations in modulating SARS-CoV-2 entry in cancers. Moreover, high expression of CTSL significantly correlated with a short overall survival (OS) in lung cancer and glioma. Thus, CTSL might play a major role in the susceptibility of lung cancer and glioma patients to SARS-CoV-2 uptake and COVID-19 severity. Furthermore, CD or m⁶₂A inhibited CTSL expression in the cancer cell lines A549, MDA-MB-231, and/or PC3 in a dose dependent manner. In conclusion, we show that CTSL is highly expressed in normal tissues and increased in most cancers, and CD or m⁶₂A could inhibit its expression, suggesting the therapeutic potential of targeting CTSL for cancer and COVID-19 treatment.

Hepatocellular carcinoma (HCC), the most prevalent liver cancer, makes patients' prognosis extremely poor due to frequent intrahepatic and distant metastasis. The epithelial-mesenchymal transition (EMT) plays an important role in this process. However, the content of NSUN5 in hepatocellular carcinoma and whether NSUN5 is involved in mediating the epithelial-mesenchymal transition of hepatocellular carcinoma remains unknown. In this study, based on clinicopathologic analyses of several independent HCC cohorts and the results of induced tumor formation in Nsun5 knockout mice, we observed that NSUN5 expression was increased in tumor tissues. The absence of Nsun5 would retard the malignant progression of hepatocellular carcinoma, suggesting that Nsun5 might be an important oncogene in HCC. Furthermore, we found that high expression of NSUN5 promotes EMT in HCC cells. After NSUN5 is knocked out, the HCC cell's ability to invade and migrate decreases in vivo and vitro conditions; on the other hand, NSUN5 overexpression in HCC cells had the opposite effect. Mechanically, the highly expressed NSUN5 in cancer tissues promotes the enrichment of Tri-Methyl-histone H3 (Lys4) in the promoter region of SMAD3 by interacting with WDR5, thereby promoting hepatocellular carcinoma metastasis by SMADd3-mediated EMT. In general, we identified NSUN5 as a novel promoter of metastasis in hepatocellular carcinoma and established an innovative theoretical foundation for treating this disease.