To investigate SARS-CoV-2-induced alterations in intracellular events, which affect rapid viral propagation during the early stage of infection, we first confirmed the abundant expression levels of endogenous ACE2 and TMPRSS2 proteins, known as key receptors for SARS-CoV-2 entry, in both human embryonic kidney 293T (HEK293T) cells and human lung epithelial Calu-3 cells through Western blot analysis (Supplementary Fig. 1a). This evidence concerns the gene TMPRSS2 and infection.