SKI-1/S1P Facilitates SARS-CoV-2 Spike Induced Cell-to-Cell Fusion via Activation of SREBP-2 and Metalloproteases, Whereas PCSK9 Enhances the Degradation of ACE2

Abstract

Proprotein convertases activate various envelope glycoproteins and participate in cellular entry of many viruses. We recently showed that the convertase furin is critical for the infectivity of SARS-CoV-2, which requires cleavage of its spike protein (S) at two sites: S1/S2 and S2′. This study investigates the implication of the two cholesterol-regulating convertases SKI-1 and PCSK9 in SARS-CoV-2 entry. The assays used were cell-to-cell fusion in HeLa cells and pseudoparticle entry into Calu-3 cells. SKI-1 increased cell-to-cell fusion by enhancing the activation of SREBP-2, whereas PCSK9 reduced cell-to-cell fusion by promoting the cellular degradation of ACE2. SKI-1 activity led to enhanced S2′ formation, which was attributed to increased metalloprotease activity as a response to enhanced cholesterol levels via activated SREBP-2. However, high metalloprotease activity resulted in the shedding of S2′ into a new C-terminal fragment (S2”), leading to reduced cell-to-cell fusion. Indeed, S-mutants that increase S2′’ formation abolished S2′ and cell-to-cell fusion, as well as pseudoparticle entry, indicating that the formation of S2′’ prevents SARS-CoV-2 cell-to-cell fusion and entry. We next demonstrated that PCSK9 enhanced the cellular degradation of ACE2, thereby reducing cell-to-cell fusion. However, different from the LDLR, a canonical target of PCSK9, the C-terminal CHRD domain of PCSK9 is dispensable for the PCSK9-induced degradation of ACE2. Molecular modeling suggested the binding of ACE2 to the Pro/Catalytic domains of mature PCSK9. Thus, both cholesterol-regulating convertases SKI-1 and PCSK9 can modulate SARS-CoV-2 entry via two independent mechanisms.

RETRACTED ARTICLE: Potential use of proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibition and prevention method in viral infection

“…Modeling at the molecular level indicated that ACE2 binds to the Pro/Catalytic domains of mature PCSK9. Thus, SARS-CoV-2 entrance can be modulated by the cholesterol-regulating convertases PCSK9 and SKI-1 via two distinct mechanisms [ 52 ]. Plasma PCSK9 levels were significantly elevated in our 156 patients with systemic inflammatory response syndrome (SIRS) or sepsis compared to the 68 healthy controls, according to the last studies.…”

“…Necessary for the transportation of the PCSK9-LDLR complex to endosomes/lysosomes for degradation is the CHRD domain of PCSK9, particularly its M2 module. Surprisingly, the catalytic and/or prodomain of PCSK9 appear to be the critical domains for PCSK9 activity on ACE2, not the CHRD; this distinguishes ACE2 as a unique and novel PCSK9 target, quite distinct from the LDLR and MHC-I, which may partially explain this observation [ 52 ] …”

“…Thus, SARS-CoV-2 entrance can be modulated by the cholesterol-regulating convertases PCSK9 and SKI-1 via two distinct mechanisms. [ 52 ] COVID-19 Patients with Gram-negative and Gram-positive infections in the non-COVID-19 SIRS/sepsis group exhibited comparable plasma PCSK9 levels to those without any discernible pathogen in their bloodstream. In conclusion, it suggested that PCSK9 may serve as a biomarker for COVID-19; however, larger cohorts are required to confirm this.…”

The rationale of PCSK9 inhibitors in sepsis: a viewpoint

“…Another potential mechanism might BE related to the effect of PCSK9 on angiotensin-converting enzyme 2 (ACE2) entry receptors in COVID-19 [8]. Likewise, it has been reported that ACE2 suppresses abdominal sepsis and severe acute lung injury [9].…”

Cholesterol and COVID-19—therapeutic opportunities at the host/virus interface during cell entry

“…Furthermore, membrane cholesterol accelerates the formation of oligomeric S protein structures that enhance S2 activity ( Meher et al, 2023 ) ( Fig 1 ). Host site-1 protease (S1P)–mediated activation of sterol regulatory element–binding protein 2 (SREBP2), the major transcription factor that controls cholesterol homeostatsis, increased SARS-CoV-2 fusion with the cell membrane due to enhanced S2 processing in a HeLa (human cervical carcinoma) cell model ( Essalmani et al, 2023 ).…”