Platelet-derived growth factor receptor and Middle East respiratory syndrome coronavirus infection

Abstract

Middle East respiratory syndrome coronavirus (MERS-CoV) is a highly pathogenic coronavirus that has the highest mortality rate among all human coronaviruses. Due to the pandemic potential of human coronavirus and the lack of MERS-specific treatment, MERS-CoV still poses a potential public health threat globally. The severe disease outcomes of MERS-CoV infection are often associated with the dysfunction of innate immune responses and the development of acute lung injury. Recent findings in platelet-derived growth factor (PDGF) have revealed that PDGF and its receptor (PDGFR) are involved in the dysfunctional endothelial cell barrier function and inflammation. In addition, proteomic analysis of the MERS-CoV infection has suggested the participation of PDGF downstream signalling pathway during MERS-CoV infection. By enhancing our understanding of the role of PDGFR in MERS-CoV infection, we aim to investigate the underlying mechanism of PDGFR in the MERS-CoV life cycle and examine the therapeutic potential of PDGFR inhibitor upon MERS-CoV infection.

Here, we repurposed an FDA-approved drug called Imatinib mesylate (STI571), a PDGFR inhibitor, against MERS-CoV infection in human alveolar epithelial cells (A549-DPP4), human airway epithelial cells (Calu-3) and human pulmonary microvascular endothelial cells. Our findings illustrate that STI571 is able to exert its antiviral effect in the in vitro cell models upon MERS-CoV infection. Time-of-drug addition assay reveals that STI571 might suppress the early and late stages of MERS-CoV replication. Additionally, the induction of pro-inflammatory cytokine and chemokine was downregulated in the presence of STI571 during MERS-CoV infection, suggesting that STI571 can modulate innate immune responses. Gene knockdown experiment using PDGFR siRNA validates the participation of PDGF and PDGFR during MERS-CoV infection, demonstrating that the antiviral effect of STI571 is not the result of the off-target mechanism. By assessing the phosphorylation of one of the PDGF signalling pathways - MAPK/ERK pathway, we successfully identified that the decrease in the phosphorylation level of p70 ribosomal S6 kinase (p70S6K) by MERS-CoV, which the phosphorylation level of p70S6K was restored by STI571 during MERS-CoV infection. Inhibition of mitogen-activated protein kinase kinase (MEK) phosphorylation by STI571 may also contribute to the effect of STI571 during MERS-CoV infection.

In addition, the therapeutic potential of STI571 was examined by using a mouse model with human DPP4 knock-in (hDPP4-KI). The findings of in vivo studies using the South Korean MERS-CoV strain showed that STI571 failed to block the replication of MERS-CoV in vivo. However, STI571 can reduce the mRNA levels and protein concentration of pro-inflammatory cytokines and chemokines, resulting in ameliorating lung injury and inflammation, as suggested by the histological staining of mouse lungs.

In conclusion, PDGF and its receptor play an essential role in the MERS-CoV replication and modulation of innate immune responses. Despite the fact that the findings of in vivo studies do not support the use of STI571 as an antiviral, the immunomodulatory and lung protective activities of STI571 suggested that STI571 can serve as a therapeutic countermeasure for the unregulated cytokine and chemokines induction.