An aberrant STAT pathway is central to COVID-19

Abstract

COVID-19 is caused by SARS-CoV-2 infection and characterized by diverse clinical symptoms. Type I interferon (IFN-I) production is impaired and severe cases lead to ARDS and widespread coagulopathy. We propose that COVID-19 pathophysiology is initiated by SARS-CoV-2 gene products, the NSP1 and ORF6 proteins, leading to a catastrophic cascade of failures. These viral components induce signal transducer and activator of transcription 1 (STAT1) dysfunction and compensatory hyperactivation of STAT3. In SARS-CoV-2-infected cells, a positive feedback loop established between STAT3 and plasminogen activator inhibitor-1 (PAI-1) may lead to an escalating cycle of activation in common with the interdependent signaling networks affected in COVID-19. Specifically, PAI-1 upregulation leads to coagulopathy characterized by intravascular thrombi. Overproduced PAI-1 binds to TLR4 on macrophages, inducing the secretion of proinflammatory cytokines and chemokines. The recruitment and subsequent activation of innate immune cells within an infected lung drives the destruction of lung architecture, which leads to the infection of regional endothelial cells and produces a hypoxic environment that further stimulates PAI-1 production. Acute lung injury also activates EGFR and leads to the phosphorylation of STAT3. COVID-19 patients' autopsies frequently exhibit diffuse alveolar damage (DAD) and increased hyaluronan (HA) production which also leads to higher levels of PAI-1. COVID-19 risk factors are consistent with this scenario, as PAI-1 levels are increased in hypertension, obesity, diabetes, cardiovascular diseases, and old age. We discuss the possibility of using various approved drugs, or drugs currently in clinical development, to treat COVID-19. This perspective suggests to enhance STAT1 activity and/or inhibit STAT3 functions for COVID-19 treatment. This might derail the escalating STAT3/PAI-1 cycle central to COVID-19.

Fig. 1. Possible differential effects of SARS-CoV-2 RNA/proteins on IFN-I and proinflammatory cytokine/chemokine production.

Molecular patterns derived from SARS-CoV-2-associated molecules, such as ssRNA, dsRNA, and viral proteins, bind to host PRRs and trigger the activation of signal transducers and transcription factors that drive the production of IFN-I and proinflammatory cytokines and chemokines. Soon after infection, the engagement of RIG-I and MDA-5 by these molecular patterns induces the activation of IRF3, or IRF7, through MAVS. In addition, viral ssRNA, dsRNA, and proteins can engage TLRs to trigger the MyD88- and TRIF-dependent pathways, primarily leading to the activation of the NF-κB (p50/p65) transcriptional complex. SARS-CoV-2 proteins that inhibit IFN-I production are indicated in black boxes, and the associated blocked pathways are indicated as dashed lines. Note that only the IFN-I production pathway, and not the secretion of proinflammatory cytokines/chemokines, is inhibited by the viral proteins. Proinflammatory cytokine/chemokine production is further activated by the engagement of TLRs by a high viral load.

Fig. 2. Possible effects of SARS-CoV-2 proteins on IFN-I signaling.

The IFN-I protein is secreted from infected cells and amplifies the IFN response by activating the ISGF3 complex (STAT1/STAT2/IRF9) to induce IFN-I-stimulated genes. The SARS-CoV-2 proteins that inhibit IFN-I signaling are indicated in black boxes, and the associated blocked pathways are indicated by dashed lines.

Fig. 3. STAT1- and STAT3-dependent drug targets in IFN-I signaling.

A IFN-I signaling before SARS-CoV-2 infection. JAK1 and TYK2 (1) are activated after IFN-I stimulation. STAT1 is normally activated in IFN-I signaling to induce ISGs (STAT1-ISGs) by ISGF3 (STAT1/STAT2/IRF9). STAT3 is also activated and becomes a homodimer, but the response is small. B IFN-I signaling with SARS-CoV-2 infection. After the infection, STAT1 activity is inhibited by the SARS-CoV-2 proteins, NSP1, and ORF6 (2). With STAT1 activity restricted, STAT3 (3) then becomes dominant and induces STAT3-ISGs. Both STAT1 and STAT3 induce SOCS1 and SOCS3 (4) that inhibit the kinase activity of JAKs for the negative feedback of IFN-I signaling. PIAS1 and PIAS3 (5) inhibit the binding of STAT1 and STAT3 to DNA, respectively, to regulate IFN-I signaling. The role of PIAS3 becomes critical when STAT3 is aberrantly activated and uncoupled from SOCSs regulation. Protein tyrosine phosphatases (PTPs, 6) have regulatory activities on activated JAKs and STATs, but their role in the viral infection needs further clarification. EGFR (7) is upregulated by acute lung injury or by reduced STAT1 activity in the SARS-CoV-2-infected lung. STAT3 is activated through directly binding to EGFR, through EGFR-activated SRC (8), or through JAK2 (data not shown). PIAS3 normally limits the activity of STAT3 but PAI-1 produced during infection blocks PIAS3 activity (9) and an escalating cascade in the STAT3/PAI-1 axis is established.

Fig. 4. A dysregulated STAT3-PAI-1 signaling node is common to COVID-19 pathophysiology. Proposed role of STAT3-PAI-1 signaling node in catastrophic cascades underlying COVID-19 pathophysiology.

Infection by SARS-CoV-2 intracellularly delivers NSP1 and ORF6, which efficiently inhibit STAT1 function (1). Repression of STAT1 increases STAT3 (2) activity. STAT3 upregulates PAI-1 (3) by repressing miR-34a, a PAI-1 inhibitor (4). This increased PAI-1 reciprocally activates STAT3 by blocking PIAS3, a STAT3 inhibitor (5). STAT3 can activate HAS2, a hyaluronic acid synthase, which produces hyaluronan (HA) and leads to diffuse alveolar damage (DAD) characterized by hyaline membrane formation (6). Fragments of HA (LMW-HA) activate PAI-1 (3). An escalating cycle of stimulation between STAT3 and PAI-1 begins a catastrophic cascade of events, resulting in combinations of coagulopathy/thrombosis (7), macrophage production of cytokines and chemokines (8), and profibrotic changes (9). Hypoxia eventually results, which further induces PAI-1 transcription through HIF-1α (10). This elevated PAI-1 activity then drives IL-6 production via TLR4, which in turn stimulates even more STAT3 activity (No. 11). Elevated STAT3 also activates PD-L1 in endothelial cells, leading to T cell lymphopenia (No. 12). Details of these events are described in the main text. Italicized outside labels are cell types, and non-italicized outside labels are locations. Bolded italicized text indicates disease states. Bent arrows indicate the transcriptional induction of the indicated target proteins. Straight arrows indicate direct activation. Dashed lines indicate direct inhibition.