Silybin attenuates microglia-mediated neuroinflammation via inhibition of STING in experimental subarachnoid hemorrhage

Background: The primary cause of subarachnoid hemorrhage (SAH) is the rupture of intracranial aneurysms. Over-activation of microglia following SAH is a primary driving force in early brain injury (EBI), which is a leading cause of poor outcomes. Silybin is a flavonoid compound extracted from Silybum marianum, a plant belonging to the Asteraceae family. Its anti-inflammatory and antioxidant properties could provide neuroprotective effects. The mechanism of silybin on EBI after SAH is unclear.

Purpose: To determine the therapeutic effect of silybin on SAH and its underlying mechanisms.

Methods: We used a prechiasmatic autologous arterial blood injection in vivo and hemoglobin in vitro to establish experimental SAH model. Dexamethasone was used as a positive control drug. We evaluated the neuroprotective effect of silybin on the in vivo SAH model by neurological function scores, rotarod test, and open field test, and explored the protective effect of silybin on neuroinflammation and Apoptosis after SAH by quantitative polymerase chain reaction (qPCR), western blot (WB), Immunofluorescence (IF) and TUNEL staining. IF staining of CD86 and CD206 was used to assess microglial phenotype polarization. Then we used WB and IF labeling of STING to explore the effect of silybin on the STING pathway after SAH, and used a combination of transcriptomics and non-targeted metabolomics to study the potential mechanism of silybin in detail, and verified the essential genes by qPCR. We also extracted cerebrospinal fluid from SAH patients and detected the expression level of STING in cerebrospinal fluid by enzyme-linked immunosorbent assay (ELISA) to clarify the association between STING and neural function.

Results: Results showed that silybin ameliorated neuronal damage and improved short-term neurological function, and reduced inflammatory damage and neuronal Apoptosis in SAH mice. Silybin inhibited the expression levels of TNF-α, IL-1β and NLRP3, and promoted the expression levels of CD206, Arg1 and IL-10. Notably, Silybin promoted M2 microglia polarization. Further studies found that silybin reduced the mRNA and protein levels of the stimulator of interferon genes (STING) in microglia. And the use of a specific activator of STING (CMA) disrupted the protective effect of silybin. A total of 358 differential expression genes were identified using transcriptomics, and 150 different metabolites abundance were identified using metabolomic screening. Analysis of the effects of STING on transcriptomics and metabolomics revealed that STING might impact metabolic pathways, including linoleic acid metabolism. The qPCR results confirmed the decreased expression of essential proteins involved in the pathway. Finally, we found that increased STING expression in the cerebrospinal fluid of SAH patients was associated with decreased neurological function scores and poor prognosis.

Conclusion: Silybin had a therapeutic effect on SAH. The underlying mechanism involves linoleic acid metabolism, which is associated with the differential genes and metabolites detected in the study. This study presented a pharmacological rationale for using silybin to treat SAH.

Neuroinflammation; STING; Silybin; Subarachnoid hemorrhage.