Gas6 or Mer deficiency ameliorates silica-induced autophagosomes accumulation in mice lung

A Review: The Potential Involvement of Growth Arrest-Specific 6 and Its Receptors in the Pathogenesis of Lung Damage and in Coronavirus Disease 2019

The tyrosine kinase receptors of the TAM family-Tyro3, Axl and Mer-and their main ligand Gas6 (growth arrest-specific 6) have been implicated in several human diseases, having a particularly important role in the regulation of innate immunity and inflammatory response. The Gas6/TAM system is involved in the recognition of apoptotic debris by immune cells and this mechanism has been exploited by viruses for cell entry and infection. Coronavirus disease 2019 (COVID-19) is a multi-systemic disease, but the lungs are particularly affected during the acute phase and some patients may suffer persistent lung damage. Among the manifestations of the disease, fibrotic abnormalities have been observed among the survivors of COVID-19. The mechanisms of COVID-related fibrosis remain elusive, even though some parallels may be drawn with other fibrotic diseases, such as idiopathic pulmonary fibrosis. Due to the still limited number of scientific studies addressing this question, in this review we aimed to integrate the current knowledge of the Gas6/TAM axis with the pathophysiological mechanisms underlying COVID-19, with emphasis on the development of a fibrotic phenotype.

Inhibition of Gas6 promotes crystalline silica-induced inflammatory response of macrophages via blocking autophagy flux

Inhalation of crystalline silica (CS) can cause silicosis, which is one of the most serious interstitial lung diseases worldwide. Autophagy dysfunction is an essential step in silicosis progression. In this study, we aim to identify the effect of growth arrest-specific protein 6 (Gas6) during autophagy induction and macrophage inflammatory response caused by CS. After RAW 264.7 macrophages exposed to CS, the levels of Gas6 and autophagy markers (p62, Beclin1, and LC3-II/LC3-I) were increased, accompanied with enhanced inflammatory cytokines secretion. Using autophagy activator (rapamycin) repressed, whereas autophagy inhibitor (3-methyladenine) promoted inflammatory cytokines release. Besides, inhibition of Gas6 aggravated CS-induced inflammatory response, and autophagy inhibition facilitated the promoted effect of Gas6 silencing, resulting in elevated expression of inflammatory cytokines. These findings reveal the protective effects of Gas6 and autophagy in macrophages in response to CS exposure, and highlight the autophagy regulated by Gas6 may be a potential prevention target for CS-induced lung inflammatory response.

Pulmonary Toxicity of Silica Linked to Its Micro- or Nanometric Particle Size and Crystal Structure: A Review

Silicon dioxide (SiO2) is a mineral compound present in the Earth’s crust in two mineral forms: crystalline and amorphous. Based on epidemiological and/or biological evidence, the pulmonary effects of crystalline silica are considered well understood, with the development of silicosis, emphysema, chronic bronchitis, or chronic obstructive pulmonary disease. The structure and capacity to trigger oxidative stress are recognized as relevant determinants in crystalline silica’s toxicity. In contrast, natural amorphous silica was long considered nontoxic, and was often used as a negative control in experimental studies. However, as manufactured amorphous silica nanoparticles (or nanosilica or SiNP) are becoming widely used in industrial applications, these paradigms must now be reconsidered at the nanoscale (<100 nm). Indeed, recent experimental studies appear to point towards significant toxicity of manufactured amorphous silica nanoparticles similar to that of micrometric crystalline silica. In this article, we present an extensive review of the nontumoral pulmonary effects of silica based on in vitro and in vivo experimental studies. The findings of this review are presented both for micro- and nanoscale particles, but also based on the crystalline structure of the silica particles.

Ulinastatin promotes macrophage efferocytosis and ameliorates lung inflammation via the ERK5/Mer signaling pathway

Acute lung injury (ALI) is a pneumonic response characterized by neutrophil infiltration. Macrophage efferocytosis is the process whereby macrophages remove apoptotic cells, and is required for ALI inflammation to subside. The glycoprotein ulinastatin (UTI) has an anti‐inflammatory effect during the acute stages of ALI, but its effect on efferocytosis and the subinflammatory stage of ALI is unclear. Extracellular signal‐regulated kinase 5 (ERK5) is a key protein in efferocytosis, and we thus hypothesized that it may be activated by UTI to regulate efferocytosis and the resolution of pneumonia. To test this hypothesis, here we monitored phagocytosis of macrophages through in vivo and in vitro experiments. Pulmonary edema, neutrophil infiltration, protein exudation, and inflammatory factor regression were observed on days 1, 3, 5, and 7 in vivo. RAW264.7 cells were pretreated with different concentrations of UTI and ERK5 inhibitors, and the expression of tyrosine‐protein kinase Mer (Mer) protein on macrophage membrane was detected. UTI increased the phagocytosis of apoptotic neutrophils by macrophages in vitro and in vivo, and promoted the resolution of pneumonia. The protein expression of ERK5 and Mer increased with UTI concentration, while the expression of Mer was down‐regulated by ERK5 inhibitors. Therefore, our results suggest that UTI enhances efferocytosis and reduces lung inflammation and injury through the ERK5/Mer signaling pathway, which may be one of the targets of UTI in the treatment of lung injury.

The Mechanism and Effect of Autophagy, Apoptosis, and Pyroptosis on the Progression of Silicosis

Silicosis remains one of the most severe pulmonary fibrotic diseases worldwide, caused by chronic exposure to silica dust. In this review, we have proposed that programmed cell death (PCD), including autophagy, apoptosis, and pyroptosis, is closely associated with silicosis progression. Furthermore, some autophagy, apoptosis, or pyroptosis-related signaling pathways or regulatory proteins have also been summarized to contribute greatly to the formation and development of silicosis. In addition, silicosis pathogenesis depends on the crosstalk among these three ways of PCD to a certain extent. In summary, more profound research on these mechanisms and effects may be expected to become promising targets for intervention or therapeutic methods of silicosis in the future.

MerTK inhibits the activation of the NLRP3 inflammasome after subarachnoid hemorrhage by inducing autophagy

The NLR family pyrin domain-containing 3 (NLRP3) multiprotein complex is associated with neuroinflammation and poor prognosis after subarachnoid hemorrhage (SAH). Accumulating evidence shows that Mer tyrosine kinase (MerTK) alleviates inflammatory responses via a negative feedback mechanism. However, the contribution and function of MerTK in SAH remain to be determined. In this study, we explored the role of MerTK during microglial NLRP3 inflammasome activation and evaluated its contribution to the outcome of SAH in mice. Activating MerTK with growth arrest-specific 6 (Gas6) alleviated brain edema, neuronal degeneration and neurological deficits after SAH by regulating neuroinflammation. Gas6 did not change the mRNA levels of Nlrp3 or Casp1 but decreased the protein expression of NLRP3, cleaved caspase1 (p20), interleukin-1β and interleukin-18. Furthermore, Gas6 increased the expression of Beclin1, the ratio of LC3-II/LC3-I and the level of autophagic flux. Inhibiting autophagy with 3-MA reversed the inhibition of NLRP3 inflammasome activation and diminished the neuroprotective effects of Gas6. Thus, MerTK activation may exert protective effects by limiting neuroinflammation and promoting neurological recovery after SAH via autophagy induction.