Role of TAM/Gas6 pathway in apoptosis in mouse lung tissues induced by silica

Background  After being inhaled into the lungs, silica dust is ingested by alveolar macrophages, which triggers endogenous or exogenous apoptotic signaling pathways and leading to lung injury. In recent years, the relationship between TAM/growth arrest-specific protein 6 (Gas6) pathway and apoptosis has become a research hotspot. Previous studies have shown that Mer, one of TAM's main receptors, may be involved in the inflammatory response and fibrosis induced by silica.

Objective  This animal experiment aims to investigate the mechanism of TAM/Gas6 pathway on the apoptosis induced by silica.

Methods  Three kinds of male C57BL/6 mice were used in the experiment, including wild-type (WT) mice, Gas6^-/- mice, and Mer^-/- mice. The mice of each kind were randomly divided into a control group and a silica group with body weight matched, and there were a total of six groups with 24 mice in each group. The mice in the control groups were subjected to 50 μL normal saline instillation, while the mice in the silica groups received 50 μL silica suspension instillation (50 μg·μL^-1). Eight mice were sacrificed on day 7, 28, and 84, respectively, and bronchoalveolar lavage fluid (BALF) and lung tissue samples were collected. Total protein (TP), lactate dehydrogenase (LDH), alkaline phosphatase (AKP), and acid phosphatase (ACP) levels were measured with corresponding kits; mRNA expression levels of apoptotic factors Bcl2 and Bax were detected by quantitative real-time PCR; protein expression levels of Bcl2, Bax, caspase-3, and cleaved-caspase-3 were detected by Western blotting.

Results  Compared with the WT control group, the levels of TP, LDH, AKP, and ACP in BALF were significantly elevated, the protein expression level of anti-apoptotic factor Bcl2 was decreased, the protein expression level of pro-apoptotic factor Bax was increased, and the mRNA and protein expression ratios of Bcl2 and Bax were decreased in the WT silica group at the three designed time points (P < 0.05). Compared with the WT silica group, the levels of TP (except day 84), LDH (except day 28 and 84), AKP, and ACP were largely decreased, the protein expression level of anti-apoptotic factor Bcl2 was increased, the protein expression level of pro-apoptotic factor Bax was decreased, the mRNA and protein expression ratios of Bcl2 to Bax were increased, and the protein expression ratio of cleaved-caspase-3 to caspase-3 was decreased in the Gas6^-/- or Mer^-/- silica group at the three designed time points (P < 0.05).

Conclusion  Silica can induce injury and apoptosis in mouse lung tissues. Blockade of Gas6 or Mer gene can partially inhibit the level of apoptosis and alleviate the lung tissue damage in mice caused by silica.