Abstract
Staphylococcus aureus is one of the major etiological pathogens of bovine mastitis. Its invasion into mammary epithelial cells has been proven to be a key event in the pathogenesis of mastitis. However, the specific pathogenic factors have not been clearly identified. Staphylococcus aureus often triggers infections by releasing virulence factor. Recent several studies reported that staphylococcal enterotoxin M was one of the most frequently found enterotoxin genes associated with bovine mastitis. Thus, the effect of staphylococcal enterotoxin M on inflammation and damage of the bovine mammary epithelial bovine mammary gland epithelial cell line (MAC-T) cells with 48 h treatment was explored in the present study. First, staphylococcal enterotoxin M protein was purified by a Ni-NTA spin column (GE Life Science, Westborough, MA). The levels of tumor necrosis factor-α, IL-6, and monocyte chemoattractant protein 1 (MCP-1) secretion were measured with the corresponding ELISA kits (R&D Systems, Abingdon, UK). Second, cell viability was assessed with a Cell Counting Kit-8 (Bioswamp, Wuhan, China) and the apoptotic percentage of cells was determined by annexin V-fluorescein isothiocyanate (FITC)/propidium iodide (PI; Beyotime, Nanjing, China) staining. Third, ATP concentration, reactive oxygen species (ROS) generation and lactate dehydrogenase (LDH) release were assayed with commercial kits, then mitochondrial membrane potential (ΔΨm) was estimated using fluorescent probe JC-1 (Beyotime). Finally, the production intercellular cell adhesion molecule-1 (ICAM-1), microtubule-associated protein 1A/1B-light chain 3 I/II (LC3 I/II), p62 (Proteintech, Rosemont, IL), and phosphorylation of IκBα, caspase 3, and mammalian target of rapamycin were detected by Western blot. The results showed that staphylococcal enterotoxin M induced inflammation of epithelial cells (upregulating tumor necrosis factor-α, IL-6, MCP-1, and ICAM-1 production) and activated NF-κB (promoting phosphorylation of IκBα). Furthermore, staphylococcal enterotoxin M impaired MAC-T cells via cell necrosis (enhancing LDH release), apoptosis (annexin V-FITC/PI stain, exacerbating oxidative stress, decreasing ΔΨm and intracellular ATP concentration, and activating caspase 3), but independent of autophagy (nonsignificantly increasing LC3-II, decreasing p62 expression, and activating mammalian target of rapamycin). Thereby, staphylococcal enterotoxin M induced the inflammatory property of bovine mammary epithelial cells by boosting cytokine, chemokine, and adhesion molecule production. Furthermore, it caused epithelial cell dysfunction via depressing cell viability and initiating cell necrosis and apoptosis. Because epithelial cells played important roles in orchestrating the inflammatory response and protecting bovine mammary tissue from mastitis, our results indicated that staphylococcal enterotoxin M may be associated with mastitis.