In vitro and ex vivo expression of serum amyloid A3 in mouse lung epithelia

Abstract

Background and Purpose

Serum amyloid A (SAA), an acute-phase protein whose level tracks infection and inflammation, is the precursor protein of amyloid A (AA) fibrils that is thought to cause AA amyloidosis in human and animals. SAA protein has several isoforms based on the difference of amino acid sequence, such as SAA1 to SAA4 in mice. AA fibrils are associated with chronic inflammation and are mainly originated from SAA1 produced in the liver. SAA3 reportedly contributes to the innate immune response in epithelia; however, little is known about its role at the lung epithelia. Therefore, we investigated SAA3 expression in the lung epithelium activated by bacterial antigens.

Materials and Methods

The expressions of SAA3 and SAA1 mRNA were investigated using quantitative real-time PCR, in vitro using mouse Clara (Club) cells and ex vivo using surgically removed mouse lungs, after their stimulation by using either lipopolysaccharide (LPS), the major outer membranous antigen of gram-negative bacteria, or lipoteichoic acid (LTA), the major outer membranous antigen of gram-positive bacteria. In addition, SAA3 and SAA1/2 proteins in treated lung samples were detected by immunohistochemistry (IHC).

Results

SAA3 mRNA expression increased in cells and lungs treated with either LPS or LTA. SAA3 mRNA was more sensitively expressed in LPS than LTA treatment. In contrast, SAA1 mRNA expression did not increase by either LPS or LTA treatment. Furthermore, SAA3 mRNA expression increased in a dose-dependent manner in cells treated with tumor necrosis factor-alpha. By IHC, SAA3 protein was highly expressed in the luminal side of the bronchial epithelium, while SAA1/2 was not expressed.

Conclusion

These results obtained from in vitro and ex vivo experiments suggest that SAA3 plays an important role in the innate immune response to bacterial infection in the lung epithelia.

Keywords:

• Clara cells
• lipopolysaccharide
• lipoteichoic acid
• lung
• serum amyloid A3

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This study was partly supported by JSPS KAKENHI under grant number 16H05027; and by the Grant for Joint Research Program of the Research Center for Zoonosis Control, Hokkaido University, from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.