Effects of cigarette smoke exposure on tartrate-resistant acid phosphatase expression in murine lung tissue
Immunological mechanisms (English)The enzyme tartrate-resistant acid phosphatase (TRAP) is highly expressed in alveolar macrophages (AM) and its expression is higher in lung tissue of patients with asthma or COPD and upon smoke exposure. Its function in AM is unclear but may be related to inhibiting antiviral responses and regulating macrophage migration. TRAP is a complex enzyme with two proteoforms: namely TRAP5a and the enzymatically more active TRAP5b.TRAP has multiple mRNA transcripts (for mice 1a, 1b and 1c), which differ in their first noncoding exon. It has been suggested that these different transcripts are involved in regulating TRAP expression. The effects of cigarette smoke (CS) on TRAP mRNA transcript expression are unknown and we therefore aimed at elucidating how CS exposure affects regulation of TRAP expression.
Female BALB/c mice were exposed to CS or air each day during 4 weeks. Lung tissue was analyzed for numbers of macrophages and neutrophils by flow cytometry and TRAP mRNA expression by PCR and TRAP protein expression by western blot. In addition, presence of TRAP protein was investigated using immunohistochemistry.
Of the three TRAP mRNA transcripts, 1b and 1c were expressed more than 1a in lung tissue. In control mice, the number of AM positively correlated with transcript 1b and with the sum of all transcripts and negatively correlated with transcript 1a, suggesting AM mainly express 1b. These correlations did not exist in CS-exposed mice. CS exposure resulted in more neutrophils and less AM in lung tissue but TRAP mRNA and TRAP5a protein expression were higher compared to air-exposed mice. This indicates either more TRAP expression per AM or expression of TRAP by other cells after smoking. All TRAP mRNA transcripts, except 1a, were expressed more after smoking. Lower 1a expression in CS-exposed mice correlated positively with TRAP5a protein expression, suggesting a regulatory role for transcript 1a. In control mice TRAP protein was mainly found in cells resembling AMs. In smoke-exposed animals, TRAP protein was also found in larger cells resembling lipid laden AM (pulmonary foam cells).
These results suggest that AM mainly express TRAP transcript 1b in normal lung and that CS exposure induces expression of both 1b and 1c and inhibits 1a expression resulting in higher TRAP5a protein. This illustrates the complex interactions between TRAP mRNA transcripts and protein synthesis, activity and cellular origin in lung tissue, which may be important for the higher expression of TRAP found in asthma and COPD.