A comparison of the binding and fate of internalized neutrophil elastase in human monocytes and alveolar macrophages
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Abstract:
Human alveolar macrophages (AM) bind and internalize neutrophil elastase (NE) in vitro by receptor-mediated endocytosis. Blood monocytes are progenitors of AM, and if they possess receptors for NE, they could bind and internalize NE in the pulmonary interstitium and may effect elastin degradation, which likely accompanies the development of emphysema. To determine whether monocytes contain receptors for NE, radioiodinated NE (I-NE) binding was assessed, and the results were compared with binding of I-NE to AM obtained concurrently from the same donors. The binding of I-NE to monocytes cultured in vitro for 8 days was also assessed. Specific binding of I-NE to monocytes and AM reached 80% of maximum in 30 min; similar quantities bound to AM and monocytes after a 2-h incubation with I-NE, and a smaller quantity bound to cultured monocytes. The estimated association constant for specific binding was 6 X 10(6)M-1 and 4 X 10(6)M-1 for AM and monocytes, respectively. The fate of I-NE in monocytes and AM at 24 h after uptake was assessed and compared using molecular sieve chromatography. Approximately 50% of the I-NE initially bound to either monocytes or AM remained cell-associated at the end of culture; 62 to 65% of this material eluted at 29,000 daltons and solubilized particulate elastin.(ABSTRACT TRUNCATED AT 250 WORDS)
Insights
Human monocytes and alveolar macrophages (AM) bind neutrophil elastase (NE) via receptor-mediated endocytosis. This suggests monocytes may play a role in the elastin degradation associated with emphysema.
Area of Science:
- Pulmonary Medicine
- Cell Biology
- Biochemistry
Background:
- Human alveolar macrophages (AM) internalize neutrophil elastase (NE) through receptor-mediated endocytosis.
- Blood monocytes are precursors to AM, and their potential to bind NE in the pulmonary interstitium could contribute to emphysema-related elastin degradation.
Purpose of the Study:
- To investigate whether monocytes possess receptors for NE.
- To compare NE binding and internalization by monocytes with that of AM.
Main Methods:
- Radioiodinated NE (I-NE) binding assays were performed on freshly isolated monocytes and AM.
- Monocytes were cultured for 8 days to assess binding in differentiated cells.
- Molecular sieve chromatography was used to analyze the fate of internalized I-NE at 24 hours.
Main Results:
- Specific binding of I-NE to both monocytes and AM reached 80% of maximum within 30 minutes.
- Similar quantities of I-NE bound to AM and freshly isolated monocytes after 2 hours, with less binding to cultured monocytes.
- Approximately 50% of internalized I-NE remained cell-associated after 24 hours, with a significant portion associated with solubilized elastin.
Conclusions:
- Monocytes possess functional receptors for NE, similar to AM.
- These findings suggest monocytes may contribute to elastin degradation in the lung interstitium, potentially playing a role in emphysema pathogenesis.