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Studies on the physiology of macrophage receptors for alpha-macroglobulin X protease complexes
Abstract:
Alveolar macrophages exhibit high-affinity receptors that effect the binding and internalization of alpha-macroglobulin X protease complexes (alpha M X P). Studies were designed to probe the mechanism of internalization of alpha M X P complexes. Macrophages also exhibit receptors capable of clearing other injurious agents such as mannose-terminal glycoproteins. We have demonstrated that uptake of mannose-terminal glycoproteins does not affect the rate of internalization of alpha M X P complexes, suggesting that "scavenger" receptors function independently, and that the machinery for endocytosis is not rate limiting. We have also demonstrated that the rate of internalization of receptor-bound alpha M X P is independent of receptor occupancy. During the internalization of alpha M X P there is a decrease in the number of surface receptors consistent with the internalization of ligand-receptor complexes. Using photoaffinity-labeled alpha M X P to inactivate surface receptors, we have demonstrated that even in the absence of ligand accumulation, pools of intracellular receptors exist which are capable of being exteriorized to the cell surface. These results suggest that concomitant with internalization of occupied receptors is the appearance of unoccupied receptors on the cell surface from an internal pool.
Insights
Alveolar macrophages internalize alpha-macroglobulin X protease complexes (alpha M X P) via specific receptors. This process is independent of other scavenger receptors and receptor occupancy, suggesting a dynamic receptor recycling mechanism.
Area of Science:
- Cell Biology
- Immunology
- Macrophage Biology
Background:
- Alveolar macrophages possess high-affinity receptors for alpha-macroglobulin X protease complexes (alpha M X P).
- Macrophages also clear other agents like mannose-terminal glycoproteins via scavenger receptors.
Purpose of the Study:
- To investigate the mechanism of alpha M X P complex internalization by macrophages.
- To determine the relationship between alpha M X P uptake, scavenger receptor activity, and endocytosis machinery.
Main Methods:
- Assessing the effect of mannose-terminal glycoprotein uptake on alpha M X P internalization rates.
- Evaluating alpha M X P internalization rate relative to receptor occupancy.
- Utilizing photoaffinity-labeled alpha M X P to inactivate surface receptors and observe receptor dynamics.
Main Results:
- Uptake of mannose-terminal glycoproteins did not alter alpha M X P internalization, indicating independent scavenger receptor function and non-rate-limiting endocytosis.
- The rate of alpha M X P internalization remained constant regardless of receptor occupancy.
- Internalization of alpha M X P led to a decrease in surface receptors, but photoaffinity labeling revealed intracellular receptor pools that replenish surface receptors.
Conclusions:
- Macrophage endocytosis machinery is not a rate-limiting factor for alpha M X P internalization.
- Receptor occupancy does not influence the rate of alpha M X P complex internalization.
- Macrophages exhibit a dynamic receptor recycling system, with internal receptor pools being exteriorized to the cell surface concurrently with occupied receptor internalization.