The inflammatory cytokine TNF has a central role in regulating the inflammatory response, and prolonged TNF release is observed in many chronic inflammatory diseases. The endocytic scavenger receptor LRP1 is involved in regulating TNF release from macrophages via the TIMP-3/ADAM17 axis. LRP1 can be shed from macrophages leading to an accumulation of TIMP-3, which inhibits ADAM17, and subsequently inhibits TNF release.
In this thesis I aimed to investigate how LRP1 was shed from human macrophages in response to inflammatory stimuli and which protease(s) were responsible for this shedding.
I first investigated LRP1 shedding in THP-1 and U937 human monocytic cell lines. However, as I was unable to detect shed LRP1 from U937 cells, and THP-1 cells were insensitive to LPS stimulation, I concluded that neither cell line was an appropriate model system for my investigations.
I next tested primary human monocyte-derived macrophages isolated from peripheral blood. Using these cells, I was able to detect constitutive LRP1 shedding after 2 hours of culture and LPS-stimulated LRP1 shedding after 6 hours of stimulation.
I used various protease inhibitors and blocking antibodies to target individual proteases to investigate which protease(s) were involved. I found that none of the broad specificity protease inhibitors significantly decreased LPS-stimulated LRP1 shedding and that a specific inhibitory blocking antibody against ADAM17 also had no effect. However, selective inhibition of ADAM10 inhibited both the constitutive and LPS-stimulated shedding of LRP1. Furthermore, I observed that activation of ADAM10 increased LRP1 shedding.
In conclusion, I have found that ADAM10 plays a key role in both the constitutive and stimulated shedding of LRP1 from human macrophages. Therefore, ADAM10 is involved in controlling the resolution of the inflammatory response by shedding LRP1 from macrophages to inhibit the continued release of TNF, which might otherwise result in chronic inflammation
