Disparate mechanisms of sICAM-1 production in the peripheral lung: contrast between alveolar epithelial cells and pulmonary microvascular endothelial cells.
Membrane-associated intercellular adhesion molecule-1 (mICAM-1; CD54) is constitutively expressed on the surface of type I alveolar epithelial cells (AEC). Soluble ICAM-1 (sICAM-1) may be produced by proteolytic cleavage of mICAM-1 or by alternative splicing of ICAM-1 mRNA. In contrast to inducible expression seen in most cell types, sICAM-1 is constitutively released by type I AEC and is present in normal alveolar lining fluid. Therefore, we compared the mechanism of sICAM-1 production in primary cultures of two closely juxtaposed cells in the alveolar wall, AEC and pulmonary microvascular endothelial cells (PVEC). AEC, but not PVEC, demonstrated high-level baseline expression of sICAM-1. Stimulation of AEC with TNFalpha or LPS resulted in minimal increase in AEC sICAM-1, whereas PVEC sICAM-1 was briskly induced in response to these signals. AEC sICAM-1 shedding was significantly reduced by treatment with a serine protease inhibitor, but not by cysteine, metalloprotease, or aspartic protease inhibitors. In contrast, none of these inhibitors effected sICAM-1 expression in PVEC. RT-PCR, followed by gel analysis of total RNA, suggests that alternatively spliced fragments are present in both cell types. However, a 16-mer oligopeptide corresponding to the juxtamembrane region of mICAM-1 completely abrogated sICAM-1 shedding in AEC but reduced stimulated PVEC sICAM-1 release by only 20%. Based on these data, we conclude that the predominant mechanism of sICAM-1 production likely differs in the two cell types from opposite sides of the alveolar wall.
Mendez
Michael P
MP
Pulmonary Section , Veterans Affairs Medical Center, Ann Arbor, MI 48105, USA. mpmendez@umich.edu
Morris
Susan B
SB
Wilcoxen
Steven
S
Du
Ming
M
Monroy
Yeni K
YK
Remmer
Henriette
H
Murphy
Hedwig
H
Christensen
Paul J
PJ
Paine
Robert
R
D000595:Amino Acid Sequence; D000818:Animals; D002478:Cells, Cultured; D018799:Intercellular Adhesion Molecule-1 / Q000096:biosynthesis*; D008070:Lipopolysaccharides / Q000494:pharmacology; D008168:Lung / Q000502:physiology*; D051379:Mice; D008810:Mice, Inbred C57BL; D008833:Microcirculation / Q000187:drug effects / Q000502:physiology*; D010446:Peptide Fragments / Q000494:pharmacology; D011650:Pulmonary Alveoli / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D011652:Pulmonary Circulation / Q000187:drug effects / Q000502:physiology*; D020545:Respiratory Mucosa / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D013047:Specific Pathogen-Free Organisms; D014409:Tumor Necrosis Factor-alpha / Q000494:pharmacology
D008070:Lipopolysaccharides; D010446:Peptide Fragments; D014409:Tumor Necrosis Factor-alpha; D018799:Intercellular Adhesion Molecule-1