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Biosynthesis and processing of the cell adhesion molecule PECAM-1 includes production of a soluble form
A Goldberger1, K A Middleton, J A Oliver
1Blood Research Institute, Blood Center of Southeastern Wisconsin, Milwaukee 55233.
Abstract:
PECAM-1 (CD31) is a 130-kDa glycoprotein found on platelets, endothelial cells, granulocytes, and monocytes, as well as on certain myelomonocytic cell lines. Recent studies have shown that PECAM-1 may be involved in activation of leukocyte integrins and may also be involved in adhesive interactions of circulating leukocytes and the vessel wall. In spite of the important functional role that PECAM-1 plays in these processes, little is known about the biosynthesis, processing, and turnover of PECAM-1 on the cell surface. We have studied the biosynthesis of PECAM-1 in the promonocytic cell line U937, and in endothelial cells, by pulse-chase labeling and immunoprecipitation. PECAM-1 was synthesized as a 110-kDa precursor form, which was processed into the 130-kDa mature form within 1-3 h, during which time it began to move to the cell surface. The protein disappeared from the cell surface in both cell types about 48 h after labeling. A soluble form of PECAM-1, which is 5-10 kDa smaller than cell-associated PECAM-1 and contains the cytoplasmic tail, was observed in the culture media of HUVECs and phorbol ester-treated U937 cells. This form of soluble PECAM-1 is encoded by an alternatively spliced mRNA from which the exon containing the transmembrane domain has been removed. Soluble PECAM-1 was also detected in normal human plasma at levels of 10-25 ng/ml. Two isoforms of plasma PECAM-1, which differed in the presence of the cytoplasmic tail, were observed by Western blot analysis. In parallel with soluble forms of other cell adhesion molecules, soluble PECAM-1 may play a role in modulating the inflammatory response.
Insights
Platelet endothelial cell adhesion molecule-1 (PECAM-1) is synthesized as a precursor and matures within hours. A soluble form of PECAM-1, detected in plasma, may modulate inflammatory responses.
Area of Science:
- Immunology
- Cell Biology
- Biochemistry
Background:
- Platelet endothelial cell adhesion molecule-1 (PECAM-1), also known as CD31, is a glycoprotein crucial for leukocyte adhesion and integrin activation.
- Despite its known functions, the biosynthesis, processing, and turnover of PECAM-1 remain incompletely understood.
Purpose of the Study:
- To investigate the biosynthesis, processing, and turnover of PECAM-1 in U937 cells and endothelial cells.
- To characterize the soluble form of PECAM-1 and its potential role in modulating inflammatory responses.
Main Methods:
- Pulse-chase labeling and immunoprecipitation were used to study PECAM-1 biosynthesis in U937 and endothelial cells.
- Western blot analysis was employed to detect and differentiate PECAM-1 isoforms in cell cultures and human plasma.
Main Results:
- PECAM-1 is synthesized as a 110-kDa precursor, maturing to a 130-kDa form within 1-3 hours and migrating to the cell surface.
- The protein has a cell surface half-life of approximately 48 hours.
- A soluble PECAM-1 form, lacking the transmembrane domain due to alternative mRNA splicing, was identified in cell culture media and human plasma (10-25 ng/ml).
- Two plasma PECAM-1 isoforms, differing in cytoplasmic tail presence, were observed.
Conclusions:
- PECAM-1 undergoes intracellular processing and cell surface expression with a defined turnover rate.
- The existence of soluble PECAM-1, potentially generated through alternative splicing, suggests a role in regulating inflammatory processes, similar to other soluble cell adhesion molecules.