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Monocyte synthesis of thrombospondin. The role of platelets
1Department of Medicine, University of Wisconsin, Madison 53706.
Abstract:
Monocytes produce thrombospondin (TSP), a trimeric glycoprotein whose primary function is not yet clear. Platelet-poor monocytes (less than 1 platelet/50 monocytes) cultured with [35S]methionine produced [35S] TSP barely detectable by immunoprecipitation with either monoclonal or polyclonal antibody to TSP. Platelet-poor monocytes that had not been so thoroughly depleted of platelets (6-12 platelets/50 monocytes) synthesized readily detectable amounts of [35S] TSP. Addition of increasing numbers of washed platelets to platelet-poor monocytes resulted in increasing synthesis of [35S]TSP. This monocyte-platelet interaction was specific for cell type; neither neutrophils nor red cells could substitute for platelets. The induction of synthesis was specific for TSP; monocyte synthesis of three other proteins was not induced upon the addition of platelets. Platelet lysate or thrombin-induced platelet releasate could not substitute for intact platelets. In fact, platelet lysate inhibited [35S]TSP synthesis by monocyte-platelet cultures. This inhibition was not due to endotoxin contamination, interference with immunoprecipitation, or dilution of the [35S]methionine pool. Platelets required contact with monocytes to exert their effect, as culturing the cell populations with a filter between them prevented increased [35S]TSP synthesis. Monocyte-platelet interactions may serve to specifically increase monocyte synthesis of the adhesive protein, TSP.
Insights
Monocyte interaction with platelets specifically increases the synthesis of thrombospondin (TSP), an adhesive glycoprotein. This interaction requires direct cell contact and is crucial for TSP production.
Area of Science:
- Immunology
- Hematology
- Cell Biology
Background:
- Monocytes are immune cells that produce thrombospondin (TSP), a glycoprotein with an unclear primary function.
- The regulation of TSP synthesis by monocytes is not well understood.
Purpose of the Study:
- To investigate the role of platelets in regulating monocyte synthesis of thrombospondin (TSP).
Main Methods:
- Monocytes were cultured under varying conditions of platelet depletion and addition.
- Radioactive labeling with [35S]methionine was used to quantify TSP synthesis.
- Immunoprecipitation assays with specific antibodies were employed to detect TSP.
Main Results:
- Platelet-poor monocytes synthesized minimal detectable TSP.
- Increasing the number of co-cultured platelets significantly enhanced monocyte TSP synthesis.
- This effect was specific to platelets and required direct cell-to-cell contact, not soluble factors.
Conclusions:
- Direct interaction between monocytes and platelets specifically stimulates monocyte production of TSP.
- This finding suggests a novel regulatory mechanism for TSP synthesis in the immune system.