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Role of stromal cells and macrophages in fibronectin biosynthesis and
matrix assembly in human long-term marrow cultures
H Lerat, JC Lissitzky, JW Singer, A Keating, P Herve and P Charbord
INSERM/CRTS, Besancon, France.
Fibronectin is a major component of the extracellular matrix of adherent
layers of human long-term marrow cultures where it may stabilize the
extracellular matrix network and provide adhesion sites for primitive
hemopoietic cells. This study was devised to analyze the role of adherent
cell populations in fibronectin synthesis, matrix assembly, and
degradation. In cultures performed under the conditions described by
Gartner and Kaplan, immunoprecipitation after metabolic labeling showed
that adherent cells synthesized a fibronectin variant comprising the EDa
domain and lacking the EDb one. Vascular smooth muscle-like stromal cells
were the cell subset responsible for this synthesis. Once synthesized by
stromal cells, EDa+fibronectin was secreted into the supernatant and
incorporated into the extracellular matrix. The cumulation in the
extracellular matrix was predominant by weeks 5 and 6 of culture, when a
decrease in the stromal cell intracytoplasmic content of fibronectin was
observed. Stromal cells from a transformed cell line, L2Ori-, were also
able to synthesize the EDa+fibronectin variant, although for these cells
the assembly into the extracellular matrix was partly impaired. Besides
stromal cells, other cell types participated in fibronectin synthesis:
early-adhering granulomonocytic cells and macrophages appearing later in
culture were able to synthesize an EDa-, EDb- fibronectin variant, clearly
distinct from the EDa+ variant produced by stromal cells. Studies on
cultures in which macrophage growth was stimulated at the expense of
stromal cells by adding granulocyte-macrophage colony-stimulating factor
(50 ng/mL) to the culture medium showed a striking decrease in amounts of
fibronectin measured in the adherent layer. This decrease was caused by a
lack of incorporation of fibronectin in the extracellular matrix,
disclosing a major difference between stromal cells and macrophages in
terms of matrix assembly. This study confirms the similarity between
stromal cells and vascular smooth muscle cells, because in vivo
subendothelial intimal aortic smooth muscle cells and cultured smooth
muscle cells from the aortic media express the EDa+, EDb- fibronectin
variant. Furthermore, our results suggest that the level of fibronectin in
adherent layers is regulated by stromal cells and macrophages. The balance
between these two cell populations may therefore be crucial for the local
control of hemopoiesis by regulating the extracellular fibronectin
available for the adhesion of hematopoietic cells. Our data indicate that
it may be essential to study the adhesion of stem cells to EDa+, EDb-
fibronectin instead of EDa-, EDb- soluble fibronectin, as found in human
plasma.
Volume 82,
Issue 5,
pp. 1480-1492,
09/01/1993
Copyright © 1993 by The American Society of Hematology
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