Glucocorticoids down-regulate the extracellular matrix proteins ®bronectin, ®bulin-1 and ®bulin-2 in bone marrow stroma Gu Y-C, Talts JF, Gullberg D, Timpl R, Ekblom M. Glucocorticoids down-regulate the extracellular matrix proteins ®bronectin, ®bulin-1 and ®bulin-2 in bone marrow stroma. Eur J Haematol 2001: 67: 176±184. # Munksgaard 2001. Abstract: Glucocorticoids regulate hematopoietic cell interactions with the bone marrow microenvironment, but the molecules involved in the regulation are still largely unknown. We have studied the effect of glucocorticoids on mRNA expression and protein synthesis of the major extracellular matrix adhesion protein ®bronectin and three other extracellular proteins, ®bulin-1, ®bulin-2 and nidogen-1, in mouse bone marrow cultures and in a hematopoiesis supporting the stromal MC3T3-G2/PA6 cell line. Glucocorticoids suppressed mRNA expression and protein synthesis of ®bronectin, ®bulin-1 and ®bulin-2, but not nidogen-1, in adherent cells of bone marrow cultures, as shown by Northern blot analysis and immunoprecipitation. mRNA levels of all four proteins were down-regulated by dexamethasone in MC3T3-G2/ PA6 cells, indicating a direct glucocorticoid effect on cells synthesizing extracellular matrix proteins. Dexamethasone down-regulated ®bronectin mRNA rapidly, within 2 h of treatment, in the stromal cells. This effect did not require mRNA or protein synthesis, as shown by Northern blot analysis after treatment by actinomycin D and cycloheximide. Interferon-a, which also has been reported to modulate haematopoietic cell±matrix interactions, did not affect mRNA expression of the proteins in MC3T3-G2/PA6 cells. Our results indicate that glucocorticoids down-regulate expression of several mesenchymal-type extracellular matrix molecules in bone marrow, but with a variable effect on different proteins. Thus one mechanism by which glucocorticoids regulate haematopoiesis may be by altering the relative proportions of extracellular matrix proteins. Yu-Chen Gu 1 , Jan Fredrik Talts 1 , Donald Gullberg 3 , Rupert Timpl 4 and Marja Ekblom 2 Departments of 1 Cell and Molecular Biology, and 2 Laboratory Medicine, Lund University and 2 University Hospital, Lund, Sweden; 3 Department of Cell and Molecular Biology, Uppsala University, Uppsala, Sweden and 4 Max-Planck-Institut fu Èr Biochemie, Martinsried, Germany Key words: ®bulin; ®bronectin; nidogen-1; glucocorticoids; bone marrow; MC3T3-G2/PA6 Correspondence: Marja Ekblom, Lund University, Department of Cell and Molecular Biology, BMC B12, SE-221 84 Lund, Sweden Tel:+46 46 2220804 Fax:+46 46 2220855 e-mail: marja.ekblom@medkem.lu.se Accepted for publication 5 Sept 2001 The bone marrow microenvironment, consisting of cytokines, growth factors, stromal cells and extracellular matrix molecules, regulates develop- ment of haematopoietic cells. Like growth factors, extracellular matrix molecules directly regulate haematopoietic cell phenotype and functions by signal transduction through cell surface adhesion receptors. Furthermore, ®ndings showing conver- gence in intracellular signalling pathways from cell adhesion and growth factor receptors indicate that co-operation of these two signalling pathways is essential for normal development and func- tions (1±4). Fibronectin is the major cell-adhesive extracellular matrix protein, which binds to several types of cellular receptors, including integ- rins and proteoglycans. The important regula- tory role for ®bronectin for survival, proliferation and differentiation of primitive and lineage- committed haematopoietic cells is well established (1, 5±9). Eur J Haematol 2001: 67: 176±184 Printed in UK. All rights reserved Copyright # Munksgaard 2001 EUROPEAN JOURNAL OF HAEMATOLOGY ISSN 0902-4441 176