Secretion of Insulinlike Growth Factor I and Insulinlike Growth Factor-binding Proteins by Murine

Insulin-like growth factor I (IGF-I) stimulates hematopoiesis. We examined whether bone marrow stromal cells synthesize IGF-I. Secretion of IGF-I immunoreactivity by cells from TC-1 murine bone marrow stromal cells was time-dependent and in-hibited by cycloheximide. Gel filtration chromatography under denaturing conditions of TC-1 conditioned medium demon-strated two major peaks of apparent IGF-I immunoreactivity with molecular weights of- 7.5-8.0 kD, the size of native IGF-I, and> 25 kD. Expression of IGF-I mRNA was identi-fied by both RNase protection assay and reverse transcription/ polymerase chain reaction. To determine whether the> 25-kD species identified by RIA possessed IGF-binding activity, a po-tential cause of artifactual IGF-I immunoreactivity, charcoal adsorption assay of these gel filtration fractions was per-formed. The peak of IGF-binding activity coeluted with appar-ent IGF-I immunoreactivity suggesting that TC-1 cells secrete IGF-binding protein(s). Unfractionated conditioned medium ex-hibited linear dose-dependent increase in specific binding of 1125I1-IGF-I with a pattern of displacement (IGF-I and IGF-II> insulin) characteristic of IGF-binding proteins. Western li-gand analysis of conditioned medium showed three IGF-I bind-ing species of 31, 38, and 40 kD. These data indicate that TC-1 bone marrow stromal cells synthesize and secrete IGF-I and IGF-binding proteins and constitute a useful model system to study their regulation and role in hematopoiesis. (J. Clin. Invest. 1991. 88:470-475.) Key words: growth factor- growth factor binding proteins- bone marrow strom