EXPRESSION AND REGULATION OF GALECTIN-3 IN RAT OSTEOBLASTIC CELLS

Galectin 3 is an endogenous soluble beta-galactoside-specific lectin o riginally identified and termed epsilon BP or IgE-binding protein in r at basophilic leukemia cells, but its wide tissue distribution and the multiple contexts in which it has been isolated have suggested that i ts function may not be limited to IgE binding but may include a role i n cell growth regulation and differentiation, neoplastic transformatio n, and cell adhesion (Liu, 1990, Crit. Rev. Immunol., 10:289-306; Baro ndes et al., 1994, J. Biol. Chem., 269:20807-20810). After immunoscree ning of a lambda gt11 cDNA expression library made from bone-nodule fo rming cultures of fetal rat calvaria (RC) cells with an antibody raise d against osteoblastic tells (Turksen et al., 1992, J. Histochem. Cyto chem., 40:1339-1352), three cDNA clones were isolated and sequenced; t he sequence matched that of rat galectin 3. Galectin 3 mRNA was detect ed in various fetal and adult rat tissues, including calvaria and cult ured RC cells. In RC cells and the rat osteosarcoma cell line ROS 17/2 .8, galectin 3 mRNA expression increased with time in culture, in cont rast to its behavior in fetal rat skin fibroblasts (RSF) in which its expression decreased with time in culture. in a second rat osteosarcom a line, UMR 106.01, galectin 3 mRNA was almost nondetectable. The synt hetic glucocorticoid dexamethasone (Dex) enhanced galectin 3 expressio n in RSF cell cultures, while 1,25-dihydroxyvitamin D-3 (1,25(OH)(2)D- 3) had no significant effect. In contrast, Dex downregulated and 1,25( OH)(2)D-3 upregulated galectin 3 expression in RC and ROS 17/2.8 cells , especially at later time points in culture when expression of osteob last-associated differentiation markers by these cell types is most ma rked. Immunolabeling with an antibody against rat galectin 3 to identi fy galectin 3 protein showed that cells labelled within both the ROS 1 7/2.8 and RC populations but with marked intercellular heterogeneity o f intensity. Our data support the conclusion that galectin 3 is a prev iously unrecognized product of osteoblastic cells, that galectin 3 mRN A and protein expression increases with time in vitro concomitant with other markers of osteogenesis, including formation of bone nodules an d expression of osteoblast-associated markers such as alkaline phospha tase, bone sialoprotein, and osteocalcin, and that its expression is r egulated by hormones such as glucocorticoids and 1,25(OH)(2)D-3 that m odulate other aspects of the osteoblast phenotype. (C) 1996 Wiley-Liss , Inc.