TGF-BETA ALTERS GROWTH AND DIFFERENTIATION-RELATED GENE-EXPRESSION INPROLIFERATING OSTEOBLASTS IN-VITRO, PREVENTING DEVELOPMENT OF THE MATURE BONE PHENOTYPE

This study examines the mechanism by which TGF-beta 1, an important me diator of cell growth and differentiation, blocks the differentiation of normal rat diploid fetal osteoblasts in vitro. We have established that the inability for pre-osteoblasts to differentiate is associated with changes in the expression of cell growth, matrix forming, and bon e related genes. These include histone, jun B, c-fos, collagen, fibron ectin, osteocalcin, alkaline phosphatase, and osteopontin. Morphologic ally, the TGF-beta 1-treated osteoblasts exhibit an elongated, spread shape as opposed to the characteristic cuboidal appearance during the early stages of growth. This is followed by a decrease in the number o f bone nodules formed and the amount of calcium deposition. These effe cts on differentiation can occur without dramatic changes in cell grow th if TGF-beta 1 is given for a short time early in the proliferative phase. However, continuous exposure to TGF-beta 1 leads to a bifunctio nal growth response from a negative effect during the proliferative ph ase to a positive growth effect during the later matrix maturation and mineralization phases of the osteoblast developmental sequence. Extra cellular matrix genes, fibronectin, osteopontin and alpha 1(I) collage n, are altered in their expression pattern which may provide an aberra nt matrix environment for mineralization and osteoblast maturation and potentiate the TGF-beta 1 response throughout the course of osteoblas t differentiation. The initiation of a TGF-beta 1 effect on cell growt h and differentiation is restricted to the proliferative phase of the culture before the cells express the mature osteoblastic phenotype. Se cond passage cells that are accelerated to differentiate by the additi on of dexamethasone or by seeding cultures at a high density are refra ctory to TGF-beta 1. These in vitro results indicate that TGF-beta 1 e xerts irreversible effects at a specific stage of osteoblast phenotype development resulting in a potent inhibition of osteoblast differenti ation at concentrations from 0.1 ng/ml. (C) 1994 Wiley-Liss, Inc.