Breen E C, Ignotz R A, McCabe L, Stein J L, Stein G S, Lian J B
University of Massachusetts Medical Center, Worcester 01655.
J Cell Physiol. 1994 Aug;160(2):323-35. doi: 10.1002/jcp.1041600214.
This study examines the mechanism by which TGF-beta 1, an important mediator 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 bone related genes. These include histone, jun B, c-fos, collagen, fibronectin, osteocalcin, alkaline phosphatase, and osteopontin. Morphologically, 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 of bone nodules formed and the amount of calcium deposition. These effects on differentiation can occur without dramatic changes in cell growth 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 bifunctional growth response from a negative effect during the proliferative phase to a positive growth effect during the later matrix maturation and mineralization phases of the osteoblast developmental sequence. Extracellular matrix genes, fibronectin, osteopontin and alpha 1(I) collagen, are altered in their expression pattern which may provide an aberrant matrix environment for mineralization and osteoblast maturation and potentiate the TGF-beta 1 response throughout the course of osteoblast differentiation. The initiation of a TGF-beta 1 effect on cell growth and differentiation is restricted to the proliferative phase of the culture before the cells express the mature osteoblastic phenotype. Second passage cells that are accelerated to differentiate by the addition of dexamethasone or by seeding cultures at a high density are refractory to TGF-beta 1. These in vitro results indicate that TGF-beta 1 exerts irreversible effects at a specific stage of osteoblast phenotype development resulting in a potent inhibition of osteoblast differentiation at concentrations from 0.1 ng/ml.
本研究探讨了细胞生长和分化的重要介质转化生长因子β1（TGF-β1）在体外阻断正常大鼠二倍体胎儿成骨细胞分化的机制。我们已经确定，前成骨细胞无法分化与细胞生长、基质形成和骨相关基因表达的变化有关。这些基因包括组蛋白、Jun B、c-fos、胶原蛋白、纤连蛋白、骨钙素、碱性磷酸酶和骨桥蛋白。形态学上，经TGF-β1处理的成骨细胞呈现出细长、铺展的形状，与生长早期典型的立方形外观不同。随后，形成的骨结节数量和钙沉积量减少。如果在增殖期早期短时间给予TGF-β1，这些对分化的影响可以在细胞生长无显著变化的情况下发生。然而，持续暴露于TGF-β1会导致双功能生长反应，从增殖期的负面影响到成骨细胞发育序列后期基质成熟和矿化阶段的正生长效应。细胞外基质基因、纤连蛋白、骨桥蛋白和α1（I）胶原蛋白的表达模式发生改变，这可能为矿化和成骨细胞成熟提供异常的基质环境，并在成骨细胞分化过程中增强TGF-β1反应。TGF-β1对细胞生长和分化的影响起始于培养的增殖期，此时细胞尚未表达成熟的成骨细胞表型。通过添加地塞米松或高密度接种培养加速分化的第二代细胞对TGF-β1不敏感。这些体外结果表明，TGF-β1在成骨细胞表型发育的特定阶段发挥不可逆作用，在浓度为0.1 ng/ml时即可有效抑制成骨细胞分化。
