Michael A. Aronow

ESTROGEN RECEPTOR-ALPHA IS DEVELOPMENTALLY REGULATED DURING OSTEOBLAST DIFFERENTIATION AND CONTRIBUTES TO SELECTIVE RESPONSIVENESS OF GENE EXPRESSION

Estrogen responsiveness of bone is a fundamental regulatory mechanism operative in skeletal homeostasis. We examined the expression of estrogen receptor-α (ER) messenger RNA (mRNA) in cultured rat calvarial-derived osteoblasts during progressive development of the osteoblast phenotype. Levels of ER message were compared with the expression of traditional osteoblastic markers that have been mapped throughout the differentiation process of these cells. ER transcripts, measured using semiquantitative RT-PCR analysis, were expressed at low levels in early stage proliferating osteoblasts and increased at confluence upon initial expression of bone cell phenotypic genes. A 23-fold up-regulation of ER mRNA expression coincided with the initiation of alkaline phosphatase activity (day 8). ER mRNA levels progressively increased 70-fold, reaching a maximum level on days 22–25 in fully differentiated osteoblasts when osteocalcin expression peaked, but declined precipitously by day 32 in osteocytic cells. Analysis of ...

Effects of 1,25(OH)2D3 and vitamin D analogs on developmental control of cell growth and tissue-specific gene expression during osteoblast differentiation

Abstract Cultured normal diploid osteoblasts provide a model for identifying selective effects of vitamin D and 1,25(OH) 2 D 3 analogs on expression of cell growth and tissue-specific genes that are dependent on the differentiated state of the bone cell. Transcription of the bone-specific osteocalcin gene is responsive to the vitamin D receptor complex together with other cellular signaling factors. Cultured normal diploid osteoblasts provide a model for identifying selective effects of vitamin D and 1,25-(OH) 2 D 3 analogs on expression of cell growth and tissue-specific genes that are dependent on the differentiated state of the bone cell. Transcription of the bone-specific osteocalcin gene is responsive to the vitamin D receptor complex together with other cellular signaling factors.

Cell Structure and the Regulation of Genes Controlling Proliferation and Differentiation: The Nuclear Matrix and Cytoskeleton

In this chapter and in the one which follows we will present concepts and experimental approaches associated with the relationship of proliferation to differentiation with emphasis on the contribution of cell structure to the regulation of cell growth and tissue specific gene expression. While these relationships are of broad biological relevance, we will focus primarily on development of the osteoblast phenotype with the understanding that analogous principles apply to the regulation of phenotype expression in general.

Cell Structure and Gene Expression: Contributions of the Extracellular Matrix to Regulation of Osteoblast Growth and Differentiation

The role of the extracellular matrix of specialized tissues in promoting cellular differentiation has long been recognized (1–2). Our studies have utilized rat osteoblast cultures (3–5) as a model system to examine a well defined extracellular matrix (ECM) and the coordinate regulation of the changes in cell structure and gene expression as related to its formation. These cells produce a mineralized ECM having a bone tissue-like organization analogous to embryonic bone (4–6). In contrast to tumor-derived or transformed osteosarcoma cell lines (7), those normal diploid bone derived cells in culture exhibit normal cell cycle regulated expression of genes which is functionally coupled to expression of differentiation specific genes (4). As the ECM develops, osteoblasts differentiate, progressing through three stages of development, a proliferation period, a period of matrix maturation, and a mineralization period (4). In the previous chapter, temporal expression of genes characterizing the osteoblast phenotype associated with this differentiation in vitro has been described in detail and is summarized in Figure 1.