Down-regulation of cell growth and cell cycle regulated genes during chick osteoblast differentiation with the reciprocal expression of histone gene variants

Abstract

Expression of cell cycle (core and H1 histone) and cell growth (c-myc and c-fos) regulated genes was examined in primary cultures of chick calvarial osteoblasts during a developmental sequence associated with the progressive maturation of the osteoblast in a bonelike mineralized extracellular matrix. We have identified a transition point early in the developmental sequence which occurs when proliferation ceases and expression of genes related to the differentiated phenotype of osteoblasts is initiated. During this transition period, cellular levels of RNA transcripts from core and H1 histone genes and the c-myc and c-fos protooncogenes decrease in a parallel and coordinate manner. The decline in histone gene transcription that accompanies the loss of accumulated histone mRNA indicates that the downregulation of histone gene expression is at least, in part, transcriptionally mediated. In contrast, persistence of c-myc and c-fos transcription following completion of the proliferation period, when the mRNAs are no longer present at detectable levels, suggests that the initial downregulation of protooncogene expression is controlled at the level of messenger RNA stability. Thus, two types of signaling mechanisms are operative in the down-regulation of cell proliferation genes during osteoblast differentiation--one that impinges on regulatory sequences that influence the interactions of transcription factors with cis-acting promoter elements and a second that modulates messenger RNA turnover. Of significance, downregulation of the cell cycle regulated histone genes is accompanied by a reciprocal increase in the expression of a structurally distinct subset of the histone genes that are not coupled with DNA replication during the period of expression of osteoblast phenotype markers.(ABSTRACT TRUNCATED AT 250 WORDS)

Modifications of protein-DNA interactions in the proximal promoter of a cell-growth-regulated histone gene during onset and progression of osteoblast differentiation.

“…Primary cultures of osteoblasts isolated from the calvaria of fetal rats undergo during a 35-day period an ordered developmental sequence in which the cells progressively acquire the phenotypic properties of mature osteoblasts in a mineralized extracellular matrix having a bone-tissue-like organization (1)(2)(3)(4). The temporal expression of genes observed in cultured osteoblasts (3,5,6) is similar to that during fetal formation of the calvarium in intact animals (7), supporting the biological relevance of the culture system.…”

“…Expression of the differentiated osteoblast phenotype in cultures of isolated fetal rat calvarial cells is associated with a temporal sequence of interrelated cellular, biochemical, and molecular events that define three periods-proliferation, matrix maturation, and mineralization-during a period in which the cells develop a bone-tissue-like organization (3)(4)(5)(6). The transition from proliferation to initiation of events that accompany the onset of extracellular matrix maturation and mineralization includes the down-regulation of cellgrowth-regulated gene expression.…”

“…Here it should be noted that not all cell-growth-regulated genes are initially down-regulated at the transcriptional level. For example, the down-regulation of c-myc and c-fos gene expression at the completion of the proliferative period in the osteoblast developmental sequence is, in contrast to that of the histone genes, initially mediated by mRNA destabilization and subsequently by decreased transcription (4). In a broader context, our results suggest that a question which must be addressed is the extent to which the progression of osteoblast differentiation is dependent on the shut-down of proliferation and on molecular mechanisms that may mediate this relationship.…”

“…Equally important, the down-regulation ofthe cell-growth-related histone genes at the completion of the proliferative period when expression of genes associated with the organization and maturation of the extracellular matrix is initiated may reflect an important regulatory event at this transition point in the developmental sequence. We have previously shown in cultured osteoblasts that the down-regulation of histone gene expression which occurs with the cessation of proliferation is transcriptionally mediated, since the decreased cellular levels of histone mRNA parallel the decline in histone gene mRNA synthesis (4). Thus, a key signaling mechanism associated with the downregulation of cell proliferation genes at the transition point early during the osteoblast developmental sequence when expression of genes related to extracellular matrix maturation and specialization is up-regulated is one that impinges on regulatory sequences that influence the interactions of transcription factors with cis-acting promoter elements.…”

Concepts of Osteoblast Growth and Differentiation: Basis for Modulation of Bone Cell Development and Tissue Formation

“…At the completion of the proliferation period, when osteoblasts cease to traverse the cell cycle, histone gene expression is down-regulated at the transcriptional level and by a selective destabilization of histone mRNA Holthuis et al, 1990;Shalhoub et al, 1989). A systematic examination of protein-DNA interactions in the proximal regulatory elements (sequences in the promoter of genes, generally 5' to the transcribed sequences) of cell-cycle-regulated H4 and H3 histone genes both in vivo (Pauli et al, 1987;Pauli et al, 1989;Pauli et al, 1988) and in vitro (Holthuis et al, 1990;van Wijnen et al, 1987;van Wijnen et al, 1988;van Wijnen et al, 1989;Kroeger et al, 1987) indicates that this down-regulation of transcription is mediated by a selective loss in the binding of a transcription factor HiNF-D ( Figure 5) to sequences (Site II) that influence both specificity and level of transcription.…”