Osteocyte Phenotype Research Articles

Isolated chick osteocytes stimulate formation and bone-resorbing activity of osteoclast-like cells

Several possible roles have been proposed for osteocytes in bone metabolism, but the precise functions of these cells are still obscure. In this study, we examined the effects of osteocytes on osteoclast formation, using isolated osteocytes from 16-day-old chick embryonic parietal bones and mouse hemopoietic blast cells from spleen cells obtained from 5-fluorouracil-treated mice. Purity of the isolated osteocytes was more than 95%, and their osteocytic phenotypes—such as those having a typical stellate shape and being immunoreactive for osteocyte-specific monoclonal antibody OB 7.3, the nonproliferative phenotype, and those negative for alkaline phosphatase (ALP) activity—were retained during 3 days of culture. When hemopoietic blast cells were cocultured with the isolated osteocytes, the number of osteoclastic tartrate-resistant acid phosphatase (TRAP)-positive multinucleate cells (MNCs) from the blast cells increased even in the absence of any osteotropic factors. This stimulatory effect depended on the number of osteocytes added, and was consistent with that obtained with bone fragments freed of nonosteocytic cells by stripping the periosteum. Conditioned medium (CM) of isolated osteocytes also had stimulatory effects on both formation and bone-resorbing activity of the MNCs, indicating that osteocytes increased osteoclastic development, at least in part, via their production of some soluble factor(s). This osteoclast-inducing activity in the osteocyte CM was observed in the fraction that eluted at around 0.3 M NaCl from a mono-Q anion-exchange column, and this elution profile was similar to that of cell lysates of freshly isolated osteocytes and of stripped bone fragments, indicating the existence of such osteoclastogenesis factor(s) in vivo. Finally, both the osteocyte CM and the active fraction of mono-Q chromatography increased the pit area on dentine slices in the cultures of unfractionated mouse bone cells as well. Thus, osteocytes may play a role in the physiological processes of osteoclastic bone resorption.

Differential expression of the chromosomal high mobility group proteins 14 and 17 during the onset of differentiation in mammalian osteoblasts and promyelocytic leukemia cells

The expression of chromosomal proteins HMG 14 and HMG 17 during proliferation and differentiation into the osteoblast and monocyte phenotypes was studied. Cellular levels of HMG 14 and HMG 17 mRNA were assayed in primary cultures of calvarial-derived rat osteoblasts under conditions that: (1) support complete expression of the mature osteocytic phenotype and development of a bone tissue-like organization; and (2) where development of osteocytic phenotypic properties are both delayed and reduced in extent of expression. HMG 14 and HMG 17 are preferentially expressed in proliferating osteoblasts and decline to basal levels post-proliferatively at the onset of extracellular matrix mineralization. In contrast, under conditions that are not conducive to extracellular matrix mineralization, HMG 14 is maximally expressed following the downregulation of proliferation. Consistent with previous reports by Bustin and co-workers [Crippa et al., 1990], HMG 14 and HMG 17 are expressed in proliferating HL-60 promyelocytic leukemia cells and downregulated post-proliferatively following phorbol ester-induced monocytic differentiation. However, differentiation into the monocyte phenotype is accompanied by reinitiation of HMG 17 gene expression. The results indicate that the levels of HMG 14 and HMG 17 mRNA are selectively down-regulated during differentiation.