Establishment from mouse growth cartilage of clonal cell lines with responsiveness to parathyroid hormone, alkaline phosphatase activity, and ability to produce an endothelial cell growth inhibitor

Chronic acidosis-induced growth retardation is mediated by proton-induced expression of Gs protein

The etiology of skeletal growth retardation accompanying metabolic acidosis is not clear. Using ex vivo models for endochondral ossification, we showed that the cAMP/PKA pathway, probably triggered by proton sensitive G-protein-coupled receptors, is responsible for impaired skeletal growth in acidosis.

INTRODUCTION

Chronic metabolic acidosis (CMA) is very often accompanied by skeletal growth retardation. We have previously shown in an ex vivo model of endochondral ossification that murine mandibular condyles subjected to acidic conditions exhibit growth retardation accompanied by a decline of insulin-like growth factor-I (IGF-I) and its receptors. PTH-induced ameliorative effects on the CMA-induced growth retardation of the mandibular condyle are partially mediated by protein kinase C (PKC). In this study we explored the mechanisms underlying the acidosis-induced growth retardation; in particular, the involvement of the cyclic adenosine monophosphate/protein kinase A (cAMP/PKA) cellular pathway in the process.

MATERIALS AND METHODS

Mandibular condyles from neonatal mice or mandibular condyle derived chondrocytes (MCDCs) were incubated for 3 days under either control or acidic conditions or in the presence of cAMP-regulating factors (cAMPrf) such as forskolin, iso-butyl methyl xanthine (IBMX), or 8-Br cAMP. The effects on proliferation and differentiation of the cultures as well as on phosphorylation of cAMP responsive element binding protein (CREB) and increased expression of the alpha subunit, Gs were determined. The intracellular pH was detected using the acridine orange assay.

RESULTS

Our results show that, under acidic conditions, PKA levels were increased. H89 abolished the adverse effects of acidosis on condylar development and restored IGF-I and IGF-I receptors (IGF-IR) levels. The inhibitory effects of acidosis on proliferation and differentiation of cartilaginous cells were mimicked by cAMPrf. We have also shown that acidosis stimulates activation of Gs trimeric protein and CREB phosphorylation. GDPbetaS--a Gs antagonist--abolished the acidosis-induced condylar growth arrest. Using an acridine orange assay, we showed that the intracellular environment is not acidified under acidic conditions.

CONCLUSIONS

Our results indicate that the adverse effects of acidosis on skeletal growth centers are mediated at least in part by the cAMP/PKA cellular pathway. We speculate that high proton concentrations exerted by acidosis conditions stimulate proton sensitive G-protein-coupled receptors, which are mediated by the cellular cAMP/PKA pathway and induce skeletal growth retardation.

Progesterone- and dexamethasone-dependent osteoprogenitors in bone cell populations derived from rat vertebrae are different and distinct

Previous experiments have demonstrated that bone cell populations derived from explants of lumbar vertebral bone of adult female rats contain osteoprogenitors that require dexamethasone (Dex) or progesterone (Prog) to proliferate and differentiate into fully differentiated bone-forming osteoblasts. We now show that the Prog-dependent population cannot be detected in male rats after sexual maturation, but is present in prepubertal rats of both sexes and can be induced in adult male-derived populations by culturing the explants in medium containing 17beta-estradiol (10(-9)-10(-8) M). This suggested that the Prog- and Dex-dependent osteoprogenitors in adult female-derived populations were probably distinct populations and that the survival of the Prog-dependent osteoprogenitors and/or their ability to proliferate are dependent on the presence of estrogen. We then proceeded to prove this by using replica plating. When one of the paired colonies duplicated was cultured in medium containing Dex (10(-8) M) and the other in medium containing Prog (10(-5) M), 5.0% of duplicates formed bone in Prog only, 11.1% formed bone in Dex only, and 3.4% formed bone in both Prog and Dex. In all cases the size of the bone-forming colonies in Dex-treated cultures was larger than that in Prog-treated cultures, indicating that the effects of Dex on osteoprogenitor proliferation are greater than those of Prog. The results demonstrate the existence of three classes ofosteoprogenitors in adult female rat-derived bone cell populations: a class responding to Dex only, a class responding to Prog only, and a class responding to both Dex and Prog. The results also indicate that the effects of Prog are not mediated by Prog binding to the glucocorticoid receptor and imply that Prog plays an important role in maintaining bone mass through regulating the class of osteoprogenitors responsive to Prog.

Effects of bone-seeking hormones on DNA synthesis, cyclic AMP level, and alkaline phosphatase activity in cultured cells from human posterior longitudinal ligament of the spine

In a study of the osteogenesis capability of the human posterior longitudinal ligament of the spine, ligament cells were isolated and cultured. The effect of bone-seeking hormones, such as parathyroid hormone (PTH), calcitonin (CT), prostaglandin E2 (PGE2), and 1,25-dihydroxycholecalciferol [1,25-(OH)2D3], on the ligament cells was investigated with respect to DNA synthesis, adenosine-3',5'-cyclic monophosphate (cAMP) levels, alkaline phosphatase (ALP) activity, and acid phosphatase (ACP) activity. Cell lines obtained from nonossified sites in patients with ossification of the posterior longitudinal ligament of the spine (OPLL) were found to have several different phenotypic characteristics for osteoblasts: high ALP activity, PTH- and PGE2-stimulated increases in cAMP, and responses to both CT and 1,25-(OH)2D3. It is clear that proliferation and differentiation in such ligament cells are controlled by various types of bone-seeking hormones, and it was suggested that many cells with osteoblast-like characteristics are present. These results are considered important with respect to the etiology of OPLL, and an experimental system using cultured ligament cells appears to be useful in research on OPLL.

Establishment of a growth hormone responsive chondrogenic cell line from fetal rat tibia

Reproducible effects of growth hormone (GH) on primary isolated cells in monolayer are highly dependent on the culture conditions and/or the fraction of GH responsive cells. To study the effect of GH at the cellular level, a homogenous cell line with both GH responsiveness and chondrogenic properties was established. Primary isolated cells from 18-day-old fetal rat tibia were subcultured using a strict protocol for passages (every third day and a seeding density of 15,000/cm2). Of six established cell lines, one fetal tibia cell line No. 5 (FTC 5) expressed adipogenic and chondrogenic properties at a low frequency. Cells from FTC 5 were subcultured in soft agar suspension with the addition of bovine GH (100 ng/ml). After 14 days in culture eight monoclonal cell lines were established from individual large colonies. Two subclones, FTC 5:3 and FTC 5:6, expressed a chondrogenic phenotype as demonstrated by chondrocyte foci, alcian blue staining and production of type II collagen. Further characterization of FTC 5:3 revealed specific binding of bovine GH with an affinity of 1.7 x 10(9) M-1, and approximately 7300 receptors/cell. Northern blot analysis of FTC 5:3 with a 32P-labeled RNA probe complementary to an extracellular part of the rat GH receptor, revealed two major labeled bands (4.0 and 1.2 kilobases). Both GH and insulin-like growth factor-I (IGF-I) stimulated 3H-thymidine uptake in FTC 5:3 (194 +/- 28% and 405 +/- 127% over control, respectively), while proteoglycan synthesis, as measured by [35S]sulphate uptake, was stimulated by IGF-I only (101 +/- 18% over control).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of tumor necrosis factor alpha on proliferation and expression of differentiated phenotypes in rabbit costal chondrocytes in culture

Tumor necrosis factor alpha (TNF alpha) decreased the synthesis of glycosaminoglycan (GAG) in rabbit costal chondrocytes in culture, but did not stimulate the release of GAG from cell layers. Like chondrocytes cultured in control medium, chondrocytes cultured in the presence of TNF alpha produced putative "cartilage-specific" proteoglycans identified by density gradient centrifugation under dissociative conditions. Although TNF alpha decreased the synthesis of the proteoglycans, it did not change their monomeric size, which is a marker of cartilage phenotypes. Moreover, TNF alpha did not affect the responsiveness to parathyroid hormone, insulin-like growth factor I, or transforming growth factor beta, which is known to stimulate GAG synthesis in cultured chondrocytes. TNF alpha decreased the alkaline phosphatase activity in the chondrocytes dose dependently. On the other hand, it stimulated their DNA synthesis slightly, but significantly. The stimulatory effect of TNF alpha on DNA synthesis was potentiated by fibroblast growth factor, epidermal growth factor, and fetal bovine serum. These findings suggest that in the presence of hormones and growth factors, TNF alpha promotes the proliferation of chondrocytes while suppressing their further differentiation at the stage of synthesis of cartilage-specific proteoglycans.