No abstract

A direct in vitro effect of 17p-estradiol Although gonadal steroids have a profound effect on skeletal tissues (1-3), and estrogen deficiency has been established as a major etiologic factor in postmenopausal osteoporosis (1, 2), their influence has been considered to be indirect. This prevailing opinion was due to a lack of evidence for either specific estradiol receptors in bone (4) or direct biologic effects of sex steroids on bone cells (5, 6). The situation has dramatically reversed since recent studies have demonstrated significant, albeit low, concentrations of 17/3-estradiol (E2) receptors (refs. 7-9; 1) as well as androgen receptors ( ¶) in bone cells. The brain type (BB) isoenzyme of creatine kinase (CK; ATP:creatine N-phosphotransferase, EC 2.7.3.2; ref. 10) involved in the "energy buffer" system, which regulates cellular concentrations of ATP and ADP, is the major component (11) of the E2-induced protein of the rat uterus (12). E2-induced protein synthesis, and more recently, modulation of CK activity, has been a useful marker for studies on the mechanism ofaction of E2 in uterus and in other tissues that contain E2 receptors (13), because of its rapid response to E2 in vivo and in vitro (14). Moreover, it is a convenient marker for estrogen-modulated gene expression, since E2 treatment increases the steady-state level of mRNA for CK BB in the rat uterus (15). The speed and sensitivity of the assay for CK activity makes CK stimulation an efficient response marker to detect the action of E2 and other hormones (16) in skeletal tissues. In this report, we present evidence that E2 acts directly on cultured osteoblasts and epiphyseal cartilage cells, leading to increased CK activity as well as increased [3H]thymidine incorporation into DNA. Moreover, we report a rapid and sex-specific action of E2 and testosterone (T) on these markers in bones of prepubertal rats. Rat epiphyseal cells were obtained from vitamin Ddeficient 16-to 18-day-old rats, which have a wider epiphyseal cartilage zone than normally fed rats. Epiphyseal cartilage plates were isolated under a binocular dissecting microscope. Cells released by digestion with 0.25% collagenase (Worthington) in phosphate-buffered saline for 60 min at 370C were cultured as described above for calvaria cells (in 2 mM Ca2+). tTo whom reprint requests should be addressed. ¶Spelsberg, T. MATERIALS AND METHODS

No abstract

A system is described for the formation of bone tissue in culture from isolated rat bone cells. The isolated bone cells were obtained from embryonic rat calvarium and periosteum or from traumatized, lifted periosteum of young rats. The cells were cultured for a period of up to 8 wk, during which time the morphological, biochemical, and functional properties of the cultures were studied. Formation of bone tissue by these isolated bone cells was shown, in that the cells demonstrated osteoblastic morphology in light and electron microscopy, the collagen formed was similar to bone collagen, there was mineralization specific for bone, and the cells reacted to the hormone calcitonin by increased calcium ion uptake. Calcification of the fine structure of the cells and the matrix is described. Three stages in the calcification process were observed by electron microscopy. It is concluded that these bone cells growing in vitro are able to function in a way similar to such cells in vivo. This tissue culture system starting from isolated bone cells is therefore suitable for studies on the structure and function of bone.

Mechanical forces applied to cultured bone cells induce the production of cAMP via stimulation of the formation of prostaglandin E2 (PGE2) and its release into the medium, resulting in stimulation of adenylate cyclase. In this paper we show that either the antibiotic gentamycin (100 micrograms/ml) or antiphospholipid antibodies (0.1%) which bind to membrane phospholipids abolish cAMP formation induced by mechanical forces; exogenously added arachidonic acid or PGE2 stimulates cAMP formation, even in the presence of these agents. Addition of exogenous phospholipase A2 (but not phospholipase C) causes an increase in the formation of cAMP in bone cells, a response that is also inhibited by gentamycin or antiphospholipase antibodies. These observations suggest that mechanical forces exert their effect on bone cells via the following chain of events: (1) activation of phospholipase A2, (2) release of arachidonic acid, (3) increased PGE synthesis, (4) augmented cAMP production.