Concerted action of androgens and mechanical strain shifts bone metabolism from high turnover into an osteoanabolic mode

Abstract

Adhesion of bone cells to the extracellular matrix is a crucial requirement for osteoblastic development and function. Adhesion receptors connect the extracellular matrix with the cyto-skeleton and convey matrix deformation into the cell. We tested the hypothesis that sex hormones modulate mechanoperception of human osteoblastic cells (HOB) by affecting expression of adhesion molecules like fibronectin and the fibronectin receptor. Only dihydrotestosterone (DHT), but not 17beta-estradiol, stimulated fibronectin (137%) and fibronectin receptor (252%) protein expression. The effects of deformation strain on HOB metabolism were investigated in a FlexerCell strain unit. Cyclically applied strain (2.5% elongation) increased DNA synthesis (125%) and interleukin-6 (IL-6) production (170%) without significantly affecting alkaline phosphatase (AP) activity, type I collagen (PICP), or osteoprotegerin (OPG) secretion. 10 nM DHT pretreatment abolished the mitogenic response of HOB to strain and increased AP activity (119%), PICP (163%), and OPG production (204%). In conclusion, mechanical strain stimulates bone remodeling by increasing HOB mitosis and IL-6 production. DHT enhances the osteoanabolic impact of deformation strain by increasing bone formation via increased AP activity and PICP production. At the same time, bone resorption is inhibited by decreased IL-6 and increased OPG secretion into the bone microenvironment.

Figure 1.

Detection of functional active β1-integrin in HOB from a 39-yr-old man by indirect immunofluorescence. While in solvent-treated cells, the β1 subunit was faintly detectable at the perinuclear region and the cytoplasm and cell margins, respectively (A, arrowheads), fluorescence appeared reinforced after 24-h DHT (10 nM) treatment (B, arrowheads). Bar, 25 μm.

Figure 2.

Comparison of FNR mRNA (Northern blot) and protein (Western blot) expression in primary HOB from five male and seven female donors, respectively. Mean ± SD, unpaired two-tailed t test. *, P (of male vs. female level) < 0.05.

Figure 3.

Effect of intermittently applied elongation strain (2.5%; 1-h strain, 3-h rest for 48 h) on HOB metabolism in vitro. Cells were submitted to strain either without any pretreatment (I), pretreatment with 10 nM DHT for 24 h (II), or pretreatment with DHT and 25 ng/ml PP1 (III), respectively. Mean ± SD (n = 6), unpaired two-tailed t test, *, P < 0.05; **, P < 0.01; ***, P < 0.001. (a) Strained cultures (I) versus unstrained controls. (b) DHT pretreatment (II) versus non–DHT-treated cells (I). (c) DHT + PP1 pretreatment (III) versus DHT pretreatment (II).