Mice lacking beta-adrenergic receptors have increased bone mass but are not protected from deleterious skeletal effects of ovariectomy

Abstract

Activation of beta2-adrenergic receptors inhibits osteoblastic bone formation and enhances osteoclastic bone resorption. Whether beta-blockers inhibit ovariectomy-induced bone loss and decrease fracture risk remains controversial. To further explore the role of beta-adrenergic signaling in skeletal acquisition and response to estrogen deficiency, we evaluated mice lacking the three known beta-adrenergic receptors (beta-less). Body weight, percent fat, and bone mineral density were significantly higher in male beta-less than wild-type (WT) mice, more so with increasing age. Consistent with their greater fat mass, serum leptin was significantly higher in beta-less than WT mice. Mid-femoral cross-sectional area and cortical thickness were significantly higher in adult beta-less than WT mice, as were femoral biomechanical properties (+28 to +49%, P < 0.01). Young male beta-less had higher vertebral (1.3-fold) and distal femoral (3.5-fold) trabecular bone volume than WT (P < 0.001 for both) and lower osteoclast surface. With aging, these differences lessened, with histological evidence of increased osteoclast surface and decreased bone formation rate at the distal femur in beta-less vs. WT mice. Serum tartrate-resistance alkaline phosphatase-5B was elevated in beta-less compared with WT mice from 8-16 wk of age (P < 0.01). Ovariectomy inhibited bone mass gain and decreased trabecular bone volume/total volume similarly in beta-less and WT mice. Altogether, these data indicate that absence of beta-adrenergic signaling results in obesity and increased cortical bone mass in males but does not prevent deleterious effects of estrogen deficiency on trabecular bone microarchitecture. Our findings also suggest direct positive effects of weight and/or leptin on bone turnover and cortical bone structure, independent of adrenergic signaling.

Figure 1

Longitudinal in vivo assessment of body composition and BMD in male WT (○) and β-less (□) mice (n = 8–9 per group). A, Body weight (grams); B, body fat (percent); C, total body BMD (grams per square centimeter); D, femoral BMD (grams per square centimeter). *, P < 0.05; **, P < 0.001 β-less vs. WT. Error bars represent sem.

Figure 2

Two-dimensional μCT-derived images of the fifth lumbar vertebrae in male 6-wk (A) and 16-wk (B) WT and 6-wk (C) and 16-wk (D) β-less mice. Scale bar, 1 mm.

Figure 3

Three-dimensional μCT-derived images of the distal femoral metaphysis in male 6-wk (A) and 16-wk (B) WT and 6-wk (C) and 16-wk (D) β-less mice.

Figure 4

Two-dimensional μCT images of the midfemoral cross-section in male 6-wk (A) and 16-wk (B) WT and 6-wk (C) and 16-wk (D) β-less mice.

Figure 5

Percent increase (mean ± se) from baseline in body weight (A), percent body fat (B), total body BMD (C), and femoral BMD (D) in β-less OVX (white bars, n = 10), β-less sham-OVX (black bars, n = 9), WT OVX (white bars, n = 11), and WT sham-OVX (black bars, n = 12) mice. *, P = 0.07; **, P < 0.05.

Figure 6

Difference (in percent) in trabecular BV/TV, TbN, and connectivity density (ConnD) between OVX and sham-OVX at the vertebrae and distal femur in wild-type and β-less mice (n = 9–12 per group). *, 0.05 < P < 0.10; **, P < 0.05 for sham vs. OVX.