Excessive growth hormone expression in male GH transgenic mice adversely alters bone architecture and mechanical strength

Abstract

Patients with acromegaly have a higher prevalence of vertebral fractures despite normal bone mineral density (BMD), suggesting that GH overexpression has adverse effects on skeletal architecture and strength. We used giant bovine GH (bGH) transgenic mice to analyze the effects of high serum GH levels on BMD, architecture, and mechanical strength. Five-month-old hemizygous male bGH mice were compared with age- and sex-matched nontransgenic littermates controls (NT; n=16/group). Bone architecture and BMD were analyzed in tibia and lumbar vertebrae using microcomputed tomography. Femora were tested to failure using three-point bending and bone cellular activity determined by bone histomorphometry. bGH transgenic mice displayed significant increases in body weight and bone lengths. bGH tibia showed decreases in trabecular bone volume fraction, thickness, and number compared with NT ones, whereas trabecular pattern factor and structure model index were significantly increased, indicating deterioration in bone structure. Although cortical tissue perimeter was increased in transgenic mice, cortical thickness was reduced. bGH mice showed similar trabecular BMD but reduced trabecular thickness in lumbar vertebra relative to controls. Cortical BMD and thickness were significantly reduced in bGH lumbar vertebra. Mechanical testing of femora confirmed that bGH femora have decreased intrinsic mechanical properties compared with NT ones. Bone turnover is increased in favor of bone resorption in bGH tibia and vertebra compared with controls, and serum PTH levels is also enhanced in bGH mice. These data collectively suggest that high serum GH levels negatively affect bone architecture and quality at multiple skeletal sites.

Figure 1.. bGH mice have increased body weights and bone lengths.

NT and bGH mice were weighed at 5 months of age and lengths of tibiae, femora, and lumbar vertebrae measured using micro-CT. Body weights (A) and lengths of tibiae (B), femora (C), and vertebrae (combined L4 and L5) (D) of bGH and NT mice are shown. Values are mean ± SD of n = 16 mice/group. ****, P < .0001 NT vs bGH mice.

Figure 2.. bGH mice have decreased cortical BMD but not trabecular BMD in vertebrae.

Cortical and trabecular BMD were assessed by micro-CT in lumbar vertebrae L5 vertebral body from NT and bGH mice aged 5 months. L5 vertebra vertebral body trabecular BMD (A) and L5 vertebra vertebral body cortical BMD (B) in bGH and NT mice. Bars represent mean ± SD of nine mice per group. ****, P < .0001 NT vs bGH mice. Tb.BMD, trabecular BMD; Ct.BMD, cortical BMD.

Figure 3.. Mechanical testing (three point bending) of femora from bGH and NT mice.

Biomechanical properties of the excised mouse femurs in NT and bGH mice aged 5 months using the three-point bending test, which tested for ultimate stress (A), Young's modulus (B), and stiffness (C). Bars represent mean ± SD of six mice per group. *, P < .05, **, P < .001 NT vs bGH mice.

Figure 4.. Bone histomorphometry parameters and serum PTH measurement in bGH and NT mice.

Histomorphometric parameters of bone formation and resorption were measured in trabecular bone on sections of tibia from bGH and NT mice. A, Trabecular bone volume measured on Goldner's trichrome-stained sections. B, Percentage of TRAP-positive surfaces/bone surfaces. C, Bone formation rate (BFR/BS). D, Serum PTH levels. Bars represent mean ± SD of five to nine mice per group. *, P < .05 NT vs bGH mice.