Rapid loss of bone mass and strength in mice after abdominal irradiation

Abstract

Localized irradiation is a common treatment modality for malignancies in the pelvic-abdominal cavity. We report here on the changes in bone mass and strength in mice 7-14 days after abdominal irradiation. Male C57BL/6 mice of 10-12 weeks of age were given a single-dose (0, 5, 10, 15 or 20 Gy) or fractionated (3 Gy × 2 per day × 7.5 days) X rays to the abdomen and monitored daily for up to 14 days. A decrease in the serum bone formation marker and ex vivo osteoblast differentiation was detected 7 days after a single dose of radiation, with little change in the serum bone resorption marker and ex vivo osteoclast formation. A single dose of radiation elicited a loss of bone mineral density (BMD) within 14 days of irradiation. The BMD loss was up to 4.1% in the whole skeleton, 7.3% in tibia, and 7.7% in the femur. Fractionated abdominal irradiation induced similar extents of BMD loss 10 days after the last fraction: 6.2% in the whole skeleton, 5.1% in tibia, and 13.8% in the femur. The loss of BMD was dependent on radiation dose and was more profound in the trabecula-rich regions of the long bones. Moreover, BMD loss in the total skeleton and the femurs progressed with time. Peak load and stiffness in the mid-shaft tibia from irradiated mice were 11.2-14.2% and 11.5-25.0% lower, respectively, than sham controls tested 7 days after a single-dose abdominal irradiation. Our data demonstrate that abdominal irradiation induces a rapid loss of BMD in the mouse skeleton. These effects are bone type- and region-specific but are independent of radiation fractionation. The radiation-induced abscopal damage to the skeleton is manifested by the deterioration of biomechanical properties of the affected bone.

FIG. 1

Changes in body weight and body mass composition after abdominal irradiation (AI). Panels A–C: Postirradiation changes in body weight. Mice were exposed to single-dose (panels A and B) or fractionated (panel C) X rays to the abdomen. Body weight was recorded daily. Weight loss was calculated individually for each mouse as percentage change from baseline values. Data points are means + SD. Panel A: n = 14 (sham), 20 (15 Gy) and 12 (20 Gy), pooled from two experiments with similar results. Panel B: n = 11 per group; experiment was performed once. Panel C: n = 8 per group; experiment was performed once. Panel D: Postirradiation changes in body mass composition. Body mass composition was determined in the same mice shown in panel A at baseline and postirradiation day 7. Data points are means ± SE. n = 14 (sham), 20 (15 Gy) and 12 (20 Gy), pooled from two experiments with similar results. #: P < 0.05 compared to sham; ##: P < 0.05 20 Gy compared to 15 Gy.

FIG. 2

Changes in bone formation and resorption activities 7 days after a single dose of radiation to the abdomen. Panel A: Serum levels of the bone formation marker osteocalcin (OCN) and bone resorption marker tartrate-resistant acid phosphatase 5b (TRAP5b) were measured by ELISA at day 7 postirradiation. Data points are means ± SE, n = 5. #: P < 0.05 compared to sham; ##: P < 0.05 20 Gy compared to 15 Gy. Panel B: Ex vivo formation of alkaline phosphatase (ALP)-positive osteoblastic colonies and tartrate-resistant acid phosphatase (TRAP)-positive multinucleated osteoclasts from bone marrows harvested at day 7 postirradiation. Data points are the mean ± SE, n = 5. #: P < 0.05 compared to sham; ##: P < 0.05 20 Gy compared to 15 Gy.

FIG. 3

Loss of total-body bone mineral density (BMD) 7 days after a single dose of radiation. Mice were subjected to a single dose of X rays to the abdomen. BMD was determined at baseline and at day 7 postirradiation. Data points are means ± SE. n = 14 (sham), 20 (15 Gy) and 12 (20 Gy), pooled from two experiments with similar results. #: P < 0.05 compared to sham.

FIG. 4

Loss of BMD in the femur, tibia and lumbar vertebra 7 days after a single dose of radiation. Mice were subjected to a single dose of X rays to the abdomen. BMD was determined at baseline and at day 7 postirradiation. Data points are means ± SE. n = 14 (sham), 20 (15 Gy) and 12 (20 Gy), pooled from two experiments with similar results. #: P < 0.05 compared to sham.

FIG. 5

Changes in BMD 14 days after a single dose of radiation. Mice were subjected to a single dose of X rays to the abdomen (AI). BMD was determined at baseline and at day 14 postirradiation. Panel A: Changes in total-body BMD. Panel B: Changes in BMD in the femur, tibia and lumbar vertebrae. Data points are means ± SE, n = 12, experiment was performed once. #: P < 0.05 compared to sham, ##: P < 0.05 compared to 5 Gy, ###: P < 0.05 compared to 10 Gy.

FIG. 6

Changes in BMD 10 days after the completion of fractionated irradiation. Mice were subjected to fractionated X irradiation of the abdomen at 3 Gy per fraction, 2 fractions per day for 7.5 consecutive days. BMD was determined at baseline and at 10 days after the last fraction. Panel A: Changes in total-body BMD. Panel B: Changes in BMD in the femur, tibia and lumbar vertebrae. Data points are means ± SE, n = 8; experiment was performed once. #: P < 0.05 compared to sham, P < 0.05 compared to 10 Gy.

FIG. 7

Changes in regional BMD 7 days after a single dose of radiation. Mice were subjected to a single-dose X irradiation of the abdomen. BMD was determined at baseline and at day 7 postirradiation. Data points are means ± SE. n = 14 (sham), 20 (15 Gy) and 12 (20 Gy), pooled from two experiments. #: P < 0.05 compared to sham.

FIG. 8

Effects of abdominal irradiation on bone strength at the mid-shaft tibia. Mice were subjected to a single-dose X irradiation of the abdomen. Tibiae were dissected at day 7 postirradiation and subjected to 3-point bending to determine the peak load and stiffness. The value for each bone was transformed to the percentage difference from the mean value of the sham control. Data points are means ± SE of the transformed values. n = 14 (sham), 20 (15 Gy) and 12 (20 Gy), pooled from 2 experiments. #: P < 0.05 compared to sham, ##: P < 0.05 20 Gy compared to 15 Gy.