A number of epidemiological studies have suggested that cigarette smoking is a risk factor for osteoporosis. Benzo(a)pyrene (BaP) and 7,12-dimethylbenz(a)anthracene (DMBA) are polycyclic aromatic hydrocarbons (PAHs) found in the tar fraction of cigarette smoke, as well as in car exhaust and furnace gases. We hypothesized that BaP and DMBA are responsible, through interaction with the aryl hydrocarbon receptor (AhR), for the bone loss and fragility seen in smoking-related osteoporosis. In this study four groups of 9-month-old Sprague-Dawley rats were examined. An intact group served as controls. A second control was the ovariectomized (ovx) group. The third group (ovx + E₂) were ovariectomized and also given a continuous basal dose of estrogen by implanted estrogen pellet (0.085 mg of 17β-estradiol per rat). The fourth group (ovx + E₂ + BaP/DMBA) was ovariectomized with an estradiol pellet, and received subcutaneous injections of 250 μg/kg of BaP/DMBA weekly for 15 weeks. The loss of ovarian function allowed the study of a direct effect of BaP/DMBA on bone while the concomitant estrogen repletion prevented ovx-related bone loss. Dual-energy X-ray absorptiometry (DEXA), histomorphometry, image analysis, and mechanical testing were used to determine the effect of the treatments on bone. The DEXA results showed a significant (p < 0.05) decrease in bone mineral density compared with intact controls with both ovx alone and with ovx + E₂ + BaP/DMBA treatment. The ovx + E₂ rats were similar to the intact controls. The osteoid parameters showed a significant increase (p < 0.05) with BaP/DMBA addition vs. intact controls, mimicking the ovx rats. The ovx + E₂ rats had osteoid parameters comparable to those of intact rats. Bone connectivity was decreased in the ovx and ovx + E₂ + BaP/DMBA animals. Connectivity of the ovx + E₂ rats was comparable to that of intact animals. A decrease in failure force was seen in three-point bending for the ovx + E₂ + BaP/DMBA group and in vertebral compression in both the ovx and ovx + E₂ + BaP/DMBA groups vs. intact controls. The mechanical properties of the ovx + E₂ rats were similar to those of intact rats. These results demonstrate that BaP/DMBA causes a loss of bone mass and bone strength, possibly through an increase in bone turnover. This is the first in vivo study linking environmental toxicants, found in the tar fraction of cigarette smoke and in urban air pollution, to loss of bone mass and strength in estrogen-replete ovx rats.
Keywords: Polycyclic aromatic hydrocarbons; Benzo(a)pyrene (BaP); 7,12-Dimethylbenz(a)anthracene (DMBA); Cigarette smoke; Ovariectomy (ovx); Estrogen; Osteoporosis