Vitamin D insufficiency reduces the protective effect of bisphosphonate on ovariectomy-induced bone loss in rats

Abstract

The present study was carried out to obtain an experimental model of vitamin D (vit D) insufficiency and established osteopenia (experiment 1) to then investigate whether vit D status, i.e. normal or insufficient, interferes with bone mass recovery resulting from bisphosphonate therapy (experiment 2). Rats (n = 40) underwent OVX (n = 32) or a sham operation (n = 8). The first 15 days post-surgery, all groups were kept under fluorescent tube lighting and fed a diet containing 200 IU% vit D (+ D). They were then assigned during an additional 45 days to receive either + D or a diet lacking vit D (− D) and kept under 12 h light/dark cycles using fluorescent or red lighting. Serum 25HOD was significantly lower in − D rats (P < 0.0001). The type of lighting did not induce differences in 25OHD, calcium (sCa), phosphorus (sP), bone alkaline phosphatase (b-AL), CTX, bone density or histology. No osteoid was observed in undecalcified bone sections. Experiment 2 (105 days): rats were fed either + D or − D according to experiment 1 and were treated with either placebo or 16 μg olpadronate (OPD)/100 g rat/week during the last 45 days. Whereas 25HOD was significantly lower (P < 0.0001) in − D/OPD than in + D/OPD rats, no significant differences in sCa, sP, b-AL or CTX were observed. OPD prevented the loss of lumbar spine (LS) and proximal tibia (PT) BMD and the decrease in bone volume (BV/TV) (P < 0.05) and in the number of trabeculae observed in untreated rats. However, + D/OPD animals presented significantly higher values of LS BMD, PT BMD and BV/TV than − D/OPD rats (P < 0.05). No osteoid was observed in undecalcified sections of bone. In summary, this is the first experimental study to provide evidence that differences in vit D status may affect the anticatabolic response to bisphosphonate treatment. However, the molecular mechanism through which vit D insufficiency reduces the effect of the aminobisphosphonate remains to be defined.

Introduction

It is now clear that vitamin D (vit D) deficiency is deleterious to bone and other body tissues [1], [2]. Indeed, since hypovitaminosis D influences calcium-regulating hormones, insufficient status of vitamin D has negative effects on skeletal metabolism. A decrease in serum concentrations of 25hydroxyvitamin D (25OHD) stimulates parathyroid hormone (PTH) secretion [3], leading to an increment in bone remodeling, particularly osteoclastic bone resorption, which increases bone loss and consequently fracture risk [4], [5].

For most mammals, including humans and rats, the skin is a physiological route of entry of vitamin D (vit D). Haddad et al. [6] showed that orally acquired vit D is absorbed with chylomicrons and taken up quickly by liver metabolism, while dermally acquired vit D is bound to vit D binding protein and metabolized gradually [6]. Because most people mainly obtain vit D from sun exposure [7] and only a small amount is obtained from food and supplements, subclinical hypovitaminosis D is a frequent phenomenon [8], even among middle-aged persons [9]. Serum 25HOD closely reflects the status of vit D [10] and is the best indicator to define vit D deficiency, insufficiency, sufficiency and toxicity [11]. The levels of 25OHD decline with age [12], [13], and although osteomalacia is uncommon, vit D deficiency may play a role in the pathogenesis of osteoporosis in the elderly [5].

It is known that estrogen deficiency is characterized by high bone remodeling. Bone resorption is greater than bone formation, increasing bone loss [14], [15]. As a result of low bone mass, fracture risk increases and treatment with anticatabolic agents is required. Ovariectomized (OVX) rats are often used as an experimental model of estrogen (Eo) deficiency [16] to test the efficacy of different therapies in preventing bone loss.

There is some evidence from clinical experience for a link between insufficiency of vit D levels and low bone mineral recovery after anticatabolic treatment with bisphosphonates. Koster et al. [17] studied the effects of intermittent cyclical etidronate therapy in postmenopausal women with and without pre-existing vit D deficiency. They found that bone mass in the lumbar spine and femoral neck was significantly lower in vit-D-deficient patients. Yamanaka et al. [18] studied the relationship between vit D status and the effect of alendronate treatment in osteoporotic postmenopausal Japanese women and concluded that adequate status of vit D is essential to the effectiveness of alendronate for the treatment of osteoporosis. Bisphosphonates are useful anticatabolic agents in several bone diseases including primary osteoporosis [19]. However, to our knowledge, there is no experimental report in the literature on the possible effects of vitamin D status on the effectiveness of bisphosphonate treatment.

It is known that: (1) a reduction in one or both sources would unavoidably induce vit D insufficiency and (2) OVX rats lose approximately 20% of their total body bone mineral content (BMC) becoming osteopenic 40 days after surgery. Based on the above, the present study was carried out to obtain an experimental model of vit D insufficiency and established osteopenia to then investigate whether vit D status, i.e. normal or insufficient, interferes with the bone mass recovery resulting from bisphosphonate therapy.