Oophorectomy leads to a selective decrease in 1,25-dihydroxycholecalciferol receptors in rat jejunal villous cells

Soy phytoestrogens: impact on postmenopausal bone loss and mechanisms of action

Due to their ability to mimic the actions of mammalian estrogens, soy phytoestrogens have been proposed as potential therapeutic agents to aid in preventing postmenopausal bone loss. In vitro, phytoestrogens promote osteoblastogenesis and inhibit osteoclastogenesis. Although a relatively large number of intervention studies have been undertaken in animals and humans, the efficacy of phytoestrogens as bone-protective agents in vivo remains unclear. Differences in the bioactivities of individual phytoestrogens, differences in phytoestrogen metabolism and bioavailability within different study populations, and imprecise reporting of the dose of phytoestrogens administered in intervention studies may have contributed to the disparity in study findings.

Dietary additions of lactose, casein and soy protein exerted only moderate effects on calcium homeostasis in calcitriol deficient piglets

It has been reported from rats and mice that blood and bone calcium can be normalised in the absence of Vitamin D hormone or its receptor by dietary means. It was the aim of this study to test, whether a similar result can be obtained with pigs. Piglets with inherited calcitriol deficiency were fed with high calcium and Pi diets and supplemented with soy protein or casein and lactose or corn starch, which have been shown to normalise plasma and bone calcium in Vitamin D deficient rats and in mice. In the calcitriol deficient piglets none of the diets was capable to prevent the development of hypocalcemia. However, additions of lactose and soy protein improved somewhat plasma calcium (P < 0.001). Feeding of soy protein also had a significant positive effect on plasma phosphate concentration (P < 0.001). The study shows that in contrast to rats, calcitriol is essential for maintaining a normal plasma and bone calcium status. Responses of this type, when obtained with rats or mice can probably not directly be transferred to pigs and perhaps also not to humans.

Oophorectomy acutely increases calcium excretion in adult rats

Estrogen deficiency-induced bone loss is associated with complex changes in the calcium fluxes that constitute calcium balance. We studied the effects of oophorectomy on calcium balance and its components within the first 9 wk after the operation. Six-day calcium balance studies were performed on 30-wk-old female Sprague-Dawley rats before either sham operation or oophorectomy (oophx) and at 3-wk intervals for 9 wk postoperation. The rats were fed a diet containing 0.4g Ca/100 g diet and 0.3 g P/100 g diet throughout the study. The postoperative changes in calcium balance (P < 0.05) and net calcium absorption (P < 0.02) were negative in the oophx group compared with the ovary-intact group. The oophx group excreted more calcium via both the kidney (urine Ca, P < 0.05) and the gastrointestinal tract (endogenous fecal Ca, P < 0.05). The postoperation endogenous fecal calcium was higher at 3 wk postoophorectomy than at later times (P < 0.05). Oophorectomy did not affect true calcium absorption up to 9 wk postoophorectomy. Oophorectomy stimulates bone metabolism and our findings indicate that within the first 9 wk after oophorectomy, bone mineral loss is associated with a transient increase in the excretion of calcium by the gastrointestinal tract and the kidney.

The polymorphism in the caudal-related homeodomain protein Cdx-2 binding element in the human vitamin D receptor gene

The major physiological activity of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] is the regulation of calcium absorption in the small intestine, and the level of vitamin D receptor (VDR) is an important factor in this regulation. In a previous study, we indicated-that the caudal-related homeodomain Cdx-2 played an important role in the intestine-specific transcription of the human VDR gene. In this study, the polymorphism was identified in the core sequence 5'-ATAAAAACTTAT-3' in the Cdx-2 binding site in the VDR gene promoter. In 261 Japanese women with genotyped VDR polymorphisms, 48 were genotype Cdx-A (adenine at -3731 nucleotides [nt] relative to the transcription start site of human VDR gene 5-ATAAAAACTTAT), 82 were genotype Cdx-G (guanine at -3731 nt, 5'-GTAAAAACTTAT-3'), and 131 were genotype Cdx-A/G (heterozygote). In postmenopausal Japanese women, the bone mineral density (BMD) in the lumbar spine (L2-L4) with the Cdx-G homozygote was 12% lower than that with the Cdx-A homozygote (p < 0.05). In electrophoretic gel mobility shift assay (EMSA), the oligonucleotide with Cdx-G allele markedly decreased the binding to Cdx-2 compared with that in the Cdx-A allele. The transcriptional activity of the VDR promoter with Cdx-G allele was decreased to 70% of the Cdx-A allele. In addition, in the herpes simplex virus thymidine kinase promoter, the Cdx-2 binding element with the G allele showed significantly lower transcriptional activity than that of the A allele. Thus, the polymorphism in the Cdx-2 binding site of the VDR gene (Cdx-polymorphism) would affect the expression of VDR in the small intestine. In addition, this polymorphism may modulate BMD in postmenopausal Japanese women.

The role of heat shock protein 70 in vitamin D receptor function

We previously demonstrated that the 1alpha,25-dihydroxyvitamin D(3) receptor (VDR) interacts with the constitutive heat shock protein, hsc70 in vitro, and with DnaK (Biochem. Biophys. Res. Commun. 260, 446-452, 1999). The biological significance of VDR-heat shock protein interactions, however, is unknown. To examine the role of such interactions in eukaryotic cells, we heterologously expressed VDR and RXRalpha together with a vitamin D-responsive reporter system in Saccharomyces cerevisiae and examined the consequences of heat shock protein 70 gene (SSA) deletion in these cells. We show that heterologously expressed VDR associates with the yeast cytosolic hsp70 protein, Ssa1p. Deletion of the SSA2, SSA3, and SSA4 genes and reduction of Ssa1p activity, reduces the intracellular concentrations of the VDR and its heterodimeric partner, RXRalpha and reduces the activity of a vitamin D-dependent gene. Hsp70-like chaperone proteins play a role in controlling concentrations of the VDR within the cell.

Estrogen increases 1,25-dihydroxyvitamin D receptors expression and bioresponse in the rat duodenal mucosa

Menopause and estrogen deficiency are associated with apparent intestinal resistance to vitamin D, which can be reversed by estrogen replacement. The in vivo influence of estrogens on duodenal vitamin D receptor (VDR) was studied in three groups of rats: ovariectomized (OVX), sham-operated, and ovariectomized rats treated daily with estrogen (40 microg/kg BW) for 2 weeks (OVX + E). Estrogen administration to OVX rats resulted in a 2-fold increase in VDR messenger RNA transcripts. 1,25(OH)2D3 was shown to bind specifically to one class of receptors in duodenal mucosal extracts, with a dissociation constant of 0.03 nM. Binding was significantly increased in duodenal extracts from OVX + E rats, compared with OVX rats (735 +/- 81 vs. 295 +/- 26 fmol/mg protein; P < 0.001); a comparable, 1.5- to 2-fold increase in VDR protein expression was observed in Western blot analyzes of the duodenal mucosa. Markers of VDR activity were increased in estrogen-exposed rats: calbindin-9k messenger RNA transcript content was 1.4- to 1.6-fold higher, and alkaline phosphatase activity was 1.4- to 3-fold higher in sham-operated and OVX + E, respectively, compared with OVX. 25(OH)D, 1,25(OH)2D, or PTH levels were not altered by estrogen treatment. Cumulatively, these findings suggest that estrogen up-regulates VDR expression in the duodenal mucosa and concurrently increases the responsiveness to endogenous 1,25(OH)2D. Modulation of intestinal VDR activity by estrogen, and subsequent influence on intestinal calcium absorption, could be one of the major protective mechanisms of estrogen against osteoporosis.

Oestrogen deficiency impairs intestinal calcium absorption in the rat

1. The effects of ovariectomy on the relationships between calcium consumption and calcium balance and its components were assessed in adult (10-14 months) sham-operated and ovariectomized (Ovx) rats fed a semi-synthetic diet with the calcium content varying between 0.02 and 0.4%. 2. Adaptation to dietary calcium restriction was monitored for 47 days from commencement of a 0.02 % Ca diet. 3. Response to 1,25-dihydroxyvitamin D (20 ng kg-1 day-1) administration in sham and Ovx rats and oestradiol (E2) (20 microgram kg-1 day-1) replacement in Ovx rats was assessed in rats fed a 0.05% Ca diet. 4. Ovx rats had lower intercepts for the relationships between calcium consumption and both calcium balance (P < 0.005) and intestinal calcium absorption (P < 0.005) compared with sham rats, but 1,25-dihydroxyvitamin D was not reduced in Ovx rats. 5. The magnitude of adaptation to dietary calcium restriction was unaffected by ovariectomy. 6. Intestinal calcium absorption was stimulated by an equivalent amount in sham and Ovx rats following 1, 25-dihydroxyvitamin D administration, although this did not reach statistical significance for sham (sham, t = 1.91, n.s.; Ovx, t = 3. 18, P < 0.05). 7. Oestradiol replacement in Ovx rats induced a marked increase in intestinal calcium absorption (t = 8.25, P < 0. 005), without stimulating circulating 1,25-dihydroxyvitamin D levels and led to a marked increase in calcium balance (t = 6.89, P < 0. 005). 8. These data indicate that the impairment of intestinal calcium absorption following ovariectomy is not the result of reduced circulating 1,25-dihydroxyvitamin D or reduced intestinal responsiveness to 1,25-dihydroxyvitamin D. Moreover E2 stimulates intestinal calcium absorption probably by a direct effect on the intestine.

Modulation of intestinal vitamin D receptor by ovariectomy, estrogen and growth hormone

Age-related decline in intestinal calcium (Ca) absorption often occur in postmenopausal osteoporotic women. The impaired Ca absorption can be corrected by estrogen (E2) therapy. Growth hormone (GH) therapy has also been reported to increase intestinal absorption of calcium. Since 1,25-dyhydroxyvitamin D (1,25(OH)2D) is the primary regulator of calcium absorption, we explored whether the mechanisms by which E2 and GH enhance Ca absorption involves the vitamin D endocrine system. We measured serum 1,25(OH)2D concentrations and determined the binding characteristics of intestinal vitamin D receptors (VDRs) in four groups of female rats: sham operated (sham), ovariectomized (ovx), ovx + E2, and ovx + GH. Serum 1,25(OH)2D levels were 42.4 +/- 3.4 and 42.5 +/- 3.2 pg/ml in sham and ovx rats, respectively, and decreased by 63 and 34% (P < 0.001) in ovx + E2 and ovx + GH-treated rats, respectively. The numbers of total, unoccupied and occupied VDRs were 116.9 +/- 2.0, 72.1 +/- 1.1 and 44.8 +/- 1.9 fmol/mg protein, respectively, in sham operated rats, and decreased significantly following ovariectomy by 24, 27 and 19% (P < 0.01), respectively. E2 therapy not only significantly increased total, unoccupied and occupied VDRs above those of ovx rats by 55, 58 and 49% respectively, but it increased the levels above those of sham operated controls as well (P < 0.01). In contrast, GH administration prevented the decrease that occurred in ovx rats in the number of total and unoccupied VDRs (111.2 +/- 3.3; 72.6 +/- 1.4 fmol/mg protein, respectively), but it had no significant effect on the number of occupied VDRs. The dissociation constant (Kd) of intestinal VDRs was unaltered by ovariectomy, E2 and GH. We conclude that down regulation of intestinal VDRs may contribute to the Ca malabsorption that occurs in ovarian hormone deficient states such as postmenopausal osteoporosis, and that the stimulation of Ca absorption by E2 and GH may result, in part, from up regulation of intestinal VDRs.

In vivo effect of 17 beta-estradiol on intestinal calcium absorption in rats

Previously we reported that intestinal cells contain estrogen receptors, and that 17 beta-estradiol enhanced calcium uptake by these cells in vitro. The current study was undertaken to examine the in vivo effects of 17 beta-estradiol on intestinal absorption of calcium and phosphorus. Three groups of rats were studied. Group 1 received solvent vehicle. Groups 2 and 3 received 5 micrograms and 40 micrograms 17 beta-estradiol/kg body weight/day, respectively, for 21 days. Hormone and solvent vehicle injections were given subcutaneously. Rats were fed a Teklad diet containing 0.4% Ca, 0.3% P and 3.0 U vitamin D/g during the study. Intestinal absorption of calcium and phosphorus was assessed over a 5-day period from day 15-19. Carmine red (25 mg/100 g diet) was added to the rat feed to mark the beginning and end of fecal collections. Administration of 17 beta-estradiol caused an increase in intestinal absorption of calcium and phosphorus. The increase was significant only for calcium, and in the animals that received high-dose 17 beta-estradiol (P < 0.05). Serum calcium and phosphorus levels were significantly greater in 17 beta-estradiol treated than in control animals. The urinary excretion of calcium and phosphorus was also increased in a dose-dependent manner by 17 beta-estradiol, and was significant for both calcium and phosphorus in animals that received high-dose 17 beta-estradiol (P < 0.05). In contrast, 17 beta-estradiol treatment did not significantly alter the serum levels of parathyroid hormone and 1,25(OH)2 vitamin D. These findings indicate that estrogen administration promotes intestinal absorption of calcium in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

A new bisphosphonate, BM 21.0955, prevents bone loss associated with cessation of ovarian function in experimental dogs

We previously found that bone loss occurs as soon as 1 month after ovariohysterectomy (OHX) in beagle dogs. Indirect evidence pointed to an early dramatic increase in bone resorption. To verify this hypothesis and evaluate the effects of a newly developed bisphosphonate, BM 21.0955 (Boehringer Mannheim), 36 beagle dogs were subjected to OHX and 12 dogs were sham operated (Sham). OHX dogs were divided into six groups (n = 6 each) and received subcutaneous injections of vehicle or BM 21.0955 at various doses (0.1, 0.3, 1, 10, and 100 micrograms/kg/day) for 1 month. Sham dogs were given vehicle (n = 6) or BM 21.0955 (1 microgram/kg/day, n = 6). Iliac crest biopsies and blood drawings were done at baseline and at month 1. OHX dogs given vehicle exhibited a decrease in cancellous bone volume associated with an increase in erosion depth and a decrease in serum levels of 1,25-dihydroxyvitamin D. BM 21.0955 prevented the bone loss at a dose > or = 1 microgram/kg and the increase in erosion depth and the decrease in serum levels of 1,25-(OH)2D at a dose > or = 0.3 microgram/kg. No osteomalacia was observed at any dose of BM 21.0955. Bone turnover was reduced only when BM 21.0955 was administered at doses of 10 or 100 micrograms/kg. There were no changes in body weight or serum levels of calcium, phosphorus, creatinine, parathyroid hormone, or osteocalcin in all groups. The increase in erosion depth in OHX dogs given vehicle proves that the early rapid bone loss after cessation of ovarian function is related to an increase in osteoclastic activity. The antiosteoclastic activity of BM 21.0955 at a dose > or = 1 microgram/kg prevents this increase and preserves bone volume. The absence of any signs of osteomalacia at any dose confers a relatively wide therapeutic margin to BM 21.0955. BM 21.0955 at a dose > or = 10 micrograms/kg also acts as an inhibitor of bone turnover. This is not observed at a dose of 1 microgram/kg, at least after 1 month of administration.

Effects of combined and separate intermittent administration of low-dose human parathyroid hormone fragment (1-34) and 17 beta-estradiol on bone histomorphometry in ovariectomized rats with established osteopenia

To evaluate the potential use of a combination of parathyroid hormone (PTH) and estrogen as therapy for osteoporosis, we examined the effects of combined and separate administration of low-dose PTH and estradiol in ovariectomized rats with established osteopenia. Ovariectomized rats were untreated for 5 weeks after surgery and then injected s.c. with vehicle (Ovx + V), 1-34 hPTH (2.5 micrograms/kg/day) (Ovx + P), 17 beta-estradiol (50 micrograms/kg/day) (Ovx + E), or a combination of these (Ovx + P + E), for a further 4 weeks. We found no differences in serum calcium, tubular reabsorption of phosphate, or 25OHD. 1,25(OH)2D levels were significantly higher in Ovx + P and lower in Ovx + E, when compared with Ovx + V. Though there was no change in bone mineral density (BMD) in the diaphysis region of femurs, reduction of BMD in the distal region of the femurs in Ovx + V was reversed in Ovx + E and Ovx + P + E. Compared with Ovx + V, Ovx + P and Ovx + P + E had significantly higher cancellous bone volume (Cn-BV/TV) whereas Ovx + E showed a nonsignificant increase. When indices of bone turnover were examined, PTH alone showed a small but not significant improvement in bone formation rate (BFR). Increased osteoclast surface (OCS), as the result of ovariectomy, was inhibited in Ovx + E and Ovx + P + E. Estrogen alone (Ovx + E) severely inhibited BFR, but co-administration of PTH and estrogen (Ovx + P + E) showed an impressive reversal of such inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the vitamin D receptor from the Caco-2 human colon carcinoma cell line: effect of cellular differentiation

The human colon carcinoma cell line, Caco-2, is the only intestinal cell line to spontaneously differentiate in culture to a population exhibiting structural and biochemical characteristics of mature enterocytes. We conducted studies to establish the presence of the vitamin D receptor (VDR), determine changes in VDR concentration and affinity with differentiation and determine whether 1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) mediates a functional response in this cell line. We found that Caco-2 cells possess a specific 1,25(OH)2D3 binding protein similar to the mammalian VDR. It has an equilibrium dissociation constant (Kd) of 0.72 nM, binds vitamin D analogues in order of their biological activities in vivo (1,25(OH)2D3 greater than 25(OH)D3 greater than 24,25(OH)2D3), sediments as a single peak on sucrose density gradients at 3.7 S, and is eluted from a DNA-cellulose column by 0.16 M KCl. The maximum number of binding sites was 2.6-fold greater in the differentiated cell (Day 15) compared to the preconfluent, undifferentiated (Day 4) cell (23 fmol/mg protein vs 56 fmol/mg protein). Cell growth was reduced 59% when exposed to 10(-7) M 1,25(OH)2D3 for 8 days. Alkaline phosphatase activity significantly increased in cultures incubated with 10(-8) M 1,25(OH)2D3 for up to 4 days when treatment was started in both undifferentiated cells (Day 5) and differentiated cells (Day 11). These findings suggest that the VDR present in undifferentiated and differentiated Caco-2 cells is functional. Caco-2 cells provide a unique in vitro model to study vitamin D-regulated functions in differentiated mammalian enterocytes.

Calcitonin, estrogens and the bone

Estrogen deficiency following natural or surgical menopause, is thought to be the main factor leading to postmenopausal bone loss. Furthermore, after estrogen failure a significant reduction of intestinal calcium absorption and a negativization of calcium balance has been observed. The mechanism of estrogen effect on skeletal tissue is not yet fully elucidated. Recently, specific receptors for estrogens in osteoblastic cells have been described; however their low density does not give a full explanation about their functional role. Therefore estrogens act, at least in part, indirectly through calciotropic hormones. In order to further elucidate this issue, we performed some studies in postmenopausal osteoporotic patients and in fertile oophorectomized women. In the first double blind placebo controlled study, after a 1-year estrogen treatment period we observed an increase in bone mineral content in the hormone-treated patients. Furthermore, in all treated patients an improvement of intestinal calcium absorption was detected, while 1,25-dihydroxy-vitamin D serum levels did not show significant changes. To further analyse the relationship between estrogens (E) and calcitonin (CT) in postmenopausal osteoporosis, we performed a double blind placebo controlled study to evaluate the effects of 1-yr estro-progestative treatment on CT secretory reserve, evaluated by calcium infusion test. Blood levels of CT showed a progressive increase during the study period in the hormone-treated group, with a significant increase in the CT response to calcium stimulation test, suggesting a modulation of CT secretion by E. Recently, we performed two studies in fertile oophorectomized women. In the first, we followed longitudinally 24 fertile women for 1 yr. In these patients we measured, before and after oophorectomy, biochemical indexes of bone metabolism and bone mass. During the observation period a significant increase in bone resorption and a significant drop in intestinal calcium absorption was observed. In the second study, performed on 14 women before and 6 months after oophorectomy, a treatment with conjugated estrogens allowed the correction of the primary intestinal defect responsible for the reduced calcium absorption.(ABSTRACT TRUNCATED AT 400 WORDS)

Normal serum concentrations of 1,25 dihydroxyvitamin D in osteoporosis

With the aim of evaluating the role of 1,25 dihydroxyvitamin D in the pathogenesis of osteoporosis, this hormone was studied in 90 subjects. They were divided into three groups: the first group consisted of 30 normal female subjects (aged 30 to 45); the second group comprised 30 elderly female subjects (aged 67 to 90) the third group consisted of female patients (aged 65 to 87) with clinical and radiological evidence of osteoporosis. Variations of serum 1,25 dihydroxyvitamin D, after stimulation with oral P04 (1,000 mg/day for 10 days) and stimulation with oral Ca (1,000 mg/day for 10 days), were studied in 16 osteoporotic females (10 of them received P04) and six Ca (Controls). This second study was performed to evaluate the renal 1-alpha-hydroxylation. No significant variation between serum 1,25 dihydroxyvitamin D of elderly and osteoporotic females was observed when compared with normal young healthy subjects. Moreover, a significant increase in serum 1,25(OH)2D after oral P04 was found in accordance with a significant increase in serum parathormone. These data demonstrate that changes in serum 1,25(OH)2D levels did not seem to play a significant part in the pathogenesis of osteoporosis in our population neither could a deficit of renal 1-alpha-hydroxylation be demonstrated.

Vertebral bone loss in menopause

A direct correlation between loss of ovarian function and reduction of bone mass is well established. The incidence of fractures sharply increases with age starting from the menopause. Therefore, it is very important to know the rate of bone loss occurring after menopause, at both trabecular and cortical levels. Several factors may contribute to the reduction of bone mass in menopause. Reduced estrogen secretion results in reduced intestinal calcium absorption, increased bone resorption, and probably a deficient production of calcitonin. Furthermore, in vivo and in vitro experimental data confirm that estrogen failure is associated with histologic changes, mirroring the biochemical changes described in postmenopausal osteoporosis.

Effects of one-year treatment with estrogens on bone mass, intestinal calcium absorption, and 25-hydroxyvitamin D-1 alpha-hydroxylase reserve in postmenopausal osteoporosis

A double-blind, placebo-controlled study on 21 postmenopausal osteoporotic women was performed in order to assess the effects of 1 year estrogen therapy (Premarin, 1.25 mg/day) on bone mass, intestinal calcium absorption, and mineral metabolism. Bone mineral content (BMC), measured by dual photon absorptiometry on the vertebral bodies and the femoral shaft, increased in both areas, but the changes were more evident at the former site, which is predominantly trabecular (+8.3%, P less than 0.05), than at the latter, which is mainly cortical (+2.6%, P less than 0.05). An improvement of intestinal calcium absorption was also detected at the end of the study (P less than 0.05) in the estrogen-treated group. Parameters of bone metabolism showed a decrease in hydroxyproline/creatinine ratio and osteocalcin, an increase in calcitonin, and no significant changes in parathyroid hormone (PTH) and alkaline phosphatase. Serum 1,25-dihydroxycholecalciferol (1,25(OH)2D3) levels increased after estrogen therapy, whereas 25-hydroxycholecalciferol (25OHD3) remained stable during the study period. Renal 25-hydroxyvitamin D 1 alpha-hydroxylase reserve, assessed by the PTH-stimulation test, showed a more rapid response in producing a 1,25(OH)2D3 peak in the estrogen-treated patients compared with the control subjects. However, estrogens did not induce an absolute improvement in the secretory reserve. This study demonstrates that 1 year treatment with estrogens improves both intestinal calcium absorption and BMC in postmenopausal osteoporotic women. The latter effect appears to be induced by an inhibition of bone resorption, associated to an increased secretion of calcitonin, whereas vitamin D metabolites do not seem to contribute substantially to the mediation of estrogen action on bone.

A longitudinal study of pre- and postmenopausal changes in calcium metabolism

Longitudinal changes in the serum concentration of calcium, phosphate, alkaline phosphatase, PTH, calcitonin and the renal handling of calcium and phosphate were studied in 19 normal women of the same age before and after the menopause. Significant increase in serum calcium, phosphate and calcitonin and urine calcium/creatinine and TmPO4/GF were shown to precede the premenopause. After cessation of the menstruation, no statistically significant further changes were observed in these variables. Changes in PTH were not observed neither during the premenopausal nor the post-menopausal period. Alkaline phosphatase increased in the postmenopausal period suggesting an increase in bone turn-over.