Loss of parathyroid hormone-stimulated 1,25-dihydroxyvitamin D3 production in aging does not involve protein kinase A or C pathways

[Pathophysiology of aging bone]

Deterioration of organ and systems function are the principal signs of aging. Aging is also believed to be a major factor in the loss of bone mass and quality, which in turn leads to an increase in the risk of fractures. Several factors seem to contribute to this scenario, with metabolic changes related to aging in the bone tissue itself being among them. Most of the current knowledge on the mechanisms associated with osteopenia/osteoporosis during aging has been generated from research in animal models (mainly rats and mice) and cell cultures derived from subjects of different ages. In this work, we have reviewed and summarised these studies, which have begun to establish the physiological and molecular basis of the bone alterations related to aging.

Ethanol extract of Fructus Ligustri Lucidi increases circulating 1,25-dihydroxyvitamin D3 by inducing renal 25-hydroxyvitamin D-1α hydroxylase activity

OBJECTIVE

The present study was designed to determine whether Fructus Ligustri Lucidi (FLL) ethanol extract can directly regulate vitamin D metabolism both in vivo and in vitro.

METHODS

Eleven-month-old, aged Sprague-Dawley sham-operated and ovariectomized (OVX) female rats were fed a normal-calcium (Ca) diet (0.6% Ca, 0.65% phosphorus) and received either FLL (700 mg/kg) or vehicle daily for 12 weeks. The in vitro effects of FLL on vitamin D metabolism were studied using primary cultures of the rat renal proximal tubules. mRNA and protein expressions of 25-hydroxyvitamin D-1α hydroxylase (1-OHase) and vitamin D receptor (VDR) in the kidney and proximal tubule were measured using real-time polymerase chain reaction and Western blotting, respectively. The concentrations of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) synthesized by renal 1-OHase were measured by a competitive enzyme immunoassay.

RESULTS

FLL treatment significantly increased serum 1,25(OH)2D3 levels in both sham (P < 0.01) and OVX (P < 0.05) rats. FLL increased renal 1-OHase and VDR protein and mRNA expressions in sham rats. Protein expression of renal 1-OHase, but not VDR, was also up-regulated in OVX rats during FLL treatment. 1-OHase mRNA and 1-OHase activity were increased by FLL treatment in primary cultures of renal proximal tubule cells.

CONCLUSIONS

FLL could increase the circulating levels of 1,25(OH)2D3 in vivo in aged female rats by directly stimulating 1-OHase activity. Thus, it might be an ideal oral agent that can help to improve the ability to induce 1,25(OH)2D3 synthesis and Ca balance in postmenopausal women who are of high risk of developing osteoporosis.

Parathyroid hormone stimulation of the renal 25-hydroxyvitamin D-1alpha-hydroxylase--effect of age and free radicals

The capacity of parathyroid hormone (PTH) to increase serum 1,25(OH)(2)D levels declines with age in both rats and humans. In young rats, PTH stimulates renal 1,25(OH)(2)D production and increases mRNA levels for the terminal mitochondrial P450 of the 1alpha-hydroxylase complex (CYP27B1 or CYP1alpha). However, in older rats PTH increases mRNA levels but not 1,25(OH)(2)D production. This suggests that in old animals there is either decreased CYP1alpha protein levels in response to PTH or that the protein produced lacks functionality. The CYP1alpha protein is located on the inner mitochondrial membrane, the site of increased free radical production with age. To study these possibilities, we examined the effect of PTH and free radicals on CYP1alpha expression in a model system-AOK-B50 renal tubular cells. PTH increased CYP1alpha mRNA and protein in a similar time-dependent manner, suggesting that CYP1alpha protein levels were largely regulated by mRNA levels. The effect of free radicals was determined by preincubation with hydrogen peroxide (H(2)O(2)), a standard model for studying free radical damage. H(2)O(2) inhibited PTH-stimulated CYP1alpha protein levels and 1,25(OH)(2)D production in a dose dependent manner. However, 1,25(OH)(2)D production was more sensitive to H(2)O(2) than was CYP1alpha protein levels. This suggests that the catalytic activity of the CYP1alpha protein may be reduced by free radical damage in these cells. Future studies will focus on detecting oxidative damage in this model system and in vivo.

Ovariectomy worsens secondary hyperparathyroidism in mature rats during low-Ca diet

Estrogen deficiency impairs intestinal Ca absorption and induces bone loss, but its effects on the vitamin D-endocrine system are unclear. In the present study, calciotropic hormones levels, renal vitamin D metabolism, 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]-dependent intestinal calcium absorption, and bone properties in 3-mo-old sham-operated (sham) or ovariectomized (OVX) rats fed either a normal-Ca (NCD; 0.6% Ca, 0.65% P) or a low-Ca (LCD; 0.1% Ca, 0.65% P) diet for 2 wk were determined. LCD increased serum 1,25(OH)2D3 levels in both sham and OVX rats. Serum parathyroid hormone [PTH(1-84)] levels were highest in OVX rats fed LCD. Renal 25-hydroxyvitamin D1alpha-hydroxylase (1-OHase) protein expression was induced in both sham and OVX rats during LCD, while renal 1-OHase mRNA expression was highest in OVX rats fed LCD. Renal vitamin D receptor (VDR) and mRNA expressions in rats were induced by ovariectomy in rats fed NCD but suppressed by ovariectomy in rats fed LCD. The induction of intestinal calcium transporter-1 and calbindin-D9k mRNA expressions by LCD were not altered by ovariectomy. As expected, bone Ca content, cancellous bone mineral density, and bone strength index in proximal metaphysis of rat tibia were reduced by both ovariectomy and LCD (P<0.05) as analyzed by two-way ANOVA. Taken together, the data demonstrate that ovariectomy alters the responses of circulating PTH levels, renal 1-OHase mRNA expression, and renal VDR expression to LCD. These results suggest that estrogen is necessary for the full adaptive response to LCD mediated by both PTH and 1,25(OH)2D3.

Prostaglandin E1 increases in vivo and in vitro calcitriol biosynthesis in rabbits

INTRODUCTION

Prostaglandins have an anabolic effect on bone. Possible mediation of this effect is via calcitriol. This study determines in vivo and in vitro effects of PGE(1) on calcitriol synthesis.

METHODOLOGY

In vivo: rabbits received intravenous vehicle or prostaglandin E(1) (50 microg/day) for 20 days before measurements of serum total and ionic calcium, magnesium and phosphorus levels, total and bone-specific alkaline phosphatases, 25(OH)D(3), calcitriol, parathyroid hormone and calcitonin. In vitro: rabbit proximal renal tubules were incubated with 25(OH)D(3) (8 microM) together with PGE(1) (2.82 x 10(-6) M) and the prostaglandin receptor inhibitor AH6809 (10(-4) M) in selected samples. After 5 or 30 min incubation, calcitriol production was measured by radioimmunoassay and data analysed statistically.

RESULTS

In vivo, in groups receiving PGE(1), levels of total Ca, Mg and calcitriol increased significantly and 25 dihydroxyvitamin D(3), parathyroid hormone and calcitonin remained unchanged. In vitro, PGE(1) increased calcitriol biosynthesis and the prostaglandin inhibitor AH6809 reduced calcitriol levels significantly after prolonged incubation.

CONCLUSIONS

In vivo and in vitro results demonstrate that PGE(1) stimulates calcitriol synthesis. This study represent a major advancement in knowledge of bone metabolism.

Age-related alteration of vitamin D metabolism in response to low-phosphate diet in rats

The responses of renal vitamin D metabolism to its major stimuli alter with age. Previous studies showed that the increase in circulating 1,25-dihydroxyvitamin D (1,25(OH)2D3) as well as renal 25-hydroxyvitamin D3 1-alpha hydroxylase (1-OHase) activity in response to dietary Ca or P restriction reduced with age in rats. We hypothesized that the mechanism involved in increasing circulating 1,25(OH)2D3 in response to mineral deficiency alters with age. In the present study, we tested the hypothesis by studying the expression of genes involved in renal vitamin D metabolism (renal 1-OHase, 25-hydroxyvitamin D 24-hydroxylase (24-OHase) and vitamin D receptor (VDR)) in young (1-month-old) and adult (6-month-old) rats in response to low-phosphate diet (LPD). As expected, serum 1,25(OH)2D3 increased in both young and adult rats upon LPD treatment and the increase was much higher in younger rats. In young rats, LPD treatment decreased renal 24-OHase (days 1-7, P<0.01) and increased renal 1-OHase mRNA expression (days 1-5, P<0.01). LPD treatment failed to increase renal 1-OHase but did suppress 24-OHase mRNA expression (P<0.01) within 7 d of LPD treatment in adult rats. Renal expression of VDR mRNA decreased with age (P<0.001) and was suppressed by LPD treatment in both age groups (P<0.05). Feeding of adult rats with 10 d of LPD increased 1-OHase (P<0.05) and suppressed 24-OHase (P<0.001) as well as VDR (P<0.05) mRNA expression. These results indicate that the increase in serum 1,25(OH)2D3 level in adult rats during short-term LPD treatment is likely to be mediated by a decrease in metabolic clearance via the down-regulation of both renal 24-OHase and VDR expression. The induction of renal 1-OHase mRNA expression in adult rats requires longer duration of LPD treatment than in younger rats.

Age-related changes in the response of intestinal cells to parathyroid hormone

The concept of the role(s) of parathyroid hormone (PTH), has expanded from that on acting on the classical target tissues, bone and kidney, to the intestine where its actions are of regulatory and developmental importance: regulation of intracellular calcium through modulation of second messengers and, activation of mitogenic cascades leading to cell proliferation. Several causes have been postulated to modify the hormone response in intestinal cells with ageing, among them, alterations of PTH receptor (PTHR1) binding sites, reduced expression of G proteins and hormone signal transduction changes. The current review summarizes the actual knowledge regarding the molecular and biochemical basis of age-impaired PTH receptor-mediated signaling in intestinal cells. A fundamental understanding of why PTH functions are impaired with age will enhance our understanding of its importance in intestinal cell physiology.

Adaptive responses of 25-hydroxyvitamin D3 1-alpha hydroxylase expression to dietary phosphate restriction in young and adult rats

Regulation of vitamin D metabolism alters with age. The present study is undertaken to investigate if the loss of renal 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) production in response to dietary phosphate (P) restriction in adult rats is due to an alteration in the renal expression of 25-hydroxyvitamin D(3) 1-alpha hydroxylase (1-OHase). Young (4-6 weeks old) and adult (12-14 weeks old) male Sprague Dawley rats were fed either normal P (NPD) or low P diet (LPD) for 0-5 days. Basal expression of 1-OHase protein was higher in adult rats. Young rats, but not adult rats, significantly increased 1-OHase protein and mRNA expressions in response to LPD in a time-dependent manner. To determine if the stability of renal 1-OHase protein changes with LPD feeding, young and adult rats fed either NPD or LPD for 5 days were injected intravenously with cycloheximide (CHX), a protein synthesis inhibitor. CHX decreased 1-OHase protein expression in young rats fed NPD. However, CHX did not alter 1-OHase protein expression in young rats fed LPD nor in adult rats fed either diet. The results indicate that the stability of renal 1-OHase protein increased with age and that LPD increased its stability only in young rats.

PTH increases renal 25(OH)D3-1alpha -hydroxylase (CYP1alpha) mRNA but not renal 1,25(OH)2D3 production in adult rats

The capacity of parathyroid hormone (PTH) to stimulate renal 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] production declines with age in the rat. The purpose of these studies was to determine whether this decline is due to a decreased capacity of PTH to increase the mRNA levels of CYP1alpha, the cytochrome P-450 component of the 25(OH)D(3)-1alpha-hydroxylase. Young (2 mo) and adult (12 mo) male Fischer 344 rats were parathyroidectomized (PTX). After 72 h, PTX rats were injected with PTH or vehicle at 24, 6, and 3 h before death, and renal CYP1alpha mRNA levels were measured by ribonuclease protection assay. In young rats, PTH markedly increased plasma 1,25(OH)(2)D(3) and renal 1,25(OH)(2)D(3) production. However, in adult rats, the response to PTH was less than 30% of that seen in young rats. Renal CYP1alpha mRNA levels, on the other hand, were increased over fivefold by PTH in both young and adult rats. In in vitro studies, PTH/forskolin increased CYP1alpha mRNA levels over twofold in renal slices from both young and adult PTX rats. These studies demonstrate that the decreased capacity of PTH to increase 1,25(OH)(2)D(3) production in adult rats is not due to decreased induction of CYP1alpha mRNA.

Insulin-like growth factor-I stimulates renal 1, 25-dihydroxycholecalciferol synthesis in old rats fed a low calcium diet

The adaptive increase in renal proximal tubule 25-hydroxyvitamin D-alpha-hydroxylase activity (1-OHase) during dietary calcium restriction is mediated by an increase in parathyroid hormone (PTH) and is inhibited by aging. Recent studies in mature (3-4 mo) rats demonstrated that insulin-like growth factor-I (IGF-I) restored stimulation of renal 1,25-dihydroxycholecalciferol [1,25(OH)(2)D(3)] production by low phosphorus diet (LPD), another major stimulus of 1-OHase. These studies were designed to determine whether IGF-I stimulates 1-OHase during low calcium intake in old rats. Male rats were fed a normal calcium diet (NCD, 6 g Ca/kg diet) or low calcium diet (LCD, 0.2 g Ca/kg diet) for 14 d, and recombinant human IGF-I [rhIGF-I, 1.4 mg/(24h 160 kg body wt)] or vehicle was administrated via miniosmotic pump for 72 h before killing. In 4-mo-old male Sprague-Dawley rats, LCD increased in vitro renal 1-OHase activity in the presence but not in the absence of rhIGF-I. LCD increased in vitro1-OHase activity in young (1-mo-old) but not old (24-mo-old) male Fischer 344 rats. RhIGF-I increased 1-OHase activity in 24 mo-old rats fed LCD to levels that were not different from those in 1-mo-old rats fed LCD. The results indicate that the adaptive increase in 1-OHase activity due to a LCD is lost by 4 mo in rats and can be restored by pharmacologic doses of rhIGF-I.

Ageing and calcium metabolism

Ageing alters the metabolism of calcium and vitamin D in a number of ways. Intake of calcium and vitamin D, exposure to sunlight, cutaneous production of vitamin D3, renal production of 1,25-dihydroxyvitamin D (1,25(OH)2D3), intestinal absorption of calcium and the ability to adapt to a low calcium diet may all be reduced in elderly subjects. As a consequence, secondary hyperparathyroidism often occurs with ageing and can contribute to accelerated bone loss. In fact, alterations in calcium and vitamin D metabolism may be widespread in the ageing population and play a central role in the pathogenesis of senile (age-related) osteoporosis. From a preventive point of view, recent intervention studies have indicated the need to optimize calcium intake and to maintain serum 25(OH)D3 levels within the normal range in elderly people.

Age-related bone loss and senile osteoporosis: evidence for both secondary hyperparathyroidism and skeletal growth factor deficiency in the elderly

Aging is characterized by a decrease in bone volume, implying that net bone resorption exceeds net bone formation. This age-related bone loss can be regarded as the main determinant of hip fracture risk in the elderly. In the concept of senile osteoporosis, a key role has been attributed to vitamin D deficiency. Lack of vitamin D activity may affect femoral strength through impaired mineralization as well as through a hyperparathyroidism-mediated increase in bone resorption. In addition to vitamin D-related mechanisms, recent evidence has indicated a decline in the skeletal content of anabolic growth factors--such as insulin-like growth factor-I (IGF-I)--in femoral (cortical) bone, suggesting that skeletal growth factor deficiency may contribute to the age-related bone loss in the proximal femur as well. It is tempting to speculate that skeletal IGF-I loss might, at least partially, be accounted for by growth hormone deficiency. However, critical evidence does not yet support the concept that the decreased activity of the growth hormone-IGF-I-axis alters bone remodeling, and the extent to which serum concentrations of growth factors are reflective of skeletal activity remains to be clarified.