The vitamin D receptor mediates rapid changes in muscle protein tyrosine phosphorylation induced by 1,25(OH)(2)D(3)

Relationship of Anabolic Hormones With Motor Unit Characteristics in Quadriceps Muscle in Healthy and Frail Aging Men

CONTEXT

Anabolic hormones are important factors in maintaining muscle mass for aging men, but their role in overall motor unit structure and function is unclear.

OBJECTIVE

The objective of this work is to determine associations of anabolic and reproductive hormone levels with motor unit characteristics in quadriceps muscle in older healthy and frail men.

DESIGN

This work is an observational cohort study of community-dwelling men.

PARTICIPANTS

Participants included healthy and frail men younger than 65 years.

INTERVENTION

No intervention was performed.

OUTCOME MEASURE

Quantitative assessments of electromyography-derived motor unit potential size (MUP) and compound muscle action potential size (CMAP) of the vastus lateralis muscle.

RESULTS

We studied 98 men (mean ± SD: age 73 ± 6 years; body mass index [BMI] 25.7 ± 4.0 kg/m2; diabetes 11%) of whom 45% were prefrail and 18% frail. After adjusting for age, BMI, and prevalent diabetes, higher total and free testosterone levels were significantly related to larger CMAP (total testosterone: β [95% CI]: 0.3 [0.08-0.53]; free testosterone: 0.34 [0.13-0.56]). Exploratory analysis showed the relationship between free testosterone and CMAP was stronger in frail rather than robust men. In univariate analyses, estradiol was associated with CMAP size (0.37 [0.16-0.57]); and vitamin D was associated with MUP size (0.22 [0.01-0.43]) but these relationships were no longer significant after adjusting for potential confounders.

CONCLUSION

Our data highlight the associations between androgen levels and the electrophysiological characteristics of older men, particularly in the frail. Clinical trials involving administration of androgens will help to elucidate the potential benefits of intervention on neuromuscular function and/or frailty status.

Mechanisms of vitamin D on skeletal muscle function: oxidative stress, energy metabolism and anabolic state

PURPOSE

This review provides a current perspective on the mechanism of vitamin D on skeletal muscle function with the emphasis on oxidative stress, muscle anabolic state and muscle energy metabolism. It focuses on several aspects related to cellular and molecular physiology such as VDR as the trigger point of vitamin D action, oxidative stress as a consequence of vitamin D deficiency.

METHOD

The interaction between vitamin D deficiency and mitochondrial function as well as skeletal muscle atrophy signalling pathways have been studied and clarified in the last years. To the best of our knowledge, we summarize key knowledge and knowledge gaps regarding the mechanism(s) of action of vitamin D in skeletal muscle.

RESULT

Vitamin D deficiency is associated with oxidative stress in skeletal muscle that influences the mitochondrial function and affects the development of skeletal muscle atrophy. Namely, vitamin D deficiency decreases oxygen consumption rate and induces disruption of mitochondrial function. These deleterious consequences on muscle may be associated through the vitamin D receptor (VDR) action. Moreover, vitamin D deficiency may contribute to the development of muscle atrophy. The possible signalling pathway triggering the expression of Atrogin-1 involves Src-ERK1/2-Akt- FOXO causing protein degradation.

CONCLUSION

Based on the current knowledge we propose that vitamin D deficiency results from the loss of VDR function and it could be partly responsible for the development of neurodegenerative diseases in human beings.

Vitamin D derivatives inhibit hepatitis C virus production through the suppression of apolipoprotein

Supplementation with vitamin D (VD) has been reported to improve the efficacy of interferon-based therapy for chronic hepatitis C. We found that 25-hydroxyvitamin D₃ (25-(OH)D₃), one of the metabolites of VD, has antiviral effects by inhibiting the infectious virus production of the hepatitis C virus (HCV). In this study, to clarify the underlying mechanisms of the anti-HCV effects, we searched VD derivatives that have anti-HCV effects and identified the common target molecule in the HCV life cycle by using an HCV cell culture system. After infection of Huh-7.5.1 cells with cell culture-generated HCV, VD derivatives were added to culture media, and the propagation of HCV was assessed by measuring the HCV core antigen levels in culture media and cell lysates. To determine the step in the HCV life cycle affected by these compounds, the single-cycle virus production assay was used with a CD81-negative cell line. Of the 14 structural derivatives of VD, an anti-HCV effect was detected in 9 compounds. Cell viability was not affected by these effective compounds. The 2 representative VD derivatives inhibited the infectious virus production in the single-cycle virus production assay. Treatment with these compounds and 25-(OH)D₃ suppressed the expression of apolipoprotein A1 and C3, which are known to be involved in infectious virus production of HCV, and the knockdown of these apolipoproteins reduced infectious virus production. In conclusion, we identified several compounds with anti-HCV activity by screening VD derivatives. These compounds reduce the infectious virus production of HCV by suppressing the expression of apolipoproteins in host cells.

Acute Effects of Vitamin D3 Supplementation on Muscle Strength in Judoka Athletes: A Randomized Placebo-Controlled, Double-Blind Trial

OBJECTIVE

Indoor athletes have been shown to be prone to vitamin D3 deficiency. The aim of the study was to examine the acute effects of vitamin D supplementation on muscle function using isokinetic dynamometry.

DESIGN

Randomized placebo-controlled, double-blind study.

SETTING

Institutional.

PARTICIPANTS

Adult male white national level judoka athletes (n = 22) who were involved in full-time training. Exclusion criteria were vitamin supplementation, overseas travel to sunny climes, and/or an injury incurred during the last 3 months before testing.

INTERVENTIONS

Subjects were randomly allocated to the treatment (150 000IU vitamin D3) or placebo and given blinded supplements by an independent researcher. Participants were tested twice, 8 days apart, on a Monday morning before the start of judo training and after 2 days of rest. A 5 to 7 mL of blood sample was collected followed by isokinetic concentric quadriceps and hamstring muscle function assessments on the right leg at 30 and 200°·s.

MAIN OUTCOME MEASURES

Repeated-measures analysis of variance was used to analyze isokinetic muscle force and serum 25(OH)D3. Regression to the mean was used to examine changes in 25(OH)D3 levels over the study period.

RESULTS

The treatment group demonstrated a significant increase in serum 25(OH)D levels (34%, P ≤ 0.001) and muscle strength (13%, P = 0.01) between days 1 and 8. No significant differences were found for the placebo group for the same period.

CONCLUSIONS

A single bolus of 150 000IU vitamin D3 had a significant positive effect on serum 25(OH)D levels and muscle function in vitamin D insufficient elite indoor athletes.

CLINICAL RELEVANCE

Serum 25(OH)D3 levels of indoor athletes should be monitored throughout the year and especially during winter months. Beneficial responses, in muscle strength and serum 25(OH)D3, to 1 dose of vitamin D3 supplementation can be observed within 1 week of ingestion. Muscle strength is linked to serum 25(OH)D levels.

Sarcopenia in post-menopausal women: Is there any role for vitamin D?

BACKGROUND

Recently, special attention has been given to the role of vitamin D on the pathogenesis and therapy of sarcopenia in postmenopausal women.

AIMS

To elucidate the role of vitamin D with respect to sarcopenia in postmenopausal women, providing current evidence from both molecular and clinical studies.

MATERIALS AND METHODS

Systematic search to PubMed and Medline databases for publications reporting data on the role of vitamin D in sarcopenia.

RESULTS

Sarcopenia has a high prevalence in postmenopausal women, leading to mobility restriction, functional impairment, physical disability and fractures. Accumulating evidence from molecular and clinical studies suggest that vitamin D deficiency is associated with sarcopenic status in elderly women independent of body composition, diet and hormonal status. Current data, but not in a uniform way, provide evidence about the beneficial effect of vitamin D supplementation on muscle strength, physical performance and prevention of falls and fractures in elderly female populations. It is still unclear if and to what extent treatment modalities, such as dose, mode of administration and duration of supplementation, could influence treatment outcome.

CONCLUSIONS

Studies with superior methodological characteristics are needed in order to establish a role for vitamin D on the treatment of sarcopenia in postmenopausal women.

Effect of Absent Immune Cell Expression of Vitamin D Receptor on Cardiac Allograft Survival in Mice

INTRODUCTION

Vitamin D (VD) has immunomodulatory properties, but whether immune cell expression of the VD receptor (VDR) impacts costimulatory blockade induced cardiac allograft survival is not known.

METHODS

To localize effects of VDR deficiency to hematopoietic cells and to avoid the metabolic consequences of systemic VDR deficiency, we produced bone marrow (BM)-chimeric mice by transplanting lethally irradiated C57BL/6 mice with congenic VDR or wild type BM. After reconstitution, we characterized baseline immune profiles and transplanted chimeras with heterotopic cardiac allografts with or without costimulatory blockade using anti-CD154 (MR1) or CTLA4Ig, the latter approved for use in human kidney transplant recipients.

RESULTS

Immune reconstitution occurred equivalently in chimeras with wild type and VDR BM. Untreated animals rejected class II disparate and fully allogeneic cardiac transplants with similar kinetics. Compared to untreated controls, treatment with either MR1 or CTLA4Ig induced significant and equivalent prolongation of graft survival in both groups of chimeric recipients. We observed no differences in induced antidonor cellular or humoral alloimmunity between groups.

CONCLUSIONS

Our findings support the conclusion that absent immune cell VDR expression (a) does not impact the strength, phenotype, or kinetics of heart transplant rejection in mice and (b) does not impact the graft-prolonging effects of costimulatory blockade including that induced by clinically used CTLA4Ig.

1,25-Dihydroxyvitamin D3 increases testosterone-induced 17beta-estradiol secretion and reverses testosterone-reduced connexin 43 in rat granulosa cells

BACKGROUND

Aromatase converts testosterone into 17beta-estradiol in granulosa cells, and the converted 17beta-estradiol contributes to follicular maturation. Additionally, excessive testosterone inhibits aromatase activity, which can lead to concerns regarding polycystic ovary syndrome (PCOS). Generally, 1,25-dihydroxyvitamin D3 (1,25D3) supplements help to improve the symptoms of PCOS patients who exhibit low blood levels of 1,25D3. Therefore, this study investigated the interaction effects of 1,25D3 and testosterone on estrogenesis and intercellular connections in rat granulosa cells.

METHODS

Primary cultures of granulosa cells were treated with testosterone or testosterone plus 1,25D3, or pre-treated with a calcium channel blocker or calcium chelator. Cell lysates were subjected to western blot analysis to determine protein and phosphorylation levels, and 17beta-estradiol secretion was examined using a radioimmunoassay technique. Cell viability was evaluated by MTT reduction assay. Connexin 43 (Cx43) mRNA and protein expression levels were assessed by qRT-PCR, western blot, and immunocytochemistry.

RESULTS

Testosterone treatment (0.1 and 1 microg/mL) increased aromatase expression and 17beta-estradiol secretion, and the addition of 1,25D3 attenuated testosterone (1 microg/mL)-induced aromatase expression but improved testosterone-induced 17beta-estradiol secretion. Furthermore, testosterone-induced aromatase phosphotyrosine levels increased at 10 min, 30 min and 1 h, whereas 1,25D3 increased the longevity of the testosterone effect to 6 h and 24 h. Within 18-24 h of treatment, 1,25D3 markedly enhanced testosterone-induced 17beta-estradiol secretion. Additionally, pre-treatment with a calcium channel blocker nifedipine or an intracellular calcium chelator BAPTA-AM reduced 1,25D3 and testosterone-induced 17beta-estradiol secretion. Groups that underwent testosterone treatment exhibited significantly increased estradiol receptor beta expression levels, which were not affected by 1,25D3. Neither testosterone nor 1,25D3 altered 1,25D3 receptor expression. Finally, at high doses of testosterone, Cx43 protein expression was decreased in granulosa cells, and this effect was reversed by co-treatment with 1,25D3.

CONCLUSIONS

These data suggest that 1,25D3 potentially increases testosterone-induced 17beta-estradiol secretion by regulating aromatase phosphotyrosine levels, and calcium increase is involved in both 1,25D3 and testosterone-induced 17beta-estradiol secretion. 1,25D3 reverses the inhibitory effect of testosterone on Cx43 expression in granulosa cells.

Vitamin D signaling in myogenesis: potential for treatment of sarcopenia

Muscle mass and strength progressively decrease with age, which results in a condition known as sarcopenia. Sarcopenia would lead to physical disability, poor quality of life, and death. Therefore, much is expected of an effective intervention for sarcopenia. Epidemiologic, clinical, and laboratory evidence suggest an effect of vitamin D on muscle function. However, the precise molecular and cellular mechanisms remain to be elucidated. Recent studies suggest that vitamin D receptor (VDR) might be expressed in muscle fibers and vitamin D signaling via VDR plays a role in the regulation of myoblast proliferation and differentiation. Understanding how vitamin D signaling contributes to myogenesis will provide a valuable insight into an effective nutritional strategy to moderate sarcopenia. Here we will summarize the current knowledge about the effect of vitamin D on skeletal muscle and myogenic cells and discuss the potential for treatment of sarcopenia.

The role of vitamin D in skeletal and cardiac muscle function

Myopathy is a feature of many inflammatory syndromes. Chronic inflammation has been linked to pathophysiological mechanisms which implicate 1,25 dihydroxyvitamin D₃ (1,25-(OH)₂D₃)-mediated signaling pathways with emerging evidence supporting a role for the vitamin D receptor (VDR) in contractile and metabolic function of both skeletal and cardiac muscle. Altered VDR expression in skeletal and cardiac muscle has been reported to result in significant effects on metabolism, calcium signaling and fibrosis in these tissues. Elevated levels of serum inflammatory cytokines, such as IL-6, TNF-α and IFNγ, have been shown to impact myogenic and nuclear receptor signaling pathways in cancer-induced cachexia. The dysregulation of nuclear receptors, such as VDR and RXRα in muscle cells, has also been postulated to result in myopathy via their effects on muscle structural integrity and metabolism. Future research directions include generating transcriptome-wide information incorporating VDR and its gene targets and using systems biology approaches to identify altered molecular networks in human tissues such as muscle. These approaches will aid in the development of novel therapeutic targeting strategies for inflammation-induced myopathies.

Role of VDR in 1α,25-dihydroxyvitamin D3-dependent non-genomic activation of MAPKs, Src and Akt in skeletal muscle cells

1α,25-dihydroxyvitamin D3 [1,25D] is recognized as a steroid hormone that rapidly elicits intracellular signals in various tissues. In skeletal myoblasts, we have previously demonstrated that one of the 1,25D-induced non-genomic effects is the upstream stimulation of MAPKs through Src activation. In this work, the data obtained suggest that the classical receptor of vitamin D (VDR) participates in non-transcriptional actions of 1,25D. We significantly reduced VDR expression by infection of C2C12 murine myoblasts with lentiviral particles containing the pLKO.1 plasmid with information to express a shRNA against mouse VDR. In these cells (C2C12-shVDR), Western blot analyses show that 1,25D-induced p38 MAPK activation and Src tyr416 phosphorylation were abolished. In addition, 1,25D-dependent activity of ERK1/2 was diminished in cells lacking VDR but to a lesser extent (∼-60%). Phosphorylation of Akt by 1,25D, recently demonstrated in C2C12 cells, in the present work also appeared to be partially dependent on VDR expression (∼50% in C2C12-shVDR cells). Our results indicate that VDR is involved in 1,25D-induced rapid events related to survival/proliferation responses in skeletal muscle cells, providing relevant information on the mechanism of initiation of the non-genomic hormone signal. The participation of a VDR-independent non-genomic mechanism of action should also be taken into consideration. This article is part of a Special Issue entitled 'Vitamin D Workshop'.

Vitamin D3 supplementation modulates inflammatory responses from the muscle damage induced by high-intensity exercise in SD rats

Vitamin D is an important factor for calcium and phosphorus homeostasis. A negative relationship has been observed between vitamin D status and diseases such as cancer, arthritis, diabetes, and muscle fiber atrophy. However, the relationship between vitamin D and prevention of skeletal muscle damage has not been clearly elucidated. The purpose of this study was to investigate the effects of vitamin D on exercise-induced muscle changes. Rats were divided into 3 groups: (1) sedentary control (C: n=10), (2) high-intensity exercise (HE: n=10), and (3) high-intensity exercise with vitamin D supplementation (HED: n=10; i.p. 1000 IU/kg body weight). Rats were trained for 30 min/day on treadmills (5 days/week for 8 weeks) with the running speed gradually increased up to 30 m/min at a 3° incline. At the end of the training period, the running speed was 38 m/min at a 5° incline. The high-intensity exercise significantly increased plasma creatine kinase (CK) and lactate dehydrogenase (LDH) activity. In addition, IL-6 and TNF-α levels as well as phosphorylation of AMPK, p38, ERK1/2, IKK, and IκB were significantly increased. Vitamin D-treated rats showed a significant decrease in plasma CK level, phosphorylation of AMPK, p38, ERK1/2, IKK, and IκB, and gene expression of IL-6 and TNF-α. Furthermore, the protein expression of vitamin D receptor (VDR) was highly increased in the muscles of HED-treated rats, respectively. Therefore, we concluded that vitamin D may play a pivotal role in exercise-induced muscle damage and inflammation through the modulation of MAPK and NF-κB involved with VDR.

Chaperone properties of pdia3 participate in rapid membrane actions of 1α,25-dihydroxyvitamin d3

Protein disulfide isomerase family A, member 3 (Pdia3) mediates many of the plasma membrane (PM)-associated rapid responses to 1α,25-dihydroxyvitamin D3 (1α,25[OH]2D3). It is not well understood how Pdia3, which is an endoplasmic reticulum (ER) chaperone, functions as a PM receptor for 1α,25(OH)2D3. We mutated 3 amino acids (K214 and R282 in the calreticulin interaction site and C406 in the isomerase catalytic site), which are important for Pdia3's ER chaperone function, and examined their role in responses to 1α,25(OH)2D3. Pdia3 constructs with and without the ER retention signal KDEL were used to investigate the PM requirement for Pdia3. Finally, we determined whether palmitoylation and/or myristoylation were required for Pdia3-mediated responses to 1α,25(OH)2D3. Overexpressing the Pdia3 R282A mutant in MC3T3-E1 cells increased PM phospholipase A2-activating protein, Rous sarcoma oncogene (c-Src), and caveolin-1 but blocked increases in 1α,25(OH)2D3-stimulated protein kinase C (PKC) seen in cells overexpressing wild-type Pdia3 (Pdia3Ovr cells). Cells overexpressing Pdia3 with K214A and C406S mutations had PKC activity comparable to untreated controls, indicating that the native response to 1α,25(OH)2D3 also was blocked. Overexpressing Pdia3[-KDEL] increased PM localization and augmented baseline PKC, but the stimulatory effect of 1α,25(OH)2D3 was comparable to that seen in wild-type cultures. In contrast, 1α,25(OH)2D3 increased prostaglandin E2 in Pdia3[±KDEL] cells. Although neither palmitoylation nor myristoylation was required for PM association of Pdia3, myristoylation was needed for PKC activation. These data indicate that both the chaperone functional domains and the subcellular location of Pdia3 control rapid membrane responses to 1α,25(OH)2D3.

Serum [25(OH)D] status, ankle strength and activity show seasonal variation in older adults: relevance for winter falls in higher latitudes

BACKGROUND

seasonal variation exists in serum [25(OH)D] and physical activity, especially at higher latitudes, and these factors impact lower limb strength. This study investigates seasonal variation in leg strength in a longitudinal repeated measures design concurrently with serum vitamin D and physical activity.

METHODS

eighty-eight community-dwelling independently mobile older adults (69.2 ± 6.5 years) were evaluated five times over a year, at the end of five consecutive seasons at latitude 41.1°S, recruited in two cohorts. Leg strength, serum [25(OH)D] and physical activity levels were measured. Time spent outside was recorded. Monthly falls diaries recorded falls. Data were analysed to determine annual means and percentage changes.

RESULTS

significant variation in [25(OH)D] (±15%), physical activity (±13%), ankle dorsiflexion strength (±8%) and hours spent outside (±20%) (all P < 0.001) was demonstrated over the year, with maximums in January and February (mid-summer). Low mean ankle strength was associated with increased incidence of falling (P = 0.047). Quadriceps strength did not change (±2%; P = 0.53).

CONCLUSION

ankle dorsiflexor strength varied seasonally. Increased ankle strength in summer may be influenced by increased levels of outdoors activity over the summer months. Reduced winter-time dorsiflexor strength may predispose older people to increased risk of tripping-related falls, and warrants investigation in a multi-faceted falls prevention programme.

Age-related changes in the response of intestinal cells to 1α,25(OH)2-vitamin D3

The hormonally active form of vitamin D(3), 1α,25(OH)(2)-vitamin D(3), acts in intestine, its major target tissue, 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 vitamin D receptor (VDR) levels and 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 1α,25(OH)(2)-vitamin D(3) receptor-mediated signaling in intestinal cells. A fundamental understanding why the hormone functions are impaired with age will enhance our knowledge of its importance in intestinal cell physiology.

VDR activation of intracellular signaling pathways in skeletal muscle

The purpose of this article is to review the activation of signal transduction pathways in skeletal muscle cells by the hormone 1α,25(OH)(2)-vitamin D(3) [1α,25(OH)(2)D(3)], focusing on the role of the vitamin D receptor (VDR). The hormone induces fast, non transcriptional responses, involving stimulation of the transmembrane second messenger systems adenylyl cyclase/cAMP/PKA, PLC/DAG+IP(3)/PKC, Ca(2+) messenger system and MAPK cascades. Short treatment with 1α,25(OH)(2)D(3) induces reverse translocation of the VDR from the nucleus to plasma membranes. Accordingly, a complex is formed in the caveolae between the VDR and TRCP3, integral protein of capacitative Ca(2+) entry (CCE), suggesting an association between both proteins and a functional role of the VDR in 1α,25(OH)(2)D(3) activation of CCE. Stimulation of tyrosine phosphorylation cascades by 1α,25(OH)(2)D(3) have demonstrated the formation of complexes between Src and the VDR. Through these mechanisms, 1α,25(OH)(2)D(3) plays an important function in contractility and myogenesis.

The Rapidly Evolving Research on Vitamin D Among HIV-Infected Populations

With ongoing improvement in antiretroviral therapy, mortality among HIV-infected persons has dramatically decreased. For HIV-infected persons who remain engaged in care on suppressive therapy, life expectancy approaches that of the general population. Additionally, we have seen increases in comorbidities traditionally associated with aging: diabetes, hypertension, dyslipidemia, ischemic heart disease, and osteoporosis. Vitamin D deficiency has also been identified as a highly prevalent entity among HIV-infected populations. The association of vitamin D deficiency with several of these comorbidities and its impact on immune function provide the impetus for well-designed studies to evaluate the impact of vitamin D supplementation on HIV disease and antiretroviral therapy. This review summarizes the role of vitamin D in several disease states that are prevalent among HIV populations, with a specific focus on bone health and the interactions with antiretroviral medications.

Activation of rapid signaling pathways does not contribute to 1 alpha,25-dihydroxyvitamin D3-induced growth inhibition of mouse prostate epithelial progenitor cells

The active form of vitamin D, 1 alpha,25-dihydroxyvitamin D(3) (1,25(OH)(2)D) inhibits the growth of prostate epithelial cells, however the underlying mechanisms have not been clearly delineated. In the current study, the impact of 1,25(OH)(2)D on the rapid activation of extracellular-regulated kinase (ERK) 1/2 and protein kinase C alpha (PKC alpha), and the role of these pathways in growth inhibition was examined in immortalized mouse prostate epithelial cells, MPEC3, that exhibit stem/progenitor cell characteristics. 1,25(OH)(2)D treatment suppressed the growth of MPEC3 in a dose and time dependent manner (e.g., 21% reduction at three days with 100 nM 1,25(OH)(2)D treatment). However, ERK1/2 activity was not altered by 100 nM 1,25(OH)(2)D treatment for time points from 1 min to 1 h in either serum-containing or serum-free medium. Similarly, PKC alpha activation (translocation onto the plasma membrane) was not regulated by short-term treatment of 100 nM 1,25(OH)(2)D. In conclusion, 1,25(OH)(2)D did not mediate rapid activation of ERK1/2 or PKC alpha in MPEC3 and therefore the growth inhibitory effect of 1,25(OH)(2)D is independent of rapid activation of these signaling pathways in this cell type.

Reshaping the way we view vitamin D signalling and the role of vitamin D in health

Although the biological requirement for vitamin D can be met by epidermal exposure to UV light, there are a number of conditions where this production does not occur or is not sufficient to meet biological needs. When this happens, vitamin D must be consumed and is a nutrient. However, two distinct observations have caused researchers to rethink certain dogma in vitamin D biology. First, it appears that in addition to the hormonally active form of 1,25 dihydroxyvitamin D (1,25(OH)2D), circulating levels of 25 hydroxyvitamin D have a critical importance for optimal human health. This and other data suggest that extra-renal production of 1,25(OH)2D contributes to Ca homeostasis and cancer prevention. Second, in addition to its role in the transcriptional activation of genes through the vitamin D receptor there is now compelling evidence that 1,25(OH)2D has a second molecular mode of action; the rapid activation of second-messenger and kinase pathways. The purpose of this second mode of action is only now being explored. The present review will discuss how these two areas are reshaping our understanding of vitamin D metabolism and action.

Vitamin D receptor levels and binding are reduced in aged rat intestinal subcellular fractions

The hormonal form of vitamin D, 1alpha,25(OH)(2)-vitaminD(3) [1alpha,25(OH)(2)D(3)], stimulates signal transduction pathways in intestinal cells. To gain insight into the relative importance of the vitamin D receptor (VDR) in the rapid hormone responses, the amounts and localization of the VDR were evaluated in young (3 months) and aged (24 months) rat intestinal cells. Immune-fluorescence and Western blot studies showed that VDR levels are diminished in aged enterocytes. Confocal microscopy assays revealed that the VDR and other immune-reactive proteins have mitochondrial, membrane, cytosol and perinuclear localization. Western blot analysis using specific antibodies detected the 60 and 50 kDa bands expected for the VDR in the cytosol and microsomes and, to a lesser extent, in the nucleus and mitochondria. Low molecular weight immune-reactive proteins were also detected in young enterocytes subcellular fractions. Since changes in hormone receptor levels appear to constitute a common manifestation of the ageing process, we also analyzed 1alpha,25(OH)(2)D(3) binding properties and VDR levels in subcellular fractions from young and aged rats. In competition binding assays, employing [(3)H]-1alpha,25(OH)(2)D(3) and 1alpha,25(OH)(2)D(3), we have detected specific binding in all subcellular fractions, with maximum binding in mitochondrial and nuclear fractions. Both, VDR protein levels and 1alpha,25(OH)(2)D(3) binding, were diminished with ageing. Age-related declines in VDR may have important consequences for correct receptor/effector coupling in the duodenal tissues and may explain age-related declines in the hormonal regulation of signal transduction pathways that we previously reported.

The classic receptor for 1alpha,25-dihydroxy vitamin D3 is required for non-genomic actions of 1alpha,25-dihydroxy vitamin D3 in osteosarcoma cells

1alpha,25-dihydroxy vitamin D3 has a major role in the regulation of the bone metabolism as it promotes the expression of key bone-related proteins in osteoblastic cells. In recent years it has become increasingly evident that in addition to its well-established genomic actions, 1alpha,25-dihydroxy vitamin D3 induces non-genomic responses by acting through a specific plasma membrane-associated receptor. Results from several groups suggest that the classical nuclear 1alpha,25-dihydroxy vitamin D3 receptor (VDR) is also responsible for these non-genomic actions of 1alpha,25-dihydroxy vitamin D3. Here, we have used siRNA to suppress the expression of VDR in osteoblastic cells and assessed the role of VDR in the non-genomic response to 1alpha,25-dihydroxy vitamin D3. We report that expression of the classic VDR in osteoblasts is required to generate a rapid 1alpha,25-dihydroxy vitamin D3-mediated increase in the intracellular Ca(2+) concentration, a hallmark of the non-genomic actions of 1alpha,25-dihydroxy vitamin D3 in these cells.

MAP kinases p38 and JNK are activated by the steroid hormone 1alpha,25(OH)2-vitamin D3 in the C2C12 muscle cell line

In chick skeletal muscle cell primary cultures, we previously demonstrated that 1alpha,25(OH)2-vitamin D3 [1alpha,25(OH)2D3], the hormonally active form of vitamin D, increases the phosphorylation and activity of the extracellular signal-regulated mitogen-activated protein (MAP) kinase isoforms ERK1 and ERK2, their subsequent translocation to the nucleus and involvement in DNA synthesis stimulation. In this study, we show that other members of the MAP kinase superfamily are also activated by the hormone. Using the muscle cell line C2C12 we found that 1alpha,25(OH)2D3 within 1 min phosphorylates and increases the activity of p38 MAPK. The immediately upstream mitogen-activated protein kinase kinases 3/6 (MKK3/MKK6) were also phosphorylated by the hormone suggesting their participation in p38 activation. 1Alpha,25(OH)2D3 was able to dephosphorylate/activate the ubiquitous cytosolic tyrosine kinase c-Src in C2C12 cells and studies with specific inhibitors imply that Src participates in hormone induced-p38 activation. Of relevance, 1alpha,25(OH)2D3 induced in the C2C12 line the stimulation of mitogen-activated protein kinase activating protein kinase 2 (MAPKAP-kinase 2) and subsequent phosphorylation of heat shock protein 27 (HSP27) in a p38 kinase activation-dependent manner. Treatment with the p38 inhibitor, SB203580, blocked p38 phosphorylation caused by the hormone and inhibited the phosphorylation of its downstrean substrates. 1Alpha,25(OH)2D3 also promotes the phosphorylation of c-jun N-terminal protein kinases (JNK 1/2), the response is fast (0.5-1 min) and maximal phosphorylation of the enzyme is observed at physiological doses of 1alpha,25(OH)2D3 (1 nM). The relative contribution of ERK-1/2, p38, and JNK-1/2 and their interrelationships in hormonal regulation of muscle cell proliferation and differentiation remain to be established.

Age-related alteration of 1alpha,25(OH)2-vitamin D3-dependent activation of p38 MAPK in rat intestinal cells

In intestinal cells, 1alpha,25(OH)(2)-vitamin D(3) (1alpha,25(OH)(2)D(3)) regulates gene expression via the specific intracellular vitamin D receptor and induces fast non-transcriptional responses involving stimulation of transmembrane signal transduction pathways. In the present study, we analyzed, for the first time, alterations in p38 MAPK response to 1alpha,25(OH)(2)D(3) in rat enterocytes with ageing. In enterocytes from young rats, the hormone increased, in a time- and dose-dependent fashion, the phosphorylation of p38 MAPK, peaking at 3 min (+2-fold). Basal levels of p38 MAPK phosphorylation were lower in enterocytes from old rats and the hormone response was greatly diminished (+0.5-fold at 3 min). p38 MAPK phosphorylation impairment in old animals was not related to significant changes of the kinase protein expression and do not explain the decreased response to 1alpha,25(OH)(2)D(3). Extracellular and intracellular Ca(2+) chelation or c-Src pharmacological inhibition suppressed hormone activation of p38 MAPK in both, young and aged rats, demonstrating that Ca(2+) and the non-receptor tyrosine kinase c-Src are required for full activation of p38 MAPK in cells stimulated with 1alpha,25(OH)(2)D(3). Two other vitamin D(3) metabolites, 25(OH)D(3) and 24,25(OH)(2)D(3, )also enhanced p38 phosphorylation, and to a similar extent than 1alpha,25(OH)(2)D(3), an ability that is lost with ageing. Enterocyte exposure to the hormone also resulted in the rapid induction of c-fos protein (peaking at 5 min, +3-fold) and to a greater extent than that of mRNA induction. With ageing, 1alpha,25(OH)(2)D(3)-dependent increase of c-fos protein level was diminished, but c-fos mRNA expression was not different from young animals. Impairment of 1alpha,25(OH)(2)D(3) activation of p38 MAPK upon ageing and abnormal hormone regulation of the c-fos oncoprotein synthesis may affect intestinal cell function.

1α,25(OH)2-Vitamin D3 stimulates intestinal cell p38 MAPK activity and increases c-Fos expression

In intestinal cells, as in other target cells, the steroid hormone 1alpha,25(OH)(2)-Vitamin D(3) (1alpha,25(OH)(2)D(3)) regulates gene expression via the specific intracellular Vitamin D receptor and induces fast non-transcriptional responses involving stimulation of transmembrane signal transduction pathways. We have previously shown that the hormone activates the extracellular signal-regulated mitogen-activated protein (MAP) kinase isoforms ERK1 and ERK2 in rat intestinal cells. In the present study, we have demonstrated that 1alpha,25(OH)(2)D(3) also induces the phosphorylation and activation of p38 MAPK in these cells. The hormone effects were time and dose-dependent, with maximal stimulation at 2min (+3-fold) and 1nM. 1alpha,25(OH)(2)D(3)-dependent p38 phosphorylation was suppressed by SB 203580, a selective inhibitor of p38 MAPK. Ca(2+) chelation with EGTA, inhibition of the c-Src-tyrosine kinase family with PP1 or protein kinase A (PKA) with Rp-cAMP, attenuated hormone activation of p38 MAPK. The physiological significance of 1alpha,25(OH)(2)D(3)-dependent activation of ERK1/2 and p38 MAP kinases was addressed by monitoring c-Fos expression. Incubation of intestinal cells with the hormone was followed by a rapid induction of c-Fos expression which was blocked by SB 203580 and partially suppressed by the ERK1/2 inhibitor PD 98059. Our results suggest that 1alpha,25(OH)(2)D(3) activates p38 MAPK, involving Ca(2+), c-Src and PKA as upstream regulators, and that p38 MAPK has a central role in hormone-induction of the oncoprotein c-Fos in rat intestinal cells.

Modulation of tyrosine phosphorylation signalling pathways by 1alpha,25(OH)2-vitamin D3

Hormonally active vitamin D(3), 1alpha,25(OH)(2)D(3), interacts with the classic vitamin D nuclear receptor that regulates gene transcription and with a putative cell membrane receptor that mediates rapid biological responses. 1alpha,25(OH)(2)D(3) actions on target tissues regulate: mineral metabolism and intracellular Ca(2+); protein kinase cascades leading to cell proliferation, differentiation and apoptosis; muscle growth and contractility; and the immune system. There is evidence for underlying 1alpha,25(OH)(2)D(3)-mediated protein tyrosine phosphorylation signalling in bone, intestine, muscle, epidermal and cancer cells. Extracellular-signal-regulated kinases-1/2, p38 and/or c-jun N-terminal kinase pathways play important roles in mediating 1alpha,25(OH)(2)D(3) actions. Studies to elucidate key regulatory metabolic steps and crosstalk sites in these pathways would enhance our understanding of the significance of tyrosine phosphorylation cascades in normal 1alpha,25(OH)(2)D(3) physiology, pathophysiology and pharmacology.

D-hormone analog alfacalcidol: an update on its role in post-menopausal osteoporosis and rheumatoid arthritis management

Alfacalcidol (1-alpha-hydroxyvitamin D3) is a non-endogenous analog of vitamin D which can bypass the renal and intestinal regulatory mechanisms that control the production of calcitriol (1,25-hydroxyvitamin D3, the active form of vitamin D, D-Hormone). Alfacalcidol may be metabolized into calcitriol with a limited risk of hypercalcemia. Alfacalcidol and calcitriol have been evaluated in animal and human studies assessing their effects on bone mineral density and fracture rates. More recently, they have been shown to produce beneficial effects in muscle, immune system, and autoimmune diseases, including rheumatoid arthritis. This paper discusses the therapeutic efficacy of alfacalcidol in reports in which it has been proposed as an interesting alternative to vitamin D or calcitriol. Some recent findings about general metabolism and regulation of vitamin D and its analogs are discussed. The biological and clinical effects of alfacalcidol in post-menopausal osteoporosis are reviewed, followed by critical appraisal of its efficacy in preventing bone loss and falls in the elderly. The last two sections discuss the role of D analogs in regulating the immune system, with particular regard to rheumatoid arthritis. The main results of this review show that alfacalcidol may have a wider range of therapeutic applicability, beyond simply restricting it to patients in hemodialysis or peritoneal dialysis with high serum levels of intact PTH.

Rapid, membrane-initiated actions of 1,25 dihydroxyvitamin D: what are they and what do they mean?

Vitamin D is a conditionally required nutrient traditionally thought to influence physiology as the metabolite 1,25-dihydroxyvitamin D [1,25(OH)(2) D] by binding to the vitamin D receptor (VDR) and stimulating the transcription of genes through direct VDR-DNA interactions. However, over the past 15 y research has demonstrated that 1,25(OH)(2) D, as well as other steroid hormones, can rapidly stimulate ion fluxes and activate protein kinases by transcription-independent mechanisms. This review summarizes recent research on the rapid actions of 1,25(OH)(2) D and identifies questions that remain to be answered in this area.

Proteins of multiple classes may participate in nongenomic steroid actions

Responses to steroids initiated from non-nuclear receptors impinge on a wide variety of cellular responses and utilize nearly all known signal transduction webs. While the mechanisms by which steroid receptors localize in the membrane are still unclear, it is apparent that this alternative localization allows steroid receptors to participate in a wide range of complex functions influencing cell proliferation, death, and differentiation. The central debate still remains the identity of the protein class or classes that mediate membrane-initiated (nongenomic) responses. The data thus far have supported several possibilities, including: nuclear steroid receptor-like forms in non-nuclear locations; other known (nonsteroid) membrane receptors or channels with additional steroid-binding sites; enzymes; transporters; receptors for serum steroid-binding proteins; unique and previously undescribed proteins; or chimeras of typical steroid receptor domains with other unique or known protein domains. Categorizing membrane steroid receptor proteins based exclusively on the actions of antagonists and agonists, without considering cell context and protein partnering issues, may mislead us into predicting more receptor subtypes than really exist. However, the plethora of signaling and functional outcomes may indicate the participation of more than one kind of steroid-binding protein. Resolving such unanswered questions will require future investigative focus on this alternative arm of steroid action, which is likely to yield as many therapeutic opportunities as have nuclear steroid mechanisms.

1,25(OH)2-vitamin D3 induces translocation of the vitamin D receptor (VDR) to the plasma membrane in skeletal muscle cells

1,25-dihydroxy-vitamin D(3) (1,25(OH)(2)D(3)), the hormonally active form of vitamin D(3), acts through two different mechanisms. In addition to regulating gene expression via the specific intracellular vitamin D receptor (VDR), 1,25(OH)(2)D(3) induces rapid, non-transcriptional responses involving stimulation of transmembrane signal transduction pathways. The activation of second messengers supports the hypothesis that a membrane-bound steroid receptor similar to those that mediate peptide hormone biology exists. Skeletal muscle is a target tissue for 1,25(OH)(2)D(3). Avian embryonic skeletal muscle cells (myoblasts/myotubes) have been shown to respond both genomically and non-genomically to the hormone. The present study provides evidence indicating that short-term treatment (1-10 min) with 1,25(OH)(2)D(3) induces translocation of the VDR from the nuclear to the microsomal fraction in chick myoblasts. This translocation is blocked by colchicine, genistein, or herbimycin, suggesting the involvement of microtubular transport and tyrosine kinase/s in the relocation of the receptor. By isolation of plasma membranes, it was demonstrated that the hormone increases the amounts of VDR specifically in this fraction. These results suggest that the nuclear VDR may be the receptor that mediates the non-genomic effects of 1,25(OH)(2)D(3) in chick myoblasts.

Alteration of cellular phosphorylation state affects vitamin D receptor-mediated CYP3A4 mRNA induction in Caco-2 cells

Expression of cytochrome P450 3A4 (CYP3A4) is induced by 1,25-dihydroxyvitamin D(3)(1,25(OH)(2)D(3)) in Caco-2 cells. However, since a typical vitamin D responsive element has not been found in the 5(')-flanking region of the CYP3A4 gene, the mechanism of 1,25(OH)(2)D(3)-induced CYP3A4 mRNA expression is poorly understood. In the present study, we demonstrated that vitamin D receptor (VDR) is a critical factor for the induction using the antisense oligonucleotide technique. In addition, we found that treatment of Caco-2 cells with the protein kinase C (PKC) inhibitors, staurosporine and GF109203X, and the tyrosine kinase inhibitor, genistein, but not with the protein kinase A inhibitor, H-89, suppressed CYP3A4 mRNA induction by 1,25(OH)(2)D(3). The depletion of PKC by prolonged treatment with phorbol ester abolished the induction. On the other hand, protein kinase inhibitors used had no effects on the constitutive expression of VDR mRNA. Therefore, these observations suggest that 1,25(OH)(2)D(3)-induced CYP3A4 mRNA expression might be involved in phosphorylation events in addition to transcriptional regulation via VDR. However, 1,25(OH)(2)D(3) did not rapidly activate PKC in the Caco-2 cells used, while the treatment with staurosporine and GF109203X, but not genistein, decreased basal PKC activity by approximately 30% of the controls. Taken together, these findings suggest that the change in the phosphorylation state via PKC and tyrosine kinase might, at least in part, modulate 1,25(OH)(2)D(3)-induced CYP3A4 mRNA expression via VDR.