Rapid stimulation of calcium uptake and protein phosphorylation in isolated cardiac muscle by 1,25-dihydroxyvitamin D3

Vitamin D: Effect on Haematopoiesis and Immune System and Clinical Applications

Vitamin D is a steroid-like hormone which acts by binding to vitamin D receptor (VDR). It plays a main role in the calcium homeostasis and metabolism. In addition, vitamin D display other important effects called “non-classical actions.” Among them, vitamin D regulates immune cells function and hematopoietic cells differentiation and proliferation. Based on these effects, it is currently being evaluated for the treatment of hematologic malignancies. In addition, vitamin D levels have been correlated with patients’ outcome after allogeneic stem cell transplantation, where it might regulate immune response and, accordingly, might influence the risk of graft-versus-host disease. Here, we present recent advances regarding its clinical applications both in the treatment of hematologic malignancies and in the transplant setting.

The Role of Inflammation in Age-Related Sarcopenia

Many physiological changes occur with aging. These changes often, directly or indirectly, result in a deterioration of the quality of life and even in a shortening of life expectancy. Besides increased levels of reactive oxygen species, DNA damage and cell apoptosis, another important factor affecting the aging process involves a systemic chronic low-grade inflammation. This condition has already been shown to be interrelated with several (sub)clinical conditions, such as insulin resistance, atherosclerosis and Alzheimer's disease. Recent evidence, however, shows that chronic low-grade inflammation also contributes to the loss of muscle mass, strength and functionality, referred to as sarcopenia, as it affects both muscle protein breakdown and synthesis through several signaling pathways. Classic interventions to counteract age-related muscle wasting mainly focus on resistance training and/or protein supplementation to overcome the anabolic inflexibility from which elderly suffer. Although the elderly benefit from these classic interventions, the therapeutic potential of anti-inflammatory strategies is of great interest, as these might add up to/support the anabolic effect of resistance exercise and/or protein supplementation. In this review, the molecular interaction between inflammation, anabolic sensitivity and muscle protein metabolism in sarcopenic elderly will be addressed.

Multiple hormonal dysregulation as determinant of low physical performance and mobility in older persons

Mobility-disability is a common condition in older individuals. Many factors, including the age-related hormonal dysregulation, may concur to the development of disability in the elderly. In fact, during the aging process it is observed an imbalance between anabolic hormones that decrease (testosterone, dehydroepiandrosterone sulphate (DHEAS), estradiol, insulin like growth factor-1 (IGF-1) and Vitamin D) and catabolic hormones (cortisol, thyroid hormones) that increase. We start this review focusing on the mechanisms by which anabolic and catabolic hormones may affect physical performance and mobility. To address the role of the hormonal dysregulation to mobility-disability, we start to discuss the contribution of the single hormonal derangement. The studies used in this review were selected according to the period of time of publication, ranging from 2002 to 2013, and the age of the participants (≥65 years). We devoted particular attention to the effects of anabolic hormones (DHEAS, testosterone, estradiol, Vitamin D and IGF-1) on both skeletal muscle mass and strength, as well as other objective indicators of physical performance. We also analyzed the reasons beyond the inconclusive data coming from RCTs using sex hormones, thyroid hormones, and vitamin D (dosage, duration of treatment, baseline hormonal values and reached hormonal levels). We finally hypothesized that the parallel decline of anabolic hormones has a higher impact than a single hormonal derangement on adverse mobility outcomes in older population. Given the multifactorial origin of low mobility, we underlined the need of future synergistic optional treatments (micronutrients and exercise) to improve the effectiveness of hormonal treatment and to safely ameliorate the anabolic hormonal status and mobility in older individuals.

The role of telomeres and vitamin D in cellular aging and age-related diseases

Aging is a complex biological process characterized by a progressive decline of organ functions leading to an increased risk of age-associated diseases and death. Decades of intensive research have identified a range of molecular and biochemical pathways contributing to aging. However, many aspects regarding the regulation and interplay of these pathways are insufficiently understood. Telomere dysfunction and genomic instability appear to be of critical importance for aging at a cellular level. For example, age-related diseases and premature aging syndromes are frequently associated with telomere shortening. Telomeres are repetitive nucleotide sequences that together with the associated sheltrin complex protect the ends of chromosomes and maintain genomic stability. Recent studies suggest that micronutrients, such as vitamin D, folate and vitamin B12, are involved in telomere biology and cellular aging. In particular, vitamin D is important for a range of vital cellular processes including cellular differentiation, proliferation and apoptosis. As a result of the multiple functions of vitamin D it has been speculated that vitamin D might play a role in telomere biology and genomic stability. Here we review existing knowledge about the link between telomere biology and cellular aging with a focus on the role of vitamin D. We searched the literature up to November 2014 for human studies, animal models and in vitro experiments that addressed this topic.

The VITAH trial VITamin D supplementation and cardiac Autonomic tone in Hemodialysis: a blinded, randomized controlled trial

Background

Patients with end-stage kidney disease (ESKD) have a high rate of mortality and specifically an increased risk of sudden cardiac death (SCD). Impaired cardiac autonomic tone is associated with elevated risk of SCD. Moreover, patients with ESKD are often vitamin D deficient, which we have shown may be linked to autonomic dysfunction in humans. To date, it is not known whether vitamin D supplementation normalizes cardiac autonomic function in the high-risk ESKD population. The VITamin D supplementation and cardiac Autonomic tone in Hemodialysis (VITAH) randomized trial will determine whether intensive vitamin D supplementation therapies improve cardiac autonomic tone to a greater extent than conventional vitamin D supplementation regimens in ESKD patients requiring chronic hemodialysis.

Methods/Design

A total of 60 subjects with ESKD requiring thrice weekly chronic hemodialysis will be enrolled in this 2x2 crossover, blinded, randomized controlled trial. Following a 4-week washout period from any prior vitamin D therapy, subjects are randomized 1:1 to intensive versus standard vitamin D therapy for 6 weeks, followed by a 12-week washout period, and finally the remaining treatment arm for 6 weeks. Intensive vitamin D treatment includes alfacalcidiol (activated vitamin D) 0.25mcg orally with each dialysis session combined with ergocalciferol (nutritional vitamin D) 50 000 IU orally once per week and placebo the remaining two dialysis days for 6 weeks. The standard vitamin D treatment includes alfacalcidiol 0.25mcg orally combined with placebo each dialysis session per week for 6 weeks. Cardiac autonomic tone is measured via 24 h Holter monitor assessments on the first dialysis day of the week every 6 weeks throughout the study period. The primary outcome is change in the low frequency: high frequency heart rate variability (HRV) ratio during the first 12 h of the Holter recording at 6 weeks versus baseline. Secondary outcomes include additional measures of HRV. The safety of intensive versus conventional vitamin D supplementation is also assessed.

Discussion

VITAH will determine whether an intensive vitamin D supplementation regimen will improve cardiac autonomic tone compared to conventional vitamin D supplementation and will assess the safety of these two supplementation regimens in ESKD patients receiving chronic hemodialysis.

Trial registration

ClinicalTrials.gov, NCT01774812

Prevalence and consequences of vitamin D insufficiency in women with takotsubo cardiomyopathy

OBJECTIVE

The purpose of this study was to identify the prevalence and clinical consequences of vitamin D insufficiency in patients with takotsubo cardiomyopathy.

BACKGROUND

Takotsubo cardiomyopathy is a syndrome of acute, transient left ventricular dysfunction seen predominantly in postmenopausal women after acute emotional or physical stress. Postmenopausal women have a high prevalence of bone and musculoskeletal consequences related to hypovitaminosis D. Although rickets is known to cause a reversible dilated cardiomyopathy in children, the importance of vitamin D for adult cardiovascular health is less understood.

METHODS

We prospectively identified patients diagnosed with takotsubo cardiomyopathy at Danbury Hospital from April 2009 through January 2011, collected demographic, clinical, laboratory, and angiographic data, and performed serum 25 hydroxyvitamin D levels during the index hospitalization. Vitamin D insufficiency was defined as serum 25-hydroxy-vitamin D less than 30 ng/mL. We compared parameters of myocardial damage and heart failure between patients with and without vitamin D insufficiency. A χ² test and a Student's t test were used for categorical and continuous variables, respectively. Statistical significance was set at P < .05 (2 tailed).

RESULTS

Twenty-seven women were diagnosed with takotsubo cardiomyopathy during the study period. The mean age was 67.4 years (SD 10.4). The serum 25-hydroxyvitamin D levels were performed on 25 patients, and 17 of these had hypovitaminosis D (68%). A comparison of laboratory and imaging parameters between the 2 groups revealed that patients with hypovitaminosis D had a slightly higher mean left ventricular end-diastolic pressure and lower mean left ventricular ejection fraction (P < .05), suggestive of slightly worse heart failure.

CONCLUSIONS

Women diagnosed with takotsubo cardiomyopathy have a high prevalence of vitamin D insufficiency. In our prospective study of 25 women with takotsubo cardiomyopathy, this was associated with worse hemodynamic parameters.

Higher serum 25-hydroxyvitamin D concentrations associate with a faster recovery of skeletal muscle strength after muscular injury

The primary purpose of this study was to identify if serum 25-hydroxyvitamin D (25(OH)D) concentrations predict muscular weakness after intense exercise. We hypothesized that pre-exercise serum 25(OH)D concentrations inversely predict exercise-induced muscular weakness. Fourteen recreationally active adults participated in this study. Each subject had one leg randomly assigned as a control. The other leg performed an intense exercise protocol. Single-leg peak isometric force and blood 25(OH)D, aspartate and alanine aminotransferases, albumin, interferon (IFN)-γ, and interleukin-4 were measured prior to and following intense exercise. Following exercise, serum 25(OH)D concentrations increased (p < 0.05) immediately, but within minutes, subsequently decreased (p < 0.05). Circulating albumin increases predicted (p < 0.005) serum 25(OH)D increases, while IFN-γ increases predicted (p < 0.001) serum 25(OH)D decreases. Muscular weakness persisted within the exercise leg (p < 0.05) and compared to the control leg (p < 0.05) after the exercise protocol. Serum 25(OH)D concentrations inversely predicted (p < 0.05) muscular weakness (i.e., control leg vs. exercise leg peak isometric force) immediately and days (i.e., 48-h and 72-h) after exercise, suggesting the attenuation of exercise-induced muscular weakness with increasing serum 25(OH)D prior to exercise. Based on these data, we conclude that pre-exercise serum 25(OH)D concentrations could influence the recovery of skeletal muscle strength after an acute bout of intense exercise.

Vitamin D and its role in skeletal muscle

This review discusses the clinical and laboratory studies that have examined a role of vitamin D in skeletal muscle. Many observational studies, mainly in older populations, indicate that vitamin D status is positively associated with muscle strength and physical performance and inversely associated with risk of falling. Clinical trials of vitamin D supplementation in older adults with low vitamin D status mostly report improvements in muscle performance and reductions in falls. The underlying mechanisms are probably both indirect via calcium and phosphate and direct via activation of the vitamin D receptor (VDR) on muscle cells by 1,25-dihydroxyvitamin D [1,25(OH)(2)D]. VDR activation at the genomic level regulates transcription of genes involved in calcium handling and muscle cell differentiation and proliferation. A putative membrane-associated VDR activates intracellular signaling pathways also involved in calcium handling and signaling and myogenesis. Additional evidence comes from VDR knockout mouse models with abnormal muscle morphology and physical function, and VDR polymorphisms which are associated with differences in muscle strength. Recent identification of CYP27B1 bioactivity in skeletal muscle cells and in regenerating adult mouse muscle lends support to the direct action of both 25-hydroxyvitamin D and 1,25(OH)(2)D in muscle. Despite these research advances, many questions remain. Further research is needed to fully characterize molecular mechanisms of vitamin D action on muscle cells downstream of the VDR, describe the effects on muscle morphology and contractility, and determine whether these molecular and cellular effects translate into clinical improvements in physical function.

Effects of alfacalcidol on muscle strength, muscle fatigue, and bone mineral density in normal and ovariectomized rats

Vitamin D affects not only bone but also muscle to prevent falls and osteoporotic fractures. However, these effects on muscle and the mechanisms of fall prevention are still unclear. The purpose of this study was to investigate the effects of alfacalcidol [1α(OH)D(3)] on muscle strength, muscle fatigue, and bone mineral density (BMD) in ovariectomized rats. Seven-month-old female Wistar rats were orally administered 1α(OH)D(3) or its vehicle everyday for 4 weeks after ovariectomy (OVX) or sham operation. Calf muscle strength and fatigue were evaluated by electrical stimulation of the sciatic nerve under general anesthesia. 1α(OH)D(3) administration significantly increased the maximum muscle strength in the sham-operated (P < 0.01) and the OVX (P < 0.01) groups compared to their respective control groups. However, 1α(OH)D(3) administration did not significantly affect muscle fatigue in these groups. The BMD of the femur in the 1α(OH)D(3)-treated OVX group was significantly higher than that in the vehicle-treated OVX group (P = 0.04). These results suggested that 1α(OH)D(3) increases muscle strength but does not affect muscle fatigue in this rat model. The effectiveness of activated vitamin D in preventing bone fractures may be partly owing to its effect on muscle strength in addition to its known effect on bone metabolism.

Vitamin D and its role in skeletal muscle

PURPOSE OF REVIEW

Vitamin D is best known for its role in regulating calcium homeostasis and in strengthening bone. However, it has become increasingly clear that it also has important beneficial effects beyond the skeleton, including muscle. This review summarizes current knowledge about the role of vitamin D in skeletal muscle tissue and physical performance.

RECENT FINDINGS

Molecular mechanisms of vitamin D action in muscle tissue include genomic and nongenomic effects via a receptor present in muscle cells. Knockout mouse models of the vitamin D receptor provide insight into understanding the direct effects of vitamin D on muscle tissue. Vitamin D status is positively associated with physical performance and inversely associated with risk of falling. Vitamin D supplementation has been shown to improve tests of muscle performance, reduce falls, and possibly impact on muscle fiber composition and morphology in vitamin D deficient older adults.

SUMMARY

Further studies are needed to fully characterize the underlying mechanisms of vitamin D action in human muscle tissue, to understand how these actions translate into changes in muscle cell morphology and improvements in physical performance, and to define the 25-hydroxyvitamin D level at which to achieve these beneficial effects in muscle.

Vitamin D and skeletal muscle tissue and function

This review aims to summarize current knowledge on the role of vitamin D in skeletal muscle tissue and function. Vitamin D deficiency can cause a myopathy of varying severity. Clinical studies have indicated that vitamin D status is positively associated with muscle strength and physical performance and inversely associated with risk of falling. Vitamin D supplementation has shown to improve tests of muscle function, reduce falls, and possibly impact on muscle fiber composition and morphology in vitamin D deficient older adults. Molecular mechanisms of vitamin D action on muscle tissue include genomic and non-genomic effects via a receptor present in muscle cells. Genomic effects are initiated by binding of 1,25-dihydroxyvitamin D [1,25(OH)(2)D] to its nuclear receptor, which results in changes in gene transcription of mRNA and subsequent protein synthesis. Non-genomic effects of vitamin D are rapid and mediated through a cell surface receptor. Knockout mouse models of the vitamin D receptor provide insight into understanding the direct effects of vitamin D on muscle tissue. Recently, VDR polymorphisms have been described to affect muscle function. Parathyroid hormone which is strongly linked with vitamin D status also may play a role in muscle function; however, distinguishing its role from that of vitamin D has yet to be fully clarified. Despite the enormous advances in recent decades, further research is needed to fully characterize the exact underlying mechanisms of vitamin D action on muscle tissue and to understand how these cellular changes translate into clinical improvements in physical performance.

The vitamin D receptor agonist elocalcitol upregulates L-type calcium channel activity in human and rat bladder

Human bladder contraction mainly depends on Ca2+ influx via L-type voltage-gated Ca2+ channels and on RhoA/Rho kinase contractile signaling, which is upregulated in overactive bladder (OAB). Elocalcitol is a vitamin D receptor agonist inhibiting RhoA/Rho kinase signaling in rat and human bladder. Since in the normal bladder from Sprague-Dawley rats elocalcitol treatment delayed the carbachol-induced contraction without changing maximal responsiveness and increased sensitivity to the L-type Ca2+ channel antagonist isradipine, we investigated whether elocalcitol upregulated L-type Ca2+ channels in human bladder smooth muscle cells (hBCs). In hBCs, elocalcitol induced a rapid increase in intracellular [Ca2+], which was abrogated by the L-type Ca2+ channel antagonist verapamil. Moreover, hBCs exhibited L-type voltage-activated Ca2+ currents (I Ca), which were selectively blocked by isradipine and verapamil and enhanced by the selective L-type agonist BAY K 8644. Addition of elocalcitol (10(-7) M) increased L-type I Ca size and specific conductance by inducing faster activation and inactivation kinetics than control and BAY K 8644, while determining a significant negative shift of the activation and inactivation curves, comparable to BAY K 8644. These effects were strengthened in long-term treated hBCs with elocalcitol (10(-8) M, 48 h), which also showed increased mRNA and protein expression of pore-forming L-type alpha(1C)-subunit. In the bladder from Sprague-Dawley rats, BAY K 8644 induced a dose-dependent increase in tension, which was significantly enhanced by elocalcitol treatment (30 microg.kg(-1).day(-1), 2 wk). In conclusion, elocalcitol upregulated Ca2+ entry through L-type Ca2+ channels in hBCs, thus balancing its inhibitory effect on RhoA/Rho kinase signaling and suggesting its possible efficacy for the modulation of bladder contractile mechanisms.

Functional vitamin D receptor (VDR) in the t-tubules of cardiac myocytes: VDR knockout cardiomyocyte contractility

We have previously shown that the active form of vitamin D, 1,25 dihydroxyvitamin D3 [1,25(OH)(2)D(3)], has both genomic and rapid nongenomic effects in heart cells; however, the subcellular localization of the vitamin D receptor (VDR) in heart has not been studied. Here we show that in adult rat cardiac myocytes the VDR is primarily localized to the t-tubule. Using immunofluorescence and Western blot analysis, we show that the VDR is closely associated with known t-tubule proteins. Radioligand binding assays using (3)H-labeled 1,25(OH)(2)D(3) demonstrate that a t-tubule membrane fraction isolated from homogenized rat ventricles contains a 1,25(OH)(2)D(3)-binding activity similar to the classic VDR. For the first time, we show that cardiac myocytes isolated from VDR knockout mice show accelerated rates of contraction and relaxation as compared with wild type and that 1,25(OH)(2)D(3) directly affects contractility in the wild-type but not the knockout cardiac myocyte. Moreover, we observed that acute (5 min) exposure to 1,25(OH)(2)D(3) altered the rate of relaxation. A receptor localized to t-tubules in the heart is ideally positioned to exert an immediate effect on signal transduction mediators and ion channels. This novel discovery is fundamentally important in understanding 1,25(OH)(2)D(3) signal transduction in heart cells and provides further evidence that the VDR plays a role in heart structure and function.

Vitamin D in the aging musculoskeletal system: An authentic strength preserving hormone

Until recently, vitamin D was only considered as one of the calciotrophic hormones without major significance in other metabolic processes in the body. Several recent findings have demonstrated that vitamin D plays also a role as a factor for cell differentiation, function and survival. Two organs, muscle and bone, are significantly affected by the presence, or absence, of vitamin D. In bone, vitamin D stimulates bone turnover while protecting osteoblasts of dying by apoptosis whereas in muscle vitamin D maintains the function of type II fibers preserving muscle strength and preventing falls. Furthermore, two major changes associated to aging: osteoporosis and sarcopenia, have been also linked to the development of frailty in elderly patients. In both cases vitamin D plays an important role since the low levels of this vitamin seen in senior people may be associated to a deficit in bone formation and muscle function. In this review, the interaction between vitamin D and the musculoskeletal components of frailty are considered from the basic mechanisms to the potential therapeutic approach. We expect that these new considerations about the importance of vitamin D in the elderly will stimulate an innovative approach to the problem of falls and fractures which constitutes a significant burden to public health budgets worldwide.

Nongenomic steroid action: controversies, questions, and answers

Steroids may exert their action in living cells by several ways: 1). the well-known genomic pathway, involving hormone binding to cytosolic (classic) receptors and subsequent modulation of gene expression followed by protein synthesis. 2). Alternatively, pathways are operating that do not act on the genome, therefore indicating nongenomic action. Although it is comparatively easy to confirm the nongenomic nature of a particular phenomenon observed, e.g., by using inhibitors of transcription or translation, considerable controversy exists about the identity of receptors that mediate these responses. Many different approaches have been employed to answer this question, including pharmacology, knock-out animals, and numerous biochemical studies. Evidence is presented for and against both the participation of classic receptors, or proteins closely related to them, as well as for the involvement of yet poorly understood, novel membrane steroid receptors. In addition, clinical implications for a wide array of nongenomic steroid actions are outlined.

Hypocalcemic heart failure masquerading as dilated cardiomyopathy

Hypocalcemia is a rare, but reversible, cause of congestive heart failure. We report a 4-month-old boy diagnosed as dilated cardiomyopathy who had prolonged QoTc with low blood levels of calcium, normal phosphate, elevated alkaline phosphatase and findings suggestive of rickets. In view of non response to calcium and vitamin D3, a possible diagnosis of VDDR I (Vitamin D-dependent rickets) was made and he was treated with calcium and calcitriol. The serum calcium levels normalised within 10 days, along with resolution of the signs and symptoms of heart failure, near normal left ventricular function and normalisation of QoTc. Pediatricians should be aware of the association of hypocalcemia with cardiac dysfunction and should keep it as a possible reversible cause of heart failure in children.

Effect of intravenous calcitriol on cardiac systolic and diastolic function in patients on hemodialysis

The systolic and diastolic function of the heart of hemodialysis (HD) patients and the effect of intravenous vitamin D therapy on cardiac function was studied by Doppler and digitized M-mode echocardiography in 10 HD patients before and after 3-4.5 months of calcitriol therapy. Calcitriol was administered intravenously 1-3 times a week at a dose of 1-2 microg after the dialysis sessions. Ten age- and sex-matched healthy controls were also examined echocardiographically. Before calcitriol therapy cardiac wall thicknesses (interventricular septum, posterior wall) and left ventricle (LV) dimensions (end diastolic, end systolic) were greater, and LV diastolic (peak late diastolic velocity, peak early diastolic velocity/peak late diastolic velocity ratio, isovolumic relaxation time) and systolic (fractional shortening) function was impaired in HD patients as compared to controls. The LV posterior wall thickness was related to plasma parathyroid hormone (PTH; r = 0. 70, p = 0.01) in the patients. Calcitriol therapy raised serum ionized Ca from 1.23+/-0.04 to 1.33 +/- 0.04 mmol/l and reduced PTH from 41.1+/-10.7 to 34.2+/-11.7 pmol/l (29+/-11%). Calcitriol therapy did not cause any significant changes in cardiac function in the whole patient group. However, in a subgroup of 5 patients with severe but controllable hyperparathyroidism (PTH >3 times upper normal margin) the LV dimensions and systolic function improved (LV end systolic dimension from 39.0 +/- 4.0 to 31.3 +/- 2.9 mm, p = 0. 03; LV end diastolic dimension from 57.7 +/- 3.1 to 53.4 +/- 3.0 mm, p = 0.06; fractional shortening from 33 +/- 4 to 42 +/- 3%, p = 0. 03). The diastolic indices improved also, but not significantly. In conclusion, left ventricle hypertrophy and systolic and diastolic dysfunction was observed in HD patients. Intravenous calcitriol therapy improved cardiac function in patients with severe secondary hyperparathyroidism.

Nongenomic effects of 1alpha,25-dihydroxyvitamin D(3)

The hormonally active form of vitamin D, 1alpha,25-dihydroxyvitamin D(3), is the key molecule of the vitamin D endocrine system, which produces biological effects in about 30 target cell systems. Growing experimental evidence supports the hypothesis that these biological effects can be generated both by a signal transduction mechanism involving a nuclear receptor (nVDR) that modulates gene transcription, and via a nongenomic receptor located in the plasma membrane (mVDR), which modulates a complex signaling system involving the rapid opening of Ca(2+) channels. Some data reviewed herein also indicate that crosstalk between genomic and nongenomic pathways operates in several cell types, and suggest that the physiological role of the rapid, nongenomic actions might involve the regulation of hormone-mediated gene activation.

Altered phosphorylation of a 91-kDa protein in particulate fractions of rat kidney after protracted 1,25-dihydroxyvitamin D3 or estrogen treatment

1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] treatment in vitamin D-deficient (-D) rats results in a dose-dependent decrease in phosphorylation of a 91-kDa protein (PP-D91) in particulate fractions of the kidney. This recently reported 1,25(OH)2D3 effect was examined in detail herein. In contrast to the pattern expected of a rapid signal transduction event, time course (4 h-7 days) experiments demonstrated that PP-D91 phosphorylation was not decreased until 3-5 days 1,25(OH)2D3 treatment, resulting in a 61 +/- 3% (P < 0.01, n = 3) decrease in PP-D91 phosphorylation by 7 days. These effects paralleled increases in plasma calcium from 9.3 +/- 0.6 to 13.9 +/- 0.7 mg/dl after 0 vs 7 days 1,25(OH)2D3 treatment, respectively. Subcellular fractionation demonstrated that the renal PP-D91 was predominantly localized and 1,25(OH)2D3-regulated in crude mitochondrial and microsomal fractions. Further, PP-D91 was present and 1,25(OH)2D3-regulated in enriched preparations of both proximal and distal renal tubule segments. Tissue distribution studies demonstrated that the PP-D91 was predominantly present and 1,25(OH)2D3 regulated in the kidney, although low levels of a vitamin D-independent phosphorylated band of similar size were observed in the lung and heart. In contrast to 1,25(OH)2D3, estradiol-17B treatment (1 mg/day x 7 day) significantly (P < 0.01) increased PP-D91 phosphorylation in kidney of both -D and +D rats (increased 118.5 +/- 10.6 and 81.9 +/- 6.3%, respectively). Phosphoamino acid analysis after PP-D91 phosphorylation, isolation, and proteolysis indicated that these hormones alter 32P incorporation into phosphoserine residues. In conclusion, the 1,25(OH)2D3 effect to reduce PP-D91 phosphorylation in particulate fractions of the rat kidney is a protracted, tissue-specific effect which parallels elevated plasma calcium levels in this model. Moreover, renal PP-D91 phosphorylation is differentially regulated by 1,25(OH)2D3 vs E2 treatment and occurs on phosphoserine residues. The parallel between decreased PP-D91 phosphorylation and 1,25(OH)2D3-induced hypercalcemia may suggest a role for PP-D91 in the renal response to hypervitaminosis D.

Effects of calcitriol and its analogues, calcipotriol (MC 903) and 20-epi-1alpha,25-dihydroxyvitamin D3 (MC 1288), on calcium influx and DNA synthesis in cultured muscle cells

The fast actions of the secosteroid hormone 1alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3; calcitriol] and the synthetic analogues calcipotriol (MC 903) and 20-epi-1alpha,25(OH)2D3 (MC 1288) on cell calcium influx were compared in rat duodenum enterocytes as well as in cells from chick embryo skeletal muscle (myoblasts) and heart (myocytes), at various concentrations (10(-12) to 10(-8) M) and treatment intervals (1-10 min). In enterocytes, at a concentration of 10(-11) M, MC 1288 was significantly more active than 1,25(OH)2D3 in rapidly stimulating 45Ca2+ uptake by enterocytes (80 vs 38% above controls, respectively), whereas MC 903 was devoid of activity. However, calcipotriol increased Ca2+ influx in myocytes and myoblasts to a greater extent than the natural hormone, whereas MC 1288 was more active only in myoblasts. Analogously to 1,25(OH)2D3, the fast MC 903- and MC 1288-induced stimulation of 45Ca2+ uptake in enterocytes and muscle cells could be blocked by both verapamil and nifedipine. In addition, MC 903 and MC 1288 were more effective than 1,25(OH)2D3 in stimulating DNA synthesis in proliferating myoblasts and in inhibiting DNA synthesis in differentiating myoblasts. The results suggest, therefore, that modifications in the side-chain of the 1,25(OH)2D3 molecule increase its ability to modulate muscle cell Ca2+ metabolism and growth. These findings are potentially relevant for the development of analogues for the treatment of vitamin D-dependent myopathies.

Homeostatic control of plasma calcium concentration

Due to the importance of Ca2+ in the regulation of vital cellular and tissue functions, the concentration of Ca2+ in body fluids is closely guarded by an efficient feedback control system. This system includes Ca(2+)-transporting subsystems (bone, and kidney), Ca2+ sensing, possibly by a calcium-sensing receptor, and calcium-regulating hormones (parathyroid hormone [PTH], calcitonin [CT], and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]). In humans and birds, acute Ca2+ perturbations are handled mainly by modulation of kidney Ca2+ reabsorption and by bone Ca2+ flow under PTH and possibly CT regulation, respectively. Chronic perturbations are also handled by the more sluggish but economic regulatory action of 1,25(OH2)D3 on intestinal calcium absorption. Peptide hormone secretion is modulated by Ca2+ and several secretagogues. The hormones' signal is produced by interaction with their respective receptors, which evokes the cAMP and phospholipase C-IP3-Ca2+ signal transduction pathways. 1,25 (OH)2D3 operates through a cytoplasmic receptor in controlling transcription and through a membrane receptor that activates the Ca2+ and phospholipase C messenger system. The calciotropic hormones also influence processes not directly associated with Ca2+ regulation, such as cell differentiation, and may thus affect the calcium-regulating subsystems also indirectly.

1,25-dihydroxyvitamin D3 inhibits proliferation of IMR-90 human fibroblasts and stimulates pyruvate kinase activity in confluent-phase cells

This study tested the hypothesis that 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) plays a role in regulating some aspects of metabolism in IMR-90 normal human fetal lung fibroblasts. Among the enzymes studied, only pyruvate kinase showed a significant increase after treatment of confluent-phase cells with 1,25(OH)2D3 at various concentrations (0.1-100 nM range) for 24 h. A parallel increase in lactate output was observed. Steroid specificity was established by the failure of 10 nM levels of 25-hydroxyvitamin D3, estradiol-17 beta and progesterone to affect pyruvate kinase activity. The determination of the time course of [3H]-2-deoxy-D-glucose transport indicated that the hormone did not influence the transmembrane transport system of D-glucose. The addition of the inhibitors cycloheximide and actinomycin D to the culture medium abolished, at least in part, the 1,25(OH)2D3 stimulation of pyruvate kinase activity, suggesting the probable dependence of the hormone effect on cellular RNA and protein synthesis. 1,25(OH)2D3 also affected fibroblast growth and DNA synthesis. Cell number significantly decreased after 2-5 days treatment with 10 nM hormone in comparison with control fibroblasts, and also the incorporation of [3H]thymidine into DNA decreased after treatment of the cells with 1 and 10 nM hormone for 48 h. In conclusion, these data demonstrate that 1,25(OH)2D3 stimulates pyruvate kinase activity in confluent-phase IMR-90 human fibroblasts by a mechanism probably dependent on de novo protein synthesis, and also affects cell growth and DNA synthesis in sub-confluent-phase cells.

Modulation by 1,25(OH)2-vitamin D3 of the adenylyl cyclase/cyclic AMP pathway in rat and chick myoblasts

We have previously reported that the calciotropic hormone 1,25(OH)2-vitamin D3 stimulates influx of Ca2+ into cultured rat and embryonic chick myoblasts via voltage sensitive Ca(2+)-channels. In the present study, we show that this effect of 1,25(OH)2D3 requires the mediation of the adenylylcyclase signalling system since the hormone-dependent Ca2+ influx is abolished by specific inhibitors of adenylylcyclase and protein kinase A and mimicked by forskolin and dibutyryl cAMP. 1,25(OH)2D3-stimulated elevations in cellular cAMP paralleled increases in Ca2+ uptake, further suggesting a coupling of adenylylcyclase activation and calcium influx. Fluoride and GTP gamma S mimicked 1,25(OH)2D3-stimulation of calcium influx while GDP beta S suppressed the effect of the hormone. Cholera toxin and Bordetella pertussis toxin both increased 45Ca2+ uptake in rat and chick myoblasts. The hormone further increased cholera toxin actions, but was unable to modify pertussis toxin-induced 45Ca2+ uptake, suggesting a similar target of action for pertussis toxin and 1,25(OH)2D3. Incubation of microsomal membranes with the sterol (10 nM, 2 min) markedly displaces (-32%) [35S]GTP gamma S binding to the membranes. ADP-ribosylation of the pertussis toxin-sensitive 41 kDa substrate was significantly increased (+40%) in 1,25(OH)2D3-pretreated cells. These results suggest that 1,25(OH)2D3-stimulated influx of Ca2+ into rat and embryonic chick cultured myoblasts sequentially requires inhibition of a pertussis toxin-sensitive G protein, accumulation of cAMP and activation of dihydropyridine-sensitive Ca(2+)-channels through PKA-mediated phosphorylation events.

Congestive heart failure caused by vitamin D deficiency?

We describe a child, 3.5 months old, with severe vitamin D deficiency, profound hypocalcaemia, hyperphosphataemia, dilated left ventricle, severely reduced myocardial contractility and congestive heart failure. She also had depressed thyroid function with subnormal thyroxine and non-detectable serum thyrotropin (TSH) levels. The child promptly responded to calcium infusions, conventional anticongestive therapy and calcitriol. She is now 3 years old and received no medication. Myocardial function is normal but she has motor delay. We believe that her transitory congestive heart failure was caused by severe vitamin D deficiency with profound hypocalcaemia.

Non-genomic signal transduction pathway of vitamin D in muscle

The secosteroid hormone 1,25(OH)2-vitamin D3 rapidly activates voltage-dependent Ca2+ channels of the L-type in skeletal and cardiac muscle cells by a non-genomic mechanism which involves guanine nucleotide binding (G) protein-medicated stimulation of the adenylate cyclase/cAMP/protein kinase A messenger system. Modifications in calmodulin intracellular distribution induced by PKA-dependent membrane protein phosphorylation may participate in the fast regulation of muscle Ca2+ influx by 1,25(OH)2D3. The protein kinase C pathway also plays a role modulating 1,25(OH)2D3 signal transduction in muscle by cross-talk with the PKA system. The hormone sequentially activates phospholipases C and D providing diacylglycerol for PKC activation and inositol triphosphate for intracellular Ca2+ mobilization. In addition, 1,25(OH)2D3 rapidly stimulates phospholipase A2 generating arachidonic acid for the eicosanoid pathway. Specificity of hormone effects suggests that binding to a muscle membrane-bound receptor mediates these events.

Involvement of protein kinase C in 1,25(OH)2-vitamin D3 regulation of calcium uptake by cultured myocytes

1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] produces an acute stimulation of calcium influx in cultured chick embryo myocytes through activation of voltage-gated Ca2+ channels and involvement of cyclic AMP-dependent protein kinase A (PKA). To investigate the participation of protein kinase C (PKC) in this hormone-induced response, calcium uptake was measured in myocytes treated with PKC activators 12-O-tetradecanoyl phorbol 13-acetate (TPA, 50 nM) or 1,2-dioctanoyl-rac-glycerol (DOG, 50 microM). TPA and DOG decreased 45Ca2+ uptake 37% below control cultures. Contrarily, the PKC inhibitors H7 and staurosporine increase myocyte Ca2+ uptake 51% and 54%, respectively. In addition, PKC activity was augmented in cytosol (39%) and membranes (31%) of myocytes after 5 min of treatment with 0.1 nM 1,25(OH)2D3. Likewise, the hormone induced a fast biphasic formation of diacylglycerol, the natural PKC activator, peaking at 30 s (26%) and 3 min (39%). On the other hand, the stimulation of Ca2+ uptake induced by compound H7 as well as 1,25(OH)2D3 was completely abolished with a specific PKA inhibitor. H7 also produced an increase in cAMP levels (172%) and PKA activity (204%). These results suggest the participation of PKC in 1,25(OH)2D3 regulated calcium influx in heart cells and the operation of a cross-talk mechanism between the PKC and PKA pathways.

Rapid 1,25(OH)2-vitamin D3 stimulation of calcium uptake by rat intestinal cells involves a dihydropyridine-sensitive cAMP-dependent pathway

The acute effects of 1,25-dihydroxy-vitamin D3 (1,25(OH)2D3) on Ca2+ influx in isolated rat enterocytes were studied. The hormone significantly increased 45Ca2+ uptake by the cells within 1-10 min in a specific dose-dependent manner (10(-11)-10(-9) M) since 25(OH)D3 and 24,25(OH)2D3 were devoid of activity. The effects of 1,25(OH)2D3 were mimicked by the Ca2+ channel agonist BAY K8644 and completely abolished by nifedipine (1 microM) and verapamil (10 microM). Incubation of duodenal cells with 1,25(OH)2D3 rapidly (1-5 min) increased cAMP levels. Forskolin caused a rapid increase in Ca2+ uptake by enterocytes which was similar to the action of the hormone. Moreover, pretreatment of cells with the specific cAMP inhibitor Rp-cAMPS suppressed the changes in 45Ca influx induced by 1,25(OH)2D3. These results provide the first evidence involving Ca2+ channel activation through the cAMP pathway by 1,25(OH)2D3 in mammalian intestinal cells.

Generation of inositol phosphates, diacylglycerol and calcium fluxes in myoblasts treated with 1,25-dihydroxyvitamin D3

We have examined the effects of the seco-steroid hormone 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] on membrane phosphoinositide metabolism, protein kinase C (PKC) activation and influx of extracellular Ca2+ in chick-embryo muscle-cell (myoblast) cultures. At physiological concentrations, the hormone induces a rapid (15 s) and transient release of inositol triphosphate (InsP3) and diacylglycerol (DAG). InsP3 release is maximal at 60 s (80% above controls) and then declines. The effects of 1,25(OH)2D3 on InsP3 production exhibited specificity, as 25-hydroxy-vitamin D3 and 24,25-dihydroxy-vitamin D3 did not alter myoblast InsP3 levels. The stimulation of DAG is biphasic, with peaks at 60 s (+105%) and 5 min (+700%). The second phase of DAG release is not associated with changes in InsP3. 1,25(OH)2D3 induces a rapid (within 60 s) accumulation of InsP2, and its effect on InsP is delayed (120 s). The hormone rapidly activates myoblast PKC, with maximal translocation of activity from the cytosol to the cell membrane occurring at 60 s. Myoblast 45Ca uptake significantly increases within 30 s of exposure to 1,25(OH)2D3. The response is time- (0.5-10 min) and dose- (1 pM-10 nM) dependent. The effects of the hormone are mimicked by the Ca(2+)-channel agonist Bay K 8644 and are effectively suppressed by nifedipine and extracellular EGTA. The results suggest that the rapid non-genomic actions of 1,25(OH)2D3 in myoblasts involve second-messenger systems associated with the generation of InsP3 and DAG and regulation of Ca2+ fluxes through voltage-operated channels.

Rapid actions of vitamin D compounds

The rapid actions of vitamin D compounds are surveyed in a variety of target tissues, including intestine, muscle, bone, hepatocytes, fibroblasts, HL-60 cells, kidney, mammary gland, and parathyroid. Evidence for non-nuclear receptors vs. membranophilic effects is discussed, followed by a consideration of signal transduction mechanisms including steroid hormone activated Ca2+ channels, phospholipid metabolism, protein kinases, and the role of G-proteins.

Regulation of Ca2+ uptake in skeletal muscle by 1,25-dihydroxyvitamin D3: role of phosphorylation and calmodulin

Experiments were carried out to obtain information about the mechanism underlying the fast action of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) in skeletal muscle. N-2'-o-dibutyryladenosine-3',5'-cyclic monophosphate (dbcAMP), similarly as 1,25(OH)2D3 (5 x 10(-10) M), rapidly increased 45Ca uptake by soleus muscle from vitamin D-deficient chicks (+25% and +98% at 3 min and 10 min, respectively) in a dose-dependent manner. The effects of the cAMP analog (10 microM) and 1,25(OH)2D3 could be abolished by the Ca(2+)-channel blocker nifedipine and the calmodulin antagonist flufenazine. Calmodulin binding by two muscle microsomal proteins of 28 kDa and 30 kDa was stimulated within 1 min of exposure of the tissue to 1,25(OH)2D3. Direct effects of the sterol on membrane calmodulin binding were shown with isolated microsomes. The 1,25(OH)2D3-mediated rise of [125I]calmodulin binding to microsomal membranes was dependent on the presence of medium ATP. Forskolin (10 microM) and cAMP (10 microM) also increased [125I]calmodulin binding (+75% and +64%, respectively, with respect to controls). Pretreatment of microsomal membranes with cAMP-dependent protein kinase inhibitor (1 microgram/ml) or addition of alkaline phosphates (1 U/ml) after hormonal treatment caused complete inhibition of 1,25(OH)2D3-induced [125I]calmodulin binding to microsomal membrane proteins. These results imply modifications of membrane protein phosphorylation through the cAMP signal pathway and in turn of calmodulin binding in the mechanism by which 1,25(OH)2D3 rapidly stimulates skeletal muscle Ca2+ uptake.

Evidence on the participation of the 3',5'-cyclic AMP pathway in the non-genomic action of 1,25-dihydroxy-vitamin D3 in cardiac muscle

Several studies have suggested that vitamin D plays a role in cardiovascular function. It has been recently shown that in vitro treatment of vitamin D-deficient chick cardiac muscle with physiological concentrations of 1,25-dihydroxy-vitamin D3 (1,25(OH)2D3) induces a rapid (1-10 min) increase of tissue 45Ca uptake which can be suppressed by Ca channel blockers. The hormone simultaneously stimulated heart microsomal membrane protein phosphorylation. Experiments were performed to investigate the existence of a relationship between these changes and to obtain information about the mechanism involved in 1,25(OH)2D3-induced modifications in cardiac protein phosphorylation. Dibutyryl cyclic AMP (10 microM) and forskolin (10 microM), known activators of the cAMP pathway, produced time courses of changes in 45Ca uptake by chick heart tissue similar to 1,25(OH)2D3 (10(-10) M). Analogously to the hormone, the effects of both compounds were abolished by nifedipine (30 microM) and verapamil (10 microM). In agreement with these observations, 1,25(OH)2D3 significantly increased (34-70%) heart muscle cAMP levels within 1-10 min of treatment. In addition, 1,25(OH)2D3 and forskolin caused similar changes in cardiac microsomal membrane protein phosphorylation (e.g. stimulation in 43 kDa and 55 kDa proteins). These changes were also evidenced by direct exposure of isolated heart microsomes to 1,25(OH)2D3, suggesting a direct membrane action of the hormone. The fast effects of 1,25(OH)2D3 on dihydropyridine-sensitive cardiac muscle Ca uptake could be reproduced in primary-cultured myocytes isolated from chick embryonic heart. Furthermore, the effects of the hormone could be suppressed by a specific protein kinase A inhibitor. These results suggest that 1,25(OH)2D3 affects heart cell calcium metabolism through regulation of Ca channel activity mediated by the cAMP pathway.