Osteomalacia associated with carbamazepine/valproate

Resurgence of vitamin D: Old wine in new bottle

There are early references of it in ancient text and physicians have discussed its importance and features of its deficiency in the past. Vitamin D has again regained interest with recent dramatic rise in the incidence of deficiency in the developing as well as developing world. In this review article, we discuss the biochemical and role of vitamin D in the skeletal system. We also discuss the recommended dietary requirements and features of skeletal deficiency. Extra-skeletal roles of vitamin D deficiency have been a matter of debate lately and it has also been discussed in detail in this article. In conclusion, it would not be wrong to label vitamin D as one of the most important vitamin involved in the metabolism of the musculoskeletal system and any clinician, especially the orthopaedician, should be well versed with its overall mechanism and roles in the human body.

Enhancement of hepatic 4-hydroxylation of 25-hydroxyvitamin D3 through CYP3A4 induction in vitro and in vivo: implications for drug-induced osteomalacia

Long-term therapy with certain drugs, especially cytochrome P450 (P450; CYP)-inducing agents, confers an increased risk of osteomalacia that is attributed to vitamin D deficiency. Human CYP24A1, CYP3A4, and CYP27B1 catalyze the inactivation and activation of vitamin D and have been implicated in the adverse drug response. In this study, the inducibility of these enzymes and monohydroxylation of 25-hydroxyvitamin D3 (25OHD3) were evaluated after exposure to P450-inducing drugs. With human hepatocytes, treatment with phenobarbital, hyperforin, carbamazepine, and rifampin significantly increased the levels of CYP3A4, but not CYP24A1 or CYP27B1 mRNA. In addition, rifampin pretreatment resulted in an 8-fold increase in formation of the major metabolite of 25OHD3, 4β,25(OH)2D3. This inductive effect was blocked by the addition of 6',7'-dihydroxybergamottin, a selective CYP3A4 inhibitor. With human renal proximal tubular HK-2 cells, treatment with the same inducers did not alter CYP3A4, CYP24A1, or CYP27B1 expression. 24R,25(OH)2 D3 was the predominant monohydroxy metabolite produced from 25OHD3, but its formation was unaffected by the inducers. With healthy volunteers, the mean plasma concentration of 4β,25(OH)2D3 was increased 60% (p < 0.01) after short-term rifampin administration. This was accompanied by a statistically significant reduction in plasma 1α,25(OH)2D3 (-10%; p = 0.03), and a nonsignificant change in 24R,25(OH)2D3 (-8%; p = 0.09) levels. Further analysis revealed a negative correlation between the increase in 4β,25(OH)2D3 and decrease in 1α,25(OH)2D3 levels. Examination of the plasma monohydroxy metabolite/25OHD3 ratios indicated selective induction of the CYP3A4-dependent 4β-hydroxylation pathway of 25OHD3 elimination. These results suggest that induction of hepatic CYP3A4 may be important in the etiology of drug-induced osteomalacia.

A potential role for adjunctive vitamin D therapy in the management of weight gain and metabolic side effects of second-generation antipsychotics

Second-generation antipsychotic (SGA) medications introduced about 20 years ago are increasingly used to treat psychiatric illnesses in children and adolescents. There has been a five-fold increase in the use of these medications in U.S. children and adolescents in the past decade. However, there has also been a parallel rise in the incidence of side effects associated with these medications, such as obesity, dyslipidemia, insulin resistance, and diabetes mellitus. Despite the severity of these complications and their financial impact on the national healthcare budget, there is neither a clear understanding of the mechanisms contributing to these side effects nor the best ways to address them. Studies that examined lifestyle modification and pharmaceutical agents have yielded mixed results. Therefore, clinical studies using agents, such as vitamin D, which are inexpensive, readily available, with low side effects profile, and have mechanisms to counteract the metabolic side effects of SGA agents, are warranted. Vitamin D is a prohormone with skeletal and extraskeletal properties that could potentially reduce the severity of these metabolic side effects. Its role as an adjunctive therapy for the management of metabolic side effects of SGA agents has not been adequately studied. Effective strategies to curb these side effects will improve the overall health of youths with psychiatric illnesses who receive SGAs. Herein we present a pilot study on the use of vitamin D in patients on treatment with SGAs.

Vitamin D in health and disease: current perspectives

Despite the numerous reports of the association of vitamin D with a spectrum of development, disease treatment and health maintenance, vitamin D deficiency is common. Originating in part from the diet but with a key source resulting from transformation by exposure to sunshine, a great deal of the population suffers from vitamin D deficiency especially during winter months. It is linked to the treatment and pathogenesis and/or progression of several disorders including cancer, hypertension, multiple sclerosis, rheumatoid arthritis, osteoporosis, muscle weakness and diabetes. This widespread deficiency of Vitamin D merits consideration of widespread policies including increasing awareness among the public and healthcare professionals.

Interplay between cholesterol and drug metabolism

Cholesterol biosynthetic and metabolic pathways contain several branching points towards physiologically active molecules, such as coenzyme Q, vitamin D, glucocorticoid and steroid hormones, oxysterols, or bile acids. Sophisticated regulatory mechanisms are involved in maintenance of the homeostasis of not only cholesterol but also other cholesterogenic molecules. In addition to endogenous cues, cholesterol homeostasis needs to accommodate also to exogenous cues that are imported into the body, such as chemicals and medications. Steroid and nuclear receptors together with sterol regulatory element-binding protein (SREBP) mediate the fine tuning of biosynthetic and metabolic routes as well as transports of cholesterol and its derivatives. Similarly, drug/xenobiotic metabolism is the subject to the feedback regulation of cytochrome P450 enzymes and transporters. The regulatory mechanisms that maintain the homeostasis of cholesterogenic molecules and are involved in drug metabolism share similarities. Cholesterol and cholesterogenic compounds (bile acids, glucocorticoids, vitamin D, etc.) regulate the xenosensor signaling in drug-mediated induction of the major drug-metabolizing cytochrome P450 enzymes. The key cellular receptors, pregnane X receptor (PXR), constitutive androstane receptor (CAR), vitamin D receptor (VDR), and glucocorticoid receptor (GR) provide a functional cross-talk between the pathways maintaining cholesterol homeostasis and controlling the expression of drug-metabolizing enzymes. These receptors serve as metabolic sensors, resulting in a coordinate regulation of cholesterogenic compounds metabolism and of the defense against xenobiotic and endobiotic toxicity. Herein we present a comprehensive review of functional interactions between cholesterol homeostasis and drug metabolism involving the main nuclear and steroid receptors.

Lithocholate--a promising non-calcaemic calcitriol surrogate for promoting human osteoblast maturation upon biomaterials

BACKGROUND AND AIMS

Calcitriol, an active vitamin D metabolite, has a limited application in bone repair because of its undesirable hypercalcaemic action. However it has emerged that lithocholic acid (LCA) is a non-calcaemic vitamin D receptor ligand but whether this steroid can support osteoblast maturation has not been reported. Using the human osteoblast cell line, MG63, we explored the potential of LCA and LCA derivatives to secure osteoblast maturation.

RESULTS

The co-stimulation of cells with LCA, LCA acetate or LCA acetate methyl ester (0.5-5 microM) and lysophosphatidic acid (LPA, 20 microM) resulted in clear, synergistic increases in MG63 maturation that was both time and dose dependent. Cells grown upon both titanium and hydroxyapatite, two widely used implant materials, responded well to co-treatment with LCA acetate (5 microM) and LPA (20 microM) as demonstrated by stark, synergistic increases in ALP activity. Evidence of activator protein-1 (AP-1) stimulation by LCA acetate (30 microM) was demonstrated using an AP-1 luciferase reporter assay. Synergistic increases in ALP activity, and therefore osteoblast maturation, were observed for MG63 cells co-stimulated with LCA acetate (5 microM) and either epidermal growth factor (10 ng/ml) or transforming growth factor-beta (10 ng/ml). Ligands acting on either the farnesoid X receptor or pregnane X receptor could not substitute for the action of LCA acetate on MG63 maturation.

CONCLUSIONS

Lithocholate is able to act as a calcitriol surrogate in generating mature osteoblasts. Given that LCA is non-calcaemic it is likely to find an application in bone repair/regeneration by aiding matrix calcification at implant sites.

The steroid and xenobiotic receptor (SXR), beyond xenobiotic metabolism

The steroid and xenobiotic receptor (SXR) (also known as pregnane X receptor or PXR) is a nuclear hormone receptor activated by a diverse array of endogenous hormones, dietary steroids, pharmaceutical agents, and xenobiotic compounds. SXR has an enlarged, flexible, hydrophobic ligand binding domain (LBD) which is remarkably divergent across mammalian species and SXR exhibits considerable differences in its pharmacology among mammals. The broad response profile of SXR has led to the development of "the steroid and xenobiotic sensor hypothesis". SXR has been established as a xenobiotic sensor that coordinately regulates xenobiotic clearance in the liver and intestine via induction of genes involved in drug and xenobiotic metabolism. In the past few years, research has revealed new and mostly unsuspected roles for SXR in modulating inflammation, bone homeostasis, vitamin D metabolism, lipid homeostasis, energy homeostasis and cancer. The identification of SXR as a xenobiotic sensor has provided an important tool for studying new mechanisms through which diet, chemical exposure, and environment ultimately impact health and disease. The discovery and pharmacological development of new PXR modulators might represent an interesting and innovative therapeutic approach to combat various diseases.

Constitutive androstane receptor-vitamin D receptor crosstalk: consequence on CYP24 gene expression

We previously reported that the pregnane X receptor (PXR) interferes with vitamin D receptor (VDR) target genes, notably CYP24, by targeting the same responsive elements. Since PXR and constitutive androstane receptor (CAR) share responsive elements in the promoter of their target genes, we wondered whether CAR also interferes with CYP24 expression. The current study shows that: (i) CAR-RXR heterodimer binds to and transactivates the proximal promoter of CYP24 (-1200/+22) and both VDRE-1 and VDRE-2 which control its expression in response to 1,25-dihydroxyvitamin D(3), (ii) androstanol an inverse agonist of hCAR inhibits transactivation of VDREs by hCAR, (iii) mutations of either VDRE-1 or -2 half sites inhibit hCAR-mediated transactivation, and (iv) in primary human hepatocytes (n =11) CITCO, a specific hCAR agonist, is an inducer of CYP24 as well as of CYP2B6 and CYP3A4 mRNAs. In conclusion, CAR/PXR and VDR bind to and transactivate the same response elements in CYP24 promoter.

Steroid and xenobiotic receptor and vitamin D receptor crosstalk mediates CYP24 expression and drug-induced osteomalacia

The balance between bioactivation and degradation of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] is critical for ensuring appropriate biological effects of vitamin D. Cytochrome P450, family 24-mediated (CYP24-mediated) 24-hydroxylation of 1,25(OH)2D3 is an important step in the catabolism of 1,25(OH)2D3. The enzyme is directly regulated by vitamin D receptor (VDR), and it is expressed mainly in the kidney, where VDR is also abundant. A recent report suggests that activation of steroid and xenobiotic receptor (SXR) also enhances the expression of CYP24, providing a new molecular mechanism of drug-induced osteomalacia. However, here we showed that activation of SXR did not induce CYP24 expression in vitro and in vivo, nor did it transactivate the CYP24 promoter. Instead, SXR inhibited VDR-mediated CYP24 promoter activity, and CYP24 expression was very low in tissues containing high levels of SXR, including the small intestine. Moreover, 1,25(OH)2D3-induced CYP24 expression was enhanced in mice lacking the SXR ortholog pregnane X receptor, and treatment of humans with the SXR agonist rifampicin had no effect on intestinal CYP24 expression, despite demonstration of marked CYP3A4 induction. Combined with our previous findings that CYP3A4, not CYP24, plays the dominant role in hydroxylation of 1,25(OH)2D3 in human liver and intestine, our results indicate that SXR has a dual role in mediating vitamin D catabolism and drug-induced osteomalacia.

Possible involvement of pregnane X receptor-enhanced CYP24 expression in drug-induced osteomalacia

Vitamin D controls calcium homeostasis and the development and maintenance of bones through vitamin D receptor activation. Prolonged therapy with rifampicin or phenobarbital has been shown to cause vitamin D deficiency or osteomalacia, particularly in patients with marginal vitamin D stores. However, the molecular mechanism of this process is unknown. Here we show that these drugs lead to the upregulation of 25-hydroxyvitamin D(3)-24-hydroxylase (CYP24) gene expression through the activation of the nuclear receptor pregnane X receptor (PXR; NR1I2). CYP24 is a mitochondrial enzyme responsible for inactivating vitamin D metabolites. CYP24 mRNA is upregulated in vivo in mice by pregnenolone 16alpha-carbonitrile and dexamethasone, 2 murine PXR agonists, and in vitro in human hepatocytes by rifampicin and hyperforin, 2 human PXR agonists. Moreover, rifampicin increased 24-hydroxylase activity in these cells, while, in vivo in mice, pregnenolone 16alpha-carbonitrile increased the plasma concentration of 24,25-dihydroxyvitamin D(3). Transfection of PXR in human embryonic kidney cells resulted in rifampicin-mediated induction of CYP24 mRNA. Analysis of the human CYP24 promoter showed that PXR transactivates the sequence between -326 and -142. We demonstrated that PXR binds to and transactivates the 2 proximal vitamin D-responsive elements of the human CYP24 promoter. These data suggest that xenobiotics and drugs can modulate CYP24 gene expression and alter vitamin D(3) hormonal activity and calcium homeostasis through the activation of PXR.

Antiepileptic drugs and reduced bone mineral density

There is a growing interest in recognizing the association between antiepileptic drugs and reduced bone mineral density. Although the literature regarding this association has been available for more than three decades, the management of this complication remains unclear. We review the relevant literature regarding antiepileptic drugs and reduction in bone mineral density with the aim of developing some guidelines for practical management of this problem. This review focuses on the mechanism of antiepileptic drug-induced bone loss, its recognition, and its management.