1,25-Dihydroxyvitamin D and 25-hydroxyvitamin D--mediated regulation of TRPV6 (a putative epithelial calcium channel) mRNA expression in Caco-2 cells

The role of the gut microbiome in neuroinflammation and chemotherapy-induced peripheral neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is one of the most debilitating adverse effects caused by chemotherapy drugs such as paclitaxel, oxaliplatin and vincristine. It is untreatable and often leads to the discontinuation of cancer therapy and a decrease in the quality of life of cancer patients. It is well-established that neuroinflammation and the activation of immune and glial cells are among the major drivers of CIPN. However, these processes are still poorly understood, and while many chemotherapy drugs alone can drive the activation of these cells and consequent neuroinflammation, it remains elusive to what extent the gut microbiome influences these processes. In this review, we focus on the peripheral mechanisms driving CIPN, and we address the bidirectional pathways by which the gut microbiome communicates with the immune and nervous systems. Additionally, we critically evaluate literature addressing how chemotherapy-induced dysbiosis and the consequent imbalance in bacterial products may contribute to the activation of immune and glial cells, both of which drive neuroinflammation and possibly CIPN development, and how we could use this knowledge for the development of effective treatment strategies.

Nongenomic Activities of Vitamin D

Vitamin D shows a variety of pleiotropic activities which cannot be fully explained by the stimulation of classic pathway- and vitamin D receptor (VDR)-dependent transcriptional modulation. Thus, existence of rapid and nongenomic responses to vitamin D was suggested. An active form of vitamin D (calcitriol, 1,25(OH)₂D₃) is an essential regulator of calcium-phosphate homeostasis, and this process is tightly regulated by VDR genomic activity. However, it seems that early in evolution, the production of secosteroids (vitamin-D-like steroids) and their subsequent photodegradation served as a protective mechanism against ultraviolet radiation and oxidative stress. Consequently, direct cell-protective activities of vitamin D were proven. Furthermore, calcitriol triggers rapid calcium influx through epithelia and its uptake by a variety of cells. Subsequently, protein disulfide-isomerase A3 (PDIA3) was described as a membrane vitamin D receptor responsible for rapid nongenomic responses. Vitamin D was also found to stimulate a release of secondary massagers and modulate several intracellular processes-including cell cycle, proliferation, or immune responses-through wingless (WNT), sonic hedgehog (SSH), STAT1-3, or NF-kappaB pathways. Megalin and its coreceptor, cubilin, facilitate the import of vitamin D complex with vitamin-D-binding protein (DBP), and its involvement in rapid membrane responses was suggested. Vitamin D also directly and indirectly influences mitochondrial function, including fusion-fission, energy production, mitochondrial membrane potential, activity of ion channels, and apoptosis. Although mechanisms of the nongenomic responses to vitamin D are still not fully understood, in this review, their impact on physiology, pathology, and potential clinical applications will be discussed.

Calcium selective channel TRPV6: Structure, function, and implications in health and disease

Calcium-selective channel TRPV6 (Transient Receptor Potential channel family, Vanilloid subfamily member 6) belongs to the TRP family of cation channels and plays critical roles in transcellular calcium (Ca2+) transport, reuptake of Ca2+ into cells, and maintaining a local low Ca2+ environment for certain biological processes. Recent crystal and cryo-electron microscopy-based structures of TRPV6 have revealed mechanistic insights on how the protein achieves Ca2+ selectivity, permeation, and inactivation by calmodulin. The TRPV6 protein is expressed in a range of epithelial tissues such as the intestine, kidney, placenta, epididymis, and exocrine glands such as the pancreas, prostate and salivary, sweat, and mammary glands. The TRPV6 gene is a direct transcriptional target of the active form of vitamin D and is efficiently regulated to meet the body's need for Ca2+ demand. In addition, TRPV6 is also regulated by the level of dietary Ca2+ and under physiological conditions such as pregnancy and lactation. Genetic models of loss of function in TRPV6 display hypercalciuria, decreased bone marrow density, deficient weight gain, reduced fertility, and in some cases alopecia. The models also reveal that the channel plays an indispensable role in maintaining maternal-fetal Ca2+ transport and low Ca2+ environment in the epididymal lumen that is critical for male fertility. Most recently, loss of function mutations in TRPV6 gene is linked to transient neonatal hyperparathyroidism and early onset chronic pancreatitis. TRPV6 is overexpressed in a wide range of human malignancies and its upregulation is strongly correlated to tumor aggressiveness, metastasis, and poor survival in selected cancers. This review summarizes the current state of knowledge on the expression, structure, biophysical properties, function, polymorphisms, and regulation of TRPV6. The aberrant expression, polymorphisms, and dysfunction of this protein linked to human diseases are also discussed.

Mitochondrial potassium channels: A novel calcitriol target

BACKGROUND

Calcitriol (an active metabolite of vitamin D) modulates the expression of hundreds of human genes by activation of the vitamin D nuclear receptor (VDR). However, VDR-mediated transcriptional modulation does not fully explain various phenotypic effects of calcitriol. Recently a fast non-genomic response to vitamin D has been described, and it seems that mitochondria are one of the targets of calcitriol. These non-classical calcitriol targets open up a new area of research with potential clinical applications. The goal of our study was to ascertain whether calcitriol can modulate mitochondrial function through regulation of the potassium channels present in the inner mitochondrial membrane.

METHODS

The effects of calcitriol on the potassium ion current were measured using the patch-clamp method modified for the inner mitochondrial membrane. Molecular docking experiments were conducted in the Autodock4 program. Additionally, changes in gene expression were investigated by qPCR, and transcription factor binding sites were analyzed in the CiiiDER program.

RESULTS

For the first time, our results indicate that calcitriol directly affects the activity of the mitochondrial large-conductance Ca²⁺-regulated potassium channel (mitoBK_Ca) from the human astrocytoma (U-87 MG) cell line but not the mitochondrial calcium-independent two-pore domain potassium channel (mitoTASK-3) from human keratinocytes (HaCaT). The open probability of the mitoBK_Ca channel in high calcium conditions decreased after calcitriol treatment and the opposite effect was observed in low calcium conditions. Moreover, using the AutoDock4 program we predicted the binding poses of calcitriol to the calcium-bound BK_Ca channel and identified amino acids interacting with the calcitriol molecule. Additionally, we found that calcitriol influences the expression of genes encoding potassium channels. Such a dual, genomic and non-genomic action explains the pleiotropic activity of calcitriol.

CONCLUSIONS

Calcitriol can regulate the mitochondrial large-conductance calcium-regulated potassium channel. Our data open a new chapter in the study of non-genomic responses to vitamin D with potential implications for mitochondrial bioenergetics and cytoprotective mechanisms.

Association between histone deacetylase activity and vitamin D-dependent gene expressions in relation to sulforaphane in human colorectal cancer cells

BACKGROUND

It is relatively unknown as to how dietary bioactive compound sulforaphane (SFN) and vitamin D regulate gene expression in colorectal cancer. We hypothesized that a combination of SFN with vitamin D would prove beneficial in colorectal cancer. A combinatorial chemo-preventive strategy was employed to investigate the impact of SFN on chromatin remodeling in colorectal carcinoma. To understand the epigenetics-mediated changes in gene expression in response to SFN and vitamin D, Caco-2 cells were exposed for 24 h to vitamin D (100 nmol L^-1 ) either alone or in combination with SFN and trichostatin A (20 and 1 μmol L^-1 , respectively) at 70% confluency (proliferating) and after 13 days post-confluency (fully differentiated). Changes to VDR, CYP24A1, CYP27B1 and TRPV6 gene expressions were quantified using real-time PCR-based assays. Histone deacetylase (HDAC) inhibitor activity was assessed using HDAC I/II assay that measured global changes in acetylation status.

RESULTS

In differentiated Caco-2 cells, none of the genes had significant changes from D alone group. D + SFN (P = 0.99) demonstrated an opposing effect from D alone and decreased VDR expression. However, in proliferating Caco-2 cells, D + SFN (P < 0.04) increased VDR expression and decreased CYP27B1 (P < 0.01) more than D alone (P = 0.38 and 0.07, respectively). Although statistically significant, D + SFN (P = 0.01) effect on HDAC inhibitor activity was less than trichostatin A alone group (P < 0.0004) or SFN alone group (P < 0.0014).

CONCLUSIONS

The data suggest that colon cancer cells respond to dietary components differently under different conditions. The effect of vitamin D and SFN is selective and gene-specific in the complex multistep process of colorectal carcinogenesis in vitro. © 2020 Society of Chemical Industry.

Endocrine disruption of vitamin D activity by perfluoro-octanoic acid (PFOA)

Perfluoroalkyl substances (PFAS) are a class of compounds used in industry and consumer products. Perfluorooctanoic acid (PFOA) is the predominant form in human samples and has been shown to induce severe health consequences, such as neonatal mortality, neurotoxicity, and immunotoxicity. Toxicological studies indicate that PFAS accumulate in bone tissues and cause altered bone development. Epidemiological studies have reported an inverse relationship between PFAS and bone health, however the associated mechanisms are still unexplored. Here, we present computational, in silico and in vitro evidence supporting the interference of PFOA on vitamin D (VD). First, PFOA competes with calcitriol on the same binding site of the VD receptor, leading to an alteration of the structural flexibility and a 10% reduction by surface plasmon resonance analysis. Second, this interference leads to an altered response of VD-responsive genes in two cellular targets of this hormone, osteoblasts and epithelial cells of the colorectal tract. Third, mineralization in human osteoblasts is reduced upon coincubation of PFOA with VD. Finally, in a small cohort of young healthy men, PTH levels were higher in the exposed group, but VD levels were comparable. Altogether these results provide the first evidence of endocrine disruption by PFOA on VD pathway by competition on its receptor and subsequent inhibition of VD-responsive genes in target cells.

Structure and function of the calcium-selective TRP channel TRPV6

Epithelial calcium channel TRPV6 is a member of the vanilloid subfamily of TRP channels that is permeable to cations and highly selective to Ca²⁺ ; it shows constitutive activity regulated negatively by Ca²⁺ and positively by phosphoinositol and cholesterol lipids. In this review, we describe the molecular structure of TRPV6 and discuss how its structural elements define its unique functional properties. High Ca²⁺ selectivity of TRPV6 originates from the narrow selectivity filter, where Ca²⁺ ions are directly coordinated by a ring of anionic aspartate side chains. Divalent cations Ca²⁺ and Ba²⁺ permeate TRPV6 pore according to the knock-off mechanism, while tight binding of Gd³⁺ to the aspartate ring blocks the channel and prevents Na⁺ from permeating the pore. The iris-like channel opening is accompanied by an α-to-π helical transition in the pore-lining transmembrane helix S6. As a result of this transition, the intracellular halves of the S6 helices bend and rotate by about 100 deg, exposing different residues to the channel pore in the open and closed states. Channel opening is also associated with changes in occupancy of the transmembrane domain lipid binding sites. The inhibitor 2-aminoethoxydiphenyl borate (2-APB) binds to TRPV6 in a pocket formed by the cytoplasmic half of the S1-S4 transmembrane helical bundle and shifts open-closed channel equilibrium towards the closed state by outcompeting lipids critical for activation. Ca²⁺ inhibits TRPV6 via binding to calmodulin (CaM), which mediates Ca²⁺ -dependent inactivation. The TRPV6-CaM complex exhibits 1:1 stoichiometry; one TRPV6 tetramer binds both CaM lobes, which adopt a distinct head-to-tail arrangement. The CaM C-terminal lobe plugs the channel through a unique cation-π interaction by inserting the side chain of lysine K115 into a tetra-tryptophan cage at the ion channel pore intracellular entrance. Recent studies of TRPV6 structure and function described in this review advance our understanding of the role of this channel in physiology and pathophysiology and inform new therapeutic design.

Oral 25-Hydroxycholecalciferol Acts as an Agonist in the Duodenum of Mice and as Modeled in Cultured Human HT-29 and Caco2 Cells

BACKGROUND

25-Hydroxycholecalciferol [25(OH)D] is the predominant circulating metabolite of vitamin D and serves as the precursor for 1α,25-dihydroxycholecalciferol [1,25(OH)2D], the hormonally active form. The presence of 1α-hydroxylase (1α-OHase) in the intestine suggests that 1,25(OH)2D can be produced from 25(OH)D, but the effects of oral 25(OH)D on the intestine have not been determined.

OBJECTIVES

We investigated the acute intestinal response to orally consumed 25(OH)D in mice by assessing mRNA induction of cytochrome p450 family 24 subfamily A member 1 (Cyp24), a vitamin D-dependent gene. The mechanism of action then was determined through in vitro analyses with Caco2 and HT-29 cells.

METHODS

Adult male C57BL6 mice were given a single oral dose of 40, 80, 200, or 400 ng 25(OH)D (n = 4 per dose) or vehicle (n = 3), and then killed 4 h later to evaluate the duodenal expression of Cyp24 mRNA by qPCR and RNA in situ hybridization. The 25(OH)D-mediated response was also evaluated with Caco2 and HT-29 cells by inhibition assay and dose-response analysis. A cytochrome p450 family 27 subfamily B member 1 (CYP27B1) knockdown of HT-29 was created to compare the dose-response parameters with wild-type HT-29 cells.

RESULTS

Oral 25(OH)D induced expression of Cyp24 mRNA in the duodenum of mice with 80 ng 25(OH)D by 3.3 ± 0.8 ΔΔCt compared with controls (P < 0.05). In vitro, both Caco2 and HT-29 cells responded to 25(OH)D treatment with 200-fold and 175-fold greater effective concentration at 50% maximal response than 1,25(OH)2D, yet inhibition of 1α-OHase and knockdown of CYP27B1 had no effect on the responses.

CONCLUSIONS

In mice, orally consumed 25(OH)D elicits a vitamin D-mediated response in the duodenum. In vitro assessments suggest that the response from 25(OH)D does not require activation by 1α-OHase and that 25(OH)D within the intestinal lumen acts as a vitamin D receptor agonist.

Ion Channel Regulation by Sex Steroid Hormones and Vitamin D in Cancer: A Potential Opportunity for Cancer Diagnosis and Therapy

Many ion channels are involved in tumor development, promoting cancer cell proliferation, migration, invasion, and survival. Accordingly, some of them have been suggested as tumor markers and novel targets for cancer therapy. Some sex steroid hormones (SSH), including estrogens and androgens, favor cancer progression. Meanwhile, other steroid hormones like vitamin D may have anticancer properties. SSH and vitamin D modulate the expression of a number of ion channels in cancer cells from hormone-sensitive tissues, including breast, ovary, prostate, and cervix. Moreover, rapid effects of SSH may be mediated by their direct action on membrane ion channels. Here, we reviewed the SSH and vitamin D regulation of ion channels involved in cancer, and analyzed the potential molecular pathways implicated. In addition, we described the potential clinical use of ion channels in cancer diagnosis and therapy, taking advantage of their regulation by SSH and vitamin D. Since SSH are considered risk factors for different types of cancer, and ion channels play important roles in tumor progression, the regulation of ion channels by SSH and vitamin D may represent a potential opportunity for early cancer diagnosis and therapeutic approaches in SSH and vitamin D sensitive tumors.

Simplified Method to Determine the Efflux Ratio on P-Glycoprotein Substrates Using Three-Compartment Model Analysis for Caco-2 Cell Assay Data

PURPOSE

Multiple time-point sampling is required in transcellular transport studies to accurately calculate the appropriate efflux ratio (ER). Our study sought to develop a simplified method to determine the ER in Caco-2 cells.

METHODS

The equation for the ER was derived from a three-compartment model of apical to basal and basal to apical transport. Transcellular transport studies were conducted with 10 non-P-glycoprotein (P-gp) and 6 P-gp substrates in Caco-2 cells, and the ER was calculated using this equation.

RESULTS

The equation for the ER used the concentration ratio in the receiver compartment at the same time-point; therefore, the ER can theoretically be calculated using only a single point. The ER of all non-P-gp substrates tested was close to 1 at all sampling times. The ERs of cyclosporine A calculated from the concentration ratio at 30, 60, 90, and 120 min incubation were 2.93, 6.43, 7.12, and 9.57, respectively, and the ER at 120 min was almost identical to the theoretical value (9.62) calculated using three-compartment model analysis. The other 5 P-gp substrates showed a similar tendency. Single-point sampling can be used to accurately calculate ER at 120 min.

CONCLUSIONS

Single-point sampling is a promising approach for calculating appropriate ERs in the drug discovery stage.

Structural bases of TRP channel TRPV6 allosteric modulation by 2-APB

Transient receptor potential (TRP) channels are involved in various physiological processes, including sensory transduction. The TRP channel TRPV6 mediates calcium uptake in epithelia and its expression is dramatically increased in numerous types of cancer. TRPV6 inhibitors suppress tumor growth, but the molecular mechanism of inhibition remains unknown. Here, we present crystal and cryo-EM structures of human and rat TRPV6 bound to 2-aminoethoxydiphenyl borate (2-APB), a TRPV6 inhibitor and modulator of numerous TRP channels. 2-APB binds to TRPV6 in a pocket formed by the cytoplasmic half of the S1-S4 transmembrane helix bundle. Comparing human wild-type and high-affinity mutant Y467A structures, we show that 2-APB induces TRPV6 channel closure by modulating protein-lipid interactions. Mutagenesis and functional analyses suggest that the identified 2-APB binding site might be present in other members of vanilloid subfamily TRP channels. Our findings reveal a mechanism of ion channel allosteric modulation that can be exploited for therapeutic design.

1α,25-Dihydroxyvitamin D3 enhances TRPV6 transcription through p38 MAPK activation and GADD45 expression

The active form of vitamin D, 1α,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃], acts as a ligand for the vitamin D receptor (VDR), and regulates various physiological processes, including calcium and bone metabolism, cellular growth and differentiation, immunity and cardiovascular function. A number of vitamin D derivatives have been synthesized for the treatment of cancer and inflammatory disease, but the adverse effect of hypercalcemic activity due to intestinal calcium absorption has limited wide clinical application. The VDR target gene product TRPV6 is essential for intestinal calcium absorption. Our prior study has demonstrated that 1,25(OH)₂D₃ induces TRPV6 mRNA expression at lower concentrations than for induction of CYP24A1, a VDR target gene involved in vitamin D inactivation, in intestinal SW480 cells, suggesting an additional mechanism for vitamin D signaling on TRPV6 induction. By searching for a signal transduction pathway involved in 1,25(OH)₂D₃-induced expression of TRPV6, we found that a p38 mitogen-activated protein kinase (MAPK) inhibitor reduces the expression of TRPV6 but not CYP24A1 in 1,25(OH)₂D₃-treated SW480 cells. Knockdown experiments showed that p38α is involved in 1,25(OH)₂D₃-induced expression of TRPV6 but not CYP24A1. Treatment with a de novo protein synthesis inhibitor suppressed 1,25(OH)₂D₃-induced TRPV6 expression. Finally, we found that 1,25(OH)₂D₃ treatment induced expression of GADD45A, which encodes the GADD45α MAPK kinase kinase activator, earlier than TRPV6 expression and that GADD45A knockdown reduced TRPV6 induction by 1,25(OH)₂D₃. These findings indicate that p38α and GADD45α are involved in an enhanced vitamin D signaling on TRPV6 expression.

Influence of 1α, 25-dihydroxyvitamin D3 [1, 25(OH)2D3] on the expression of Sox 9 and the transient receptor potential vanilloid 5/6 ion channels in equine articular chondrocytes

BACKGROUND

Sox 9 is a major marker of chondrocyte differentiation. When chondrocytes are cultured in vitro they progressively de-differentiate and this is associated with a decline in Sox 9 expression. The active form of vitamin D, 1, 25 (OH)2D3 has been shown to be protective of cartilage in both humans and animals. In this study equine articular chondrocytes were grown in culture and the effects of 1, 25 (OH)2D3 upon Sox 9 expression examined. The expression of the transient receptor potential vanilloid (TRPV) ion channels 5 and 6 in equine chondrocytes in vitro, we have previously shown, is inversely correlated with de-differentiation. The expression of these channels in response to 1, 25 (OH)2D3 administration was therefore also examined.

RESULTS

The active form of vitamin D (1, 25 (OH)2D3) when administered to cultured equine chondrocytes at two different concentrations significantly increased the expression of Sox 9 at both. In contrast 1, 25 (OH)2D3 had no significant effect upon the expression of either TRPV 5 or 6 at either the protein or the mRNA level.

CONCLUSIONS

The increased expression of Sox 9, in equine articular chondrocytes in vitro, in response to the active form of vitamin D suggests that this compound could be utilized to inhibit the progressive de-differentiation that is normally observed in these cells. It is also supportive of previous studies indicating that 1α, 25-dihydroxyvitamin D3 can have a protective effect upon cartilage in animals in vivo. The previously observed correlation between the degree of differentiation and the expression levels of TRPV 5/6 had suggested that these ion channels may have a direct involvement in, or be modulated by, the differentiation process in vitro. The data in the present study do not support this.

TRPV6 channels

TRPV6 (former synonyms ECAC2, CaT1, CaT-like) displays several specific features which makes it unique among the members of the mammalian Trp gene family (1) TRPV6 (and its closest relative, TRPV5) are the only highly Ca(2+)-selective channels of the entire TRP superfamily (Peng et al. 1999; Wissenbach et al. 2001; Voets et al. 2004). (2) Translation of Trpv6 initiates at a non-AUG codon, at ACG, located upstream of the annotated AUG, which is not used for initiation (Fecher-Trost et al. 2013). The ACG codon is nevertheless decoded by methionine. Not only a very rare event in eukaryotic biology, the full-length TRPV6 protein existing in vivo comprises an amino terminus extended by 40 amino acid residues compared to the annotated truncated TRPV6 protein which has been used in most studies on TRPV6 channel activity so far. (In the following numbering occurs according to this full-length protein, with the numbers of the so far annotated truncated protein in brackets). (3) Only in humans a coupled polymorphism of Trpv6 exists causing three amino acid exchanges and resulting in an ancestral Trpv6 haplotype and a so-called derived Trpv6 haplotype (Wissenbach et al. 2001). The ancestral allele encodes the amino acid residues C197(157), M418(378) and M721(681) and the derived alleles R197(157), V418(378) and T721(681). The ancestral haplotype is found in all species, the derived Trpv6 haplotype has only been identified in humans, and its frequency increases with the distance to the African continent. Apparently the Trpv6 gene has been a strong target for selection in humans, and its derived variant is one of the few examples showing consistently differences to the orthologues genes of other primates (Akey et al. 2004, 2006; Stajich and Hahn 2005; Hughes et al. 2008). (4) The Trpv6 gene expression is significantly upregulated in several human malignancies including the most common cancers, prostate and breast cancer (Wissenbach et al. 2001; Zhuang et al. 2002; Fixemer et al. 2003; Bolanz et al. 2008). (5) Male mice lacking functional TRPV6 channels are hypo-/infertile making TRPV6 one of the very few channels essential for male fertility (Weissgerber et al. 2011, 2012).

Importance of apical membrane delivery of 1,25-dihydroxyvitamin D3 to vitamin D-responsive gene expression in the colon

Synthetic conjugation of a glucuronide to 1,25-dihydroxyvitamin D3 (1,25D3) to produce β-25-monoglucuronide-1,25D3 (βGluc-1,25D3) renders the hormone biologically inactive and resistant to mammalian digestive enzymes. However, β-glucuronidase produced by bacteria in the lower intestinal tract can cleave off the glucuronide, releasing the active hormone. In mice given a single oral dose of 1,25D3, 24-hydroxylase (Cyp24a1) gene expression was strongly enhanced in the duodenum, but not in the colon, despite circulating concentrations of 1,25D3 that peaked at ∼3.0 nmol/l. In contrast, in mice treated with an equimolar dose of βGluc-1,25D3, Cyp24a1 gene expression increased 700-fold in the colon but was significantly weaker in the duodenum compared with mice treated with 1,25D3. Similar results were observed with another vitamin D-dependent gene. When administered subcutaneously, 1,25D3 weakly stimulated colon Cyp24a1 gene expression while βGluc-1,25D3 again resulted in strong enhancement. Surgical ligation to block passage of ingesta beyond the upper intestinal tract abolished upregulation of colon Cyp24a1 gene expression by orally and subcutaneously administered βGluc-1,25D3. Feeding βGluc-1,25D3 for 5 days revealed a linear, dose-dependent increase in colon Cyp24a1 gene expression but did not significantly increase plasma 1,25D3 or calcium concentrations. This study indicates that the colon is relatively insensitive to circulating concentrations of 1,25D3 and that the strongest gene enhancement occurs when the hormone reaches the colon via the lumen of the intestinal tract. These findings have broad implications for the use of vitamin D compounds in colon disorders and set the stage for future therapeutic studies utilizing βGluc-1,25D3 in their treatment.

A challenging case of hypocalcemia supporting the concept that 25-hydroxyvitamin D status is important for intestinal calcium absorption

CONTEXT

Intestinal mucosa seems to be responsive not only to circulating calcitriol but also to serum 25-hydroxyvitamin D concentrations.

OBJECTIVE

We report a complex patient with chronic kidney disease who presented with symptomatic hypocalcemia (ionized calcium, 0.77 mmol/liter) despite regular calcitriol and calcium supplementation.

METHODS

Case history, laboratory evaluation, and bone biopsies are discussed.

RESULTS

Only vigorous treatment with im cholecalciferol led to a significant improvement of serum calcium, a decrease in PTH levels, and histological improvement of osteomalacic bone disease. However, oral anticoagulation became necessary for advanced peripheral artery disease, which precluded further im injections. Therefore, UVB phototherapy was initiated to treat vitamin D deficiency.

CONCLUSION

This case is clinically relevant because it demonstrates that efficient calcium absorption is markedly reduced in profound vitamin D deficiency, even with normal active vitamin D levels. An important consequence is to stay aware of vitamin D deficiency in patients with compromised kidney function irrespective of regular calcitriol replacement. Second, when both parenteral and oral vitamin D administration are contraindicated, ineffective, or unavailable, UVB phototherapy is an effective option to treat vitamin D deficiency. Third, this case underlines the importance of obtaining regular 25-hydroxyvitamin D levels in complex clinical cases when prediction of individual response is unreliable.

Human PXR-mediated induction of intestinal CYP3A4 attenuates 1α,25-dihydroxyvitamin D₃ function in human colon adenocarcinoma LS180 cells

Oxidative catabolism of 1α,25-dihydroxyvitamin D(3) [1α,25(OH)(2)D(3)] is mediated by either CYP24A1 or CYP3A4. In this paper, we tested whether induction of CYP3A4 in the LS180 intestinal cell model enhances clearance of 1α,25(OH)(2)D(3) and blunts its hormonal effect on expression of the apical membrane calcium transport protein, TRPV6. Treatment with the hPXR agonist rifampin significantly increased CYP3A4 mRNA content and catalytic activity, but had no effect on CYP24A1 or TRPV6 mRNA content. Pre-treating cells with rifampin for 48h, prior to a 24h 1α,25(OH)(2)D(3) treatment phase, was associated with a subsequent 48% increase in the elimination of 1α,25(OH)(2)D(3) and a 35% reduction of peak TRPV6 mRNA. Introduction of the CYP3A4 inhibitor, 6',7'-dihydroxybergamottin, an active inhibitor in grapefruit juice, reversed the effects of rifampin on 1α,25(OH)(2)D(3) clearance and TRPV6 expression. Over-expression of hPXR in LS180 cells greatly enhanced the CYP3A4 responsiveness to rifampin pretreatment, and elicited a greater relative suppression of TRPV6 expression and an increase in 1α,25(OH)(2)D(3) disappearance rate, compared to vector expressed cells, following hormone administration. Together, these results suggest that induction of CYP3A4 in the intestinal epithelium by hPXR agonists can result in a greater metabolic clearance of 1α,25(OH)(2)D(3) and reduced effects of the hormone on the intestinal calcium absorption, which may contribute to an increased risk of drug-induced osteomalacia/osteoporosis in patients receiving chronic therapy with potent hPXR agonists. Moreover, ingestion of grapefruit juice in the at-risk patients could potentially prevent this adverse drug effect.

The transient receptor potential ion channel TRPV6 is expressed at low levels in osteoblasts and has little role in osteoblast calcium uptake

Background

TRPV6 ion channels are key mediators of regulated transepithelial absorption of Ca²⁺ within the small intestine. Trpv6^-/- mice were reported to have lower bone density than wild-type littermates and significant disturbances in calcium homeostasis that suggested a role for TRPV6 in osteoblasts during bone formation and mineralization. TRPV6 and molecules related to transepithelial Ca²⁺ transport have been reported to be expressed at high levels in human and mouse osteoblasts.

Results

Transmembrane ion currents in whole cell patch clamped SaOS-2 osteoblasts did not show sensitivity to ruthenium red, an inhibitor of TRPV5/6 ion channels, and ⁴⁵Ca uptake was not significantly affected by ruthenium red in either SaOS-2 (P = 0.77) or TE-85 (P = 0.69) osteoblastic cells. In contrast, ion currents and ⁴⁵Ca uptake were both significantly affected in a human bronchial epithelial cell line known to express TRPV6. TRPV6 was expressed at lower levels in osteoblastic cells than has been reported in some literature. In SaOS-2 TRPV6 mRNA was below the assay detection limit; in TE-85 TRPV6 mRNA was detected at 6.90±1.9 × 10⁻⁵ relative to B2M. In contrast, TRPV6 was detected at 7.7±3.0 × 10⁻² and 2.38±0.28 × 10⁻⁴ the level of B2M in human carcinoma-derived cell lines LNCaP and CaCO-2 respectively. In murine primary calvarial osteoblasts TRPV6 was detected at 3.80±0.24 × 10⁻⁵ relative to GAPDH, in contrast with 4.3±1.5 × 10⁻² relative to GAPDH in murine duodenum. By immunohistochemistry, TRPV6 was expressed mainly in myleocytic cells of the murine bone marrow and was observed only at low levels in murine osteoblasts, osteocytes or growth plate cartilage.

Conclusions

TRPV6 is expressed only at low levels in osteoblasts and plays little functional role in osteoblastic calcium uptake.

TRPV5 and TRPV6 in transcellular Ca(2+) transport: regulation, gene duplication, and polymorphisms in African populations

TRPV5 and TRPV6 are unique members of the TRP super family. They are highly selective for Ca(2+) ions with multiple layers of Ca(2+)-dependent inactivation mechanisms, expressed at the apical membrane of Ca(2+) transporting epithelia, and robustly responsive to 1,25-dihydroxivitamin D(3). These features are well suited for their roles as Ca(2+) entry channels in the first step of transcellular Ca(2+) transport pathways, which are involved in intestinal absorption, renal reabsorption of Ca(2+), placental transfer of Ca(2+) to fetus, and many other processes. While TRPV6 is more broadly expressed in a variety of tissues such as esophagus, stomach, small intestine, colon, kidney, placenta, pancreas, prostate, uterus, salivary gland, and sweat gland, TRPV5 expression is relatively restricted to the distal convoluted tubule and connecting tubule of the kidney. There is only one TRPV6-like gene in fish and birds in comparison to both TRPV5 and TRPV6 genes in mammals, indicating TRPV5 gene was likely generated from duplication of TRPV6 gene during the evolution of mammals to meet the needs of complex renal function. TRPV5 and TRPV6 are subjected to vigorous regulations under physiological, pathological, and therapeutic conditions. The elevated TRPV6 level in malignant tumors such as prostate and breast cancers makes it a potential therapeutic target. TRPV6, and to a lesser extent TRPV5, exhibit unusually high levels of single nucleotide polymorphisms (SNPs) in African populations as compared to other populations, indicating TRPV6 gene was under selective pressure during or after humans migrated out of Africa. The SNPs of TRPV6 and TRPV5 likely contribute to the Ca(2+) conservation mechanisms in African populations.

Human duodenum responses to vitamin D metabolites of TRPV6 and other genes involved in calcium absorption

Calcium absorption by the intestine is necessary for bone mineralization. Much has been learned about this process and the role of vitamin D metabolites in gene transcription from animal studies, but the molecular mechanisms in humans are less well understood. We have used samples of normal human duodenal mucosa, obtained at endoscopy, to investigate the effects of the vitamin D metabolites, 1alpha-dihydroxycholecalciferol [1,25(OH)(2)D(3)] and 25-hydroxycholecalciferol (25OHD), on transcripts on genes involved in calcium absorption and vitamin D metabolism. TRPV6 transcripts were significantly higher after incubation for 6 h with 1,25(OH)(2)D(3) (10(-9) mol/l) than after control incubations (median difference 3.1-fold, P < 0.001). Unexpectedly, TRPV6 expression was also higher (2.4-fold, P < 0.02) after incubation with 25OHD (10(-7) mol/l). Transcripts for the calcium-ATPase, PMCA1, were significantly higher with 1,25(OH)(2)D(3); CYP24 transcripts were reliably detected after incubation with either metabolite, but calbindin-D9k transcripts were unaffected. The response of TRPV6 to 25OHD and the expression of transcripts for CYP27B1, the 25OHD-1alpha-hydroxylase, were significantly correlated (r = 0.82, P < 0.02). Basal duodenal expression of TRPV6 and CYP27B1 were significantly associated (r = 0.72, P < 0.001) in a separate previously reported series of subjects. Multiple regression analysis of the associations with basal duodenal TRPV6 expression identified CYP27B1 expression and serum 1,25(OH)(2)D as major factors. Expression of the CYP27B1 protein was demonstrated immunohistochemically in duodenal mucosa. This study has shown that human duodenal TRPV6, PMCA1, and CYP24 transcripts respond rapidly to 1,25(OH)(2)D(3) and provides evidence suggesting that local duodenal production of 1,25(OH)(2)D(3) by 25OHD-1alpha-hydroxylase may have a role in human calcium absorption.

Up-regulation of transporters and enzymes by the vitamin D receptor ligands, 1alpha,25-dihydroxyvitamin D3 and vitamin D analogs, in the Caco-2 cell monolayer

The effects of 1alpha,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] on gene expression and function were studied in Caco-2 cells. Microarray analyses, real-time quantitative polymerase chain reactions, and Western blotting were used to determine the mRNA and protein expression of transporters and enzymes after 1,25(OH)(2)D(3) or vehicle (0.1% ethanol) treatment for 1, 3, 6, and 10 days. The mRNA and protein expressions of the apical sodium-dependent bile acid transporter, oligopeptide transporter 1, multidrug resistance-associated protein (MRP) 3, and sulfotransferase 1E1 remained unchanged with 1,25(OH)(2)D(3) treatment, whereas those for CYP3A4, multidrug resistance protein 1, and MRP2 were significantly increased (P < 0.05). 1,25(OH)(2)D(3) treatment significantly enhanced MRP4 protein expression by increasing protein stability without affecting mRNA expression, as confirmed in cycloheximide experiments. Marked increase in 6beta-hydroxylation of testosterone by CYP3A4 was also observed in the 6-day 1,25(OH)(2)D(3)-treated (100 nM) cell lysate. The transport of [(3)H]digoxin, the P-glycoprotein (P-gp) substrate, after treatment with 100 nM 1,25(OH)(2)D(3) for 3 days revealed a higher apparent permeability (P(app)) value in the basal (B)-to-apical (A) direction over that of vehicle treatment (15.1 +/- 0.53 x 10(-6) versus 11.8 +/- 0.58 x 10(-6) cm/s; P < 0.05), whereas the P(app) in the A-to-B direction was unchanged; the efflux ratio was increased (from 5.8 to 8.0). Reduced cellular retention of 5-(and-6)-carboxy-2',7'-dichlorofluorescein, suggestive of higher MRP2 activity, was observed in the 3-day 100 nM 1,25(OH)(2)D(3)-treated cells over controls. Higher protein expression of CYP3A4, MRP2, P-gp, and MRP4 was also observed after a 6-day treatment with other vitamin D analogs (100 nM 1alpha-hydroxyvitamin D(3),1alpha-hydroxyvitamin D(2) or Hectorol, and 25-hydroxyvitamin D(3)) in Caco-2 cells, suggesting a role of 1,25(OH)(2)D(3) and analogs in the activation of enzymes and transporters via the vitamin D receptor.

The effect of trans-10, cis-12 conjugated linoleic acid on gene expression profiles related to lipid metabolism in human intestinal-like Caco-2 cells

We conducted an in-depth investigation of the effects of conjugated linoleic acid (CLA) on the expression of key metabolic genes and genes of known importance in intestinal lipid metabolism using the Caco-2 cell model. Cells were treated with 80 mumol/L of linoleic acid (control), trans-10, cis-12 CLA or cis-9, trans-11 CLA. RNA was isolated from the cells, labelled and hybridized to the Affymetrix U133 2.0 Plus arrays (n = 3). Data and functional analysis were preformed using Bioconductor. Gene ontology analysis (GO) revealed a significant enrichment (P < 0.0001) for the GO term lipid metabolism with genes up-regulated by trans-10, cis-12 CLA. Trans-10, cis-12 CLA, but not cis-9, trans-11 CLA, altered the expression of a number of genes involved in lipid transport, fatty acid metabolism, lipolysis, beta-oxidation, steroid metabolism, cholesterol biosynthesis, membrane lipid metabolism, gluconeogenesis and the citrate cycle. These observations warrant further investigation to understand their potential role in the metabolic syndrome.

The effect of differentiation on 1,25 dihydroxyvitamin D-mediated gene expression in the enterocyte-like cell line, Caco-2

We examined 1,25 dihydroxyvitamin D (1,25(OH)(2)D(3))-induced expression of 25-hydroxyvitamin D(3) 24-hydroxylase (CYP24) and apical calcium channel (TRPV6) mRNA levels in 2-, 9-, and 15-day cultures Caco-2 cells that model proliferating, post-proliferative, and differentiated enterocytes. 1,25(OH)(2)D(3)-induced (10 nM, 8 h) CYP24 and TRPV6 mRNA levels were significantly greater in differentiated and post-proliferative than proliferating Caco-2 cells (>16X and >3X, respectively). Neither CYP24 mRNA half-life nor induction of a -298 bp rat CYP24 promoter-luciferase reporter construct (10 nM 1,25(OH)(2)D(3), 24 h) were different between proliferating and post-proliferating Caco-2 cells. We next tested whether the blunted response of natural genes to 1,25(OH)(2)D(3) in proliferating Caco-2 cells is due to altered chromatin remodeling. VDR and coactivator protein levels do not increase with differentiation but the level of the co-repressor Alien falls by 50% with differentiation. Over-expression of Alien reduced 1,25(OH)(2)D(3)-induced activity of a minimal VDRE containing promoter-luciferase construct by more than 60% in differentiated Caco-2 cells while siRNA knockdown of Alien in proliferating Caco-2 cells increased 1,25(OH)(2)D(3)-induced CYP24 mRNA level by 40%. These observations suggest that Alien is a regulator of VDR-mediated gene transcription in Caco-2 cells. In addition, we found that 1,25(OH)(2)D(3)-induced association of VDR with chromatin and with the CYP24 promoter was lower in proliferating cells. This suggests that decreased recruitment of VDR to vitamin D response elements also contributes to the blunted transcriptional responsiveness to 1,25(OH)(2)D(3) in proliferating Caco-2 cells.

Endocrine regulation of calcium transport in epithelia

1. Calcium (re)absorption occurs in epithelia, including the intestine, kidney, mammary glands, placenta and gills (in the case of fish). 2. Calcium is transported across epithelia by two transport mechanisms, paracellular and transcellular, and the movement is regulated by a complex array of transport processes that are mediated by hormonal, developmental and physiological factors involving the gastrointestinal tract, bone, kidney and the parathyroids. 3. Clear understanding of the calcium transport pathways and their endocrine regulation is critical for minimizing various metabolic and health disorders at different physiological stages. Here, we first briefly review the calcium transport mechanisms before discussing in detail the endocrine factors that regulate calcium transport in the epithelia.

The annexin 2-S100A10 complex and its association with TRPV6 is regulated by cAMP/PKA/CnA in airway and gut epithelia

The formation of a heterotetrameric complex between annexin 2 (anx 2) and S100A10 plays an important role in regulating the cellular distribution and biochemical properties of anx 2. A major distinction between the anx 2-S100A10 complex and other annexin-S100 complexes is that S100A10 binding to anx 2 occurs independently of calcium. Here we describe a cyclic 3',5'-adenosine monophosphate (cAMP) and cAMP-dependent protein kinase (PKA, EC 2.7.1.37)-dependent mechanism regulating anx 2-S100A10 complex formation and its interaction with the transient receptor potential vanilloid type 6 channel (TRPV6) in airway and gut epithelia. In both 16HBE14o- and Caco-2 cells, forskolin (FSK) stimulated increased anx 2-S100A10 complex formation, which was attenuated by either PKA inhibitors or calcineurin A (CnA) inhibitors. The anx 2-S100A10 complex association with TRPV6 was dependent on FSK-induced CnA-dependent dephosphorylation of anx 2. Analysis of the significance of the cAMP/PKA/CnA pathway on calcium influx showed that both PKA and CnA inhibitors attenuated Ca(45) uptake in Caco-2, but not 16HBE14o-, cells. Thus, the cAMP/PKA/CnA-induced anx 2-S100A10/TRPV6 complex may require additional factors for calcium influx or play a role independent of calcium influx in airway epithelia. In conclusion, our data demonstrates that cAMP/PKA/CnA signalling is important for anx 2-S100A10 complex formation and interaction with target molecules in both absorptive and secretory epithelia.

Modulation of the intestinal Ca2+ uptake by a cheese whey protein digest

We have previously constructed a system which enables the search for factors that could modulate the intestinal calcium transporter, CaT1 (TRPV6; Takano et al., Cytotechnology, 43, 113 (2003)). This system evaluates the CaT1-mediated calcium uptake by using CHO cells stably expressing human CaT1 (CHO-hCaT1 cells). We found that a cheese whey protein digest (CWP-D) increased the calcium uptake by the CHO-hCaT1 cells. CWP-D also enhanced the calcium uptake in human intestinal Caco-2 cells. The in vivo effects of CWP-D were then measured by using rats with enteral feeding. Although enteral feeding decreased the portal calcium concentration, CWP-D partially suppressed the decrease, suggesting that CWP-D could be used for food to enhance calcium absorption.

TRP channels in cancer

The progression of cells from a normal differentiated state in which rates of proliferation and apoptosis are balanced to a tumorigenic and metastatic state involves the accumulation of mutations in multiple key signalling proteins and the evolution and clonal selection of more aggressive cell phenotypes. These events are associated with changes in the expression of numerous other proteins. This process of tumorigenesis involves the altered expression of one or more TRP proteins, depending on the nature of the cancer. The most clearly described changes are those involving TRPM8, TRPV6 and TRPM1. Expression of TRPM8 is substantially increased in androgen-dependent prostate cancer cells, but is decreased in androgen independent and metastatic prostate cancer. TRPM8 expression is regulated, in part, by androgens, most likely through androgen response elements in the TRPM8 promoter region. TRPM8 channels are involved in the regulation of cell proliferation and apoptosis. Expression of TRPV6 is also increased in prostate cancer and in a number of other cancers. In contrast to TRPM8, expression of TRPV6 is not directly regulated by androgens. TRPM1 is highly expressed in early stage melanomas but its expression declines with increases in the degree of aggressiveness of the melanoma. The expression of TRPV1, TRPC1, TRPC6, TRPM4, and TRPM5 is also increased in some cancers. The level of expression of TRPM8 and TRPV6 in prostate cancer, and of TRPM1 in melanomas, potentially provides a good prognostic marker for predicting the course of the cancer in individuals. The Drosophila melanogaster, TRPL, and the TRPV1 and TRPM8 proteins, have been used to try to develop strategies to selectively kill cancer cells by activating Ca(2+) and Na(+) entry, producing a sustained increase in the cytoplasmic concentration of these ions, and subsequent cell death by apoptosis and necrosis. TRPV1 is expressed in neurones involved in sensing cancer pain, and is a potential target for pharmacological inhibition of cancer pain in bone metastases, pancreatic cancer and most likely in other cancers. Further studies are required to assess which other TRP proteins are associated with the development and progression of cancer, what roles TRP proteins play in this process, and to develop further knowledge of TRP proteins as targets for pharmaceutical intervention and targeting in cancer.

The effects of Vitamin D metabolites on expression of genes for calcium transporters in human duodenum

The role of Vitamin D in the regulation of calcium absorption in the intestine is well recognized but the mechanisms of the effects on human genes are surprisingly poorly understood. We have determined the expression of transcripts of the apical membrane calcium transporter TRPV6, the cytoplasmic calcium binding protein calbindin-D9k, the basolateral plasma membrane Ca(2+)-ATPase (PMCA1) and the Vitamin D receptor (VDR) in normal endoscopic duodenal mucosal biopsies using quantitative real-time RT-PCR and related baseline expression to Vitamin D metabolites. TRPV6 transcript levels have been shown to be significantly correlated with serum 1,25(OH)(2)D levels in men, but not overall in women, where negative effects of age predominate. TRPV6 and VDR expression were significantly related in both men and women, but were significantly lower in older women. Associations with bone mineral density and fractional calcium absorption were also studied. In a second series of subjects, duodenal biopsies were incubated in organ culture for 6h with Vitamin D metabolites. TRPV6 expression was significantly increased by 1,25(OH)(2)D(3) (10(-9)mol/l) as was PMCA1 to a much smaller extent. TRPV6 expression also increased with 25(OH)D(3). CYP27B1 expression was found in all samples, and CYP24 transcripts were detected after incubation with 1,25(OH)(2)D(3) or 25(OH)D(3).