Vitamin D and calcium insufficiency-related chronic diseases: molecular and cellular pathophysiology

A compromised vitamin D status, characterized by low 25-hydroxyvitamin D (25-(OH)D) serum levels, and a nutritional calcium deficit are widely encountered in European and North American countries, independent of age or gender. Both conditions are linked to the pathogenesis of many degenerative, malignant, inflammatory and metabolic diseases. Studies on tissue-specific expression and activity of vitamin D metabolizing enzymes, 25-(OH)D-1 alpha-hydroxylase and 25-(OH)D-24-hydroxylase, and of the extracellular calcium-sensing receptor (CaR) have led to the understanding of how, in non-renal tissues and cellular systems, locally produced 1,25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)) and extracellular Ca(2+) act jointly as key regulators of cellular proliferation, differentiation and function. Impairment of cooperative signalling from the 1,25-(OH)(2)D(3)-activated vitamin D receptor (VDR) and from the CaR in vitamin D and calcium insufficiency causes cellular dysfunction in many organs and biological systems, and, therefore, increases the risk of diseases, particularly of osteoporosis, colorectal and breast cancer, inflammatory bowel disease, insulin-dependent diabetes mellitus type I, metabolic syndrome, diabetes mellitus type II, hypertension and cardiovascular disease. Understanding the underlying molecular and cellular processes provides a rationale for advocating adequate intake of vitamin D and calcium in all populations, thereby preventing many chronic diseases worldwide.

Vitamin D receptor ablation alters skin architecture and homeostasis of dendritic epidermal T cells

BACKGROUND

1alpha,25-dihydroxyvitamin D(3)[1,25(OH)(2)D(3)], the active metabolite of vitamin D, exerts its activities by binding to the vitamin D receptor (VDR) with subsequent function as a transcription factor. Targeted ablation of the VDR in mice results in rickets and alopecia.

OBJECTIVES

To study the consequences of VDR deficiency for skin physiology, and to investigate the mechanisms of the immunosuppressive effect of 1,25(OH)(2)D(3) on LC.

METHODS

We studied the structural, phenotypic and functional properties of skin and individual skin leucocyte populations in VDR(-/-) mice.

RESULTS

The lack of VDR induced a wide spectrum of pathologies including dermal deposition of collagen, enlargement of sebaceous glands, dilation of the hair follicles, development of epidermal cysts, increased numbers of dendritic epidermal T cells (DETC) and hyperkeratosis. Ageing aggravated these changes. Intriguingly, Langerhans cells (LC) were indistinguishable in distribution, morphology and number compared with controls. In vitro, LC underwent a maturation/migration process similar to LC from control mice. Pretreatment of epidermal cells or LC-enriched epidermal cell suspensions with 1,25(OH)(2)D(3) impaired LC maturation and T-cell stimulatory capacity from VDR(+/+) but not VDR(-/-) mice, demonstrating that LC are targets of vitamin D(3) and that interaction between vitamin D(3) and LC results in a suppression of LC activity.

CONCLUSIONS

Our data imply that VDR expression controls dermal collagen production, hair development and growth, proliferation of sebaceous glands and the homeostasis of DETC. Surprisingly, VDR deficiency does not influence LC phenotype and function.

Vitamin D and calcium deficits predispose for multiple chronic diseases

There is evidence from both observational studies and clinical trials that calcium malnutrition and hypovitaminosis D are predisposing conditions for various common chronic diseases. In addition to skeletal disorders, calcium and vitamin D deficits increase the risk of malignancies, particularly of colon, breast and prostate gland, of chronic inflammatory and autoimmune diseases (e.g. insulin-dependent diabetes mellitus, inflammatory bowel disease, multiple sclerosis), as well as of metabolic disorders (metabolic syndrome, hypertension). The aim of the present review was to provide improved understanding of the molecular and cellular processes by which deficits in calcium and vitamin D cause specific changes in cell and organ functions and thereby increase the risk for chronic diseases of different aetiology. 1,25-Dihydroxyvitamin D(3) and extracellular Ca(++) are both key regulators of proliferation, differentiation and function at the cellular level. However, the efficiency of vitamin D receptor-mediated intracellular signalling is limited by the negative effects of hypovitaminosis D on extrarenal 25-hydroxyvitamin D-1alpha-hydroxylase activity and thus on the production of 1,25-dihydroxyvitamin D(3). Calcium malnutrition eventually causes a decrease in calcium concentration in extracellular fluid compartments, resulting in organ-specific modulation of calcium-sensing receptor activity. Hence, attenuation of signal transduction from the ligand-activated vitamin D receptor and calcium-sensing receptor seems to be the prime mechanism by which calcium and vitamin D insufficiencies cause perturbation of cellular functions in bone, kidney, intestine, mammary and prostate glands, endocrine pancreas, vascular endothelium, and, importantly, in the immune system. The wide range of diseases associated with deficits in calcium and vitamin D in combination with the high prevalence of these conditions represents a special challenge for preventive medicine.

Vitamin D receptor activity and prevention of colonic hyperproliferation and oxidative stress

Unimpaired vitamin D action has been implicated in human cancer prevention. We have previously demonstrated the effectiveness of 1 alpha-dihydroxyvitamin D3 (1,25-D3) to reduce proliferation and increase differentiation in human colon cancer cells. The aim of this study was to investigate, on the one hand, expression of the vitamin D receptor (VDR) and of 25-hydroxyvitamin D(3)-1 alpha-hydroxylase (1 alpha-hydroxylase) in human normal and malignant colonic tissue and, on the other hand, to determine consequences of reduced or lacking VDR action in a VDR knockout mouse model. In low-grade malignancies of the human colon we found increased VDR and 1 alpha-hydroxylase mRNA expression. However, in late-stage high-grade tumors the vitamin D system is severely compromised. In the mouse colon we found an inverse relationship between VDR levels and proliferation in colon descendens, a tissue known to be specifically affected by nutrients during carcinogenesis. Expression of 8-hydroxy-2'-deoxyguanosine (8-OHdG), a marker of oxidative DNA damage, was significantly augmented with complete loss of VDR. These data suggest that genomic 1,25-D(3) action is necessary to protect against nutrition-linked hyperproliferation and oxidative DNA damage.

Characterization of a vitamin D receptor knockout mouse as a model of colorectal hyperproliferation and DNA damage

The vitamin D receptor knockout (VDR-KO) mouse presents with a skeletal phenotype typical for complete lack of genomic 1,25-dihydroxycholecalciferol effects. Our previous data from human colorectal tissue suggest that the steroid hormone and its receptor may have protective function against tumour progression. In order to investigate the relevance of the vitamin D system for pre-malignant site-directed changes in the colon, we characterized the amount and site-specific distribution of the VDR along the large intestine in wild-type (WT), heterozygote (HT) and KO mice. We also evaluated expression of proliferating cell nuclear antigen (PCNA), of cyclin D1 and the levels of 8-hydroxy-2'-deoxyguanosine (8-OHdG), a marker of oxidative stress. In colon ascendens, proliferative cells were dispersed all along the crypt and expression levels of all three markers were high in WT mice. A decrease of VDR expression did not affect expression significantly. In colon descendens, however, fewer proliferative cells were solely located in the lower third of the crypt, and an inverse relationship between VDR reduction, PCNA positivity and cyclin D1 expression was found in HT and KO mice. In parallel to enhanced proliferation a highly significant increase of 8-OHdG positivity occurred. Therefore, the sigmoid colon of VDR-KO mice, fed on an appropriate lactose/calcium-enriched diet to alleviate impaired calcium homeostasis-related phenotypic changes, is an excellent model for investigating induction and prevention of pre-malignant changes in one of the hotspots for human colorectal cancer incidence.

25-hydroxy-vitamin d metabolism in human colon cancer cells during tumor progression

RT-PCR analysis showed elevated expression of 25-hydroxyvitamin D-1alpha-hydroxylase (1alpha-OHase) and of 25-hydroxyvitamin D-24-hydroxylase (24-OHase) in well differentiated human colon carcinomas in comparison to normal mucosa. Further tumor progression is associated with a rise in 1alpha-OHase but with no significant change in 24-OHase mRNA expression. Accordingly, HPLC analysis of 25-hydroxy-vitamin D3 metabolism in freshly isolated tumor cells indicated that well to moderately differentiated colon cancers in situ are able to produce 1alpha,25-dihydroxyvitamin D3 (1alpha,25-(OH)2D3) and convert it through 24-OHase activity into side-chain modified metabolites, 1,24,25-(OH)3-D3 and 1,25-(OH)2- 24-oxo-D3. Likewise, 25-(OH)-D3 is metabolized into 24,25-(OH)2D3, 23,25-(OH)2D3, and 23,25-(OH)2-24-oxo-D3. Poorly-differentiated cancers expressed low levels of 1alpha-OHase mRNA, whereas 24-OHase was even over-expressed. RT-PCR and HPLC analysis of vitamin D metabolism in primary culture cell clones strongly suggested that the extent of endogenously produced 1alpha,25-(OH)2-D3 was inversely related to 24-OHase activity, which could thus limit the antimitotic efficacy of 1alpha,25-(OH)2-D3 particularly at late stages of colon cancer progression.

25-Hydroxyvitamin D(3)-1alpha-hydroxylase and vitamin D receptor gene expression in human colonic mucosa is elevated during early cancerogenesis

Human colorectal cancer cells not only express the nuclear vitamin D receptor (VDR) but are also endowed with 25-hydroxy-vitamin D(3)-1alpha-hydroxylase activity and therefore are able to produce the specific ligand for the VDR, the hormonally active steroid 1alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)). In the present study we show by semiquantitative reverse transcriptase polymerase chain reaction (RT-PCR) as well as by Western blotting and immunohistochemical methods, that in human large intestinal carcinomas expression of the genes encoding the 25-(OH)D(3)-1alpha-hydroxylase as well as the VDR increases in parallel with ongoing dedifferentiation in the early phase of cancerogenesis, whereas in poorly differentiated late stage carcinomas only low levels of the respective mRNAs can be detected. This indicates that, through up-regulation of this intrinsic 1alpha,25(OH)(2)D(3)/VDR system which mediates the anti-mitotic effects of the steroid hormone, colorectal cancer cells are apparently able to increase their potential for an autocrine counter-regulatory response to neoplastic cell growth, particularly in the early stages of malignancy.

Dietary calcium and growth modulation of human colon cancer cells: role of the extracellular calcium-sensing receptor

Using the human colon adenocarcinoma-derived cell line Caco-2, we investigated the possible role of the Ca2+-sensing receptor (CaR) in mediating effects of extracellular Ca2+ on cellular proliferation. Caco-2 cells respond to low ambient [Ca2+]o by activation of the protein kinase C-signaling pathway, leading to upregulation of c-myc mRNA expression and thereby, finally, to alleviation from the G1/S phase control of the cell cycle. This proliferative response can be reverted by activation of the CaR either through raising [Ca2+]o or, respectively, by using the CaR agonist Gd3+ as a substitute for Ca2+. The inhibitory effect of [Ca2+]o on cell replication exhibits saturation kinetics (IC50 = 0.045 mM), indicating the existence of a highly sensitive CaR operating at low ambient [Ca2+]o. Specific immunostaining revealed the presence of CaR-positive cells in the crypt epithelium of normal human colonic mucosa as well as in glandular (i.e., differentiated structures) of carcinomatous lesions. This could provide a rationale for use of calcium supplements for intervention in early phases of colon tumorigenesis.

The mRNA of L-type calcium channel elevated in colon cancer: protein distribution in normal and cancerous colon

Previous reports indicate that the mRNA for the cardiac isoform of the voltage-gated L-type calcium channel (alpha(1C)) is elevated in colon cancer. The aim of these experiments was to verify that the mRNA for alpha(1C) was significantly increased in tumors of two separate populations of patients when compared to normal adjacent mucosa. The second aim was to measure the distribution of alpha(1C) using immunocytochemistry in normal human colon and in colon cancer and to determine what might regulate the channel expression. Biopsies were taken from patients with various stages of colon cancer and nearby normal mucosa were used as control. RNA was prepared and mRNA level measured by semiquantitative reverse transcriptase-polymerase chain reaction. The mRNA of the calcium channel was compared with other markers including beta-actin. The mRNA for alpha(1C) was increased significantly in colon cancers compared to nearby adjacent mucosa. Using confocal microscopy alpha(1C) was localized mainly at the apical membrane in the surface epithelium of normal human colon with less distribution on the lateral and basal membranes. The channel was localized on the lateral and basal membranes in crypt cells. Calcium channel localization appeared to be nearer nuclei in colon cancer samples, in part because of the smaller size of the cells. Likewise, cultured Caco-2 and T84 cells showed a membrane distribution. Western blotting indicated that alpha(1C) protein was increased in nonconfluent cultures of colonic carcinoma cells compared to confluent cells and immunocytochemistry confirms that there is more calcium channel protein in cells that are nonconfluent. We conclude that the increase in mRNA of alpha(1) subunit of the cardiac isoform of the L-type calcium channel may be a useful marker of colon cancer compared to other markers because the increase is large and this increase can be documented on small samples using a simple semiquantitative reverse transcriptase-polymerase chain reaction. We found that alpha(1C) protein is increased when colonic cells are nonconfluent or dividing which may account for the increase in cancer.

In situ mRNA hybridization analysis and immunolocalization of the vitamin D receptor in normal and carcinomatous human colonic mucosa: relation to epidermal growth factor receptor expression

There is evidence that vitamin D receptor (VDR)-mediated action of 1 alpha,25-dihydroxyvitamin D3 (1 alpha,25-(OH)2D3) could limit colon cancer cell growth particularly when induced by activation of the epidermal growth factor receptor (EGFR). We therefore wanted to ascertain the relevance of this observation for human colon cancerogenesis. Utilizing in situ mRNA hybridization and immunocytochemical techniques, we analyzed cell-specific expression of VDR and EGFR in normal and malignant human colonic mucosa. In normal mucosa, VDR positivity is weak and observed only in a small number of luminal surface colonocytes. In contrast, EGFR expression at a relatively high level is also found in cells at the crypt base. The number of VDR-positive colonocytes increases remarkably during tumor progression. It reaches its maximum in low grade adenocarcinomas and returns to lower levels in highly malignant cancers. In both low- and high grade carcinomas, the great majority of tumor cells contain the EGFR message. The relative abundance of EGFR over VDR in normal mucosa and in high grade carcinomas would create a situation in which mitogenic effects from EGFR activation are only ineffectively counteracted by signaling from 1 alpha,25-(OH)2D3/VDR. In contrast, in well to moderately differentiated tumors, upregulation of VDR could retard further tumor progression.

The pattern of prion-related protein expression in the gastrointestinal tract

Prion diseases or transmissible spongiform encephalopathies have been shown to be communicated by oral ingestion of the infectious agent. However, the exact route of transmission is still unknown. In order to better understand the pathophysiology of these diseases, it is crucial to identify cell types of peripheral tissues in which the infectious agent may propagate. Since expression of cellular prion protein (PrPc) is a prerequisite for prion replication, we determined the expression of PrPc in the mucosa of the gastrointestinal tract using immunohistochemistry. Expression of PrPc was negative or weak in the neck region of the gastric mucosa and moderate to strong in crypts of both the small and the large bowel. PrPc was found to be upregulated in the mucosa of patients with Helicobacter pylori gastritis. In contrast, PrPc staining appeared to be downregulated in patients with inflammatory disorders of the large bowel and it remained moderate to strong in inflammatory disorders of the small bowel. Our results support the notion that epithelial cells of the gastrointestinal tract may represent a possible target for prion entry and replication.

Immunocytochemical localization of the extracellular calcium-sensing receptor in normal and malignant human large intestinal mucosa

We identified the parathyroid type Ca(2+)-sensing receptor (CaR) in normal human colon mucosa and in cancerous lesions at the mRNA and protein level. Polymerase chain reaction produced an amplification product from reverse-transcribed large intestinal RNA which corresponded in size and length to a 537-bp sequence from exon 7 of the CaR gene. With a specific antiserum against its extracellular domain, the CaR could be detected by immunostaining in normal human colon mucosa in cells preferentially located at the crypt base. The CaR protein was also expressed in tumors of the large bowel in all 20 patients examined. However, the great majority of CaR-positive cells in the adenocarcinomas inspected were confined to more differentiated areas exhibiting glandular-tubular structures. Poorly or undifferentiated regions were either devoid of specific immunoreactivity or contained only isolated CaR-positive cells. In the normal mucosa and in glandular-tubular structures of cancerous lesions, the CaR was exclusively expressed in chromogranin A-positive enteroendocrine cells and in only a small fraction of PCNA-positive cells.

Biological effects of 1alpha-hydroxy- and 1beta-(hydroxymethyl)-vitamin D compounds relevant for potential colorectal cancer therapy

1alpha,25-dihydroxyvitamin D(3) and two synthetic analogs, 1alpha, 25-dihydroxy-16-ene-23-yne-vitamin D(3) (Ro 23-7553) and 1alpha, 25-dihydroxy-16-ene-24-oxo-vitamin D(3) (JK-1624-3), were tested for their ability to specifically inhibit growth and promote differentiation of human colon cancer cells in comparison with a series of 1beta-(hydroxymethyl) congeners of the natural hormone, such as 1beta-(hydroxymethyl)-3alpha,25(OH)(2)-16-ene,24-oxo-vitamin D(3) (JK-1624-2), 1beta-(hydroxymethyl)-3alpha, 25-dihydroxy-16-ene-26,27-dihomo vitamin D(3) (JK-1626-2), and 1beta-(hydroxymethyl)-3alpha,25-dihydroxy-22,24-diene-26,27- dihomo vitamin D(3) (MCW-EE). Western blot analysis revealed that reduction of cyclin D1 levels is a key mechanism by which the vitamin D compounds under investigation inhibit Caco-2 tumor cell growth. Both the 1alpha-hydroxy- as well as the 1beta-hydroxymethyl-type vitamin D compounds, which exhibit only low affinity for the vitamin D receptor, significantly reduced [(3)H]thymidine DNA labeling in confluent Caco-2 cell cultures. This suggests that high-affinity binding to the vitamin D receptor is not an absolute prerequisite for genomic action on tumor cell growth. Hybrid analogs JK-1624-2 and MCW-EE, although antimitotically active, were rather ineffective in promoting phenotypic differentiation of human colon cancer cells. However, because both compounds also do not promote osteoclast differentiation from hematopoetic bone marrow cells, they still could be used as antimitotic agents in cancer therapy, even at dose levels that, with other analogs, could cause hypercalcemia.

Mechanism of antimitogenic action of vitamin D in human colon carcinoma cells: relevance for suppression of epidermal growth factor-stimulated cell growth

Because the efficacy of 1alpha,25-dihydroxyvitamin D3 [1alpha,25-(OH)2D3] in treatment of colon cancer might critically depend on its ability to specifically counteract epidermal growth factor (EGF)-stimulated tumor cell growth, we utilized human colon adenocarcinoma-derived cells in primary culture as well as the Caco-2 cell line to elucidate possible sites of interaction of 1alpha,25-(OH)2D3 with signaling from EGF receptor activation. In both types of colon cancer cells investigated, 10(-8) M 1alpha,25-(OH)2D3 reduced basal cell proliferation by about 50%, and prevented any rise in proliferation when colon cancer cells were treated with 25 ng/ml EGF: this can be explained by a marked inhibitory effect of 1alpha,25-(OH)2D3 on EGFR mRNA and protein expression. The steroid hormone also seemingly promotes EGF-induced internalization of apical and basolateral membrane EGFR. In addition, 1alpha,25-(OH)2D3 significantly reduced basal and EGF-stimulated expression of cyclin D1 at the mRNA and protein level in primary cultures as well as in the Caco-2 cell line. The ability of 1alpha,25-(OH)2D3 to interfere with a key event in cell cycle control and thereby to block mitogenic signaling from EGF could be seen as advantageous for the potential use of vitamin D compounds in colon cancer therapy.

Induction of epidermal growth factor receptor expression and mitogenesis by alcohol in human colon adenocarcinoma-derived Caco-2 cells

Epidemiologic studies suggest that alcohol may be an inducing factor in human colon tumorigenesis. As colon cells are frequently under autocrine control by growth factors, involvement of the EGFR pathway in alcohol-related colon tumor progression was investigated in the human colon adenocarcinoma-derived cell line Caco-2 which shows EGFR distribution mainly in basolateral cell membranes. EGF treatment results in almost complete downregulation of the basolateral receptor. Low concentrations of ethanol (0.22 mM, 0.1%) however, lead to significantly increased EGFR mRNA and protein expression and a raised mitotic rate mainly in basolaterally treated cells. Alcohol-induced overexpression of EGFR is paralleled by increased cyclin D1 expression. This suggests a possible mechanism for low blood levels of alcohol to stimulate in vivo proliferation of colonocytes by elevating transcription of a growth factor receptor as well as by modifying expression of a cell cycle regulator.

Growth regulation of human colon cancer cells by epidermal growth factor and 1,25-dihydroxyvitamin D3 is mediated by mutual modulation of receptor expression

The human colon adenocarcinoma-derived cell line Caco-2 was used as a model system to study the interaction of epidermal growth factors (EGF) and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) in control of colorectal cancer cell growth. The mitogenic stimulus of EGF was rapidly transduced via apical and basal membrane receptors alike into elevation of c-myc expression, causing a shift of Caco-2 cells from the G0/G1 into the S phase of the cell cycle. The stimulatory effect of EGF on cell division was effectively counteracted by 1,25(OH)2D3: the presence of the steroid hormone prevents the negative effect of EGF on vitamin D receptor abundance and concurrently minimises ligand-occupied EGF receptor numbers on both sides of Caco-2 cell monolayers. Our data suggest that EGF and 1,25-(OH)2D3 actions on mutual receptor levels represent a specific feature of the potent antimitogenic effect of the steroid hormone on colon cancer cells.

Epidermal growth factor receptor expression in primary cultured human colorectal carcinoma cells

In situ hybridization on human colon tissue demonstrates that epidermal growth factor receptor (EGFR) mRNA expression is strongly increased during tumour progression. To obtain test systems to evaluate the relevance of growth factor action during carcinogenesis, primary cultures from human colorectal carcinomas were established. EGFR distribution was determined in 2 of the 27 primary cultures and was compared with that in well-defined subclones derived from the Caco-2 cell line, which has the unique property to differentiate spontaneously in vitro in a manner similar to normal enterocytes. The primary carcinoma-derived cells had up to three-fold higher total EGFR levels than the Caco-2 subclones and a basal mitotic rate at least fourfold higher. The EGFR affinity constant is 0.26 nmol l(-1), which is similar to that reported in Caco-2 cells. The proliferation rate of Caco-2 cells is mainly induced by EGF from the basolateral cell surface where the majority of receptors are located, whereas primary cultures are strongly stimulated from the apical side also. This corresponds to a three- to fivefold higher level of EGFR at the apical cell surface. This redistribution of EGFR to apical plasma membranes in advanced colon carcinoma cells suggests that autocrine growth factors in the colon lumen may play a significant role during tumour progression.

Differentiation-related pathways of 1 alpha,25-dihydroxycholecalciferol metabolism in human colon adenocarcinoma-derived Caco-2 cells: production of 1 alpha,25-dihydroxy-3epi-cholecalciferol

We used the human colon adenocarcinoma-derived cell line Caco-2, which spontaneously differentiates in vitro, as a model system to investigate the metabolism of 1 alpha,25-dihydroxycholecalciferol in colon cancer cells. Subconfluent proliferating and confluent differentiating cells were incubated with 1 microM 1 alpha,25-dihydroxycholecalciferol for a period of 24 to 48 h. HPLC analysis of the lipid extract of both cells and media was performed to isolate and identify the various metabolites of 1 alpha,25-dihydroxycholecalciferol. Undifferentiated, highly proliferating Caco-2 cells metabolized 1 alpha, 25-dihydroxycholecalciferol into several side chain modified metabolites formed through the C-24 oxidation pathway. In contrast, no metabolites of the C-24 oxidation pathway were identified in differentiated Caco-2 cells. However, differentiated cells produced significant amounts of a metabolite which was less polar than 1 alpha, 25-dihydroxycholecalciferol on a straight phase HPLC system. This metabolite was identified as 1 alpha,25-dihydroxy-3alpha-cholecalciferol by comigration with a synthetic standard on two different HPLC systems and gas chromatography--mass spectrometry. Thus, we were able to demonstrate that the state of differentiation has a profound influence on 1 alpha,25-dihydroxycholecalciferol metabolism in colon cancer cells.

Vitamin D increases tight-junction conductance and paracellular Ca2+ transport in Caco-2 cell cultures

We investigated the effects of 1 alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3] on paracellular intestinal Ca2+ absorption by determination of transepithelial electric resistance (TEER), as a measure of tight-junction ion permeability and bidirectional transepithelial 45Ca2+ fluxes in confluent Caco-2 cell cultures. The rise of TEER to steady-state levels of approximately 2,000 omega.cm2 was significantly attenuated by 1,25(OH)2D3 (by up to 50%) in a dose-dependent fashion between 10(-11) and 10(-8) M. Synthetic analogs of 1,25(OH)2D3, namely, 1 alpha,25-dihydroxy-16-ene,23-yne-vitamin D3 and 1 alpha,25-dihydroxy-26,27-hexafluoro-16-ene,23-yne-vitamin D3, exhibited similar biopotency, whereas their genomically inactive 1-deoxy congeners were only marginally effective. Enhancement of transepithelial conductance of Caco-2 cell monolayers by vitamin D was accompanied by a significant increase in bidirectional transepithelial 45Ca2+ fluxes. Although 1,25(OH)2D3 also induced cellular 45Ca2+ uptake from the apical aspect of Caco-2 cell layers and upregulated the expression of calbindin-9kDa mRNA, no significant contribution of the Ca(2+)-adenosinetriphosphatase-mediated transcellular pathway to transepithelial Ca2+ transport could be detected. Therefore stimulation of Ca2+ fluxes across confluent Caco-2 cells very likely results from a genomic effect of vitamin D sterols on assembly and permeability of tight-junctional complexes.

Establishment of primary cultures from human colonic tissue during tumor progression: vitamin-D responses and vitamin-D-receptor expression

Primary cultures derived from pre-cancerous and cancerous human colon tissue are essential for understanding normal and abnormal growth function in the large intestine. Here presented are (i) the methodology for routine establishment of primary cultures of normal, adenoma- and carcinoma-derived cells, and (ii) data for the apparently protective role of vitamin-D compounds in colon carcinogenesis. The steroid hormone 1,25-dihydroxyvitamin D3 and some non-hypercalcemic analogs reduce the high mitotic rate of adenoma cells to that of normal colonocytes. After vitamin-D treatment, tumor cells are less proliferative and differentiation is enhanced. Primary-colon-cancer cultures display a mosaic pattern of vitamin-D-receptor expression, at the mRNA level and at the protein level, with varying intensity of expression in positive cells. This suggests that, in human colorectal tumors in vivo, a large fraction of cells will respond to genomic action of vitamin-D compounds.

Vitamin D metabolism in human colon adenocarcinoma-derived Caco-2 cells: expression of 25-hydroxyvitamin D3-1alpha-hydroxylase activity and regulation of side-chain metabolism

1Alpha,25-dihydroxyvitamin D3 (1alpha,25(OH)2D3) and its synthetic analogues exhibit structure-related variations in their growth inhibitory actions in human colon adenocarcinoma-derived Caco-2 cells. Because this might be caused by differences in resistance against metabolic degradation, we used high performance liquid chromatography (HPLC) analysis to investigate pathways of vitamin D metabolism in two different Caco-2 cell clones. Importantly, when Caco-2 cells were incubated with tritium-labelled 25-hydroxyvitamin D3 (25(OH)D3) for up to 2 h they produced almost exclusively a metabolite, which was identified as 1alpha,25(OH)2D3 by co-chromatography with the synthetic standard in two different HPLC systems, and by a radioligand assay showing an identical binding affinity to the intestinal nuclear vitamin D receptor. Expression of the 25(OH)D3-1alpha-hydroxylase appears to be constitutive because almost identical enzyme activities are observed in any growth phase. 1Alpha,25(OH)2D3 can also activate side chain metabolism in Caco-2 cells: thereby, 1alpha,25(OH)2D3 or 25(OH)D3 are metabolized through the C-24 oxidative pathway into 1alpha,24(R),25(OH)3D3 and 24(R),25(OH)2D3, respectively, which undergo sequential metabolism into 1alpha,25(OH)2-24oxo-D3 and 24-oxo-25(OH)D3. Through C-23 oxidation these intermediary metabolites are further converted into 1alpha,23,25(OH)3-24-oxo-D3 and 23,25(OH)2-24-oxo-D3. Also direct C-23 oxidation of the substrates 1alpha,25(OH)2D3 and 25(OH)D3 generates 1alpha,23(S),25(OH)3D3 and 23(S),25(OH)2D3, respectively. In summary, our results demonstrated the presence of distinct pathways of vitamin D metabolism in Caco-2 cells: apart from metabolizing 1alpha,25(OH)2D3 along the C-24 and C-23 oxidative pathways, Caco-2 cells are able to synthesize 1alpha,25(OH)2D3 from 25(OH)D3 through constitutive expression of 25(OH)D3-1alpha-hydroxylase activity. The relevance of this finding for the intrinsic growth control of neoplastic colonocytes is discussed.

Calcium-dependent c-myc proto-oncogene expression and proliferation of Caco-2 cells: a role for a luminal extracellular calcium-sensing receptor

The human colonic cell line Caco-2 responds to a reduction of ambient [Ca++]o to levels at and below 0.25 mM by a twofold increase in [3H]thymidine labelling of their DNA. [Ca++]o signals sensed preferentially across the luminal aspect of Caco-2 cells, are rapidly (4 h) transduced via PKC activation into up to sixfold increases in c-myc expression. This suggests the presence of a [Ca++]o-sensing membrane receptor (CaR) similar to that described by Brown et al. (1) in parathyroid and kidney cells. By RT-PCR we were able to amplify a 426 bp fragment from Caco-2 mRNA with 98% nucleotide identity to a part of the coding region for the extracellular domain of the parathyroid CaR. Immunohistochemical staining with a monoclonal antiparathyroid CaR antibody demonstrates CaR protein at the plasma membrane of confluent Caco-2 cells. Our results imply that the intestinal CaR is a potential mediator for the transduction of low luminal [Ca++]o into tumor promoting signals in human colonocytes.

Vitamin D receptor and cytokeratin expression may be progression indicators in human colon cancer

Epidemiological data suggest the protective role of vitamin D against the development of colorectal carcinoma in man. This could be due to the anti-mitogenic effect of the steroid hormone on human colon carcinoma cells which is mediated by a specific nuclear vitamin D receptor (VDR). Western blot analysis showed that VDR expression increases during the transition from normal mucosa to polyps and later to pT3 tumors. In later stages, however, VDR is dramatically reduced. Cytokeratin 20, which was monitored as a differentiation marker, decreases in parallel with advancing proliferation and disappears from "normal" mucosa adjacent to later stage carcinoma. Interestingly, VDR density was conspicuously higher in all tumors tested when compared to adjacent "normal" tissue. This suggest that, up to a certain degree of dedifferentiation, malignant colonocytes can upregulate the VDR, probably as a counteractive measure in response to tumor cell growth, but that this ability is finally lost in highly undifferentiated carcinoma cells.

Growth inhibitory effects on human colon adenocarcinoma-derived Caco-2 cells and calcemic potential of 1 alpha,25-dihydroxyvitamin D3 analogs: structure-function relationships

A panel of synthetic analogs of 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3] bearing one or multiple structural modifications at functionally or metabolically sensitive positions of the molecule, i.e., C-1, 16, 23, 26 and 27, were tested for their growth inhibitory and prodifferentiating potency in human colon adenocarcinoma-derived Caco-2 cells. With respect to the peak response elicited at 10(-8) M, 1 alpha,25-dihydroxy-16-ene-vitamin D3, 1 alpha,25-dihydroxy-23-yne-vitamin D3 and 1 alpha,25-dihydroxy-16,23Z-diene-vitamin D3 suppressed [3H]thymidine incorporation in confluent Caco-2 cells less than 1 alpha,25(OH)2D3. 1 alpha,25-dihydroxy-16,23e-diene-vitamin D3 was at least equipotent to the parent compound, whereas 1 alpha,25-dihydroxy-16-ene-23-yne-vitamin D3 and most conspicuously 1 alpha,25-dihydroxy-26,27-hexafluoro-16-ene-23-yne- vitamin D3 reduced growth of Caco-2 cells to significantly (P < .05) lower levels than 1 alpha,25(OH)2D3. The same rank order was obtained for the ability of the vitamin D compounds to induce activity of the differentiation marker enzyme, alkaline phosphatase, in quiescent Caco-2 cells. Whereas the effect of the synthetic analogs on calcium uptake by cultured embryonic chick duodenum in general was less pronounced than that of 1 alpha,25(OH)2D3, the two most potent antimitogenic compounds, 1 alpha,25-dihydroxy-16-ene-23-yne-vitamin D3 and 1 alpha,25-dihydroxy-26,27-hexafluoro-16-ene-23-yne-vitamin D3, elicited calcium mobilization from cultured neonatal mouse calvaria at a 10-fold lower concentration than the parent compound.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth control of human colon-adenocarcinoma-derived Caco-2 cells by vitamin-D compounds and extracellular calcium in vitro: relation to c-myc-oncogene and vitamin-D-receptor expression

The human colon-cancer cell line Caco-2, though of malignant origin, is still able to express the c-myc proto-oncogene in a regulable fashion. Transition from the logarithmic growth phase into the quiescent, i.e., confluent state, is accompanied by a significant increase in the number of cells in the G0/G1 phase of the cell cycle and a concomitant reduction of c-myc mRNA and of nuclear association of c-myc protein. Conversely, growth stimulation by lowering extracellular [Ca++]0 to 0.25 mM results in up-regulation of c-myc expression levels and consequently inhibition of re-entry of Caco-2 cells into the G0/G1 phase. In contrast, regulation of c-myc in Caco-2 cells is completely resistant to vitamin-D sterols, since the anti-mitogenic action of I alpha, 25-dihdroxyvitamin D3 (I alpha, 25(OH)2D3) and of 2 synthetic analogs, I alpha, 25(OH)2-16-ene-23-yne-D3 and I alpha, 25(OH)2-26, 27-F6-16-ene-23-yne-D3, occurred independently of any change in c-myc mRNA and nuclear protein levels. Although the antiproliferative effect of the vitamin-D sterols requires high-affinity binding to the cytoplasmic vitamin-D receptor (VDR), vitamin-D sterols have no effect on VDR mRNA levels in Caco-2 cells. However, VDR mRNA expression changed in an antiparallel fashion to c-myc regulation upon transition between different growth states. This suggests that VDR mRNA abundance could nevertheless be important for vitamin-D-related c-myc-independent growth control in Caco-2 cells.

Growth regulation of human colon adenocarcinoma-derived cells by calcium, vitamin D and epidermal growth factor

The Caco-2 cell line was utilized to analyze the role of nutrient factors such as calcium, vitamin D and epidermal growth factor (EGF) in epigenetic control of human colon carcinoma cell growth. Proliferative signals from either low extracellular calcium or EGF, respectively, are transduced in Caco-2 cells via an increase in c-myc proto-oncogene mRNA and nuclear protein expression levels. Activation of the EGF receptor is associated also with down-regulation of the cytoplasmic high-affinity vitamin D receptor (VDR). This would allow colon carcinoma cells to escape from the VDR-mediated anti-mitogenic action of 1 alpha, 25-dihydroxyvitamin D3 (1 alpha,25(OH)2D3). However, Caco-2 cells have the unique property to synthesize the vitamin D hormone from 25-hydroxyvitamin D3. 1 alpha,25(OH)2D3, in turn, counteracts the negative effect of EGF on VDR abundancy and slow down tumor cell proliferation through a c-myc-independent pathway.

Interspecies comparative pathology of colorectal neoplasms: relevance for treatment

Comparative pathology may serve as a practical tool for therapy by comparison of normal and abnormal structures of the digestive tract in animals and men. A better understanding of colon cancer as the most common solid neoplasm after lung cancer in the industrialized world is sought. In the so-called developed nations and in animals colon cancer is less frequent. The pathogenesis of colon cancer involves environmental and genetic factors. Several types of colorectal cancer can be discerned and the species distribution ranges from invertebrates to man. Colorectal neoplastic progression is species-specific. An intraspecies-specific comparison of large bowel cancer is also valuable. Alteration of signal transduction pathways and somatic mutations of oncogenes are described, as well as the occurrence, research and current treatment. Metastasis of neoplasms of the colon and of the rectum can be studied by intraspecies-specific comparison. Sections of this review deal with vitamin D and cancer and close with present therapies for colorectal cancer.

Suramin is a potent inhibitor of calcemic hormone- and growth factor-induced bone resorption in vitro

Suramin, a polysulfonated naphtylurea compound that has been used in the past for treatment of trypanosomiasis and onchocerciasis, is also an effective antitumor agent. Its marked antiproliferative potential probably resides in the ability of the drug to interfere with various growth factor signaling mechanisms. We were interested in whether suramin could also interact with signal transduction in bone cells, leading to osteoclast proliferation and, consequently, bone resorption. Utilizing organ-cultured neonatal mouse calvaria, we studied the effect of suramin on bone resorption induced by, for example, parathyroid hormone, 1,25-dihydroxycholecalciferol, epidermal growth factor or thrombin. In the 1 to 100 microM concentration range, in which no toxic effect on bone cells was observed, suramin effectively suppressed bone resorption regardless of whether it was mediated by endogenous prostaglandin production or induced by parathyroid hormone (Ki = 70 microM), 1,25-dihydroxycholecalciferol (Ki = 70 microM), epidermal growth factor (Ki = 5 microM) or thrombin (Ki = 5 microM). The profound inhibitory effect of suramin on various bone resorptive processes around 100 microM, which is regarded as the minimally effective concentration for successful anticancer treatment, could be exploited for the treatment particularly of tumors associated with hypercalcemia.

Growth inhibition of human colon adenocarcinoma-derived Caco-2 cells by 1,25-dihydroxyvitamin D3 and two synthetic analogs: relation to in vitro hypercalcemic potential

The effect of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and of two synthetic analogs, 1,25S,26-tri-hydroxy-delta 22-vitamin D3 (1,25,26(OH)3-22ene-D3, Ro 23-4319) and 1,25-dihydroxy-delta 16-23yne-vitamin D3 (1,25(OH)2-16ene-23yne-D3, Ro 23-7553) on cell growth was evaluated by determination of [3H]thymidine incorporation into DNA of human colon adenocarcinoma-derived Caco-2 cells. The extent of growth inhibition by the vitamin D compounds varied between 20-40% (at 10(-8) M), depending on particular growth conditions of Caco-2 cells as well as on the molecular structure of the vitamin D sterols. In confluent, i.e., rather quiescent cells, all three vitamin D compounds were equipotent in suppressing growth. In rapidly dividing log phase cells, 1,25(OH)2-16ene-23yne-D3 or 1,25,26(OH)3-22ene-D3 were ten or five times, respectively, more efficient than 1,25(OH)2D3. A substantial effect on induction of the colonocyte differentiation marker alkaline phosphatase was only elicited by 1,25(OH)2-16ene-23yne-D3. The ability of the vitamin D compounds to raise intestinal calcium absorption was evaluated by determination of 45Ca2+ accumulation in embryonic chick duodenal explants. In this assay, both synthetic analogs were less effective than 1,25(OH)2D3 by a factor of 20. The intrinsic bone resorbing activities of the vitamin D analogs were compared in organ-cultured neonatal mouse calvariae. The most effective antiproliferative compound, 1,25(OH)2-16ene-23yene-D3, stimulated calcium release from cultured bones at concentrations less than 10(-11) M, and was thus ten times more potent than 1,25(OH)2D3 and hundred times more than 1,25,26(OH)3-22ene-D3.

Inhibition of sucrose-isomaltase expression by EGF in the human colon adenocarcinoma cells Caco-2

To investigate the role and mechanism of action of epidermal growth factor (EGF) in the intestinal epithelium, we have studied its influence on proliferation and differentiation of Caco-2 cells, a human colon adenocarcinoma cell line exhibiting several characteristics of adult small intestinal enterocytes. A clone of Caco-2 cells synthesizing minimal amounts of transforming growth factor-alpha (TGF-alpha)/epidermal growth factor (EGF)-like activity was used in these studies. Cells grown in the presence of 20-200 ng EGF/ml exhibited increased DNA synthesis and proliferation; formation of morphologically poorly differentiated multilayers was observed at 200 ng EGF/ml. At all concentrations tested EGF produced a significant and marked reduction in sucrase activity, whereas other brush-border enzymes (aminopeptidase N, alkaline phosphatase, dipeptidylpeptidase IV) were only marginally affected. EGF influenced sucrase expression at two different levels. At 20 ng/ml, it affected primarily sucrase-isomaltase processing in the endoplasmic reticulum and/or increased its degradation. At 200 ng EGF/ml, a significant and marked reduction in sucrase-isomaltase mRNA levels and biosynthesis was observed. These results demonstrated that EGF has important and selective effects on Caco-2 cell proliferation and differentiation and may affect different cellular activities depending on its concentration.

1,25-Dihydroxycholecalciferol-related Na+/D-glucose transport in brush-border membrane vesicles from embryonic chick jejunum. Modulation by triiodothyronine

1,25-Dihydroxycholecalciferol, when present at and above 10 nM in an organ-culture system of embryonic chick jejunum, approximately doubled the rate of Na(+)-gradient-driven D-glucose uptake by brush-border membrane vesicles, but had no effect on Na(+)-independent D-glucose transfer. The sterol also had no effect on Na+ influx along an outside/inside Na+ gradient ([Na+]o = 100 mM; [Na+]i = 0 mM). This renders it unlikely that in embryonic intestine, calcitriol raises Na(+)-dependent D-glucose transport through changes in the electrochemical Na+ gradient. D-[U-14C]Glucose tracer exchange, measured under voltage-clamp condition at Na+/D-glucose equilibrium, revealed that addition of calcitriol to the culture medium approximately doubled the activity of the Na+/D-glucose transporter in the brush-border membrane. This was also reflected by an corresponding increase in the maximal velocity of the transfer process. Increased [3H]phlorizin binding after calcitriol treatment suggests that the steroid hormone activates Na+/D-glucose transport through increasing the number of carrier molecules in the brush-border membrane. 10 nM triiodothyronine, which by itself has no effect on Na(+)-dependent D-glucose transport, potentiated the effect of 1,25-dihydroxycholecalciferol such that in the presence of both hormones, Na+/D-glucose-carrier activity was increased fourfold above control levels.

Antiproliferative effect of 1,25-dihydroxyvitamin D3 and its analogs on human colon adenocarcinoma cells (CaCo-2): influence of extracellular calcium

Depending on culture in either "low Ca++" (0.25 mM) or "normal Ca++" (1.8 mM) medium, human colon adenocarcinoma-derived CaCo-2 cells exhibit differential sensitivity to the antiproliferative action of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and of two side-chain modified analogs, 1,25S,26-trihydroxy-delta 22-vitamin D3 (Ro 23-4319) and 1,25-dihydroxy-delta 16-23yne-vitamin D3 (Ro 23-7553). CaCo-2 cells cultured under low Ca++ conditions exhibit a high proliferative potential, and in these cells, all vitamin D compounds under investigation significantly inhibit [3H]thymidine incorporation into cellular DNA at greater than or equal to 10(-10) M. The rank order of biopotency is: Ro 23-7553 greater than or equal to Ro 23-4319 greater than 1,25(OH)2D3. At 1.8 mM Ca++, only Ro 23-7553 is able to inhibit proliferation of CaCo-2 cells. Parallel to their antiproliferative action, all three vitamin D compounds stimulate akaline phosphatase activity in CaCo-2 cells, indicating their ability to induce differentiated functions at the same time as they reduce neoplastic cell growth.

Differentiation-dependent expression of calcitriol actions on absorptive processes in cultured chick intestine: modulation by triiodothyronine

Embryonic chick jejunum maintained in organ-culture exhibits a characteristic stage-specific pattern of responses to calcitriol and T3. Whereas induction of luminal Na+/inorganic phosphate and Na+/D-glucose transport by calcitriol was only possible at an advanced state of differentiation prior to hatching on day 20, the sterol induced cellular calcium transport with high efficiency even in undifferentiated enterocytes in day 15 embryonic intestine. T3 had no effect at all on calcium transport, but induced Na+/inorganic phosphate transport at all stages of epithelial maturation. In contrast, Na+/D-glucose transport was effectively induced by T3 only in relatively immature intestinal epithelium. T3, at a medium concentration of 10(-8) mol/l, in a permissive fashion potentiated the effects of calcitriol (10(-10)-10(-7) mol/l) on calcium transport as well as on Na+/inorganic phosphate and Na+/D-glucose transfer. Thereby, T3 facilitated induction of transport activities by calcitriol against differentiation-related restraints. By facilitating the expression of genomic actions of calcitriol, T3 may thus play an important role in the regulation of calcium and phosphate metabolism.

A high yield preparation of brush border membrane vesicles from organ-cultured embryonic chick jejunum: demonstration of insulin sensitivity of Na(+)-dependent D-glucose transport

Although embryonic chick small intestinal segments provide a very limited amount of tissue for preparation of enterocyte brush border membrane vesicles (BBMV), we were able to develop a procedure for isolation of BBMV from cultured 20-d-old embryonic chick jejunum in high yield by modifying a divalent cation precipitation method. Total yield of the brush border marker enzyme alkaline phosphatase in the vesicle fraction as compared to the crude homogenate was approximately 40%, and the specific activity of the enzyme was increased 25-fold on the average. The brush border membrane vesicle fraction was only contaminated with other cellular organelles (basolateral membranes, mitochondria, lysosomes or endoplasmic reticulum) to a minor extent. Functional integrity of the brush border vesicles was indicated by Na+ gradient-driven electrogenic D-glucose transport leading to concentrative transfer (overshoot) of the sugar into an osmotically active intravesicular space. When jejuna were cultured for 48 h in the presence of 10(-6) mol/L insulin, the initial rate of Na(+)-dependent D-glucose uptake by brush border membrane vesicles as well as Na(+)-dependent [3H]phlorizin binding to brush border membranes was approximately twice as high as in vesicles from untreated controls. This strongly suggests that insulin could enhance intestinal absorption of D-glucose by increasing the intrinsic activity of the Na(+)-dependent D-glucose transport system at the luminal membrane of enterocytes.

Triiodothyronine stimulates 2-deoxy-D-glucose uptake by organ cultured embryonic chick small intestine

The possible contribution of increased D-glucose absorption from the intestine to the impairment of oral glucose tolerance in hyperthyroidism was evaluated by investigating the influence of T3 on different pathways of D-glucose transport, utilizing an organ culture system of embryonic chick small intestinal explants. T3, when present in the culture medium at a concentration between 10(-10)-10(-8) mol/l, had no effect on uptake of alpha-methyl-D-glucoside, but stimulated uptake of 2-deoxy-D-glucose by the intestinal epithelium in a dose-dependent fashion. T3 thereby enhanced the maximal velocity of a saturable, cytochalasin B-sensitive but phloretin-insensitive 2-deoxy-D-glucose transport system with an apparent Km of 7 mmol/l. The combined data are consistent with the assumption that T3 can enhance D-glucose entry into the intestinal epithelium through stimulation of a low-affinity transport system at the brush-border membrane of enterocytes. Our findings provide a basis for the explanation of adaptive modulation of intestinal glucose absorption in hyperthyroidism.

D-glucose uptake is increased in jejunal brush-border membrane vesicles from hyperthyroid chicks

Jejunal brush-border membrane vesicles were harvested from 4-week old chicks whose thyroid status had been altered either by a daily injection of 20 micrograms T3 for 1 week or which through the preceding 4 weeks had received propylthiouracil and that had been repleted with either 20 or 80 micrograms T3 in divided doses within 48 h. T3 markedly stimulated D-glucose uptake in brush-border membrane vesicles in the presence of an outside/inside (100/0 mmol/l) Na+ gradient. T3 administration had no detectable influence on the Na+ permeability of the isolated vesicles. The effect of the thyroid hormone on Na+ gradient-driven D-glucose uptake was fully preserved at zero transmembrane potential difference. These findings exclude that T3 stimulates Na+-dependent D-glucose transport in the small intestine through changes of the electrochemical Na+ gradient or through alteration of the transmembrane potential difference. Tracer exchange experiments under equilibrium and voltage-clamp conditions revealed a significantly shorter half-time of D-glucose uptake in brush-border membrane vesicles from T3-treated chicks. Kinetic analysis showed that T3 administration significantly increases the apparent maximal velocity of D-glucose transport in brush-border membrane vesicles, whereas the apparent Km values were virtually unaltered. From these data we conclude that T3 increases the activity of Na+-dependent D-glucose carriers in the brush-border membrane. This is interpreted as consistent with a greater rate of D-glucose absorption from the intestinal lumen under conditions of hyperthyroidism.

Calcium and inorganic phosphate transport in embryonic chick intestine: triiodothyronine enhances the genomic action of 1,25-dihydroxycholecalciferol

The influence of triiodothyronine (T3) on the induction of intestinal calcium and inorganic phosphate (Pi) transport by 1,25-dihydroxycholecalciferol (1,25-(OH)2D3) was studied in 48 h cultures of embryonic chick jejunum. While T3 alone had no effect on calcium uptake by gut segments cultured on d 20 of embryonic development, the thyroid hormone amplified the effect of 1,25-(OH)2D3 on calcium transport and effectively shifted the dose-response curve to lower 1,25-(OH)2D3 concentrations. T3 had a dual effect on Pi uptake by cultured jejunum: It induced transport activity even in the absence of the steroid hormone, and, in addition, synergistically raised 1,25-(OH)2D3-related Pi uptake. In d 17 embryonic small intestine, which does not respond to 1,25-(OH)2D3 by a significant increase in Pi transport, T3 permitted the induction of Pi transport by the sterol. In general, the thyroid hormone enhanced the responsiveness of cultured embryonic intestine toward 1,25-(OH)2D3 by two orders of magnitude, resulting in facilitated induction of calcium and Pi transport by the sterol and, in particular, modulated the stage-specific expression of 1,25-(OH)2D3 action on intestinal Pi transport.

Calcitriol-dependent, paracellular sodium transport in the embryonic chick intestine

1,25-Dihydroxyvitamin D3 (calcitriol), or vitamin D3 itself, when added to cultures of 20-day-old embryonic chick small intestine, stimulated sodium (Na+) uptake from the mucosal surface. The calcitriol-mediated increase in Na+ uptake appeared to be related to increased tight-junctional or paracellular permeability. Support for this conclusion was, first, that the uptake of other ions, potassium (K+) and rubidium (Rb+), with tight-junctional permeabilities greater than Na+, was also stimulated by calcitriol, and second, perturbation of cellular Na+ and K+ fluxes by inhibition of Na+/K+-ATPase activity did not affect calcitriol-stimulated Na+, K+, or Rb+ transport. Calcitriol stimulation of Na+ fluxes across the brush border as an alternate possibility is unlikely for the following reason: the calcium ionophore A23187, while mimicking the stimulatory action of calcitriol on calcium (Ca2+) uptake, reduced epithelial Na+ uptake. It is therefore suggested that calcitriol, by virtue of its effect on Ca2+ transport, reduces rather than stimulates cellular Na+ uptake.

Intestinal phosphate and calcium absorption: joint regulation by thyroid hormones and 1,25-dihydroxyvitamin D3

Thyroxine (T4) and triiodothyronine (T3) activate Na+-dependent inorganic phosphate (Pi) transport in organ-cultured embryonic chick small intestine. Induction of transport activity requires intact protein synthesis and can be expressed in enterocytes with varying degrees of differentiation. T3 and T4 exert their effect independent of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), which is shown to stimulate Pi uptake only in the final stage of embryonic differentiation. At this time point, a potentiating effect of 1,25(OH)2D3 and T4 on Pi transport in cultured jejunum can be demonstrated. Thyroid hormones appear to stimulate Na+ gradient-driven Pi transport without concomitantly raising (Na+-K+)-ATPase activity. T4 has no influence whatsoever on calcium uptake by cultured embryonic small intestine while 1,25(OH)2D3 is effective at all stages of embryonic development investigated (day 15-20). However, when both hormones were present in the culture medium, the effect of 1,25(OH)2D3 on calcium transport is doubled. Our results suggest that the hyperphosphataemia associated with hyperthyroidism is likely to result, at least in part, from the independent effect of thyroid hormones as well as from their potentiation of the 1,25(OH)2D3 action on Na+-dependent intestinal Pi transport. In addition, their permissive effect on 1,25(OH)2D3-induced calcium absorption provides an explanation for unaltered calcium absorption in a number of hyperthyroid patients, although reduced plasma levels of 1,25(OH)2D3 are generally observed in this condition.

Vitamin D stimulates (Na+ + K+)-ATPase activity in chick small intestine

Vitamin D3 and 1,25-dihydroxyvitamin D3 raise (Na+ + K+)-ATPase activity (ouabain-sensitive 86Rb+ uptake) in cultured embryonic and 4-week-old chick small intestine. Vitamin D stimulation of the sodium pump, which requires genomic action of the sterol, may lead to enhanced Ca2+ extrusion via a basolateral Na+/Ca2+ exchange mechanism, and, in addition, may provide a proliferative signal in undifferentiated enterocytes.

Differential response of enterocytes to vitamin D during embryonic development: induction of intestinal inorganic phosphate, D-glucose and calcium uptake

Chick intestine maintained in organ culture at day 20 of embryonic life responds to appropriate dose levels of 1,25-dihydroxyvitamin D3 or vitamin D3 in the culture medium by stimulation of Na+-dependent uptake of inorganic phosphate (Pj) and D-glucose as well as by increased calcium absorption. The intensity of each single response to vitamin D varies along the small intestine: Only the jejunum shows high vitamin D increments of all three transport systems under investigation. Inhibition of vitamin D responses by actinomycin D suggests the involvement of gene activation in the sterol's action of raising intestinal transport activities. Determination of the developmental pattern of each vitamin D-dependent transport system showed that only calcium absorption could be induced in undifferentiated cells on day 15, while complete expression of vitamin D action, including elevation of Na+-dependent uptake of D-glucose and Pj, required a fairly advanced state of epithelial maturation and was thus not observed before the final day 20 of embryonic development. This indicates a progressive change of hormone sensitivity of absorptive cells during differentiation.

Stimulation of D-glucose transport. A novel effect of vitamin D on intestinal membrane transport

Vitamin D stimulates absorption of D-glucose in chick jejunum and ileum by a specific action on the maximal velocity of Na+-gradient driven D-glucose transport across the brush-border membrane of intestinal cells. Induction of D-glucose transport by either vitamin D-3 or 1,25-dihydroxyvitamin D-3 in embryonic intestine can be blocked by inhibitors of RNA and protein synthesis.