Vitamin D Impact on the Liver and Kidney

Vitamin D bushel on the Liver and KidneyRevised vitamin D copySources and forms of vitamin DVitamin D, excessively termed calciferol, is a fat-soluble secosteroid compound that is an essential restrictive work out for atomic number 20 and ortho inorganic orthophosphate metamorphosis in humans and animals. Its biological functions involve a physiological save in tog out formation and mineralization, muscle contraction, nerve head modulation and transmission as sanitary as m both cellular metabolic government issueuates in various variety meat. There atomic number 18 ii forms of vitamin D that argon metabolically important vitamin D2 or ergocalciferol and vitamin D3 or cholecalciferol. The nutritional sources of both forms are limited to certain types of fodders that naturally stick out vitamin D and thence it is added to some foods as a supplement.1.1.1- Exogenous (Diet) some(prenominal) forms of vitamin D (D2 and D3) are exogenously obtained in showtime quantit ies from some types of food in the diet. Vitamin D2 is rare as it is produced from fungal and plant sources much(prenominal) as mushrooms and cereals, as a allow for of irradiation, by ultraviolet photons, of the plant sterol ergosterol. When these foods are ingested, ergocalciferol is absorbed into the gillyflower. Vitamin D3 , on the opposite hand, is available in very(prenominal) low amounts from animal sources including oily fish much(prenominal) as pink-orange and mackerel other sources include meat, colorful, cheese, cod liver oil, eggs and fortified foods much(prenominal) as margarine and milk (Holick, 2006 Engelsen et al., 2005 Nowson et al., 2004). Farmed salmon, for example, contains only 25% of the vitamin D levels birth in wild salmon, however, the amount of vitamin D in plunderned food may affected by modern demonst respecting methods (Chen et al., 2007).1.1.2- EndogenousIn humans the principal precursor of vitamin D3 is cholesterol which is obtained from the diet. Cholesterol is initially converted to 7-dehydrocholesterol, provitamin D3, finished the action of enzymes termed the mucosal dehydrogenase complex, present in the abject intestine. Provitamin D3, is thence incorpo wood pussyed within chylomicrons and transported to the skin where temperature dependent photoisomerisation cognitive processing of 7-dehydrocholesterol takes place in cuticular cells resulting in the toil of D3. Within the epidermal cells, vitamin D3 undergoes photocoversion to its isomers 5,6-transvitamin D3 and suprasterol, a process which relies on the amount of ultraviolet radiation absorbed inadequate sunshine exposure compromises this process (Holick, 2003 Iqbal, 1994). Sunlight exposure is therefore a of import element in the regulation and parentment of endogenous cholecalciferol achievement (Dusso, et al., 2005 Iqbal, 1994 Reichel, et al., 1989 Smith, 1988). erstwhile photoconversion is completed, cholecalciferol binds to Vitamin D Binding Prote in (VDBP) and transported to the liver for further metabolic processing.Vitamin D metamorphosisBoth forms of vitamin D (D2 and D3) undergo similar metabolic activation in the liver and kidney respectively to produce the physiologically active form 1,25-dihydroxyvitamin D3.1.2.1- undressThe skin is characterized by two layers, the outer epidermal region, consisting of some(prenominal) strata, and the inner(a) dermal layer. Skin exposure to UVB rays in sunlight, characterized by a wavelength of 290 nm to 315 nm, allows the initial steps of vitamin D price reduction to occur using the substratum 7-dehydrocholesterol (7-DHC) as illustrated in step 1 of icon 1. UVB absorption by 7-DHC is thought to occur actively in the stratum basale and stratum spinosum regions of the epidermal layer. The substrate 7-DHC is an important intermediate of cholesteryl ester biosynthesis from squalene. Du ingroup the reaction, 7-DHC forms procholecalciferol done B ring opening of the steroid body construction. This transition state is relatively unstable and shag further undergo photocatalyzed reactions to form lumisterol and tachysterol (Wolpowitz and Gilchrest, 2006). Lumisterol and tachysterol feed been shown to prevent vitamin D attain intoxicating levels and do non brace any transfer vitamin D onuss (Bouillon et al., 1998). In addition to this protective utensil, previtamin D poisoning is likewise prevented because this is an equilibrium reaction that allows cholecalciferol to revert back to 7-DHC (Webb, 2006). Cholecalciferol (previtamin D3) is produced upon double bond rearrangement of procholecalciferol and body in the extracellular space where it becomes bound to the ubiquitous VDBP (Holick, 2005).Figure1. Sources and steps of vitamin D synthesis in the three major sites skin, liver and kidney (Figure obtained from Wolpowitz and Gilchrest, 2006).1.2.2- LiverCholecalciferol that has been transported to the liver undergoes the first step of its bioactivatio n, the hydroxylation of carbon 25 (Dusso, et al., 2005) by two liverwort enzymes the microsomal and mitochomdrial 25-hydroxylases (Deluca et al., 1990). In hepatic cellular microsomes and mitochondria, vitamin D3 is hydroxylated at carbon 25 and transformed to 25-hydroxyvitamin D3 by both 25-hydroxylase enzymes. This enzyme complex requires the presence of essential catalytic cofactors including nicotinamide adenine dinucleotide phosphate (NAPDH), flavin adenine dinucleotide (FAD), ferredoxin and molecular oxygen for this reaction to occur (Sahota and Hosking, 1999 Ohyama et al., 1997 Kumar, 1990). Recently, large numbers of hepatic cytochrome P-450 enzymes exhibiting 25-hydroxylase action have been set in vitamin D activation nerve tracts these enzymes include CYP27A1, CYP3A4, CYP2D25 and CYP2R1 (Dusso, et al., 2005 Cheng et al., 2003 Sawada et al., 2000). However, CYP2R1 is believed to be the principal enzyme in the hepatic pathway and the presence of a ingredienttic mutat ion in its gene may compromise the outcome of this process both CYP27A1 and CYP2D25 demonstrate noble cleverness and low affinity features, therefore, their action is considered in pregnant in this pathway (Dusso, et al., 2005 Cheng et al., 2003 Sawada et al., 2000). This metabolic step is inefficiently regulated, i.e. the levels of 25-hydroxy vitamin D are elevated as dietetical inlet of vitamin D profits. Consequently, over 95% of 25-hydroxyvitamin D in serum circulates as 25-hydroxyvitamin D3 which has a half-life of approximately three weeks, and is therefore used in the assessment of vitamin D status (Dusso, et al., 2005 Reichel et al., 1989). The metabolically apathetic 25-hydroxyvitamin D3 is then transported to the kidney for the second step of its bioactivation.1.2.3- KidneyThe second step of vitamin D3 bioactivation takes place at the proximal convoluted tubule of the kidney. Hydroxylation occurs at C-1 of 25-hydroxyvitamin D3 whereby the spicyly active 25-hydroxyv itamin D3 1--hydroxylase (CYP27B1) incorporates a hydroxyl group to deoxycytidine monophosphate-1 of the first ring to form the biologically active metabolite 1,25-dihydroxyvitamin D3 (Holick,2006 Dusso, et al., 2005 Deluca et al, 1990 Reichel, et al., 1989). The high activeness of 1--hydroxylase (CYP27B1) present in kidney is non unique to this organ and can in any case be found in some other organs (Bouillon, 1998). The nephritic hydroxylation of 25-hydroxyvitamin D3 is the rate-limiting step in the turnout of 1,25-dihydroxyvitamin D3 and is well regulated. An alternative pathway of hydroxylation of 25-hydroxyvitamin D3 within nephritic mitochondria takes place at Carbon-24 to form 24,25-dihydroxyvitamin D3 which is metabolically inert. This process is catalyzed by nephritic 24--hydroxylase in reaction to 1--hydroxylase suppression. However, 24--hydroxylase not only gos the addendum of the hydroxyl group at Carbon-24 but also levys the dehydrogenation of 24,25-dihydroxy vitamin D3 and hydroxylation at Carbon 23 and 26 (Sahota and Hosking, 1999 Bouillon, 1998 Reichel, et al., 1989). Renal hydroxylases require the presence of catalytic cofactors that enhance their synthetic activities during this process. Figure 2 shows the details of vitamin synthesis including the enzymes and cofactors required for distributively step.Figure2. Enzymes, cofactor and intermediates compounds of vitamin D metabolism (Bouillon et al. 1998)1.2.4- Regulation of vitamin D metabolismNumbers of factors have been demonstrated to be important in the regulation of vitamin D metabolism particularly significant its regulation through renal achievement. The factors involved in this regulation comprise parathyroid hormone (PTH), calcitonin, dietetical calcium and phosphate, insulin and insulin-like ontogeny factor and 1,25-dihydroxyvitamin D3 itself (Holick,2006 Deluca, 2004 Sahota and Hosking, 1999). Key fundamental interactions of vitamin D with its sense organ are cognise t o initiate gene regulation. These mechanisms have been studied using vitamin D analogues which have revealed the mechanism of assembly of transcriptions factors and promotion of gene regulation by this scrap (Cheng et al., 2004 Wu et al., 2002). Figure 3 shows the effect of various regulators on vitamin D metabolism.Figure 3 Alternate pathway for vitamin D3 under different metabolic conditions of low mineral Ca and P levels, PTH concentration and secretion of GH / IGH (Figure obtained from Gomez, 2006).1.2.4.1- parathyroid secreter HormoneParathyroid hormone (PTH) is the primary regulator of renal 1,25-dihydroxyvitamin D3 formation (Holick, 2006 Dusso et al., 2005 Bouillon et al., 1998 Issa et al., 1998). PTH regulates 1,25-dihydroxyvitamin D3 production instantaneously through enhancing 1--hydroxylase application within kidney cells and increasing the genetic transcription rate of renal proximal tubular 1--hydroxylase both of which result in an improver in the renal 1,25-dihyd roxyvitamin D3 production rate. High levels of 1,25-dihydroxyvitamin D3 suppress the enzyme transcription activeness and PTH concentration. Thus, renal 1,25-dihydroxyvitamin D3 has a negative feedback reply on PTH secretion, providing an efficient restrictive control of renal 1,25-dihydroxyvitamin D3 homeostasis (Dusso, et al., 2005 Holick,2003 Sahota and Hosking, 1999 Reichel, et al., 1989 Iqbal, 1994).1.2.4.2- atomic number 20Dietary calcium exhibits a direct regulatory influence on renal 1--hydroxylase body process via fluctuating serum calcium concentration and indirectly via its effect on serum PTH concentration. Calcium exerts its effect through calcium-sensing receptor (CaR) activation within the parathyroid gland and renal proximal tubules cells in result to low calcium concentration. Thus, the low intracellular calcium levels premise to increased production of 1,25-dihydroxyvitamin D3 within renal cells (Ramasamy, 2006 Bland et al., 1999 Chattopadhyay et al., 1996). On the other hand, it has been shown that high calcium concentrations markedly flub renal 1,25-dihydroxyvitamin D3 formation in human nephrotic cell cultures and in parathyroidectomised animals (Bland et al., 1999 Chattopadhyay et al., 1996). An increase in extracellular calcium indirectly suppresses 1,25-dihydroxyvitamin D3 production at the proximal convoluted tubule by inhibiting PTH release (Deluca, 2004 Carpenter, 1990). However, the detailed mechanism of calcium-sensing receptors (CaR) activation is not yet fully understood (Dusso, et al., 2005 Hewison, et al., 2000).1.2.4.3- PhosphateDietary phosphate intake and serum phosphate concentrations exhibit regulatory effects on 1,25-dihydroxyvitamin D3 production in proximal renal tubules. This effect has been demonstrated in several studies which showed that a decrease in dietary phosphate accelerated renal formation of 1,25-dihydroxyvitamin D3, but did not directly affect 1, 25-dihydroxyvitamin D3 catabolism. Conversely, elevate d serum phosphate and increased phosphate intake led to decreased production of 1, 25-dihydroxyvitamin D3 (Carpenter, 1989 Reichel et al., 1989). Several studies have shown that inorganic phosphate levels have no significant direct effect on mitochondrial 1--hydroxylase body process in polite renal cells in the short term, suggesting that the action of inorganic phosphate is not liaise via changes in PTH and Calcium concentrations and is possibly inducted by other hormones such(prenominal) as growth hormone, insulin and insulin-like growth factor (Khanal et al., 2006 Dusso et al., 2005 Carpenter, 1989). In recent studies, fibroblast growth factor 23 (FGF-23), frizzled-related protein 4 (FRP-4) and matrix extracellular phosphoglycoprotein (MEPE) have all been identified as soused and key regulatory factors of 1--hydroxylase activity in renal cells. These factors act through a biphasic mechanism on renal phosphate homeostasis and modulate the spread levels of 1, 25-dihydroxyvitam in D3 produced by proximal renal tubules (Dusso et al., 2005 Inoue et al., 2005 Mirams et al., 2004).1.2.4.4- calcitoninCalcitonin belongs to a family of calcium regulating hormones that is produced in the parafollicular cells of the thyroid gland, also cognize as C cells. It is a short and linear polypeptide with a molecular weight of only 3.7 kD. It is characterized by 32 amino group acids and a disulfide bridge over in the N terminal portion of the peptide. Calcitonin is secreted in response to increased free Ca2+ in blood and acts on osteoclasts, the bone resorbing cells, as a suppressor of bone dissolution. Although calcitonin decreases Ca+2 and inorganic phosphate in blood, it also has the dexterity to recruit the Tempter into other cells. In addition to these metabolic functions, it is also involved in the upregualtion of CYP27B hydroxylase through the protein kinase C pathway (Yoshida et al., 1999) via a phosphorylation cascade that activates cAMP and induces the contem plation of hydroxylase thereby activating the transformation of 25(OH) D3 to 1,25(OH)2 D3.In addition to the significant role as a calcium regulating hormone, calcitonin is also known to catch the production of vitamin D in tandem with PTH (Yoshida et al., 1999 Wongsurawat and Armbrecht, 1991). Previous studies revealed that 1--hydroxylase mRNA expression, 1--hydroxylase activity and the production of 25(OH)D and 1,25(OH)2D3 all increased in rat kidney cells spare-time activity the political science of calcitonin (Yoshida et al., 1999 Galante et al., 1972 Rasmussent et al., 1972). However, in cases of diabetes, it is postulated that the kidney becomes immune to the effect of this hormone in diabetic rats which lead to increase vitamin D production (Wongsurawat and Ambrecht, 1991).1.2.4.5- harvest-tide hormone, Insulin and Insulin-like growth factor-1Growth hormone (GH) has many regulatory actions in various metabolic processes in humans and mammals and its effect on mineral home ostasis in target organs such as bone and renal cells is well documented. While the regulatory effects of GH on dietary calcium and phosphate metabolism in different tissues have been established, its effect on vitamin D metabolism carcass controversial. However, many studies have shown that GH increases the expression of 1--hydroxylase and 1, 25-dihydroxyvitamin D3 in cultured cells and experimental animals (Gomez, 2006). Wu and colleagues inform that serum1, 25-dihydroxyvitamin D3 increases after GH administration in hypophysectomized rats fed with a phosphate depleted diet. Short-term studies in healthy humans have shown that GH raises 1--hydroxylase enzyme activity and promotes 1, 25-dihydroxyvitamin D3 synthesis without changes in PTH, calcium and phosphate concentrations, suggesting that the increasing circulating levels of 1, 25-dihydroxyvitamin D3 following GH administration is not mediated by PTH action (Wu et al., 1997 Bianda et al., 1997 Wright et al., 1996). GH has als o been shown to lead to increased production and serum concentration of 1, 25-dihydroxyvitamin D3 in pigs and in renal impaired prepubescent children. These are thought to be a result of the direct and indirect effects of GH on 1--hydroxylase expression, and on calcium and inorganic phosphate homeostasis in renal tubules cells (Strife and Hug, 1996 Denis et al., 1995). However, the action of GH on vitamin D metabolism in vitro re main(prenominal)s uncertain and may involve other regulatory factors such as PTH and Insulin-like growth factor-1 (IGF-1). It has been shown that GH does not raise 1, 25-dihydroxyvitamin D3 levels directly in cultured cells obtained from aged-rats yet it stimulates calcium absorption and the expression of calcium ski concealment proteins in vitro indicating that the effect of GH is mediated through the action of other factors such as IGF-1 (Fleet et al., 1991).Insulin is another key factor with a role in vitamin D homeostasis. Insulin significantly decrea ses renal hydroxylase activity and renal synthetic capacity of 1, 25-dihydroxyvitamin D3 in insulin deficient patients or those receiving insulin therapy (Armbrecht et al., 1996). However, a involve of different routes of alterative insulin administration in human diabetic subjects concluded that insulin induces the hepatic hydroxylation of 25-hydroxyvitamin D3. This effect is related to the fact that insulin is a potent inducer of the vast absolute majority of liver hydroxylases enzymes (Colette et al., 1989). This study also showed that there was no significant expiration in circulating levels of 1,25-dihydroxyvitamin D3 in the midst of different methods of insulin administration. Serum 1,25-dihydroxyvitamin D3 is maintained at normal concentrations in those subjects on long term insulin therapy however, continuous intraperitioneal excerpt procedure (CPII) may augment hepatic 25-hydroxlase activity (Colette et al., 1989). Similarly insulin has shown a significant effect on affect 1,25-dihydroxyvitamin D3 production through 1,25-dihydroxyvitamin D3 and PTH comment with no concomitant action on 24-hydroxylase expression in rat osteoblast cells when these cells were cultured with known concentrations of 1,25-dihydroxyvitamin D3 and PTH (Armbrecht et al., 1996).Insulin-like growth factor-1 (IGF-1) is a relatively small peptide that is primarily expressed in hepatic cells and to a lesser outcome in some other cells and tissues. It has been identified as one of the potent regulatory components of mineral metabolism in humans and mammals. Recent studies on the metabolic effect of IGF-1 revealed that the administration of IGF-1 to aged laboratory animals, fed on a calcium- and phosphate- deficient diet, can restore 1--hydroxylase activity and enhance the production of 1,25-dihydroxyvitamin D3. In contrast, there was no significant effect of IGF-1 on enzyme activity and 1,25-dihydroxyvitamin D3 levels in adolescent or elderly rats fed on a calcium and phosph ate fortified diet concluding that the expression of IGF-1 is not age related but related to the dietary calcium and phosphorus status. (Gomez, 2006 Wong et al., 1997 Wu et al., 1997). In healthy human subjects, a significant effect of IGF-1 on renal 1,25-dihydroxyvitamin D3 synthesis was observed after short term excerption with IGF-1. There was no noticeable alteration of the levels of circulating calcium, phosphate and PTH play up the role of IGF-1 in stimulating renal expression of 1--hydroxylase and 1,25-dihydroxyvitamin D3 formation in conjunction with GH, independently from PTH (Bianda et al., 1997). In vitro studies have shown that IGF-1 influences the expression of 1--hydroxylase and 1,25-dihydroxyvitamin D3 synthesis in cells cultured from non renal human tissues. Halhali and colleagues demonstrated that IGF-1 noticeably elevates both the enzyme activity and 1,25-dihydroxyvitamin D3 levels when added into cultured syncytiotrophoblast cells obtained from human placental s ources. This study demonstrated that IGF-1 strongly enhances the ability of non renal cells to produce 1,25-dihydroxyvitamin D3 without involvement of GH and PTH (Halhali et al., 1997).1.2.4.6- 1, 25-dihyroxy vitamin D3The circulating levels of 1,25-dihydroxyvitamin D3 modulate its production by renal cells through an indirect negative feedback mechanism. This mechanism appears to reduce the likelihood of vitamin D toxicity by inhibiting 1,25-dihydroxyvitamin D3 synthesis by an indirect mechanism that controls the 1--hydroxylase gene expression at the molecular level rather than inhibiting 1,25-dihydroxyvitamin D3 synthesis directly. However, the exact mechanism is not yet fully understood (Dusso et al., 2005 Deluca et al., 1990). A recent study examined the effect of 1,25-dihydroxyvitamin D3 on 1--hydroxylase production by cultured human keratinocytes. Keratinocytes were cultured with labeled 25-hydroxyvitamin D3 and different concentrations of 1--hydroxylase mRNA and 24-hydroxylas e- suppressed proteins. The 1,25-dihydroxyvitamin D3 did not suppress either the 1--hydroxylase activity or the rate of gene transcription. The study implied that metabolic regulation of 1,25-dihydroxyvitamin D3 is related to the molecules biodegradation in response to augmented 24-hydroxylase activity rather than 1,25-dihydroxyvitamin D3 formation by 1--hydroxylase (Xie et al., 2002). In addition, Wu and colleagues demonstrated a possible alternative mechanism of 1,25-dihydroxyvitamin D3 synthesis linked to the fact that both 24-hydroxylase and 1--hydroxylase enzymes care equivalent metabolic capability and they proposed the possibility of protein- protein interaction between intracellular vitamin D dressing protein and 1--hydroxylase (Wu et al., 2002).1.2.5- Vitamin D Transport, receptors and mechanism of actionVitamin D receptor (VDR), also known as calcitriol receptor, is a member of the steroid family and belongs to the atomic receptor superfamily (NHR). Human VDR until rec ently was thought to comprises four functional units with a total of 427 amino acids residues with an estimated molecular weight of about 48 kDa. These units are the DNA backbone landed estate (DBD) or C reach, the D domain and the ligand binding domain (LBD) or E domain. More recently, a carboxy-group with undefinable function, known as the F region has been identified (Christakos et al., 2003 Aranda and Pascual, 2001 Rastinejad et al., 2000). These units as, shown in record 4, are also known as A/B domain. The A/B region of VDR contains a low number of amino acids that participates in essential ligand-independent receptor stimulation (Aranda and Pascual, 2001 Issa et al., 1998). It is not yet clear if the stinger of A/B domain from VDR go forth compromise ligand binding, DNA binding or its transactivation features (Issa et al., 1998). In contrast, the structure of the DNA binding domain or C region among NHRs comprises 40% unique amino acids sequences and a domain of more t han 67 resemble amino acids residues (Rastinejad et al., 2000). Moreover, the core structure of DBD comprises between 22 and 114 amino acid residues, nine of them are cysteines. Eight of cysteine residues take with zinc atoms in tetrahedral fashion to form a dual zinc-like find DNA binding anatomys containing approximately 70 amino acids with a carboxy-terminal offstage (CTE). This encloses T and A boxes in a dual curl molecule in which one helix is essential for definitive interaction with the main domain on DNA while the second helix takes a part in receptors structural properties (i.e. receptor dimerization) (Aranda and Pascual, 2001 Issa et al., 1998). However, the integration of the structural amino acids of the DBD -helix one, at the site of the first zinc atom, determines the selectivity and specificity of recognition of DBD and forms an area known as the P Box. Similarly the integration of amino acids at the position of the second zinc atom modulates the formation of a configuration termed the D Box which forms a dimerization interface zone (Aranda and Pascual, 2001 Rastinejad et al., 2000 Issa et al., 1998). Furthermore the vast majority of DBD amino acid units are basic amino acids which enhance the non-covalent binding of the DNA helix at the negatively charged phosphate group (Issa et al., 1998). The ligand binding domain (LBD) or E domain has a spherical configuration with many functional regions composed of 12 cohered helix anchors defined as H1 to H12. LBD itself comprises a net of 427 amino acids which contribute to homodimerization and heterodimerization and the interaction of hormones and costimulaotors by a crucial transactivational mechanism (Aranda and Pascual, 2001 Weatherman et al., 2000 Issa et al., 1998). Crystallographic studies show that LBD have two cohered and integrated domains, the Ti or signature motif and the carboxy or C terminal AF-2 providing the self-ligand transcriptional properties hence a higher degree of standoff of 1,25 dihydroxyvitamin D3 binding is observed at 382 to 402 of LBD amino acid sequence and any genetic aberration at this particular amino acids sequence will diminish the interaction capability of LBD (Aranda and Pascual, 2001 Issa et al., 1998).Figure 4 The primary structure of the vitamin D receptor (VDR) and the binding of retinoid X receptor (RXR)-VDR heterodimers to vitamin D response elements (VDREs) in the form of DR3 and ER6 motifs. (Figure from Lin and White, 2003)1,25-dihydroxyvitamin D3, has been identified as steroid hormone with a mechanism of action similar to other steroid hormones, causing upstart protein expression in various target organs. Based on the thermonuclear receptors structural studies, calcitriol is known to exert its biological action through binding with VDR in the cell nucleus to mediate a cascade of transcriptional and translational processes resulting in either the regulation or inhibition of new protein expression in target tissues or the bindi ng to plasma membrane receptors without stimulating new protein synthesis (Nezbedova and Brtko, 2004 Reichel and Norman, 1989). Two different receptors for 1,25-dihydroxyvitamin D3 have been recognized in different target cells identified as genomic VDRnuc and typical VDRmem .These receptors provide the beaver dynamical conformational forms for calcitriol interaction and to evoke its genomic and non-genomic effects (Norman et al., 2002). The binding of 1,25-dihydroxyvitamin D3 to VDRnuc enhances the interaction with an undistinguished protein known as the nuclear accessory factor (NAF) and to the caroxy-terminal of VDR. This interaction leads to a structural conversion pattern of the C-terminal of VDR allowing the AF-2 domain to attach with other transcriptional elements such as SCR-1, calcium binding protein (CBP) and P300. This promotes the binding of the heterodimer molecule with DNA at the vitamin D response sites (VDRE) and directs its transcriptional gene activity (Jones et al., 1998 Iqbal, 1994). In addition, these coactivators play a role in DNA configurational changes through histone acetyl transferase activation pathway of the core components of histones. This results in mechanical instability of the DNA structure and enhances the net binding capacity of the coactivators with their corresponding receptors at nucleosomal histone level and leads to the upregulation of these transcriptional coactivators which in trun, accelerate the net gene transcriptional rate to promote the synthesis of the alike protein (Lipkin and Lamprech, 2006 Jones et al., 1998).Conversely, the non-genomic or classical effect of 1,25-dihydroxyvitamin D3 is modulated through its binding with the approach cellular membrane receptor known as mVDR which initiates an immediate response in various target tissues with no genomic transcriptional activity. Many studies demonstrate the speedy effect of calcitriol in rapidly increasing both the level of circulating calcium and its abs orption rate in animal intestines, evoking phosphoinoisitide bioactivation, cyclic deoxyguanosine monophosphate (cGMP) elevation, activation of protein kinase C and triggering the mitogen activated protein kinase pathways and involving the chloride gates action potential in different organs (Dusso et al., 2005 Nezbedova and Brtko, 2004 Boyan and Schwartz, 2004 Norman et al., 2002). The entire mechanism, as shown in interpret 5, for the rapid effect of calcitriol remains doubtful, however the proposed mechanism is mediated through the interaction with mVDR leading to a series of intracellular signaling events. signal is orchestrated by the activation of various metabolic pathways involving different transfer mechanisms of certain mineral components of target organs. (Pedrozo et al., 1999 Norman et al., 1999 Revelli et al., 1998). However, other studies reveal that the genomic effect of 1,25-dihydroxyvitamin D3 is independent of its non-genomic mechanism (Dusso et al., 2005).Figur e 5 Cellular mechanism of action of 1,25(OH)2D3 (Figure from Horst et al., 1997)1.3- Biological actions of Vitamin D on target tissues and SystemsThe active form of vitamin D, 1,25-dihydroxyvitamin D3 is well recognized as a member of steroid hormones that mediates several metabolic and non-metabolic processes in various organs in human and animals as shown in figure 6.1.3.1- IntestineMineral absorption in the intestines is increased in the presence of the hormone 1,25(OH) vitamin D. However without this, only 10 to 15% of dietary calcium and 60% of phosphorus is absorbed from the diet (De Luca, 2004). Ca2+ and HPO42- are also absorbed when intestinal cells interact with the vitamin D- VDR- RXR complex. The latter enhances the expression of the epithelial calcium channel and calcium-binding protein which recruits calcium and phosphorus (Holick, 2007). Knock out mice experiments studying the effect of VDR gene deletions also show that the size of the small intestines is related to th e levels of calcitriol and dietary calcium availability. Vitamin D deficient mice fed with diets low in calcium exhibited the largest small intestine to large intestine ratio (Cantorna et al., 2004). VDR knock-out mice experiments also incite in the discovery of calcium channels, the route for Ca absorption, in the intestine (Peng et al., 1999). Calbindin is a potent calcium transporter in mammals which characterized by a high affinity for calcium ions. Therefore, the binding of vitamin D to VDR and RXR signals an increased production of calbindin which facilitates systemic Ca2+ ions transportation and prevent the occurrence of calcium toxicity in the intestines.Figure 6 Schematic diagram of the effects of Vitamin D on different tissues and organs (Figure from Holick, 2007).1.3.2- BoneTakeda et al. (1999) studied the role of vitamin D and VDR in bone cells using knock out mice experiments. Their results showed that bone cells formation triggering mechanisms such as cell to cell int eraction between osteoblast and osteoclast progenitors and stromal cells bring forth by 1,25(OH)2 vitamin D3 and provoke the formation of osteoclasts. In their capacity as bone resorbing cells, osteoclasts can be triggered by low serum calcium levels, to let on down bone and free calcium back in to the blood thus redistributing calcium throughout the body. However, this does not occur without the expression of VDR and without vitamin D complexing with its receptor. This study emphasizes the important role of recognition sites on the VDR and the structural implications that the receptor-ligand binding has on VDREs and transcription initiation.Although the effects of PTH