Ligand binding, involving the AF2 domain, imparts a conforma- tional change in the secondary structure of the VDR which trig­gers the recruitment of motor proteins responsible for cyto­plasmic VDR transition to the nucleus along microtubules. VDR heterodimerization with its protein partner RXR confers a conformational structure to the receptor which is critical for transactivation function by high affinity interaction with VDREs in the promoter region of vitamin D-responsive genes (Figure 3). During VDR-RXR interaction with the most common VDRE type, DR3, RXR binds the 5′ half-site and the VDR oc­cupies the 3′ one. According to mutagenesis experiments in the VDRE of avian parathyroid hormone (PTH) gene promot­er, the switch between VDR-RXR-activated or -repressed gene transcription depends on the polarity of the VDR/RXR-VDRE complex. However recent experimental evidences regard­ing VDR interactions with nuclear coregulator molecules and their action on VDR-mediated regulation of gene transcription have provided further complexity to the entire mechanism. In fact RXR heterodimerization and AF2 domains are both in­volved in interactions with nuclear proteins which serves as VDR-coregulators (36). In details the ligand-induced conforma­tional change in AF2 domain is critical for the receptor to inter­act with components of the transcription initiation complex, RNA polymerase II and nuclear transcription coactivators which promote chromatin remodelling necessary to gene tran­scription. Some of them in fact, such as SRC-1 and CBP/p300 are histone acetylases which determines the recruitment of a second complement of transcription coactivators, the DRIP- TRAP complex. This is a 15 proteins-complex which facilitates the assembly of the preinitiation complex thus strengthening VDR-induced gene transcription. Binding of VDR-RXR complex to a negative VDRE promotes recruitment of corepressors which have histone deacetylase activity and then inhibit bind­ing of proteins promoting the assembly of the transcription ma­chinery. The situation is further complicated by comod- ulator proteins, such as NCoA62/Skip which can act as core- pressor or coactivator depending on the specific cell context of coregulator molecules. The NH₂-terminal region of Skip protein is targeted by both nuclear corepressors, such as NCoR, and coactivators, like CBP/p300, and more importantly some of them are under VDR-mediated transcriptional regula­tion. Recently an ATP-dependent chromatin remodel­ling complex has been demonstrated to amplify VDR activation and repression of transcription.

…..Click here to read more

Introduction

Vitamin D receptor (VDR) is the responsible of much of the vi­tamin D [1,25-(OH)₂D₃] signalling. It is a direct regulator of gene transcription, belonging to the nuclear receptor family. Within this family, it has sequence and structure resemblance with the sub­family that includes retinoic acid, thyroid hormone and peroxi­some proliferator activator receptor (PPAR) receptors. VDR is a protein of 427 amino acids, with a molecular mass of approximately 48 kDa. Similarly to the other nuclear receptors VDR consists of several distinct functional domains, namely the NH2 terminus A/B domain whose function is still unclear, a DNA binding domain (DBD), termed C domain, spanning from amino acid 20 to 90, a hinge or D domain (amino acids 90 to 130) and the ligand binding domain (LBD), or E domain, span­ning from amino acid 130 to 423. The last one is the most complex domain of the protein as it is responsible for the spe­cific binding of VDR ligands, for heterodimerization with retinoid X receptor (RXR) and for interactions with transcription factors (Figure 1).

VDR cDNA was firstly cloned from chicken and shortly thereafter from human. Genomic organization of the human VDR gene, which locates on chromosome 12q13-14, is similar to other nuclear receptor genes. The gene is made up of 11 ex- ons spanning approximately 75 kb (Figure 2). The non coding 5′ end of the gene includes several exon 1 isoforms (from 1A to 1E) while exons 2-9 encode the structural portion of the VDR gene product. Alternative splicing of exon 1 provides at least five different VDR mRNA transcripts, while the presence of a polymorphic sequence in exon 2 determines the presence or absence of an alternative start translation site. Most of the promoter sequence is upstream exon 1 and has high GC con­tent but does not contain an apparent TATA box. The promoter is capable to generate multiple tissue-specific transcripts and is positioned just downstream from collagen type II alpha 1 gene. A unique feature for this gene is the presence of an additional exon that is not found in other nuclear hormone re­ceptors family members. It encodes for an insertion peptide of about 40 amino acid that locates in the LBD of the receptor. VDR binds its physiological ligand 1,25-(OH)₂D₃ with high affin­ity (about 10-¹⁰ M) and both hydroxyl groups seem to be relevant in determining it as the absence of either results in ap­proximately a 500-fold decrease in affinity. Although widely expressed, VDR protein levels are usually low and reach relatively high values only in targets tissues such as bone, kidney and intestine (3,000-6,000 fmol/mg protein). In contrast to other nuclear receptors which are usually bound to cytoplasmic proteins before hormone binding, VDR is predominantly nuclear.

…..Click here to read more

Clinical and laboratory data of patients and controls are report­ed in Table I.

BMD was reduced in 16 (64%) out of 25 patients: 5 patients (20%) could be classified as osteoporotic and 11 patients (44%) osteopenic on the basis of at least one measurement. Table II reports mean BMD values for lumbar spine (LS) and femoral neck (FN) in patients and controls, showing a de­creased BMD in patients in both sides of measurements even if only for the LS it reaches the significance. In order to compare the BMD of our patients with reference population data we calculated the 95% CI of the mean Z-score at the LS and at the FN. Data showed that the mean Z-score at LS but not at FN among liver patients was significantly lower as compared to the age- and sex-matched general population (lumbar spine mean Z-score 95% CI = -1.06 to -0.920; femoral neck mean Z-score 95% CI = -0.54 to + 0.53). Using multiple linear regression analysis we investigated the relationship between BMD (gr/cm²) and the variables consid­ered, including the time of detection of abnormal tests of liver function. No correlation was found between BMD and the vari­ables studied.

…..Click here to read more

All patients lived in the same geographical area from Southern Italy. Their diet was a tipical mixed, mediterranean diet, con­taining milk and cheese products, without alcohol intake; only five patients consumed less than 20 g of alcohol three or four times a week. Information about dietary habits and alcohol in­take were carefully obtained from a food frequency question­naire.

Exclusion criteria were: female sex, age > 60 years, alcohol abuse, drug addiction, obesity, previous bone fractures or his­tory of recent immobilization, autoimmune, endocrinological or rheumatological diseases, treatment with corticosteroids, di­uretics, drugs affecting bone metabolism or interferon for more than three months over the past three years. Advanced liver cirrhosis (B or C Child’s class), hepatocellular carcinoma, pri­mary biliary cirrhosis, cholestatic liver diseases, genetic he- mochromatosis were also considered exclusion criteria. The selected patients had positive serological markers for viral hepatitis (2 for hepatitis B virus and 23 for hepatitis C virus) and biopsy-proven diagnosis of chronic liver disease: 12 pa­tients were affected by chronic hepatitis and 13 patients by liv­er cirrhosis, A Child’s class. Liver function was well preserved and renal function was normal in all patients. None exhibited indices of cholestasis nor experienced previous decompensa­tion of cirrhosis.

Thirthy healthy male volunteers, living in the same geographi­cal area, age, and BMI-matched, were also examined as con­trol group.

…..Click here to read more

Introduction

Metabolic bone disease is a complication of chronic liver dis­ease (CLD) and is well known as “hepatic osteodistrophy”. Osteoporosis accounts for the majority of cases, whereas osteomalacia is rare in the absence of advanced liver disease and severe malabsorption. The prevalence is the same in men and women, however the published prevalence of os­teoporosis considerably differs and ranges from 20% to 100%, depending on patients selection and different diagnostic criteria. Many reports are referred to a broad spectrum of liver disease of different aetiology and severity: in patients with ad­vanced liver disease candidates for or treated with orthotopic liver transplantation, osteoporosis is prevalent and contributes to a major source of morbidity preceding and following trans­plantation. Nevertheless, little is known about the preva­lence among patients with non-advanced liver disease. The ae­tiology and pathogenesis of osteoporosis in these patients also remain undefined, even though its histology is quite similar to post-menopausal and age-related bone loss, affecting trabecu­lar bone more rapidly than cortical bone. Finally, there is controversy about risk factors for osteoporosis in CLD: liver cir­rhosis, cholestasis, hypogonadism, corticosteroid or immuno- suppressive treatment, alcohol consumption, malnutrition and malabsorption, sex, physical activity, subnormal vitamin D lev­els and/or vitamin D receptor genotype, insulin growth factor 1 (IGF-1) deficiency, as well as country and nationality are all re­ported factors affecting bone metabolism. However, whether non-advanced liver disease “per se” could be a risk factor for osteoporosis still remains uncertain; more­over there are few reports about the effects of viral liver dis­ease on bone turnover.

…..Click here to read more

However, some other aspects seem to link the kidney to the complex relationships occurring between hypercalciuria and bone. Increased urinary phosphate excretion was found in hy- percalciuric patients as compared to normal subjects, irrespec­tive of the presence of a true form of absorptive hypercalciuria with renal calcium leak. It was suggested that an exces­sive excretion of phosphate may be present in the majority of patients with PH, then concurring to the development of the hy- percalciuric state in the whole population of PH patients. Simi­lar findings were reported by Prie et al. in hypercalciuric stone formers. They observed that the distribution of renal phosphate threshold normalized for glomerular filtration rate (TmPi) was quite different between patients and controls, with hypercalciuric patients showing a decreased value of TmPi in approximately 20% of cases. No assessment of bone status was made in the two papers. However, it could be hypothe­sized that the alteration in phosphate metabolism seen in these patients may play a role also in the pathogenesis of bone dam­age in hypercalciuric patients. Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is a paradigm of this patho­physiology. This disease shares some clinical aspects with the disorder seen in the Npt2 knockout mice, carrying the dele­tion of the gene of kidney-specific Na-Pi cotransporter, in which a delay in bone mineralization is seen 21 days after birth. These bone alterations may resemble those observed in hyper- calciuric patients by histomorphometric studies, in which an alteration in bone mineralization process was report­ed. Accordingly, a mutation of NPT2 gene was found in nephrolithiasic patients with decreased bone density by Prie and coworkers. In conclusion, even if no clear evidence supports the hypothesis that renal phosphate leak may be at least in part responsible for bone disease in PH patients, this research field appears as one of the most promising to better elucidate the role of kidney in the pathogenesis of bone loss in PH.

…..Click here to read more