Genetic factors associated with absolute and relative plasma concentrations of calcitriol

BACKGROUND

Little is known regarding factors associated with calcitriol and a relative measure of calcitriol, the calcitriol-24,25-dihydroxyvitamin D3-calcifediol proportion ratio (C24CPR).

METHODS

Using a cross-sectional study design, healthy young adults of African and European descent, matched (1:1) on age (+/- 5 years) provided a blood sample in non-summer months (N=376). Vitamin D metabolites were measured in plasma with high performance liquid chromatography/tandem mass spectrometry (HPLC/MS-MS). West African genetic ancestry proportion (WGA) was estimated using STRUCTURE modeling of genetic ancestry-informative markers. Multivariable regression models were used to estimate the association of WGA and vitamin D-pathway gene variants with calcitriol and C24CPR, controlling for days from summer solstice, age, sex, blood pressure, body mass index, dietary vitamin D intake, oral contraceptive/medroxyprogesterone acetate use, smoking, tanning bed use, and time of day.

RESULTS

Calcitriol and C24CPR were not highly correlated (rho=0.14), although both were significantly, positively, and monotonically associated with WGA (p-trends 0.025 and <0.001, respectively). In fully adjusted models, genetic factors explained a greater proportion of variability in C24CPR (R2=0.121 and 0.310, respectively). Variants in genes with associated with calcitriol (CALB1, CYP27B1, GC, PPARGC1A) differed from those associated with C24CPR (CYP3A43, FGF23, KL, VDR).

CONCLUSIONS

Both absolute and relative measures of calcitriol were significantly higher among African Americans. Otherwise, these biomarkers appear to be genetically distinct.

IMPACT

C24CPR may be better suited to personalized medicine, due to a higher proportion of population variability explained by genetic variation and a less skewed distribution.

Tumor necrosis factor-α treatment of HepG2 cells mobilizes a cytoplasmic pool of ERp57/1,25D₃-MARRS to the nucleus

ERp57/PDIA3/1,25-MARRS has diverse functions and multiple cellular locations in various cell types. While classically described as an endoplasmic reticulum (ER) resident protein, ERp57 has a nuclear location sequence (NLS) and can enter the nucleus from the cytosol to alter transcription of target genes. Dysregulation and variable expression of ERp57 is associated with a variety of cancers including hepatocellular carcinoma (HCC). We investigated the dynamic mobility of ERp57 in an HCC cell line, HepG2, to better understand the movement and function of the non-ER resident pool of ERp57. Subcellular fractionation indicated ERp57 is highly expressed in the ER with a smaller cytoplasmic pool in HepG2 cells. Utilizing an ERp57 green fluorescent protein fusion construct created with and without a secretory signal sequence, we found that cytoplasmic ERp57 translocated to the nucleus within 15 min after tumor necrosis factor-α (TNF-α) treatment. Protein kinase C activators including 1,25-dihydroxyvitamin D(3) and phorbol myristate acetate did not trigger nuclear translocation of ERp57, indicating translocation is PKC independent. To determine if an interaction between the rel homology binding domain in ERp57 and the nuclear factor-κB subunit, p65, occurred after TNF-α treatment and could account for nuclear movement, co-immunoprecipitation was performed under control and conditions that stabilized labile disulfide bonds. No support for a functional interaction between p65 and ERp57 after TNF-α treatment was found in either case. Immunostaining for both ERp57-GFP and p65 after TNF-α treatment indicated that nuclear translocation of these two proteins occurs independently in HepG2 cells.

Regulation of expression of 1,25D3-MARRS/ERp57/PDIA3 in rat IEC-6 cells by TGF beta and 1,25(OH)2D3

We examined the transcriptional regulation of expression of the redox-sensitive Membrane-Associated-Rapid Response, Steroid-binding (1,25D(3)-MARRS) protein specific for 1,25(OH)(2)D(3) in a rat small intestinal cell line, IEC-6, that demonstrates rapid responses to 1,25(OH)(2)D(3). 1,25D(3)-MARRS binds and is activated by 1,25(OH)(2)D(3), but is not itself up-regulated by treatment with 1,25(OH)(2)D(3), nor is there a Vitamin D response element (VDRE) in its proximal promoter. We previously reported that transforming growth factor beta (TGFbeta) increased steady state levels of 1,25D(3)-MARRS transcript and protein approximately two-fold [Rohe B, Safford SE, Nemere I, Farach-Carson, MC. Identification and characterization of 1,25D(3)-membrane-associated rapid response, steroid (1,25D(3)-MARRS)-binding protein in rat IEC-6 cells. Steroids 2005;70:458-63]. To determine if this up-regulation could be attributed to the function of a highly conserved consensus smad 3 binding element present in the proximal promoter of the 1,25D(3)-MARRS gene, we created a promoter-reporter [SEAP] construct that was responsive to TGFbeta (200 pM). Deletion or mutation of the smad 3 element greatly reduced the response of the 1,25D(3)-MARRS promoter to TGFbeta. Subsequent studies found that the smad 3 response element is bound by a protein found in the IEC-6 nuclear extract, most likely smad 3. Interestingly, although 1,25(OH)(2)D(3) alone did not increase expression of the 1,25D(3)-MARRS promoter-reporter, co-treatment of transfected IEC-6 cells with 1,25(OH)(2)D(3) and TGFbeta shifted the dose-response curve to a lower effective concentration (100 pM peptide). We conclude that TGFbeta is a transcriptional regulator of 1,25D(3)-MARRS expression via a functional smad 3 element and that cross-talk with non-classical 1,25(OH)(2)D(3)-stimulated pathways occurs. The findings have broad implications for redox-sensitive signaling phenomena including those that regulate phosphate transport in the intestine.

Identification and characterization of 1,25D3-membrane-associated rapid response, steroid (1,25D3-MARRS)-binding protein in rat IEC-6 cells

We report the presence of a mammalian equivalent of the avian Membrane-Associated Rapid Response, Steroid (1,25D3-MARRS)-binding protein specific for 1,25(OH)2D3 in a rat small intestinal cell line, IEC-6, that demonstrates rapid responses to the steroid hormone. Identification of transcript and protein was achieved using RT-PCR with several specific primer sets, Western blot analysis with two separate antibodies recognizing distinct regions of the protein, ribozyme knockdown and immunohistochemistry. Promoter analysis of the 1000-bp upstream region of the 1,25D3-MARRS gene in several species revealed the presence of a conserved smad-3 element in the 5' proximal promoter region, but no classical vitamin D response element (VDRE). Treatment of IEC-6 cells with transforming growth factor beta1 (TGFbeta1) increased steady-state levels of 1,25D3-MARRS (mRNA and protein) approximately two-fold over a 24-h period. In contrast, treatment with 1,25(OH)2D3 failed to significantly change 1,25D3-MARRS protein or mRNA levels. Localization studies showed rapid nuclear translocation of a pool of 1,25D3-MARRS protein after 1,25(OH)2D3 treatment, suggesting that the protein is subject to membrane-initiated signal pathway activation. Together these data point to complex interactions between the two important 1,25(OH)2D3 sensitive response systems in intestinal cells, 1,25D3-MARRS protein and the well-studied nVDR, that together work to fine tune intestinal Ca2+ absorption in a variety of avian and mammalian species.

Identification and characterization of 1,25D3-membrane-associated rapid response, steroid (1,25D3-MARRS) binding protein

1,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) operates through pharmacologically distinct nuclear receptor-mediated and plasma membrane-initiated mechanisms. The nuclear receptor is well described, but the membrane receptor identity remains unproven. A 66 kDa protein from chick intestinal basolateral membranes was isolated previously and identified as a candidate receptor (now termed 1,25D(3)-MARRS). A chicken cDNA library was screened for clones encoding the N-terminal sequence of 1,25D(3)-MARRS. An exact match was found with an insert containing an open coding region for the full-length candidate 1,25D(3)-MARRS protein. Analysis reveals a 5' untranslated region, a precursor translation product with methionine start site, a signal peptide and a translation product of 505 amino acids prior to translation termination site. Prosite analysis predicts potential sites for phosphorylation by casein kinase II cAMP-dependent kinase, protein kinase C, and tyrosine kinase and an N-myristoylation site with high probability of occurrence. Additionally, two conserved domains capable of interacting with Rel homology domains (RHD) are present. Oligonucleotide primers sets designed to amplify unique regions of the sequence produced amplimers of the predicted size from both chicken and rat intestinal cells. Transcription-translation produced a protein that was recognized in Western blot analysis by Ab099, a polyclonal antibody recognizing the N-terminus of the 66 kDa MARRS protein.

Ribozyme knockdown functionally links a 1,25(OH)2D3 membrane binding protein (1,25D3-MARRS) and phosphate uptake in intestinal cells

We used a ribozyme loss-of-function approach to demonstrate that the protein product of a cDNA encoding a multifunctional membrane-associated protein binds the seco-steroid 1,25(OH)(2)D(3) and transduces its stimulatory effects on phosphate uptake. These results are paralleled by studies in which the ability of the hormone to stimulate phosphate uptake in isolated chick intestinal epithelial cells is abolished by preincubation with Ab099 directed against the amino terminus of the protein. We now report the complete sequence of the cloned chicken cDNA for the 1,25D(3)-MARRS (membrane-associated, rapid-response steroid-binding) protein and reveal it to be identical to the multifunctional protein ERp57. Functional studies showed that active ribozyme, but not a scrambled control, decreased specific membrane-associated 1,25(OH)(2)D(3) binding, but did not affect binding to the nuclear receptor for 1,25(OH)(2)D(3). Seco-steroid-dependent stimulation of protein kinase C activity was diminished as 1,25D(3)-MARRS protein levels were reduced in the presence of the ribozyme, as judged by Western blot analyses. Phosphate uptake in isolated cells is an index of intestinal phosphate transport that occurs during growth and maturation. Whereas cells and perfused duodena robustly responded to 1,25(OH)(2)D(3) in preparations from young birds, older animals no longer responded with stimulated phosphate uptake or transport. The age-related decline was accompanied by a decrease in 1,25D(3)-MARRS mRNA that was apparent up to 1 year of age. Together, these studies functionally link phosphate transport in the chick duodenum with the 1,25D(3)-MARRS protein and point to a previously uncharacterized role for this multifunctional protein class.

Suppression of fibrosarcoma metastasis by elevated expression of manganese superoxide dismutase

A mouse fibrosarcoma cell line (FSa-II), which exhibits low endogenous levels of manganese superoxide dismutase, was transfected with a human manganese superoxide dismutase complementary DNA. Fifty clones were screened for manganese superoxide dismutase activity by the superoxide dismutase activity gel assay. Activity of the positive clones was measured by the nitro blue tetrazolium-reduction assay in the presence of cyanide. Three cell lines exhibiting a range of activity were chosen to be transplanted into syngeneic mice. The results indicated that the metastasis rate for all transfected cells was significantly less than that of control cells.

Radioimmunoassay for detection of antibody to hepatitis A virus. Results of clinical evaluation

The performance of a solid-phase radioimmunoassay (HAVAB) for the detection of antibody to hepatitis A virus (HAV) was evaluated in clinical studies. The procedure was reproducible by eight investigators, and laboratory-to-laboratory variations were minimal. The sensitivity of the test was about equal to or slightly greater than that of immune adherence hemagglutination (IAHA) for detecting antibody in serum, but IAHA gives somewhat higher titer values than HAVAB. A survey of the incidence of anti-HAV in selected populations revealed an overall frequency of 48% and a correlation with age and lower socioeconomic status. The specificity of the test was demonstrated with specimens from patients who had clinical hepatitis A. Seroconversion to anti-HAV positivity was demonstrated with HAVAB to coincide with onset of illness, about two to three weeks earlier than it was detected by IAHA. HAVAB provided a convenient test for demonstrating the immune status of subjects, and was useful as an aid in diagnosing hepatitis A.