Biomedical subjects

J C Fleet

Publications and source records attributed to J C Fleet.

At least 19 recordsLinked to original sources

PubMed2001

DASH without the dash (of salt) can lower blood pressure.

While the debate regarding the effectiveness of dietary sodium restriction raged, the Dietary Approaches to Stop Hypertension (DASH) research group showed that a diet rich in fruits, vegetables, and low-fat dairy products can reduce blood pressure in the general population and people with Stage I hypertension. The original DASH diet did not require either sodium restriction or weight loss--the two traditional dietary tools to control blood pressure--to be effective. A recent study from the DASH research group now finds that coupling the original DASH diet with sodium restriction is more effective than either dietary manipulation alone.

Blood Pressure↗

PubMed2001

Increased vitamin D receptor level enhances 1,25-dihydroxyvitamin D3-mediated gene expression and calcium transport in Caco-2 cells.

Altered vitamin D receptor (VDR) level has been proposed to explain differences in intestinal responsiveness to 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. We tested whether the enterocyte VDR level influences 1,25(OH)2D3-mediated gene expression and transepithelial calcium (Ca) transport in the human intestinal cell line Caco-2. Cells were stably transfected with a human metallothionein (hMT) IIA promoter-human VDR (hVDR) complementary DNA (cDNA) transgene that overexpressed hVDR in response to heavy metals. In MTVDR clones, induction of 25-hyroxyvitamin D3-24-hydroxylase (24-OHase) messenger RNA (mRNA) expression by 1,25(OH)2D3 (10(-9) M, 4 h) was correlated to metal-induced changes in nuclear VDR level (r2 = 0.99). In MTVDR clones, basal VDR level was 2-fold greater and 1,25(OH)2D3-mediated Ca transport (10(-7) M, 24 h) was 43% higher than in parental Caco-2 cells. Treatment of MTVDR clones with Cd (1 microM, 28 h) increased VDR level by 68%, significantly enhanced 1,25(OH)2D3-mediated Ca transport by 24%, and increased accumulation of calbindin D9K mRNA by 76% relative to 1,25(OH)2D3 alone. These observations support the hypothesis that the enterocyte VDR level is an important modulator of intestinal responsiveness to 1,25(OH)2D3.

Caco-2 Cells↗

PubMed2000

Leptin and bone: does the brain control bone biology?

The means by which obesity leads to high bone density and protects individuals from osteoporosis is not known. The study of bone biology in two mouse models of obesity, leptin-deficient (ob/ob) and leptin receptor-deficient (db/db) mice, points to a role for leptin in the control of bone density. When leptin action is missing in these mice, bone density is high. This is true despite concurrent hypogonadism and hypercortisolism, two strong proresorptive signals that would normally lead to low bone density. Curiously, leptin does not have a direct effect on osteoblasts, which suggests the existence of a central, neuroendocrine pathway that controls bone mass.

Animals↗

PubMed1999

Reciprocal regulation of HFE and NNamp2 gene expression by iron in human intestinal cells.

The newly identified hemochromatosis gene, HFE, and a candidate iron transporter gene, Nramp2, have been proposed as key factors responsible for the regulation of intestinal iron absorption. Although the exact functions of these proteins in intestinal iron absorption are unknown, HFE may be required for the down-regulation of iron absorption that occurs with increasing iron status, and Nramp2 may up-regulate iron absorption when iron status is low. Thus, we examined whether the expression of the HFE and Nramp2 genes are regulated by iron status in the human intestinal cell line Caco-2. HFE mRNA and HFE protein were increased and Nramp2 mRNA was decreased by increasing cellular iron status in Caco-2 cells. This iron-mediated modulation of mRNA levels was specific to iron. Moreover, super-induction of HFE mRNA in the presence of cycloheximide suggests that HFE gene expression may be controlled by a short-lived repressor protein. HFE and Nramp2 mRNA levels also changed in opposite directions during cellular differentiation. This reciprocal modification of the HFE and Nramp2 gene expression during both iron treatment and cell differentiation in Caco-2 cells is consistent with an opposing role for these proteins in homeostatic regulation of human intestinal iron absorption.

Caco-2 Cells↗

PubMed1999

Vitamin D receptors: not just in the nucleus anymore.

Calcitriol, the active metabolite of vitamin D, is a steroid hormone that regulates calcium metabolism and cell differentiation by interacting with its nuclear receptor--the vitamin D receptor (VDR)--and by stimulating gene transcription. During the last decade, calcitriol also has been shown to stimulate rapid signal transduction pathways. This observation supports the hypothesis that a membrane-bound receptor similar to those that mediate peptide hormone biology exists. Recent research provides evidence for such a unique membrane VDR. Future research challenges are to integrate the membrane VDR into existing knowledge of vitamin D biology.

Animals↗

PubMed1999

Specific 1,25(OH)2D3-mediated regulation of transcellular calcium transport in Caco-2 cells.

Calcium transport in the apical-to-basolateral (A-to-B) or B-to-A direction was examined in cells treated with 10 nM 1, 25-dihydroxyvitamin D3 [1,25(OH)2D3, calcitriol] for up to 72 h. Net A-to-B calcium transport was positive at all time points and increased from 0.14 +/- 0.06 to 0.50 +/- 0.01 nmol. well-1. min-1 after 72 h of calcitriol treatment. Neither phenol red transport nor transepithelial electrical resistance was altered by calcitriol treatment, suggesting that the increase in net A-to-B calcium transport was not due to paracellular movement. Neither 25-hydroxyvitamin D3 nor 24,25-dihydroxyvitamin D3 (100 nM, 48 h) alters basal or calcitriol-stimulated A-to-B calcium transport. Treatment with the calmodulin antagonist trifluoperazine (50 microM) reduced calcitriol-stimulated A-to-B Ca transport by 56%. The transcription inhibitor actinomycin D inhibited calcitriol-regulated A-to-B calcium transport as well as calbindin D9k and 24-hydroxylase mRNA accumulation. These data demonstrate that calcitriol-mediated A-to-B calcium transport in Caco-2 cells is a specific, transcellular process that requires transcriptional events normally mediated through the vitamin D receptor.

24,25-Dihydroxyvitamin D 3↗

PubMed1998

Identification of Nramp2 as an iron transport protein: another piece of the intestinal iron absorption puzzle.

Although a number of iron-binding proteins have been identified, the roles for specific proteins in mediating iron absorption have not been definitively assigned. Two recent papers report the identification of an iron transport protein that may be responsible for movement of iron from the intestinal lumen into the enterocyte. Coupled with the recent identification of the protein mutated in hemochromatosis, researchers are now establishing a clearer picture of the mechanism of intestinal iron absorption.

Animals↗

PubMed1998

The genetics of osteoporosis: vitamin D receptor polymorphisms.

Osteoporosis is a metabolic bone disease characterized by low bone mass and deterioration of bone tissue that leads to bone fragility and an increase in fracture risk. It is a disease with a complex etiology that includes genetic and environmental contributors. Environmental factors that influence bone density include dietary factors-such as intakes of calcium, alcohol, and caffeine-and lifestyle factors-such as exercise and smoking. Ethnic differences in the propensity to nontraumatic bone fracture suggest that genetic factors are important. Recently, common allelic variations in he vitamin D receptor gene have been found to be associated with bone mineral density in racially diverse population groups, as well as in prepubertal girls, young adult and postmenopausal women, and men. However, many studies have not been able to find this association. Additional approaches, such as sib-pair analysis, will probably be necessary in the future to identify the important determinants of osteoporosis.

Adult↗

PubMed1998

Intestinal calcium absorption in the aged rat: evidence of intestinal resistance to 1,25(OH)2 vitamin D.

We investigated the role of circulating 1,25-dihydroxycholecalciferol (1,25(OH)2D) and intestinal resistance to 1,25(OH)2D in the diminished intestinal calcium absorption capacity of the senescent rat. We measured plasma 1,25(OH)2D, total and unoccupied duodenal vitamin D receptor, duodenal calbindin D9k protein (calbindin D), and net dietary calcium absorption in rats at several ages. As expected, circulating 1,25(OH)2D, calbindin D, and net calcium absorption decreased with age. However, no age-related changes were evident in intestinal vitamin D receptor levels. We then measured duodenal calcium absorption from in situ intestinal loops after continuous s.c. infusion of 1,25(OH)2D for up to 6 days and found that despite a marked elevation of plasma 1,25(OH)2D duodenal calcium absorption was significantly lower in old compared with young rats. To assess calcium absorption over a wide physiological range of plasma 1,25(OH)2D, in a dose-response study we altered plasma 1,25(OH)2D by continuous infusion of 1,25(OH)2D (at 0, 4, or 14 ng/100 g BW/day) for 9 days. We found that the slope of the linear regression between plasma 1,25(OH)2D and duodenal Ca transport in old rats was only 46% of that observed in young rats, suggesting an age-related resistance of the duodenal calcium transport process to the hormonal action of 1,25(OH)2D. Collectively, our observations suggest a dual defect in vitamin D metabolism in old animals: one defect related to the low circulating levels of 1,25(OH)2D and a second defect related to a relative intestinal resistance to the action of 1,25(OH)2D, which is apparently not due to a reduction in intestinal vitamin D receptor levels.

Absorption↗

PubMed1997

Dietary selenium repletion may reduce cancer incidence in people at high risk who live in areas with low soil selenium.

Studies examining the relationship between dietary selenium intake and risk of various cancers have shown that low selenium intake is associated with higher cancer rates. A recent well-controlled intervention trial studied whether selenium supplementation can prevent cancer in subjects who have a history of skin cancer and live in areas of the United States with low soil selenium levels. Selenium supplementation did not reduce skin cancer rates, but the incidence of total, lung, colorectal, and prostate cancers was significantly reduced by the intervention. Although these data need confirmation, they suggest that adequate selenium intake is essential for cancer prevention.

Diet↗

PubMed1996

1 alpha,25-(OH)2-vitamin D3 analogs with minimal in vivo calcemic activity can stimulate significant transepithelial calcium transport and mRNA expression in vitro.

Several 1 alpha,25-(OH)2-vitamin D3 (1 alpha,25-(OH)2-D3) analogs have significant antiproliferative effects in vitro but do not elevate serum calcium in vivo. We tested whether the lack of a calcemic response of a vitamin D analog in vivo is due to its inability to stimulate intestinal calcium absorption by examining the effect of several such compounds on transepithelial calcium transport in the human colonic carcinoma cell line Caco-2. The relative stimulations of calcium transport by the four A-ring diastereomers of 1 alpha, 25-(OH)2-D3 (1 alpha,3 beta) and a 3 beta-bromoacetate analog (1 alpha,3 beta-BrAc) of the vitamin following 48-h treatment of cells at 10 nM were 1 alpha,3 beta (=100%), 1 alpha,3 alpha (+45.2%), 1 beta,3 beta (-15.6%), 1 beta,3 alpha (+6.5%), and 1 alpha,3 beta-BrAc (+50.6%). This was similar to the reported affinity of these compounds for the vitamin D receptor (VDR) and suggests that VDR binding predicts calcium transport. In contrast, three noncalcemic, sidechain- or D-ring-modified analogs of vitamin D, 1 alpha,25-(OH)2-16-ene-D3, 1 alpha,25-(OH)2-16-ene-23-yne-D3, and 1 alpha,25,28-(OH)3-D2 (at 10 nM for 48 h), showed a different relationship between VDR affinity (150, 60, and 63% of 1 alpha, 25-(OH)2-D3, respectively) and calcium transport (74.1, 126, and 10%, respectively). Elevated calcium transport was accompanied by higher 24-hydroxylase and calbindin D9k mRNA levels. The data demonstrate that although some vitamin D compounds cannot stimulate calcium transport due to an inability to interact with the VDR (e.g., 1 beta isomers), other factors, e.g., differential cellular metabolism, may account for variations in biological response in vivo to various vitamin D analogs.

Biological Transport↗

PubMed1996

Bone lead as a risk factor for hypertension in men.

While some have observed a positive relationship between blood lead levels and blood pressure, this relationship has been controversial. A recent study examined the hypothesis that bone lead levels, used as a measure of long-term lead accumulation, are associated with an increased risk of hypertension in men. Using a backward elimination logistic regression model, the researchers found that tibia lead, body mass index, and family history of hypertension contributed to the development of hypertension. An increase in tibia lead from 8 to 37 micrograms per gram of bone mineral was associated with an increased odds ratio of hypertension of 1.5. This is an interesting preliminary finding that requires support from additional experimentation.

Adult↗

PubMed1996

Discovery of the hemochromatosis gene will require rethinking the regulation of iron metabolism.

The identity of the protein responsible for hemochromatosis, the iron overload disease, has eluded scientists for years. However, a recent report identifies the gene where the hemochromatosis defect lies. It is a gene that encodes a major histocompatibility complex (MHC) class-1-like protein called HLA-H. The mechanism by which an HLA-H defect alters iron metabolism is still unidentified. However, this new discovery will certainly ignite a new wave of study into the physiology of iron metabolism and its regulation.

Animals↗

PubMed1996

The effects of aging on the bone inductive activity of recombinant human bone morphogenetic protein-2.

We examined the effects of gain on the ectopic bone-forming ability of recombinant human BMP-2 (rhBMP-2) in rats and investigated the mechanism by which aging might affect this type of bone. Bone formation induced after 12 days of sc implantation of 5 micrograms rhBMP-2 was reduced as animals aged from 1-16 months. The osteocalcin messenger RNA levels of implants also declined in aging animals. When the implant period was doubled, 16-month-old rats formed amounts of bone equivalent to those in 3-month-old rats. Increasing the dose of rhBMP-2 increased bone formation in older rats. To get a response comparable to that seen in 1-month-old rats given 5 micrograms rhBMP-2 for 12 days, 3-month-old rats required 30 micrograms rhBMP-2, whereas 16-month-old rats required 60 micrograms. Treatment with either GH or 1,25-dihydroxyvitamin D3 during the 12-day implantation period returned the bone formation in 16-month-olds rats to that in 3-month-old rats. These studies show that aging blunts rhBMP-2 inducted bone formation in rats. We speculate that the decreased response may be due in part to a decrease in the number of mesenchymal stem cells present in order rats or to a change in the responsiveness of these target cells to rhBMP-2.

Aging↗

PubMed1995

The BsmI vitamin D receptor restriction fragment length polymorphism (BB) predicts low bone density in premenopausal black and white women.

We conducted a study to determine whether a recently described restriction fragment length polymorphism in the vitamin D receptor gene (VDR-RFLP) predicts bone mineral density (BMD) in unrelated, premenopausal women as well as to determine the racial contribution to any genotypic influences on BMD. White (n = 83) and black (n = 72) women between 20 and 40 years of age were genotyped based on the presence (b) or absence (B) of a BsmI restriction enzyme site in the VDR gene, and BMD in the lumbar spine and femur neck was determined for each subject. There were 16 BB, 73 Bb, and 66 bb women. No significant difference was observed in genotypic distribution between the racial groups. The interaction of race by genotype on age- and body mass index (BMI)-adjusted BMD was not significant at either site. Age- and BMI-adjusted BMD was higher in black women at the spine (by 7.2%, p = 0.046) and femur neck (7.3% higher, p = 0.004). In the group as a whole, mean BMD in the femur neck was lower in the BB women that the bb (by 8.1%, p = 0.034) or Bb women (by 9.3%, p = 0.015) after controlling for age, BMI, race, and the race by genotype interaction. Adjusted lumbar spine BMD was lower in the BB women than the Bb women (6.4% lower, p = 0.036) in the group as a whole. No difference were detected between Bb and bb women at either site. A similar pattern of low BMD at the femur neck and the lumbar spine was seen in BB women of both races.(ABSTRACT TRUNCATED AT 250 WORDS)

Absorptiometry, Photon↗

PubMed1995

Moderate folate deficiency does not cause global hypomethylation of hepatic and colonic DNA or c-myc-specific hypomethylation of colonic DNA in rats.

Global and gene-specific DNA hypomethylation is considered to be an important early epigenetic event in several human neoplasms. A growing body of evidence has suggested that DNA methylation can be altered by dietary manipulation of methyl group donors. This study investigated whether moderate depletion of folate, a dietary component needed for the synthesis of methyl groups, would cause decreased hepatic and colonic S-adenosylmethionine concentrations, and thereby lead to global and/or protooncogene-specific DNA hypomethylation. Weanling rats were fed an amino acid-defined diet containing either 0 or 8 mg folate/kg diet for 15 or 24 wk. Significantly lower systemic, hepatic and colonic folate concentrations were observed in the moderately folate-depleted rats than in controls at both 15 and 24 wk (P < 0.005). Although hepatic S-adenosylmethionine was significantly lower in the moderately folate-depleted rats than in controls at the two time points (P < 0.03), colonic S-adenosylmethionine concentrations were not significantly different between the two groups at either time point. No significant differences between the folate-depleted and control animals could be detected with regard to global DNA methylation in the liver or colonic mucosa. Furthermore, c-myc protooncogene-specific DNA methylation in the colonic mucosa was not significantly different between these two groups of animals. These results indicate that moderate folate depletion does not cause a significant reduction in global DNA methylation in liver or colonic mucosa or in c-myc-specific colonic mucosal DNA methylation in this rat model.

Animals↗

PubMed1995

Are low-sodium diets appropriate for treated hypertensive men?

Because a low-sodium diet is often recommended as part of the treatment to control hypertension, a recent study examined the relationship between sodium intake, 24-hour urinary sodium excretion, and subsequent cardiovascular disease in treated hypertensive men and women. Contrary to expectations, the men with the lowest urinary sodium excretion had the highest incidence of myocardial infarction. Concerns about the group studied and some of the measurements used raise doubt about the results of this study, especially in light of the weight of data from population studies supporting a link between low blood pressure and low sodium intake. However, should future studies corroborate this one, clinicians will need to re-evaluate the simple notion that severe dietary sodium restriction is always beneficial for people with hypertension.

Blood Pressure↗

PubMed1995

A new role for lactoferrin: DNA binding and transcription activation.

The specific function of lactoferrin, an iron-binding protein found predominantly in milk and in granulocytes, is unclear. However, researchers now believe that it may modulate immune function. Recent research shows that lactoferrin enters the cell from the serum and is transported into the nucleus where it binds DNA. Specific DNA sequences that can confer lactoferrin-induced gene transcription of a reporter gene have now been identified. This potential direct transcriptional function of lactoferrin is unique and remains to be confirmed in whole cells or tissues. These observations have great potential for explaining the molecular basis for lactoferrin's role in the inflammatory response and in the transfer of immunity from mother to child.

Base Sequence↗