Thyroid hormone and the gut: selective transcriptional activation of a villus-enterocyte marker

Novel Therapeutic Approaches for Mitigating Complications in Short Bowel Syndrome

Short bowel syndrome (SBS) is a particularly serious condition in which the small intestine does not absorb sufficient nutrients for biological needs, resulting in severe illness and potentially death if not treated. Given the important role of the gut in many signaling cascades throughout the body, SBS results in disruption of many pathways and imbalances in various hormones. Due to the inability to meet sufficient nutritional needs, an intravenous form of nutrition, total parental nutrition (TPN), is administered. However, TPN presents difficulties such as severe liver injury and altered signaling secondary to the continued lack of luminal contents. This manuscript aims to summarize relevant studies into the systemic effects of TPN on systems such as the gut-brain, gut-lung, and gut-liver axis, as well as present novel therapeutics currently under use or investigation as mitigation strategies for TPN induced injury.

A curious case of refractory hypothyroidism due to selective malabsorption of oral thyroxine

There are very few cases in the literature in which refractory persistent hypothyroidism responded only to parenteral doses of levothyroxine and no evidence of any malabsorptive disorder could be identified. Here, we present a rare case of a 35-year-old woman with refractory hypothyroidism who responded only to intravenous doses of levothyroxine. We also discuss possible causes for the same.

Intestinal alkaline phosphatase: multiple biological roles in maintenance of intestinal homeostasis and modulation by diet

The diverse nature of intestinal alkaline phosphatase (IAP) functions has remained elusive, and it is only recently that four additional major functions of IAP have been revealed. The present review analyzes the earlier literature on the dietary factors modulating IAP activity in light of these new findings. IAP regulates lipid absorption across the apical membrane of enterocytes, participates in the regulation of bicarbonate secretion and of duodenal surface pH, limits bacterial transepithelial passage, and finally controls bacterial endotoxin-induced inflammation by dephosphorylation, thus detoxifying intestinal lipopolysaccharide. Many dietary components, including fat, protein, and carbohydrate, modulate IAP expression or activity and may be combined to sustain a high level of IAP activity. In conclusion, IAP has a pivotal role in intestinal homeostasis and its activity could be increased through the diet. This is especially true in pathological situations (e.g., inflammatory bowel diseases) in which the involvement of commensal bacteria is suspected and when intestinal AP is too low to detoxify a sufficient amount of bacterial lipopolysaccharide.

Nuclear receptor co-repressor is required to maintain proliferation of normal intestinal epithelial cells in culture and down-modulates the expression of pigment epithelium-derived factor

Stem cells of the gut epithelium constantly produce precursors that progressively undergo a succession of molecular changes resulting in growth arrest and commitment to a specific differentiation program. Few transcriptional repressors have been identified that maintain the normal intestinal epithelial cell (IEC) proliferation state. Herein, we show that the nuclear receptor co-repressor (NCoR1) is differentially expressed during the proliferation-to-differentiation IEC transition. Silencing of NCoR1 expression in proliferating cells of crypt origin resulted in a rapid growth arrest without associated cell death. A genechip profiling analysis identified several candidate genes to be up-regulated in NCoR1-deficient IEC. Pigment epithelium-derived factor (PEDF, also known as serpinf1), a suspected tumor suppressor gene that plays a key role in the inhibition of epithelial tissue growth, was significantly up-regulated in these cells. Chromatin immunoprecipitation experiments showed that the PEDF gene promoter was occupied by NCoR1 in proliferating epithelial cells. Multiple retinoid X receptor (RXR) heterodimers interacting sites of the PEDF promoter were confirmed to interact with RXR and retinoid acid receptor (RAR). Cotransfection assays showed that RXR and RAR were able to transactivate the PEDF promoter and that NCoR1 was repressing this effect. Finally, forced expression of PEDF in IEC resulted in a slower rate of proliferation. These observations suggest that NCoR1 expression is required to maintain IEC in a proliferative state and identify PEDF as a novel transcriptional target for NCoR1 repressive action.

Upregulation of Slc39a10 gene expression in response to thyroid hormones in intestine and kidney

A novel zinc transporter has been purified and cloned from rat renal brush border membrane. This transporter was designated as Zip10 encoded by Slc39a10 gene and characterized as zinc importer. Present study documents the impact of thyroid hormones on the expression of Zip10 encoded by Slc39a10 gene in rat model of hypo and hyperthyroidism. Serum T(3) and T(4) levels were reduced significantly in hypothyroid rats whereas these levels were significantly elevated in hyperthyroid rats as compared to euthyroid rats thereby confirming the validity of the model. Kinetic studies revealed a significant increase in the initial and equilibrium uptake of Zn(++) in both intestinal and renal BBMV of hyperthyroid rats in comparison to hypothyroid and euthyroid rats. By RT-PCR, Slc39a10 mRNA expression was found to be significantly decreased in hypothyroid and increased in hyperthyroid as compared to euthyroid rats. These findings are in conformity with the immunofluorescence studies that revealed markedly higher fluorescence intensity at periphery of both intestinal and renal cells isolated from hyperthyroid rats as compared to hypothyroid and euthyroid rats. Higher expression of Zip10 protein in hyperthyroid group was also confirmed by western blot. These findings suggest that expression of zinc transporter protein Zip10 (Slc39a10) in intestine and kidney is positively regulated by thyroid hormones.

When normal is not: The dilemma of interpreting laboratory averages of bioactive molecules subject to heterogeneous regulatory feedback and epigenetic mosaicism

Complex regulatory systems control the levels of many bioactive molecules in the serum. These systems involve the integration of feedback responses from numerous tissues. End-organs and tissues can manifest epigenetic mosaicism, particularly with aging or disease states. We propose that an isolated lab value may reflect a blended average of inhomogeneous feedback responses from target tissues in various states of dysfunction. Reliance on such data may underestimate the state of systemic dysfunction. Yet in clinical practice, normal serum levels of a given molecule and its associated regulatory machinery are often assumed to reflect normal body homeostasis and tissue function. Organism-wide integration of abnormally high and low levels of bioactivity of a molecule in different tissues may yield apparently normal serum values of the bioactive molecule and known components of its regulatory system. We specifically discuss thyroid hormone regulation and function as a case example. Epigenetic reprogramming of either regulatory loops or tissue responses represents another way in which normal serum levels of the molecule may obscure target-organ dysfunction. The proposed idea has broad implications for disease pathogenesis, diagnosis, and therapies. A model where individual tissues employ illegitimate signaling to subvert the concerns of the organism as a whole is also proposed.

Thyroid hormones increase Na+–Pi co-transport activity in intestinal brush border membrane: Role of membrane lipid composition and fluidity

In the present study, we documented the promising role of thyroid hormones status in animals in modulation of Na+-Pi transport activity in intestinal brush border membrane vesicles (BBMV) which was accompanied with alterations in BBM lipid composition and fluidity. Augmentation of net Pi balance in hyperthyroid (Hyper-T) rats was fraternized with accretion of Pi transport across BBMV isolated from intestine of Hyper-T rats as compared to hypothyroid (Hypo-T) and euthyroid (Eu-T) rats while Na+-Pi transport across BBMV was decreased in Hypo-T rats relative to Eu-T rats. Increment in Na+-Pi transport in intestinal BBMV isolated from Hyper-T rats was manifested as an increase in the maximal velocity (Vmax) of Na+-Pi transport system. Furthermore, BBMV lipid composition profile in intestinal BBM from Hyper-T was altered to that of Hypo-T rats and Eu-T rats. The molar ratio of cholesterol/phospholipids was higher in intestinal BBM from Hypo-T rats. Fluorescence anistropy of diphenyl hexatriene (rDPH) and microviscosity were significantly decreased in the intestinal BBM of Hyper-T rats and decreased in Hypo-T rats as compared to Eu-T rats which corroborated with the alteration in membrane fluidity in response to thyroid hormone status of animals. Therefore, thyroid hormone mediated change in membrane fluidity might play an important role in modulating Na+-Pi transport activity of intestinal BBM.

Differential regulation of intestinal alkaline phosphatase gene expression by Cdx1 and Cdx2

We have examined the role that the caudal-related homeobox transcription factors Cdx1 and Cdx2 play in activating the enterocyte differentiation marker gene intestinal alkaline phosphatase (IAP). Human colon cancer Caco-2 cells were transiently transfected with Cdx1 and/or Cdx2, and semiquantitative RT-PCR was used to study the effects on IAP mRNA expression. Transfections with a variety of IAP-luciferase reporter constructs were used to identify a Cdx response element located within the human IAP gene promoter. Protein-DNA interactions were examined by EMSA. Results showed that Cdx1 markedly induced IAP mRNA expression, whereas Cdx2 did not, and, in fact, inhibited the Cdx1 effects. Functional analysis revealed that Cdx1 transactivates (fourfold, P < 0.05) the IAP promoter through a novel Cdx response element (GTTTAGA) located between -2369 and -2375 upstream of the translational start site. EMSA showed that both Cdx1 and Cdx2 could bind to the cis element, but in cotransfection experiments, Cdx2 inhibited the Cdx1 effects by approximately 50%. Thus we have identified a previously unrecognized interaction between two important gut transcription factors, Cdx1 and Cdx2, in the context of IAP gene regulation. Cdx1 activates the IAP gene via a novel cis element, whereas Cdx2 inhibits the Cdx1 effects.

Thyroid hormones stimulate Na+-Pi transport activity in rat renal brush-border membranes: role of membrane lipid composition and fluidity

Antecedent studies have suggested that lipid composition and fluidity of cellular membranes of various organs are altered in response to thyroid hormone status. To date, the effects of thyroid hormone status on these parameters have not been examined in rat renal apical membrane in regard to sodium-dependent phosphate transport. In the present study, we determined the potential role of alterations in cortical brush-border membrane lipid composition and fluidity in modulation of Na+-Pi transport activity in response to thyroid hormone status. Thyroid hormone status influences the fractional excretion of Pi, which is associated with alteration in renal brush-border membrane phosphate transport. The increment in Na+-Pi transport in renal BBMV isolated from Hyper-T rats is manifested as an increase in the maximal velocity (Vmax) of Na+-Pi transport. Further, the cholesterol content was significantly increased in renal BBM of Hypo-T rats and decreased in Hyper-T rats as compared to the Eu-T rats. The molar ratio of cholesterol/phospholipids was also higher in renal BBM from hypo-T rats. Subsequently, fluorescence anisotropy of diphenyl hexatriene (rDPH) and microviscosity were significantly decreased in the renal BBM of the Hyper-T rats and increased in the Hypo-T rats as compared to Eu-T rats. The result of this study, therefore, suggest that alteration in renal BBM cholesterol, cholesterol/phospholipid molar ratio, and membrane fluidity play an important role in the modulation of renal BBM Na+-Pi transport in response to thyroid hormone status of animals.

Thyroid hormones stimulate calcium transport systems in rat intestine

Thyroid hormone status influences calcium metabolism. To elucidate the mechanism of action of thyroid hormones on transcellular transport of calcium in rat intestine, Ca(2+) influx and efflux studies were carried out in brush border membrane vesicles (BBMV) and across the basolateral membrane (BLM) of enterocytes, respectively. Steady-state uptake of Ca(2+) into BBMV as well as Ca(2+) efflux from the BLM enterocytes was significantly increased in hyperthyroid (Hyper-T) rats and decreased in hypothyroid (Hypo-T) rats as compared to euthyroid (Eu-T) rats. Kinetic studies revealed that increase in steady state Ca(2+) uptake into BBMV from hyper-T rats was fraternized with decrease in Michaelis Menten Constant (K(m)), indicating a conformational change in Ca(2+) transporter. Further, this finding was supported by significant changes in transition temperature and membrane fluidity. Increased Ca(2+) efflux across enterocytes was attributed to sodium-dependent Ca(2+) exchange activity which was significantly higher in Hyper-T rats and lower in Hypo-T rats as compared to Eu-T rats. However, there was no change in Ca(2+)-ATPase activity of BLMs of all groups. Kinetic studies of Na(+)/Ca(2+) exchanger revealed that alteration in Na(+)-dependent Ca(2+) efflux was directly associated with maximal velocity (V(max)) of exchanger among all the groups. cAMP, a potent activator of Na(+)/Ca(2+) exchanger, was found to be significantly higher in intestinal mucosa of Hyper-T rats as compared to Eu-T rats. Therefore, the results of this study suggest that Ca(2+) influx across BBM is possibly modulated by thyroid hormones by mediating changes in membrane fluidity. Thyroid hormones activated the Na(+)/Ca(2+) exchange in enterocytes possibly via cAMP-mediated pathway.

The effect of early feeding on growth and small intestinal development in the posthatch poult

Poults with early access to feed in the hatchery or turkey house grew more than those reared under standard commercial practice. During 48 h posthatch, fed poults utilized yolk and exogenous feed to increase BW by 11 g. The small intestine increased from 3.8% of BW at hatch to 8.9% after 48 h. In contrast, BW in feed-deprived poults decreased by 10 g, whereas the small intestine increased slightly in weight and composed 4.5% of BW after 48 h. The number of cells per villus and the villus surface area increased dramatically posthatch in the duodenum but more slowly in the jejunum and ileum. Enterocyte width changed little, but length increased more than twofold in the duodenum and by approximately 50% in the jejunum and ileum by 6 d posthatch. Lack of access to feed depressed the rate of growth of villi and enterocyte length in all intestinal segments until 6 d posthatch. All intestinal epithelial cells were proliferating at hatch, which changed rapidly within 48 h posthatch, with proliferating cells becoming located mainly in the intestinal crypts where about half of the cells were proliferating. In feed-deprived poults, the decrease in the proportion of proliferating cells in the crypt was greater than that of fed poults; after refeeding, an increase in the rate of proliferation was observed in feed-deprived poults. Plasma concentrations of Na, glucose, triglycerides, and phospholipids were not affected by feed deprivation; however, nonesterified fatty acid concentrations were enhanced in feed-deprived poults, indicating a greater use of fatty acids for energy. Plasma triiodothyronine (T3) concentrations, which may mediate some of the intestinal effects of feed deprivation, were depressed in poults without access to feed.

Functional interference between thyroid hormone receptor alpha (TRalpha) and natural truncated TRDeltaalpha isoforms in the control of intestine development

Thyroid hormone is known to participate in the control of intestine maturation at weaning. Its action is mediated by the thyroid hormone nuclear receptors, encoded by the TRalpha and TRbeta genes. Since previous studies have shown that TRbeta plays a minor role in the gut, we focused here our analysis on the TRalpha gene. The TRalpha locus generates the TRalpha1 receptor together with the splicing variant TRalpha2 and the truncated products TRDeltaalpha1 and TRDeltaalpha2, which all lack an intact ligand binding domain. The TRDeltaalpha isoforms are transcribed from an internal promoter located in intron 7, and their distribution is restricted to a few tissues including those of the intestine. In order to define the functions of the different isoforms encoded by the TRalpha locus in the intestinal mucosa, we produced mice either lacking all known TRalpha products or harboring a mutation which inactivates the intronic promoter. We performed a detailed analysis of the intestinal phenotypes in these mice and compared it to that of the previously described TRalpha(-/-) mice, in which TRalpha isoforms are abolished but the TRDeltaalpha isoforms remain. This comparative analysis leads us to the following conclusions: (i) the TRalpha1 receptor mediates the T3-dependent functions in the intestine at weaning time and (ii) the TRDeltaalpha products negatively control the responsiveness of the epithelial cells to T3. Moreover, we show that TRDeltaalpha proteins can interfere with the transcription of the intestine-specific homeobox genes cdx1 and cdx2 and that their activity is regulated by TRalpha1. Altogether these data demonstrate that cooperation of TRalpha and TRDeltaalpha products is essential to ensure the normal postnatal development of the intestine and that mutations in the TRalpha locus can generate different phenotypes caused by the disruption of the equilibrium between these products.

Changes in growth and function of chick small intestine epithelium due to early thermal conditioning

The effect of exposure to heat at 3 d of age on small intestine functionality and development was assayed by measuring villus size, proliferating enterocytes, and brush-border membrane (BBM) enzyme expression and activity. Results showed that thermal conditioning caused an immediate effect characterized by lowered triiodothyronine (T3) level, reduced feed intake, and depressed enterocyte proliferation and BBM enzyme activity. A second series of effects, observed 48 h posttreatment, was characterized by elevated T3, increased feed intake, increased enterocyte proliferation, and higher expression and activity of BBM enzymes. The association between ambient temperature, feed intake, growth rate, and plasma T3 levels was reflected in the structure and function of the intestinal tract. The results suggest that thermal conditioning at an early age influences T3 concentrations, which in turn alter the intestinal capacity to proliferate, grow, and digest nutrients. However, these experiments were not able to discriminate between effects due to feed intake and those due to thermal conditioning. The treatments modulated changes in the intestinal tract following thermal treatment.

Unusual malabsorption of levothyroxine

We report a 50-year-old woman, with overt hypothyroidism undergoing thyrotropin (TSH)-stimulating hormone suppressive levothyroxine (LT4) treatment after subtotal thyroidectomy. At her first visit to our department, the laboratory results revealed a borderline low free thyroxine (FT4) level accompanied by a clearly elevated TSH level. Both parameters did not significantly change during therapy with an oral dose of 500 microg of LT4. Investigations revealed malabsorption of oral administrated LT4. Thyroid serum hormone levels only became normal during parenteral therapy with LT4.

Thyroid hormones modulate zinc transport activity of rat intestinal and renal brush-border membrane

Thyroid hormone status influences the Zn2+ and metallothionein levels in intestine, liver, and kidney. To evaluate the impact of thyroid hormones on Zn2+ metabolism, Zn2+ uptake studies were carried out in intestinal and renal brush-border membrane vesicles (BBMV). Steady-state Zn2+ transport in intestinal and renal cortical BBMV was increased in hyperthyroid (Hyper-T) rats and decreased in the hypothyroid (Hypo-T) rats relative to euthyroid (Eu-T) rats. In both the intestinal and renal BBMV, Hyper-T rats showed a significant increase in maximal velocity compared with Eu-T and Hypo-T rats. Apparent Michaelis constant was unaltered in intestinal and renal BBMV prepared from the three groups. Fluorescence anisotropy of diphenyl hexatriene was decreased significantly in intestinal and renal brush-border membrane (BBM) isolated from Hyper-T rats compared with Hypo-T and Eu-T rats. A significant reduction in the microviscosity and transition temperature for Zn2+ uptake in intestinal and renal BBM from Hyper-T rats is in accordance with the increased fluidity of these BBMs. These findings suggest that the increased rate of Zn2+ transport in response to thyroid hormone status could be associated with either an increase in the number of Zn2+ transporters or an increase in the active transporters due to alteration in the membrane fluidity. Thus the thyroid hormone-mediated change in membrane fluidity might play an important role in modulating Zn2+ transport activity of intestinal and renal BBM.