A possible role of a nuclear factor NF-LPH1 in the regional expression of lactase-phlorizin hydrolase along the small intestine

Thyroid and glucocorticoid hormones induce expression of lactase-phlorizin hydrolase gene in CDX-2/HNF-1α co-transfected IEC-6 cells

Thyroid and glucocorticoid hormones and several transcriptional factors such as caudal type homeobox (CDX)-2 and hepatocyte nuclear factor (HNF)-1α are important for the differentiation of small intestinal absorptive cells and the consequent expression of genes related to the digestion/absorption of carbohydrates. In this study, we investigated whether thyroid and glucocorticoid hormones enhanced the expression of lactase-phlorizin hydrolase (LPH) gene, an intestine-specific gene that encodes an enzyme for lactose digestion, in small intestinal stem-like IEC-6 cells co-transfected with CDX-2 and HNF-1α using a retrovirus system. Changes in expression of intestine-specific genes caused by treatment with thyroid and/or glucocorticoid hormones were monitored in empty vector-transfected cells and in CDX-2/HNF-1α co-transfected cells by qRT-PCR. Stable co-transfection with CDX-2 and HNF-1α evoked the expression of the LPH gene in IEC-6 cells. Furthermore, treatment with a thyroid hormone, triiodothyronine, and a glucocorticoid receptor agonist, dexamethasone, significantly enhanced expression of the LPH, CDX-2 and HNF-1α genes in CDX-2/HNF-1α co-transfected IEC-6 cells. These results suggest that thyroid and glucocorticoid hormones synergistically enhance expression of the LPH gene in CDX-2/HNF-1α co-transfected IEC-6 cells.

The critical period for thyroid hormone responsiveness through thyroid hormone receptor isoform alpha in the postnatal small intestine

During second and third weeks after birth in rats, serum thyroid hormone level is elevated. In this study, we investigated the jejunal expression of thyroid hormone receptor (TR) alpha in developing rats. The TRalpha-1 mRNA level and TRalpha-1/TRalpha-2 mRNA ratio increased two-fold from 5 to 13 days after birth. This high level of TRalpha-1 mRNA was maintained until 20 days and then decreased to the basal level by the end of weaning period at 27 days; however, the level of TRalpha-2 mRNA remained unchanged throughout the developmental period. The increase in the TRalpha-1/TRalpha-2 mRNA ratio from 5 to 13 days was accompanied by an initial rise in the levels of mRNA for hexose transporters in the jejunum. Administration of T(3) during the suckling period (8-13 days) caused a 50% increase in the TRalpha-1/TRalpha-2 mRNA ratio, while administration of T(3) on days 12-17 and days 16-21, but not on days 22-27, caused a two to four-fold increase in the levels of mRNA for hexose transporters. These results suggest that a transient variation in the TRalpha-1/TRalpha-2 expression ratio is closely related to the critical period of thyroid hormone responsiveness for hexose transporters expression in the developing rat jejunum.

The possible roles of homeobox protein, Cdx-2 for the expression of LPH gene during postnatal development

The expression of intestinal lactase-phlorizin hydrolase (LPH) gene normally decreases after completion of weaning in almost all mammals. To elucidate the mechanism whereby LPH gene expression is regulated during the suckling-weaning period, we studied the effects of the thyroid hormone (T(3)) on LPH gene expression in the small intestine during postnatal development in the rat. Firstly, we measured LPH mRNA level in rat jejunum at 5, 13, 20 and 27 days after birth. The amount of LPH mRNA at 27 days was significantly lower than that at 5 days. The transcript level of Cdx-2, which is a putative transcriptional factor for regulation of LPH gene expression, was also significantly decreased after 21 days. The binding of nuclear protein to the cis element CE-LPH1 on the promoter region of the LPH gene was reduced at the end of the weaning period. Daily intraperitoneal (i.p.) injection of T(3) for 6 days during days 22-27 significantly reduced LPH mRNA level by day 27 (50%, P<0.01), but injection of T(3) during days 8-13 did not. Moreover, i.p. T(3) injection during days 22-27 was accompanied by a reduction in the level of Cdx-2 mRNA. Our study suggests that the decrease in the LPH gene expression during the weaning period is associated with a reduction of Cdx-2 expression caused by thyroid hormone.

Mechanisms responsible for poor oral bioavailability of paeoniflorin: Role of intestinal disposition and interactions with sinomenine

PURPOSE

To determine the intestinal disposition mechanisms of paeoniflorin, a bioactive glucoside, and to investigate the mechanisms by which sinomenine increases paeoniflorin bioavailability.

MATERIALS AND METHODS

A single-pass "four-site" rat intestinal perfusion model and a cultured Caco-2 cell model were employed.

RESULTS

In both model systems, paeoniflorin permeability was poor. In the perfusion model, maximal absorption and metabolism of paeoniflorin occurred in duodenum and jejunum, which were significantly decreased by a glucosidase inhibitor gluconolactone (20 mM). On the other hand, paeoniflorin absorption in terminal ileum increased significantly but its metabolism did not in the presence of sinomenine and cyclosporine A. In the Caco-2 cell model, paeoniflorin was transported 48-fold slower than its aglycone (paeoniflorigenin). Absorptive transport of paeoniflorin was significantly (p < 0.05) increased by sinomenine (38%), verapamil (27%), and cyclosporine A (41%), whereas its secretory transport was significantly (p < 0.01) decreased by sinomenine (50%), verapamil (35%) and cyclosporine A (37%). In contrast, MRP inhibitors MK-571 and leukotriene C4 did not affect transport of paeoniflorin. Lastly, sinomenine was also shown to significantly increase the absorptive transport of digoxin (a prototypical p-glycoprotein substrate) and to significantly decrease its secretory transport.

CONCLUSIONS

Poor permeation, p-gp-mediated efflux, and hydrolysis via a glucosidase contributed to the poor bioavailability of paeoniflorin. Sinomenine (an inhibitor of the p-gp-mediated digoxin efflux) increased paeoniflorin's bioavailability via the inhibition of p-gp-mediated paeoniflorin efflux in the intestine.

Dietary Sucrose Enhances Intestinal Lactase Gene Expression in Euthyroid Rats

It is postulated that dietary carbohydrates and thyroid hormones are major regulators for expression of the lactase/phlorizin hydrolase (LPH) gene in rat jejunum. In this study, we investigated the effects of thyroid hormones and dietary sucrose on LPH gene expression and lactase activity in starved rats. Firstly, animals at 8 wk of age were fed a low-starch diet (5.5% energy as cornstarch) or high-starch diet (71% energy as cornstarch) for 7 d (experiment 1). The mRNA level of LPH as well as lactase activity significantly decreased in rats fed the low-starch diet as compared to those fed the high-starch diet. To investigate the effects of thyroid hormone status, the animals previously fed the low-starch diet were starved for 3 d, and half of the animals were given intraperitoneal (i.p.) injections of 20 microg/ 100 g body weight triiodothyronine (T3) twice daily (experiment 2). The LPH mRNA level and lactase activity were elevated by starvation for 3 d, but they were repressed by the injection of T3 during starvation. To investigate the effects of dietary sucrose in starved rats, they were force-fed a sucrose diet for 6 h (experiment 3). The LPH gene expression and lactase activity were up-regulated by force-feeding a sucrose diet, only when the animals were kept in euthyroid status by daily T3 administrations. In contrast, the sucrase-isomaltase mRNA levels and sucrase activity were unaffected by force-feeding the sucrose diet for both T3-treated and untreated starved rats. Our work suggests that dietary sucrose is capable of enhancing lactase gene expression in starved rats when they have a sustainable thyroid hormone level.

Adult-type hypolactasia and regulation of lactase expression

A common genetically determined polymorphism in the human population leads to two distinct phenotypes in adults, lactase persistence and adult-type hypolactasia (lactase non-persistence). All healthy newborn children express high levels of lactase and are able to digest large quantities of lactose, the main carbohydrate in milk. Individuals with adult-type hypolactasia lose their lactase expression before adulthood and consequently often become lactose intolerant with associated digestive problems (e.g. diarrhoea). In contrast, lactase persistent individuals have a lifelong lactase expression and are able to digest lactose as adults. Lactase persistence can be regarded as the mutant phenotype since other mammals down-regulate their lactase expression after weaning (the postweaning decline). This phenomenon does not occur in lactase persistent individuals. The regulation of lactase expression is mainly transcriptional and it is well established that adult-type hypolactasia is inherited in an autosomal recessive manner, whereas persistence is dominant. The recent findings of single nucleotide polymorphisms associated with lactase persistence have made it possible to study the potential mechanisms underlying adult-type hypolactasia. This work has led to the identification of gene-regulatory sequences located far from the lactase gene (LCT). The present review describes the recent advances in the understanding of the regulation of lactase expression and the possible mechanisms behind adult-type hypolactasia.

Intestinal brush border membrane catalyzes hydrolysis of pyridoxine-5'-beta-D-glucoside and exhibits parallel developmental changes of hydrolytic activities toward pyridoxine-5'-beta-D-glucoside and lactose in rats

Pyridoxine-5'-beta-D-glucoside (PNG) is a major form of vitamin B-6 in plant foods that exhibits partial bioavailability as vitamin B-6 in humans. We previously identified an intestinal mucosal cytosolic PNG hydrolase that catalyzes the partial hydrolysis of PNG absorbed without prior deglycosylation. Recent observations that the brush border membrane also catalyzes PNG hydrolysis led to the hypothesis that PNG hydrolysis may be another function of the beta-glucosidase lactase-phlorizin hydrolase (LPH) and, thus, brush border PNG hydrolysis would undergo a developmental decline similar to that of lactose hydrolysis. In this study, the relationships among hydrolytic activities in small intestinal cytosolic and brush border fractions in rats (n = 9 per group) of various ages (1-2 d and 2, 4, 8, 12 and 24 wk) were examined. In vitro specific activities toward PNG and lactose were greater in brush border than cytosol, and these were greater in newborn rats than in all other age groups (P < 0.01). Brush border activities toward PNG and lactose and were closely correlated (r = 0.84; P < 0.0001). These findings suggest that the hydrolysis of PNG is catalyzed at least partially at the brush border and that the bioavailability of PNG may be influenced by the residual LPH activity in children and adults.