1,25(OH)2D3-dependent regulation of calbindin-D28k mRNA requires ongoing protein synthesis in chick duodenal organ culture

Influence of feeding and UVB exposition on the absorption mechanisms of calcium in the gastrointestinal tract of veiled chameleons (Chamaeleo calyptratus)

The purpose of this study was to investigate the influence of feeding and UVB exposition on the occurrence and distribution patterns of vitamin D receptors (VDR) and calbindin D28k (Cb-D28k) in the gastrointestinal tract of veiled chameleons. Thus, 56 veiled chameleon hatchlings were divided into six treatment groups: UV (with UVB exposure); No (no supplements, no UVB exposure); CaAUV (with calcium (Ca), vitamin A supplementation, UVB exposure); CaA (with Ca, vitamin A supplementation); CaADUV (with Ca, vitamin A, vitamin D supplementation, UVB exposure); and CaAD (with Ca, vitamin A, vitamin D supplementation). Animals were reared under the suspected conditions for 6 months on locust-based diets. Tissue samples of stomach, duodenum, ileum and colon were taken, and semi-quantitative immunohistochemical methods (IHC) were performed to detect Cb-D28k and VDR. VDR immunoreactions were higher in the luminal epithelium of the duodenum than in that of the ileum. VDR immunoreactions in the luminal epithelium were higher at the base of the villi of the duodenum as compared to the tip. Cb-D28k immunoreactions were mainly observed in the luminal epithelium of the duodenum. The two groups treated with all dietary supplements (CaADUV, CaAD) exhibited a higher Cb-D28k immunoreaction as those with no supplements and UVB exposure only. No immunoreaction for both proteins could be detected in the stomach. This study suggests that the duodenum plays an important role in the active transcellular absorption of Ca in veiled chameleons as shown by the immunohistochemical detection of VDR and Cb-D28k. Expression of Cb-D28k, in particular, appears to be regulated by dietary supplementation of vitamin D and vitamin A. VDRs, however, tended to be upregulated when animals were not supplemented with Ca, vitamin D and vitamin A. This may be due to the decreased Ca concentrations which caused vitamin D activation in the skin without any supplementation, but UVB exposure.

Protein synthesis is required for optimal induction of 25-hydroxyvitamin D(3)-24-hydroxylase, osteocalcin, and osteopontin mRNA by 1,25-dihydroxyvitamin D(3)

The regulation of the 25-hydroxyvitamin D(3)-24-hydroxylase gene by 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) has been extensively studied. It is well established that two vitamin D response elements in the promoter are responsible for the 1,25(OH)(2)D(3) induction of transcription. Surprisingly, this induction is blocked by the protein synthesis inhibitor, cycloheximide (CHX). In AOK-B50 cells, 1,25(OH)(2)D(3) caused a large induction of 24-hydroxylase mRNA by 7h; however, the addition of CHX simultaneously or 2h after 1,25(OH)(2)D(3) addition caused 76.4+/-13.0 and 37.1+/-18.8% reductions in the mRNA, respectively. Addition of CHX 4h after 1,25(OH)(2)D(3) had the opposite effect, and 21.7+/-17.2% more mRNA was observed after 7h. Similar patterns of mRNA expression were observed in other cell lines. CHX also decreased the induction by 1,25(OH)(2)D(3) of osteocalcin and osteopontin mRNA in ROS17/2.8 cells when added together with 1,25(OH)(2)D(3). The effect of CHX on the expression of a stably transfected luciferase construct under the control of 1400bp of 24-hydroxylase promoter indicates that a 1,25(OH)(2)D(3)-inducible transcription factor(s) that acts in the promoter region may at least in part be responsible for the effect of CHX on mRNA production of target genes.

Aluminum toxicity alters the regulation of calbindin-D28k protein and mRNA expression in chick intestine

Previous studies have shown that aluminum inhibits vitamin D-dependent calcium absorption. The mechanism involves reduced sensitivity to 1,25-dihydroxycholecalciferol and reduced expression of the calcium transport protein, calbindin-D28k. Reduced expression of calbindin protein may be due to decreased levels of calbindin mRNA. To test this hypothesis, we measured calbindin mRNA levels in chicks fed diets with and without added aluminum. Groups of chicks were fed one of four diets: control, control plus aluminum, low calcium, or low calcium plus aluminum. A fifth group was fed a vitamin D-free diet as a negative control. Calbindin protein was measured by immunoblotting. Serum calcium and inorganic phosphorus were determined. Intestinal mRNA was isolated and assayed by slot-blot hybridization to a fluorescein-conjugated oligonucleotide probe complementary to calbindin-D28k mRNA. Antifluorescein antibodies conjugated to alkaline phosphatase were used to detect hybrids and mRNA levels were quantified by densitometry. Specificity of the probe was verified by Northern analysis. Intestinal calbindin protein was greater in the control plus aluminum group than in controls, but no difference in calbindin mRNA was observed. These changes were associated with small decreases in serum phosphorus and calcium, suggesting a postranscriptional effect of aluminum. Chicks fed the low calcium diet had greater intestinal calbindin protein and mRNA levels relative to the control group in association with a 45% decrease in serum calcium. In contrast, no difference in calbindin protein, and significantly less mRNA were found in the low calcium plus aluminum group compared with controls, despite a decrease in serum calcium similar to that of chicks fed the low calcium diet without aluminum. These results show that in chicks fed a low calcium diet, aluminum intake decreases transcription and/or stability of intestinal calbindin mRNA, and that aluminum may inhibit the expression of vitamin D-dependent genes.

Effect of 1,25(OH)(2)-vitamin D(3) on the activation of natural killer cells: role of protein kinase C and extracellular calcium

As a first approach for studying the implication of PKC and the steroid hormone 1,25(OH)(2)-vitamin D(3) [1,25(OH)(2)D(3)] on natural killer cell (NK) activity, we analyzed in the YT NK cell line the expression of PKC isoforms and the effects of 1, 25(OH)(2)D(3) on BLT-esterase (a marker of NK lytic granules) activity. Western blot and RT-PCR showed a greater extent of PKC alpha, beta, delta, zeta, epsilon, theta, and lambda and lower levels of PKC mu and eta. In a dose-dependent manner 1, 25(OH)(2)D(3) induced significant increases in BLT-esterase and PKC activities and the stimulatory effect on BLT-esterase activity was mimicked and blocked, respectively, by the PKC activator phorbol ester PMA and PKC inhibitors (H7, PKC(19-36), and N-myristoylated PKC(19-31) peptides). Moreover, the effects of 1,25(OH)(2)D(3) on BLT-esterase could be blocked in a Ca(2+)-free (+EGTA) medium and mimicked by the Ca2+ ionophore A23187. The results suggest that 1, 25(OH)(2)D(3) is a stimulatory factor of NK activity acting through a mechanism involving PKC and extracellular Ca2+.