Regulation of the epithelial Ca2+ channels in small intestine as studied by quantitative mRNA detection
Monique van Abel1, Joost G J Hoenderop, Annemiete W C M van der Kemp
1Department of Cell Physiology, Niujmegen Center for Molecular Life Sciences, University Medical Center Nijmegen, P. O. Box 9101, NL-6500 HB Nijmegen, The Netherlands.
Abstract:
The epithelial Ca2+ channels TRPV5 and TRPV6 are localized to the brush border membrane of intestinal cells and constitute the postulated rate-limiting entry step of active Ca2+ absorption. The aim of the present study was to investigate the hormonal regulation of these channels. To this end, the effect of 17beta-estradiol (17beta-E2), 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], and dietary Ca2+ on the expression of the duodenal Ca2+ transport proteins was investigated in vivo and analyzed using realtime quantitative PCR. Supplementation with 17beta-E2 increased duodenal gene expression of TRPV5 and TRPV6 but also calbindin-D9K and plasma membrane Ca2+-ATPase (PMCA1b) in ovariectomized rats. 25-Hydroxyvitamin D3-1alpha-hydroxylase (1alpha-OHase) knockout mice are characterized by hyperparathyroidism, rickets, hypocalcemia, and undetectable levels of 1,25(OH)2D3 and were used to study the 1,25(OH)2D3-dependency of the stimulatory effects of 17beta-E2. Treatment with 17beta-E2 upregulated mRNA levels of duodenal TRPV6 in these 1alpha-OHase knockout mice, which was accompanied by increased serum Ca2+ concentrations from 1.69 +/- 0.10 to 2.03 +/- 0.12 mM (P < 0.05). In addition, high dietary Ca2+ intake normalized serum Ca2+ in these mice and upregulated expression of genes encoding the duodenal Ca2+ transport proteins except for PMCA1b. Supplementation with 1,25(OH)2D3 resulted in increased expression of TRPV6, calbindin-D9K, and PMCA1b and normalization of serum Ca2+. Expression levels of duodenal TRPV5 mRNA are below detection limits in these 1alpha-OHase knockout mice, but supplementation with 1,25(OH)2D3 upregulated the expression to significant levels. In conclusion, TRPV5 and TRPV6 are regulated by 17beta-E2 and 1,25(OH)2D3, whereas dietary Ca2+ is positively involved in the regulation of TRPV6 only.
Insights
17beta-estradiol (17beta-E2) and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] regulate epithelial calcium channels TRPV5 and TRPV6 in the intestine. Dietary calcium influences TRPV6 expression, impacting calcium absorption.
Area of Science:
- Endocrinology
- Molecular Biology
- Gastroenterology
Background:
- Epithelial calcium channels TRPV5 and TRPV6 are crucial for intestinal calcium absorption.
- These channels are located in the brush border membrane of intestinal cells.
- Their regulation is key to maintaining calcium homeostasis.
Purpose of the Study:
- To investigate the hormonal regulation of TRPV5 and TRPV6 by 17beta-estradiol (17beta-E2), 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], and dietary calcium.
- To elucidate the roles of these factors in duodenal calcium transport protein expression.
Main Methods:
- In vivo studies using ovariectomized rats and 1alpha-hydroxylase knockout mice.
- Real-time quantitative PCR to analyze gene expression of duodenal calcium transport proteins.
- Measurement of serum calcium concentrations.
Main Results:
- 17beta-E2 upregulated duodenal TRPV5, TRPV6, calbindin-D9K, and PMCA1b expression in ovariectomized rats.
- In 1alpha-OHase knockout mice, 17beta-E2 upregulated TRPV6 and increased serum calcium.
- Dietary calcium normalized serum calcium and upregulated duodenal calcium transport protein genes (except PMCA1b).
- 1,25(OH)2D3 supplementation increased TRPV6, calbindin-D9K, PMCA1b, and TRPV5 expression, normalizing serum calcium.
Conclusions:
- TRPV5 and TRPV6 expression is regulated by both 17beta-E2 and 1,25(OH)2D3.
- Dietary calcium positively influences TRPV6 expression.
- These findings highlight the complex hormonal and dietary regulation of intestinal calcium absorption.
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