Intestinal calcium transporter genes are upregulated by estrogens and the reproductive cycle through vitamin D receptor-independent mechanisms.
1alpha,25(OH)2-vitamin D strongly regulates the expression of the epithelial calcium channel CaT1. CaT1 expression is reduced in ERKOalpha mice and induced by estrogen treatment, pregnancy, or lactation in VDR WT and KO mice. Estrogens and vitamin D are thus independent potent regulators of the expression of this calcium influx mechanism, which is involved in active intestinal calcium absorption.


BACKGROUND
Active duodenal calcium absorption consists of three major steps: calcium influx into, transfer through, and extrusion out of the enterocyte. These steps are carried out by the calcium transport protein 1 (CaT1), calbindin-D9K, and the plasma membrane calcium ATPase (PMCA1b), respectively. We investigated whether estrogens or hormonal changes during the female reproductive cycle influence the expression of these genes, and if so, whether these effects are vitamin D-vitamin D receptor (VDR) dependent.


METHODS
We evaluated duodenal expression patterns in estrogen receptor (ER)alpha and -beta knockout (KO) mice, as well as in ovariectomized, estrogen-treated, pregnant, and lactating VDR wild-type (WT) and VDR KO mice.


RESULTS
Expression of calcium transporter genes was not altered in ERKObeta mice. CaT1 mRNA expression was reduced by 55% in ERKOalpha mice, while the two other calcium transporter genes were not affected. Ovariectomy caused no change in duodenal expression pattern of VDR WT and KO mice, whereas treatment with a pharmacologic dose of estrogens induced CaT1 mRNA expression in VDR WT (4-fold) and KO (8-fold) mice. Pregnancy enhanced CaTI expression equally in VDR WT and KO mice (12-fold). Calbindin-D9K and PMCA1b expression increased to a lesser extent and solely in pregnant VDR WT animals. In lactating VDR WT and KO mice, CaT1 mRNA expression increased 13 times, which was associated with a smaller increase in calbindin-D9K protein content and PMCA1b mRNA expression.


CONCLUSIONS
Estrogens or hormonal changes during pregnancy or lactation have distinct, vitamin D-independent effects at the genomic level on active duodenal calcium absorption mechanisms, mainly through a major upregulation of the calcium influx channel CaT1. The estrogen effects seem to be mediated solely by ERalpha.
Laboratory of Experimental Medicine and Endocrinology (Legendo), Katholieke Universiteit Leuven, Leuven, Belgium.
S J
SJ
Van Cromphaut
K
K
Rummens
I
I
Stockmans
E
E
Van Herck
F A
FA
Dijcks
A G H
AG
Ederveen
P
P
Carmeliet
J
J
Verhaeghe
R
R
Bouillon
G
G
Carmeliet
D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000502:physiology*; D020895:Enterocytes / Q000378:metabolism; D047628:Estrogen Receptor alpha; D047629:Estrogen Receptor beta; D004967:Estrogens / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D008957:Models, Genetic; D016296:Mutagenesis; D012333:RNA, Messenger / Q000378:metabolism; D018167:Receptors, Calcitriol / Q000378:metabolism*; D011960:Receptors, Estrogen / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction; D050916:TRPV Cation Channels; D015854:Up-Regulation; D014807:Vitamin D / Q000378:metabolism
D015220:Calcium Channels; D047628:Estrogen Receptor alpha; D047629:Estrogen Receptor beta; D004967:Estrogens; D012333:RNA, Messenger; D018167:Receptors, Calcitriol; D011960:Receptors, Estrogen; D050916:TRPV Cation Channels; C120776:TRPV6 channel; D014807:Vitamin D; D002118:Calcium