17β-Estradiol Upregulated Expression of α and β Subunits of Larger-Conductance Calcium-Activated K(+) Channels (BK) via Estrogen Receptor β

A direct estrogenic involvement in the expression of human hypocretin

Hypocretin is synthesized exclusively in the hypothalamus and distributes inputs to several areas of the brain, which may play an important role in depression. Our previous study showed that hypocretin-1 was increased in the lateral hypothalamus in female patients with depression compared to female controls. Estrogen acts through estrogen receptor (ER)α and ERβ. We studied the possibility of a direct action of estrogen receptors on the expression of human hypocretin. We found that hypocretin-1 plasma levels were significantly higher in female patients with depression than in female controls. Female depression estrogen receptors and hypocretin are colocalized in the human lateral hypothalamus, PC12, and SK-N-SH cells. The estrogen receptor response elements (ERE) that exist in the hypocretin promoter region may directly regulate the gene expression of hypocretin. The synchronicity of change of hypocretin and estradiol both in hypothalamus and plasma was verified in female rats. In the presence of estradiol, specific binding occurs between the recombinant human ER and hypocretin-ERE. Expression of ER combined with estradiol repressed hypocretin promoter activity via the ERE. In conclusion, we found that estradiol may directly affect hypocretin neurons in the human hypothalamus via ER binding to the hypocretin-ERE, which may lead to the sex-specific pathogenesis of depression.

Estrogen inhibits colonic smooth muscle contractions by regulating BKβ1 signaling

The estrogen inhibits colonic smooth muscle contractions, which may lead to constipation. However, the mechanisms of inhibition are poorly understood. Therefore, the present study examined the effect of estrogen on rat colonic smooth muscle contractions and its potential association with the large-conductance Ca2+-activated K+ channels β1 (BKβ1) subunit. Twenty-four female Sprague Dawley rats were randomly assigned to 4 groups. After 2 weeks of intervention, the contraction activity of isolated colonic smooth muscle and the expression of BKβ1 in colonic smooth muscle of rats were detected. Additionally, in order to investigate the effects of estrogen on BKβ1 expression and calcium mobilization, in vitro experiments were conducted using rat and human colonic smooth muscle cells (SMCs). BKβ1 shRNA was used to investigate whether calcium mobilization is affected by BKβ1 in colonic SMCs. To explore the relationship between ERβ and BKβ1, serial deletions, site-directed mutagenesis, a dual-luciferase reporter assay, and chromatin immunoprecipitation assays were employed. In response to E2, colonic smooth muscle strips showed a decrease in tension, while IBTX exposure transiently increased tension. Furthermore, in these muscle tissues, BKβ1 and α-SMA were found to be co-expressed. The E2 group showed significantly higher BKβ1 expression. In cultured colonic SMCs, the expression of BKβ1 was found to increase in the presence of E2 or DPN. E2 treatment reduced Ca2+ concentrations, while BKβ1 shRNA treatment increased Ca2+ concentrations relative to the control. ERβ-initiated BKβ1 expression appears to occur via binding to the BKβ1 promoter. These results indicated that E2 may upregulate BKβ1 expression via ERβ and inhibit colonic smooth muscle contraction through ERβ by directly targeting BKβ1.

Estrogen increases the expression of BKCa and impairs the contraction of colon smooth muscle via upregulation of sphingosine kinase 1

Estrogen (E2) may impair the contraction of colonic smooth muscle (SM) leading to constipation. Large conductance Ca2+ -activated K+ channels (BKCa ) are widely expressed in the smooth muscle cells (SMCs) contributing to hyperpolarization and relaxation of SMCs. Sphingosine kinase 1 (SphK1) is known to influence the expression of BKCa . We aimed to elucidate the potential underlying molecular mechanism of BKCa and SphK1 that may influence E2-induced colonic dysmotility. In ovariectomized rats, SM contraction and expression of BKCa , SphK1, sphingosine-1-phosphate receptor (S1PR) were analyzed after the treatment with vehicle, BSA-E2, E2, and E2 receptor antagonist. The role of BKCa , SphK1, and S1PR in E2-induced SM dysmotility was investigated in rat colonic SMCs. The effect of SphK1 on SM contraction as well as on the expression of BKCa and S1PR was analyzed in SphK1 knock-out mutant mice and wild-type (WT) mice treated with or without E2. The E2-treated group exhibited a weak contraction of colonic SM and a delayed colonic transit. The treatment with E2 significantly upregulated the expression of BKCa , SphK1, S1PR1, and S1PR2, but not S1PR3, in colon SM and SMCs. Inhibition of BKCa , SphK1, S1PR1, and S1PR2 expression attenuated the effect of E2 on Ca2+ mobilization in rat colon SMCs. WT mice treated with E2 showed impaired gastrointestinal motility and enhanced expression of BKCa , S1PR1, and S1PR2 compared with those without E2 treatment. Conversely, in SphK1 knock-out mice treated with E2, these effects were partially reversed. E2 increased the release of S1P which in turn could have activated S1PR1 and S1PR2. Loss of SphK1 attenuated the effect of E2 on the upregulation of S1PR1 and S1PR2 expression. These findings indicated that E2 impaired the contraction of colon SM through activation of BKCa via the upregulation of SphK1 and the release of S1P. In the E2-induced BKCa upregulation, S1PR1 and S1PR2 might also be involved. These results may provide further insights into a therapeutic target and optional treatment approaches for patients with constipation.

Sex differences in pituitary corticotroph excitability

Stress-related illness represents a major burden on health and society. Sex differences in stress-related disorders are well documented, with women having twice the lifetime rate of depression compared to men and most anxiety disorders. Anterior pituitary corticotrophs are central components of the hypothalamic-pituitary-adrenal (HPA) axis, receiving input from hypothalamic neuropeptides corticotrophin-releasing hormone (CRH) and arginine vasopressin (AVP), while regulating glucocorticoid output from the adrenal cortex. The dynamic control of electrical excitability by CRH/AVP and glucocorticoids is critical for corticotroph function; however, whether corticotrophs contribute to sexually differential responses of the HPA axis, which might underlie differences in stress-related disorders, is very poorly understood. Using perforated patch clamp electrophysiology in corticotrophs from mice expressing green fluorescent protein under the control of the Pomc promoter, we characterized basal and secretagogue-evoked excitability. Both male and female corticotrophs show predominantly single-spike action potentials under basal conditions; however, males predominantly display spikes with small-amplitude (<20 mV) afterhyperpolarizations (B-type), whereas females displayed a mixture of B-type spikes and spikes with a large-amplitude (>25 mV) afterhyperpolarization (A-type). In response to CRH, or CRH/AVP, male cells almost exclusively transition to a predominantly pseudo-plateau bursting, whereas only female B-type cells display bursting in response to CRH±AVP. Treatment of male or female corticotrophs with 1 nM estradiol (E2) for 24-72 h has no effect on the proportion of cells with A- or B-type spikes in either sex. However, E2 results in the cessation of CRH-induced bursting in both male and female corticotrophs, which can be partially reversed by adding a BK current using a dynamic clamp. RNA-seq analysis of purified corticotrophs reveals extensive differential gene expression at the transcriptional level, including more than 71 mRNAs encoding ion channel subunits. Interestingly, there is a two-fold lower level (p < 0.01) of BK channel pore-forming subunit (Kcnma1) expression in females compared to males, which may partially explain the decrease in CRH-induced bursting. This study identified sex differences at the level of the anterior pituitary corticotroph ion channel landscape and control of both spontaneous and CRH-evoked excitability. Determining the mechanisms of sex differences of corticotroph and HPA activity at the cellular level could be an important step for better understanding, diagnosing, and treating stress-related disorders.

Estrogenic Modulation of Ionic Channels, Pumps and Exchangers in Airway Smooth Muscle

To preserve ionic homeostasis (primarily Ca²⁺, K⁺, Na⁺, and Cl^-), in the airway smooth muscle (ASM) numerous transporters (channels, exchangers, and pumps) regulate the influx and efflux of these ions. Many of intracellular processes depend on continuous ionic permeation, including exocytosis, contraction, metabolism, transcription, fecundation, proliferation, and apoptosis. These mechanisms are precisely regulated, for instance, through hormonal activity. The lipophilic nature of steroidal hormones allows their free transit into the cell where, in most cases, they occupy their cognate receptor to generate genomic actions. In the sense, estrogens can stimulate development, proliferation, migration, and survival of target cells, including in lung physiology. Non-genomic actions on the other hand do not imply estrogen's intracellular receptor occupation, nor do they initiate transcription and are mostly immediate to the stimulus. Among estrogen's non genomic responses regulation of calcium homeostasis and contraction and relaxation processes play paramount roles in ASM. On the other hand, disruption of calcium homeostasis has been closely associated with some ASM pathological mechanism. Thus, this paper intends to summarize the effects of estrogen on ionic handling proteins in ASM. The considerable diversity, range and power of estrogens regulates ionic homeostasis through genomic and non-genomic mechanisms.

Er-xian decoction drug-containing serum promotes Mc3t3-e1 cell proliferation and osteogenic differentiation via regulating BK channel

ETHNOPHARMACOLOGICAL RELEVANCE

Er-xian Decoction (EXD) is a well-known prescription widely used to prevent and treat climacteric syndrome and osteoporosis in China. BK channel positively affects osteoblast bone formation in vitro. However, it is still unclear whether the effect of EXD on promoting osteoblasts osteogenic differentiation is related to BK channel.

AIM OF THE STUDY

The study is aimed at determining whether the EXD-containing serum promotes the proliferation of osteoblasts and their differentiation through BK channel.

MATERIALS AND METHODS

The chemical compounds of EXD were analyzed by UPLC-Q-TOF/MS. An osteogenic induction medium (OM) was used to induce MC3T3-E1 cells' osteogenic differentiation. The effects of EXD-containing serum and tetraethylammonium (TEA) on the proliferation activity of Mc3t3-e1 cells were detected by CCK-8 assay. ALP activity was determined by an alkaline phosphatase kit. The protein expression (BMP2, OPG, and COL1) was analyzed by Western blot, and the mRNA expression (Runx2, OPG, and BMP2) was assessed by real-time PCR. Alizarin red was used to stain the mineralized region of osteoblasts. In addition, we analyzed the relationship between BK channel and its downstream PTEN/Akt/Foxo1 signaling pathway.

RESULTS

72 compounds were identified by UPLC-Q-TOF/MS analysis in EXD. Mangiferin, ferulic acid, berberine, and icariin were main active components of EXD. EXD-containing serum could enhance the cell viability of MC3T3-E1 cells by decreasing the expression of BKα protein. EXD-containing serum increased ALP activity and calcium nodule formation of Mc3t3-e1 cells, promoted the protein expression of BKα, COL1, BMP2, OPG, and the mRNA expression of RUNX2, OPG, and BMP2, however, these effects can be reversed after adding TEA. In addition, EXD-containing serum could upregulate phosphorylation of Akt and Foxo1 in osteogenic-induced Mc3t3-e1 cells, and lower the expression of PTEN. And these effects of EXD-containing serum could be reduced by TEA.

CONCLUSIONS

The effect of EXD-containing serum on promoting cell proliferation and osteogenic differentiation of Mc3t3-e1 cells might be linked to the regulation of BK channel.

Cerebral arteriolar and neurovascular dysfunction after chemically induced menopause in mice

Cognitive decline is linked to decreased cerebral blood flow, particularly in women after menopause. Impaired cerebrovascular function precedes the onset of dementia, possibly because of reduced functional dilation in parenchymal arterioles. These vessels are bottlenecks of the cerebral microcirculation, and dysfunction can limit functional hyperemia in the brain. Large-conductance Ca2+-activated K+ channels (BKCa) are the final effectors of several pathways responsible for functional hyperemia, and their expression is modulated by estrogen. However, it remains unknown whether BKCa function is altered in cerebral parenchymal arterioles after menopause. Using a chemically induced model of menopause, the 4-vinylcyclohexene diepoxide (VCD) model, which depletes follicles while maintaining intact ovaries, we hypothesized that menopause would be associated with reduced functional vasodilatory responses in cerebral parenchymal arterioles of wild-type mice via reduced BKCa function. Using pressure myography of isolated parenchymal arterioles, we observed that menopause (Meno) induced a significant increase in spontaneous myogenic tone. Endothelial function, assessed as nitric oxide production and dilation after cholinergic stimulation or endothelium-dependent hyperpolarization pathways, was unaffected by Meno. BKCa function was significantly impaired in Meno compared with control, without changes in voltage-gated K+ channel activity. Cerebral functional hyperemia, measured by laser-speckle contrast imaging during whisker stimulation, was significantly blunted in Meno mice, without detectable changes in basal perfusion. However, behavioral testing identified no change in cognition. These findings suggest that menopause induces cerebral microvascular and neurovascular deficits.NEW & NOTEWORTHY Cerebral parenchymal arterioles from menopause mice showed increased myogenic tone. We identified an impairment in smooth muscle cell BKCa channel activity, without a reduction in endothelium-dependent dilation or nitric oxide production. Microvascular dysfunction was associated with a reduction in neurovascular responses after somatosensory stimulation. Despite the neurovascular impairment, cognitive abilities were maintained in menopausal mice.

The modulation of potassium channels by estrogens facilitates neuroprotection

Estrogens, the sex hormones, have the potential to govern multiple cellular functions, such as proliferation, apoptosis, differentiation, and homeostasis, and to exert numerous beneficial influences for the cardiovascular system, nervous system, and bones in genomic and/or non-genomic ways. Converging evidence indicates that estrogens serve a crucial role in counteracting neurodegeneration and ischemic injury; they are thereby being considered as a potent neuroprotectant for preventing neurological diseases such as Alzheimer's disease and stroke. The underlying mechanism of neuroprotective effects conferred by estrogens is thought to be complex and multifactorial, and it remains obscure. It is well established that the K+ channels broadly expressed in a variety of neural subtypes determine the essential physiological features of neuronal excitability, and dysfunction of these channels is closely associated with diverse brain deficits, such as ataxia and epilepsy. A growing body of evidence supports a neuroprotective role of K+ channels in malfunctions of nervous tissues, with the channels even being a therapeutic target in clinical trials. As multitarget steroid hormones, estrogens also regulate the activity of distinct K+ channels to generate varying biological actions, and accumulated data delineate that some aspects of estrogen-mediated neuroprotection may arise from the impact on multiple K+ channels, including Kv, BK, KATP, and K2P channels. The response of these K+ channels after acute or chronic exposure to estrogens may oppose pathological abnormality in nervous cells, which serves to extend our understanding of these phenomena.

Estrogen-induced downregulation of TASK-1 expression through estrogen receptor β in N2A cells

BACKGROUND

Our previous data revealed that reduction of TASK-1 expression, as a consequence of exposure to 17β-estradiol, could participate in neuroprotective effects in N2A cells. However, it is unclear which estrogen receptor underlies these effects of 17β-estradiol.

METHODS AND RESULTS

In this study, the knockdown experiments are carried out to clarify the estrogen receptor responsible for effects of estrogen on TASK-1 channels. Subsequently, data from QPCR measurements reveal that estrogen receptor β (ERβ), but not estrogen receptor α, serves as a binding target for 17β-estradiol after a 48-h treatment.

CONCLUSIONS

The current result suggests the implication of the ERβ-dependent manner in the pro-proliferative action of estrogen via TASK-1 channels.

Subunits of BK channels promote breast cancer development and modulate responses to endocrine treatment in preclinical models

BACKGROUND AND PURPOSE

Pore-forming α subunits of the voltage- and Ca²⁺ -activated K⁺ channel with large conductance (BKα) promote malignant phenotypes of breast tumour cells. Auxiliary subunits such as the leucine-rich repeat containing 26 (LRRC26) protein, also termed BKγ1, may be required to permit activation of BK currents at a depolarized resting membrane potential that frequently occur in non-excitable tumour cells.

EXPERIMENTAL APPROACH

Anti-tumour effects of BKα loss were investigated in breast tumour-bearing MMTV-PyMT transgenic BKα knockout (KO) mice, primary MMTV-PyMT cell cultures, and in a syngeneic transplantation model of breast cancer derived from these cells. The therapeutic relevance of BK channels in the context of endocrine treatment was assessed in human breast cancer cell lines expressing either low (MCF-7) or high (MDA-MB-453) levels of BKα and BKγ1, as well as in BKα-negative MDA-MB-157.

KEY RESULTS

BKα promoted breast cancer onset and overall survival in preclinical models. Conversely, lack of BKα and/or knockdown of BKγ1 attenuated proliferation of murine and human breast cancer cells in vitro. At low concentrations, tamoxifen and its major active metabolites stimulated proliferation of BKα/γ1-positive breast cancer cells, independent of the genomic signalling controlled by the oestrogen receptor. Finally, tamoxifen increased the relative survival time of BKα KO but not of wild-type tumour cell recipient mice.

CONCLUSION AND IMPLICATIONS

Breast cancer initiation, progression, and tamoxifen sensitivity depend on functional BK channels thereby providing a rationale for the future exploration of the oncogenic actions of BK channels in clinical outcomes with anti-oestrogen therapy.

Sex differences in the vascular function and related mechanisms: role of 17β-estradiol

The incidence of cardiovascular disease (CVD) is lower in premenopausal women but increases with age and menopause compared with similarly aged men. Based on the prevalence of CVD in postmenopausal women, sex hormone-dependent mechanisms have been postulated to be the primary factors responsible for the protection from CVD in premenopausal women. Recent Women’s Health Initiative studies, Cochrane Review studies, the Early Versus Late Intervention Trial with Estradiol Study, and the Kronos Early Estrogen Prevention Study have suggested that beneficial effects of hormone replacement therapy (HRT) are seen in women of <60 yr of age and if initiated within <10 yr of menopause. In contrast, the beneficial effects of HRT are not seen in women of >60 yr of age and if commenced after 10 yr of menopause. The higher incidence of CVD and the failure of HRT in postmenopausal aged women could be partly associated with fundamental differences in the vascular structure and function between men and women and in between pre- and postmenopausal women, respectively. In this regard, previous studies from human and animal studies have identified several sex differences in vascular function and associated mechanisms. The female sex hormone 17β-estradiol regulates the majority of these mechanisms. In this review, we summarize the sex differences in vascular structure, myogenic properties, endothelium-dependent and -independent mechanisms, and the role of 17β-estradiol in the regulation of vascular function.

Sex differences in lower urinary tract biology and physiology

Females and males differ significantly in gross anatomy and physiology of the lower urinary tract, and these differences are commonly discussed in the medical and scientific literature. However, less attention is dedicated to investigating the varied development, function, and biology between females and males on a cellular level. Recognizing that cell biology is not uniform, especially in the lower urinary tract of females and males, is crucial for providing context and relevance for diverse fields of biomedical investigation. This review serves to characterize the current understanding of biological sex differences between female and male lower urinary tracts, while identifying areas for future research. First, the differences in overall cell populations are discussed in the detrusor smooth muscle, urothelium, and trigone. Second, the urethra is discussed, including anatomic discussions of the female and male urethra followed by discussions of cellular differences in the urothelial and muscular layers. The pelvic floor is then reviewed, followed by an examination of the sex differences in hormonal regulation, the urinary tract microbiome, and the reticuloendothelial system. Understanding the complex and dynamic development, anatomy, and physiology of the lower urinary tract should be contextualized by the sex differences described in this review.

Beta-Estradiol Regulates Voltage-Gated Calcium Channels and Estrogen Receptors in Telocytes from Human Myometrium

Voltage-gated calcium channels and estrogen receptors are essential players in uterine physiology, and their association with different calcium signaling pathways contributes to healthy and pathological conditions of the uterine myometrium. Among the properties of the various cell subtypes present in human uterine myometrium, there is increasing evidence that calcium oscillations in telocytes (TCs) contribute to contractile activity and pregnancy. Our study aimed to evaluate the effects of beta-estradiol on voltage-gated calcium channels and estrogen receptors in TCs from human uterine myometrium and to understand their role in pregnancy. For this purpose, we employed patch-clamp recordings, ratiometric Fura-2-based calcium imaging analysis, and qRT-PCR techniques for the analysis of cultured human myometrial TCs derived from pregnant and non-pregnant uterine samples. In human myometrial TCs from both non-pregnant and pregnant uterus, we evidenced by qRT-PCR the presence of genes encoding for voltage-gated calcium channels (Cav3.1, Ca3.2, Cav3.3, Cav2.1), estrogen receptors (ESR1, ESR2, GPR30), and nuclear receptor coactivator 3 (NCOA3). Pregnancy significantly upregulated Cav3.1 and downregulated Cav3.2, Cav3.3, ESR1, ESR2, and NCOA3, compared to the non-pregnant condition. Beta-estradiol treatment (24 h, 10, 100, 1000 nM) downregulated Cav3.2, Cav3.3, Cav1.2, ESR1, ESR2, GRP30, and NCOA3 in TCs from human pregnant uterine myometrium. We also confirmed the functional expression of voltage-gated calcium channels by patch-clamp recordings and calcium imaging analysis of TCs from pregnant human myometrium by perfusing with BAY K8644, which induced calcium influx through these channels. Additionally, we demonstrated that beta-estradiol (1000 nM) antagonized the effect of BAY K8644 (2.5 or 5 µM) in the same preparations. In conclusion, we evidenced the presence of voltage-gated calcium channels and estrogen receptors in TCs from non-pregnant and pregnant human uterine myometrium and their gene expression regulation by beta-estradiol in pregnant conditions. Further exploration of the calcium signaling in TCs and its modulation by estrogen hormones will contribute to the understanding of labor and pregnancy mechanisms and to the development of effective strategies to reduce the risk of premature birth.

Chronic interval exercise training prevents BKCa channel-mediated coronary vascular dysfunction in aortic-banded miniswine

Conventional treatments have failed to improve the prognosis of heart failure with preserved ejection fraction (HFpEF) patients. Thus, the purpose of this study was to determine the therapeutic efficacy of chronic interval exercise training (IT) on large-conductance Ca²⁺-activated K⁺ (BK_Ca) channel-mediated coronary vascular function in heart failure. We hypothesized that chronic interval exercise training would attenuate pressure overload-induced impairments to coronary BK_Ca channel-mediated function. A translational large-animal model with cardiac features of HFpEF was used to test this hypothesis. Specifically, male Yucatan miniswine were divided into three groups ( n = 7/group): control (CON), aortic banded (AB)-heart failure (HF), and AB-interval trained (HF-IT). Coronary blood flow, vascular conductance, and vasodilatory capacity were measured after administration of the BK_Ca channel agonist NS-1619 both in vivo and in vitro in the left anterior descending coronary artery and isolated coronary arterioles, respectively. Skeletal muscle citrate synthase activity was decreased and left ventricular brain natriuretic peptide levels increased in HF vs. CON and HF-IT animals. A parallel decrease in NS-1619-dependent coronary vasodilatory reserve in vivo and isolated coronary arteriole vasodilatory responsiveness in vitro were observed in HF animals compared with CON, which was prevented in the HF-IT group. Although exercise training prevented BK_Ca channel-mediated coronary vascular dysfunction, it did not change BK_Ca channel α-subunit mRNA, protein, or cellular location (i.e., membrane vs. cytoplasm). In conclusion, these results demonstrate the viability of chronic interval exercise training as a therapy for central and peripheral adaptations of experimental heart failure, including BK_Ca channel-mediated coronary vascular dysfunction. NEW & NOTEWORTHY Conventional treatments have failed to improve the prognosis of heart failure with preserved ejection fraction (HFpEF) patients. Our findings show that chronic interval exercise training can prevent BK_Ca channel-mediated coronary vascular dysfunction in a translational swine model of chronic pressure overload-induced heart failure with relevance to human HFpEF.

Down-Regulation of Ca2+-Activated K⁺ Channel KCa1.1 in Human Breast Cancer MDA-MB-453 Cells Treated with Vitamin D Receptor Agonists

Vitamin D (VD) reduces the risk of breast cancer and improves disease prognoses. Potential VD analogs are being developed as therapeutic agents for breast cancer treatments. The large-conductance Ca²⁺-activated K⁺ channel K_Ca1.1 regulates intracellular Ca²⁺ signaling pathways and is associated with high grade tumors and poor prognoses. In the present study, we examined the effects of treatments with VD receptor (VDR) agonists on the expression and activity of K_Ca1.1 in human breast cancer MDA-MB-453 cells using real-time PCR, Western blotting, flow cytometry, and voltage-sensitive dye imaging. Treatments with VDR agonists for 72 h markedly decreased the expression levels of K_Ca1.1 transcripts and proteins in MDA-MB-453 cells, resulting in the significant inhibition of depolarization responses induced by paxilline, a specific K_Ca1.1 blocker. The specific proteasome inhibitor MG132 suppressed VDR agonist-induced decreases in K_Ca1.1 protein expression. These results suggest that K_Ca1.1 is a new downstream target of VDR signaling and the down-regulation of K_Ca1.1 through the transcriptional repression of K_Ca1.1 and enhancement of K_Ca1.1 protein degradation contribute, at least partly, to the antiproliferative effects of VDR agonists in breast cancer cells.