Potential Role of the Vitamin D Receptor in Control of Cholesterol Levels

Abstract

See “Vitamin D receptor activation down-regulates the small heterodimer partner and increases CYP7A1 to lower cholesterol,” by Chow ECY, Magomedova L, Quach HP, et al, on page 1048.CYP7A1, a cholesterol 7α-hydroxylase, is the rate-limiting enzyme in bile acid synthesis and its expression is thought to control levels of serum cholesterol.1Chiang J.Y. Bile acids: regulation of synthesis.J Lipid Res. 2009; 50: 1955-1966Abstract Full Text Full Text PDF PubMed Scopus (1068) Google Scholar, 2Jelinek D.F. Andersson S. Slaughter C.A. et al.Cloning and regulation of cholesterol 7α-hydroxylase, the rate-limiting enzyme in bile acid biosynthesis.J Biol Chem. 1990; 265: 8190-8197Abstract Full Text PDF PubMed Google Scholar Humans deficient in CYP7A1 have increased serum cholesterol,3Pullinger C.R. Eng C. Salen G. et al.Human cholesterol 7α -hydroxylase (CYP7A1) deficiency has a hypercholesterolemic phenotype.J Clin Invest. 2002; 110: 109-117Crossref PubMed Scopus (440) Google Scholar and mice having constitutive expression of a human CYP7A1 transgene are protected from a high-fat-diet–induced hypercholesterolemia.4Li T. Matozel M. Boehme S. et al.Overexpression of cholesterol 7α-hydroxylase promotes hepatic bile acid synthesis and secretion and maintains cholesterol homeostasis.Hepatology. 2011; 53: 996-1006Crossref PubMed Scopus (155) Google Scholar The Cyp7a1 gene is primarily regulated through activation of the ligand-dependent nuclear receptor farnesoid X receptor (FXR) by endogenous bile acids.5Forman B.M. Goode E. Chen J. et al.Identification of a nuclear receptor that is activated by farnesol metabolites.Cell. 1995; 81: 687-693Abstract Full Text PDF PubMed Scopus (951) Google Scholar, 6Seol W. Choi H.S. Moore D.D. Isolation of proteins that interact specifically with the retinoid X receptor: two novel orphan receptors.Mol Endocrinol. 1995; 9: 72-85Crossref PubMed Google Scholar, 7Makishima M. Okamoto A.Y. Repa J.J. et al.Identification of a nuclear receptor for bile acids.Science. 1999; 284: 1362-1365Crossref PubMed Scopus (2103) Google Scholar Bile acids are able to suppress their synthesis and to increase their secretion from the liver into the bile ducts. Suppression of Cyp7a1 by hepatic FXR is due in part to an induction of the small heterodimer protein (SHP), a member of the nuclear receptor superfamily that lacks a DNA binding domain but has the domain required for dimerization with other nuclear receptors.8Seol W. Choi H.S. Moore D.D. An orphan nuclear hormone receptor that lacks a DNA binding domain and heterodimerizes with other receptors.Science. 1996; 272: 1336-1339Crossref PubMed Scopus (438) Google Scholar SHP is able to form a complex with the liver receptor homolog-1 (LRH-1), a positive regulator of Cyp7a1, rendering it unable to activate the Cyp7a1 gene.9Gupta S. Pandak W.M. Hylemon P.B. LXR alpha is the dominant regulator of CYP7A1 transcription.Biochem Biophys Res Commun. 2002; 293: 338-343Crossref PubMed Scopus (104) Google Scholar Intriguingly, the importance of the intestine in the regulation of hepatic Cyp7a1 has been highlighted by the discovery of the FXR-mediated induction in the ileum of fibroblast growth factor (FGF) 19/15, a hormone that is secreted in the portal blood and signals to the liver to repress Cyp7A1 expression synergistically with SHP.10Inagaki T. Choi M. Moschetta A. et al.Fibroblast growth factor 15 functions as an enterohepatic signal to regulate bile acid homeostasis.Cell Metab. 2005; 2: 217-225Abstract Full Text Full Text PDF PubMed Scopus (1286) Google Scholar Thus, hepatic and intestinal FXR cooperate to down-regulate Cyp7A1 expression (Figure 1). The use of tissue-specific Fxr-null or transgenic mice demonstrated a much more prominent role for the intestinal FXR-FGF15 pathway, with respect to the hepatic FXR-SHP pathway, in repressing Cyp7a1 expression.11Kim I. Ahn S.H. Inagaki T. et al.Differential regulation of bile acid homeostasis by the farnesoid X receptor in liver and intestine.J Lipid Res. 2007; 48: 2664-2672Abstract Full Text Full Text PDF PubMed Scopus (404) Google Scholar, 12Modica S. Petruzzelli M. Bellafante E. et al.Selective activation of nuclear bile acid receptor FXR in the intestine protects mice against cholestasis.Gastroenterology. 2012; 142 (e1–e4): 355-365Abstract Full Text Full Text PDF PubMed Scopus (209) Google ScholarVitamin D receptor (VDR) is also reported to have effects on bile acid synthesis and cholesterol levels in hepatocytes and serum. VDR is activated by 1α, 25-dihydroxyvitamin D3 (1,25(OH)2D3), an active form of vitamin D3, and mediates vitamin D signaling in numerous physiologic and pharmacologic processes.13Haussler M.R. Haussler C.A. Jurutka P.W. et al.The vitamin D hormone and its nuclear receptor: molecular actions and disease states.J Endocrinol. 1997; 154: S57-S73PubMed Google Scholar VDR is expressed in kidney, intestine, and bone, and at low levels in most other tissues.14Bookout A.L. Jeong Y. Downes M. et al.Anatomical profiling of nuclear receptor expression reveals a hierarchical transcriptional network.Cell. 2006; 126: 789-799Abstract Full Text Full Text PDF PubMed Scopus (775) Google Scholar It is not expressed to a significant extent in liver and thus its physiologic role in this tissue is questionable. Deficiency of VDR leads to hypocalcemia, hyperparathyroidism, rickets, osteomalacia, alopecia, uterine hypoplasia, and growth retardation, which might result from repression of calcium absorption owing to the down-regulation of duodenal epithelial calcium channels.15Li Y.C. Pirro A.E. Amling M. et al.Targeted ablation of the vitamin D receptor: an animal model of vitamin D-dependent rickets type II with alopecia.Proc Natl Acad Sci U S A. 1997; 94: 9831-9835Crossref PubMed Scopus (799) Google Scholar, 16Yoshizawa T. Handa Y. Uematsu Y. et al.Mice lacking the vitamin D receptor exhibit impaired bone formation, uterine hypoplasia and growth retardation after weaning.Nat Genet. 1997; 16: 391-396Crossref PubMed Scopus (965) Google Scholar, 17Li Y.C. Kong J. Wei M. et al.1,25-Dihydroxyvitamin D(3) is a negative endocrine regulator of the renin-angiotensin system.J Clin Invest. 2002; 110: 229-238Crossref PubMed Scopus (1723) Google Scholar, 18Lieben L. Masuyama R. Torrekens S. et al.Normocalcemia is maintained in mice under conditions of calcium malabsorption by vitamin D-induced inhibition of bone mineralization.J Clin Invest. 2012; 122: 1803-1815Crossref PubMed Scopus (251) Google Scholar, 19Bouillon R. Carmeliet G. Verlinden L. et al.Vitamin D and human health: lessons from vitamin D receptor null mice.Endocr Rev. 2008; 29: 726-776Crossref PubMed Scopus (1287) Google Scholar VDR could affect the levels of cholesterol in liver and serum, possibly through controlling expression of genes involved in bile acid synthesis from cholesterol. However, earlier studies in mouse and human hepatoma cells revealed that activation of the VDR actually blocked the expression of FXR, which would result in suppression of Cyp7a1 expression leading to decreased cholesterol levels.20Honjo Y. Sasaki S. Kobayashi Y. et al.1,25-dihydroxyvitamin D3 and its receptor inhibit the chenodeoxycholic acid-dependent transactivation by farnesoid X receptor.J Endocrinol. 2006; 188: 635-643Crossref PubMed Scopus (49) Google Scholar, 21Jiang W. Miyamoto T. Kakizawa T. et al.Inhibition of LXRalpha signaling by vitamin D receptor: possible role of VDR in bile acid synthesis.Biochem Biophys Res Commun. 2006; 351: 176-184Crossref PubMed Scopus (36) Google ScholarTo clarify the role of VDR in control of Cyp7a1 and serum cholesterol levels, the current study by Chow et al22Chow E.C.Y. Magomedova L. Quach H.P. et al.Vitamin D receptor activation down-regulates the small heterodimer partner and increases CYP7A1 to lower cholesterol.Gastroenterology. 2014; 146: 1048-1059Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar was carried out using in vivo experimentation and various gene knockout mouse models to investigate the mechanism by which VDR affects cholesterol levels. They found that VDR is expressed at low but measurable levels in the livers of mice and is able to activate its main target gene Cyp24a1 upon injection of 1,25(OH)2D3. In contrast with earlier studies in cultured tumor cells, they show that activation of VDR by 1,25(OH)2D3 represses SHP expression through suppression of Shp gene transcription. They also convincingly demonstrate that 1,25(OH)2D3 reduces both liver and serum cholesterol that they ascribe to the activation of Cyp7a1; this change was not observed in mice lacking expression of SHP in the liver (Figure 1). This study illustrates the importance of in vivo experimentation to determine the effects of drugs and other compounds on hepatic gene expression. Tumor cells frequently lack all of the transcription machinery found in the liver and even liver cells in culture differ from intact liver in response to stimulation because hepatocyte function depends on liver architecture and the presence of other nonhepatocytes, such as stromal cells and Kupffer cells.Among the unresolved issues in this study is the translatability to humans. This is of great importance because oral vitamin D3 and its derivatives are used as dietary supplements and are frequently prescribed to patients with low vitamin D3 levels. However, a randomized, placebo-controlled trial revealed that short-term treatment with 50,000 IU of vitamin D3 weekly for 8 weeks had no effect on serum cholesterol levels23Ponda M.P. Dowd K. Finkielstein D. et al.The short-term effects of vitamin D repletion on cholesterol: a randomized, placebo-controlled trial.Arterioscler Thromb Vasc Biol. 2012; 32: 2510-2515Crossref PubMed Scopus (71) Google Scholar; similar results were obtained with a long-term treatment.24Zittermann A. Frisch S. Berthold H.K. et al.Vitamin D supplementation enhances the beneficial effects of weight loss on cardiovascular disease risk markers.Am J Clin Nutr. 2009; 89: 1321-1327Crossref PubMed Scopus (441) Google Scholar It should be noted in this context, that these studies were done with oral dosing, whereas Chow et al22Chow E.C.Y. Magomedova L. Quach H.P. et al.Vitamin D receptor activation down-regulates the small heterodimer partner and increases CYP7A1 to lower cholesterol.Gastroenterology. 2014; 146: 1048-1059Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar treated mice by injecting 1,25(OH)2D3. Injection of humans with vitamin D3 is not practical as a means for therapeutic delivery for lowering cholesterol, even if it is shown to be efficacious by this route of administration. However, the present work does give a hint that endogenously produced vitamin D3 and hepatic VDR could affect cholesterol levels in humans.The Cyp7a1 gene is positively regulated by several transcription factors including LRH-1, liver X receptor (LXR), and hepatocyte (HNF4α). SHP represses Cyp7a1 by binding to and inactivating the transcription potential of LRH-1 and this pathway is conserved in humans and mice.25Lu T.T. Makishima M. Repa J.J. et al.Molecular basis for feedback regulation of bile acid synthesis by nuclear receptors.Mol Cell. 2000; 6: 507-515Abstract Full Text Full Text PDF PubMed Scopus (1208) Google Scholar SHP could also affect signaling by HNF4α.26Chen J. Cooper A.D. Levy-Wilson B. Hepatocyte nuclear factor 1 binds to and transactivates the human but not the rat CYP7A1 promoter.Biochem Biophys Res Commun. 1999; 260: 829-834Crossref PubMed Scopus (38) Google Scholar, 27Han S. Chiang J.Y. Mechanism of vitamin D receptor inhibition of cholesterol 7α -hydroxylase gene transcription in human hepatocytes.Drug Metab Dispos. 2009; 37: 469-478Crossref PubMed Scopus (76) Google Scholar There exists a species difference in control of the mouse Cyp7a1 and human CYP7A1 genes. LXR activates the mouse Cyp7a1 gene, but does not activate the human CYP7A1 gene owing to the lack of an LXR binding site (direct repeat 4 element) in the CYP7A1 promoter.1Chiang J.Y. Bile acids: regulation of synthesis.J Lipid Res. 2009; 50: 1955-1966Abstract Full Text Full Text PDF PubMed Scopus (1068) Google Scholar, 26Chen J. Cooper A.D. Levy-Wilson B. Hepatocyte nuclear factor 1 binds to and transactivates the human but not the rat CYP7A1 promoter.Biochem Biophys Res Commun. 1999; 260: 829-834Crossref PubMed Scopus (38) Google Scholar HNF4α activates human CYP7A1 but not mouse Cyp7a128Inoue Y. Yu A.M. Yim S.H. et al.Regulation of bile acid biosynthesis by hepatocyte nuclear factor 4α.J Lipid Res. 2006; 47: 215-227Abstract Full Text Full Text PDF PubMed Scopus (107) Google Scholar or rat26Chen J. Cooper A.D. Levy-Wilson B. Hepatocyte nuclear factor 1 binds to and transactivates the human but not the rat CYP7A1 promoter.Biochem Biophys Res Commun. 1999; 260: 829-834Crossref PubMed Scopus (38) Google Scholar CYP7A1 transcription. Does the lack of an LXR binding site in the human CYP7A1 promoter affect the suppression of this gene by SHP? The effect of SHP on HNF4α and the CYP7A1 gene could also impact the signaling in humans. Chow et al22Chow E.C.Y. Magomedova L. Quach H.P. et al.Vitamin D receptor activation down-regulates the small heterodimer partner and increases CYP7A1 to lower cholesterol.Gastroenterology. 2014; 146: 1048-1059Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar did show that 1,25(OH)2D3 induced CYP7A1 mRNA in human hepatocytes, although they did not see an effect on SHP expression which was attributed to the short half-life on SHP mRNA in these cells. Promoter studies revealed a conserved VDR binding site (direct repeat 3 element) in both the mouse Cyp7a1 and human CYP7A1 promoters, suggesting that this pathway is also active in humans.Others found that mice lacking VDR had a decrease in Shp RNA and an increase in Cyp7a1 mRNA and that injection of 1,25(OH)2D3 increased expression of Fgf15 mRNA in the intestine with subsequent inhibition of hepatic Cyp7a1 mRNA.29Schmidt D.R. Holmstrom S.R. Fon Tacer K. et al.Regulation of bile acid synthesis by fat-soluble vitamins A and D.J Biol Chem. 2010; 285: 14486-14494Abstract Full Text Full Text PDF PubMed Scopus (165) Google Scholar In contrast with these data, administration of 1,25(OH)2D3 in the present study induced Cyp7a1 mRNA. However, as the authors’ noted, the dose used by Schmidt et al29Schmidt D.R. Holmstrom S.R. Fon Tacer K. et al.Regulation of bile acid synthesis by fat-soluble vitamins A and D.J Biol Chem. 2010; 285: 14486-14494Abstract Full Text Full Text PDF PubMed Scopus (165) Google Scholar was an order of magnitude higher than that employed in the present study although they used a single IP injection and the studies were short term. Thus, it is possible that a dose-dependent effect could characterize the apparent discrepancy in entero-hepatic VDR driven regulation of Cyp7a1 expression.Among the most interesting findings in this paper is that activated VDR repressed expression of the Shp gene. The authors provide evidence using reporter assays in cultured tumor cells and chromatin immunoprecipitation studies in 1,25(OH)2D3-treated mice that VDR binds to an upstream element in the Shp gene. Gene repression by nuclear receptors was found in earlier studies, but the molecular mechanism of repression of transcription had not been flushed out. VDR binds to upstream sites of Cyp24a1 as a heterodimer with the retinoid X receptor (RXR) and activates transcription.30Zierold C. Darwish H.M. DeLuca H.F. Two vitamin D response elements function in the rat 1,25-dihydroxyvitamin D 24-hydroxylase promoter.J Biol Chem. 1995; 270: 1675-1678Abstract Full Text Full Text PDF PubMed Scopus (182) Google Scholar, 31Meyer M.B. Zella L.A. Nerenz R.D. et al.Characterizing early events associated with the activation of target genes by 1,25-dihydroxyvitamin D3 in mouse kidney and intestine in vivo.J Biol Chem. 2007; 282: 22344-22352Abstract Full Text Full Text PDF PubMed Scopus (81) Google Scholar In the current study, VDR/RXR bound to an upstream element of the Shp gene but instead of activating transcription, the heterodimer represses transcription. What element of the Shp promoter mediates this differential response? For nuclear receptors to activate transcription, co-activators must be recruited along with other transcription machinery that modifies chromatin (methylation and acetylation) and attracts RNA polymerase 2. Does binding of VDR/RXR to the Cyp7a1 element recruit a co-repressor instead of a co-activator? Is chromatin modified to a form that promotes transcription or that suppresses transcription after receptor binding? Finally, regulation of the Cyp7a1 gene involves many transcription factors (Figure 1). How does VDR influence the FXR-mediated activation of the Shp gene? In the presence of increased hepatic bile acids, does FXR signaling negate the effects of VDR on Shp? These are questions that can be addressed experimentally. See “Vitamin D receptor activation down-regulates the small heterodimer partner and increases CYP7A1 to lower cholesterol,” by Chow ECY, Magomedova L, Quach HP, et al, on page 1048. See “Vitamin D receptor activation down-regulates the small heterodimer partner and increases CYP7A1 to lower cholesterol,” by Chow ECY, Magomedova L, Quach HP, et al, on page 1048. See “Vitamin D receptor activation down-regulates the small heterodimer partner and increases CYP7A1 to lower cholesterol,” by Chow ECY, Magomedova L, Quach HP, et al, on page 1048. CYP7A1, a cholesterol 7α-hydroxylase, is the rate-limiting enzyme in bile acid synthesis and its expression is thought to control levels of serum cholesterol.1Chiang J.Y. Bile acids: regulation of synthesis.J Lipid Res. 2009; 50: 1955-1966Abstract Full Text Full Text PDF PubMed Scopus (1068) Google Scholar, 2Jelinek D.F. Andersson S. Slaughter C.A. et al.Cloning and regulation of cholesterol 7α-hydroxylase, the rate-limiting enzyme in bile acid biosynthesis.J Biol Chem. 1990; 265: 8190-8197Abstract Full Text PDF PubMed Google Scholar Humans deficient in CYP7A1 have increased serum cholesterol,3Pullinger C.R. Eng C. Salen G. et al.Human cholesterol 7α -hydroxylase (CYP7A1) deficiency has a hypercholesterolemic phenotype.J Clin Invest. 2002; 110: 109-117Crossref PubMed Scopus (440) Google Scholar and mice having constitutive expression of a human CYP7A1 transgene are protected from a high-fat-diet–induced hypercholesterolemia.4Li T. Matozel M. Boehme S. et al.Overexpression of cholesterol 7α-hydroxylase promotes hepatic bile acid synthesis and secretion and maintains cholesterol homeostasis.Hepatology. 2011; 53: 996-1006Crossref PubMed Scopus (155) Google Scholar The Cyp7a1 gene is primarily regulated through activation of the ligand-dependent nuclear receptor farnesoid X receptor (FXR) by endogenous bile acids.5Forman B.M. Goode E. Chen J. et al.Identification of a nuclear receptor that is activated by farnesol metabolites.Cell. 1995; 81: 687-693Abstract Full Text PDF PubMed Scopus (951) Google Scholar, 6Seol W. Choi H.S. Moore D.D. Isolation of proteins that interact specifically with the retinoid X receptor: two novel orphan receptors.Mol Endocrinol. 1995; 9: 72-85Crossref PubMed Google Scholar, 7Makishima M. Okamoto A.Y. Repa J.J. et al.Identification of a nuclear receptor for bile acids.Science. 1999; 284: 1362-1365Crossref PubMed Scopus (2103) Google Scholar Bile acids are able to suppress their synthesis and to increase their secretion from the liver into the bile ducts. Suppression of Cyp7a1 by hepatic FXR is due in part to an induction of the small heterodimer protein (SHP), a member of the nuclear receptor superfamily that lacks a DNA binding domain but has the domain required for dimerization with other nuclear receptors.8Seol W. Choi H.S. Moore D.D. An orphan nuclear hormone receptor that lacks a DNA binding domain and heterodimerizes with other receptors.Science. 1996; 272: 1336-1339Crossref PubMed Scopus (438) Google Scholar SHP is able to form a complex with the liver receptor homolog-1 (LRH-1), a positive regulator of Cyp7a1, rendering it unable to activate the Cyp7a1 gene.9Gupta S. Pandak W.M. Hylemon P.B. LXR alpha is the dominant regulator of CYP7A1 transcription.Biochem Biophys Res Commun. 2002; 293: 338-343Crossref PubMed Scopus (104) Google Scholar Intriguingly, the importance of the intestine in the regulation of hepatic Cyp7a1 has been highlighted by the discovery of the FXR-mediated induction in the ileum of fibroblast growth factor (FGF) 19/15, a hormone that is secreted in the portal blood and signals to the liver to repress Cyp7A1 expression synergistically with SHP.10Inagaki T. Choi M. Moschetta A. et al.Fibroblast growth factor 15 functions as an enterohepatic signal to regulate bile acid homeostasis.Cell Metab. 2005; 2: 217-225Abstract Full Text Full Text PDF PubMed Scopus (1286) Google Scholar Thus, hepatic and intestinal FXR cooperate to down-regulate Cyp7A1 expression (Figure 1). The use of tissue-specific Fxr-null or transgenic mice demonstrated a much more prominent role for the intestinal FXR-FGF15 pathway, with respect to the hepatic FXR-SHP pathway, in repressing Cyp7a1 expression.11Kim I. Ahn S.H. Inagaki T. et al.Differential regulation of bile acid homeostasis by the farnesoid X receptor in liver and intestine.J Lipid Res. 2007; 48: 2664-2672Abstract Full Text Full Text PDF PubMed Scopus (404) Google Scholar, 12Modica S. Petruzzelli M. Bellafante E. et al.Selective activation of nuclear bile acid receptor FXR in the intestine protects mice against cholestasis.Gastroenterology. 2012; 142 (e1–e4): 355-365Abstract Full Text Full Text PDF PubMed Scopus (209) Google Scholar Vitamin D receptor (VDR) is also reported to have effects on bile acid synthesis and cholesterol levels in hepatocytes and serum. VDR is activated by 1α, 25-dihydroxyvitamin D3 (1,25(OH)2D3), an active form of vitamin D3, and mediates vitamin D signaling in numerous physiologic and pharmacologic processes.13Haussler M.R. Haussler C.A. Jurutka P.W. et al.The vitamin D hormone and its nuclear receptor: molecular actions and disease states.J Endocrinol. 1997; 154: S57-S73PubMed Google Scholar VDR is expressed in kidney, intestine, and bone, and at low levels in most other tissues.14Bookout A.L. Jeong Y. Downes M. et al.Anatomical profiling of nuclear receptor expression reveals a hierarchical transcriptional network.Cell. 2006; 126: 789-799Abstract Full Text Full Text PDF PubMed Scopus (775) Google Scholar It is not expressed to a significant extent in liver and thus its physiologic role in this tissue is questionable. Deficiency of VDR leads to hypocalcemia, hyperparathyroidism, rickets, osteomalacia, alopecia, uterine hypoplasia, and growth retardation, which might result from repression of calcium absorption owing to the down-regulation of duodenal epithelial calcium channels.15Li Y.C. Pirro A.E. Amling M. et al.Targeted ablation of the vitamin D receptor: an animal model of vitamin D-dependent rickets type II with alopecia.Proc Natl Acad Sci U S A. 1997; 94: 9831-9835Crossref PubMed Scopus (799) Google Scholar, 16Yoshizawa T. Handa Y. Uematsu Y. et al.Mice lacking the vitamin D receptor exhibit impaired bone formation, uterine hypoplasia and growth retardation after weaning.Nat Genet. 1997; 16: 391-396Crossref PubMed Scopus (965) Google Scholar, 17Li Y.C. Kong J. Wei M. et al.1,25-Dihydroxyvitamin D(3) is a negative endocrine regulator of the renin-angiotensin system.J Clin Invest. 2002; 110: 229-238Crossref PubMed Scopus (1723) Google Scholar, 18Lieben L. Masuyama R. Torrekens S. et al.Normocalcemia is maintained in mice under conditions of calcium malabsorption by vitamin D-induced inhibition of bone mineralization.J Clin Invest. 2012; 122: 1803-1815Crossref PubMed Scopus (251) Google Scholar, 19Bouillon R. Carmeliet G. Verlinden L. et al.Vitamin D and human health: lessons from vitamin D receptor null mice.Endocr Rev. 2008; 29: 726-776Crossref PubMed Scopus (1287) Google Scholar VDR could affect the levels of cholesterol in liver and serum, possibly through controlling expression of genes involved in bile acid synthesis from cholesterol. However, earlier studies in mouse and human hepatoma cells revealed that activation of the VDR actually blocked the expression of FXR, which would result in suppression of Cyp7a1 expression leading to decreased cholesterol levels.20Honjo Y. Sasaki S. Kobayashi Y. et al.1,25-dihydroxyvitamin D3 and its receptor inhibit the chenodeoxycholic acid-dependent transactivation by farnesoid X receptor.J Endocrinol. 2006; 188: 635-643Crossref PubMed Scopus (49) Google Scholar, 21Jiang W. Miyamoto T. Kakizawa T. et al.Inhibition of LXRalpha signaling by vitamin D receptor: possible role of VDR in bile acid synthesis.Biochem Biophys Res Commun. 2006; 351: 176-184Crossref PubMed Scopus (36) Google Scholar To clarify the role of VDR in control of Cyp7a1 and serum cholesterol levels, the current study by Chow et al22Chow E.C.Y. Magomedova L. Quach H.P. et al.Vitamin D receptor activation down-regulates the small heterodimer partner and increases CYP7A1 to lower cholesterol.Gastroenterology. 2014; 146: 1048-1059Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar was carried out using in vivo experimentation and various gene knockout mouse models to investigate the mechanism by which VDR affects cholesterol levels. They found that VDR is expressed at low but measurable levels in the livers of mice and is able to activate its main target gene Cyp24a1 upon injection of 1,25(OH)2D3. In contrast with earlier studies in cultured tumor cells, they show that activation of VDR by 1,25(OH)2D3 represses SHP expression through suppression of Shp gene transcription. They also convincingly demonstrate that 1,25(OH)2D3 reduces both liver and serum cholesterol that they ascribe to the activation of Cyp7a1; this change was not observed in mice lacking expression of SHP in the liver (Figure 1). This study illustrates the importance of in vivo experimentation to determine the effects of drugs and other compounds on hepatic gene expression. Tumor cells frequently lack all of the transcription machinery found in the liver and even liver cells in culture differ from intact liver in response to stimulation because hepatocyte function depends on liver architecture and the presence of other nonhepatocytes, such as stromal cells and Kupffer cells. Among the unresolved issues in this study is the translatability to humans. This is of great importance because oral vitamin D3 and its derivatives are used as dietary supplements and are frequently prescribed to patients with low vitamin D3 levels. However, a randomized, placebo-controlled trial revealed that short-term treatment with 50,000 IU of vitamin D3 weekly for 8 weeks had no effect on serum cholesterol levels23Ponda M.P. Dowd K. Finkielstein D. et al.The short-term effects of vitamin D repletion on cholesterol: a randomized, placebo-controlled trial.Arterioscler Thromb Vasc Biol. 2012; 32: 2510-2515Crossref PubMed Scopus (71) Google Scholar; similar results were obtained with a long-term treatment.24Zittermann A. Frisch S. Berthold H.K. et al.Vitamin D supplementation enhances the beneficial effects of weight loss on cardiovascular disease risk markers.Am J Clin Nutr. 2009; 89: 1321-1327Crossref PubMed Scopus (441) Google Scholar It should be noted in this context, that these studies were done with oral dosing, whereas Chow et al22Chow E.C.Y. Magomedova L. Quach H.P. et al.Vitamin D receptor activation down-regulates the small heterodimer partner and increases CYP7A1 to lower cholesterol.Gastroenterology. 2014; 146: 1048-1059Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar treated mice by injecting 1,25(OH)2D3. Injection of humans with vitamin D3 is not practical as a means for therapeutic delivery for lowering cholesterol, even if it is shown to be efficacious by this route of administration. However, the present work does give a hint that endogenously produced vitamin D3 and hepatic VDR could affect cholesterol levels in humans. The Cyp7a1 gene is positively regulated by several transcription factors including LRH-1, liver X receptor (LXR), and hepatocyte (HNF4α). SHP represses Cyp7a1 by binding to and inactivating the transcription potential of LRH-1 and this pathway is conserved in humans and mice.25Lu T.T. Makishima M. Repa J.J. et al.Molecular basis for feedback regulation of bile acid synthesis by nuclear receptors.Mol Cell. 2000; 6: 507-515Abstract Full Text Full Text PDF PubMed Scopus (1208) Google Scholar SHP could also affect signaling by HNF4α.26Chen J. Cooper A.D. Levy-Wilson B. Hepatocyte nuclear factor 1 binds to and transactivates the human but not the rat CYP7A1 promoter.Biochem Biophys Res Commun. 1999; 260: 829-834Crossref PubMed Scopus (38) Google Scholar, 27Han S. Chiang J.Y. Mechanism of vitamin D receptor inhibition of cholesterol 7α -hydroxylase gene transcription in human hepatocytes.Drug Metab Dispos. 2009; 37: 469-478Crossref PubMed Scopus (76) Google Scholar There exists a species difference in control of the mouse Cyp7a1 and human CYP7A1 genes. LXR activates the mouse Cyp7a1 gene, but does not activate the human CYP7A1 gene owing to the lack of an LXR binding site (direct repeat 4 element) in the CYP7A1 promoter.1Chiang J.Y. Bile acids: regulation of synthesis.J Lipid Res. 2009; 50: 1955-1966Abstract Full Text Full Text PDF PubMed Scopus (1068) Google Scholar, 26Chen J. Cooper A.D. Levy-Wilson B. Hepatocyte nuclear factor 1 binds to and transactivates the human but not the rat CYP7A1 promoter.Biochem Biophys Res Commun. 1999; 260: 829-834Crossref PubMed Scopus (38) Google Scholar HNF4α activates human CYP7A1 but not mouse Cyp7a128Inoue Y. Yu A.M. Yim S.H. et al.Regulation of bile acid biosynthesis by hepatocyte nuclear factor 4α.J Lipid Res. 2006; 47: 215-227Abstract Full Text Full Text PDF PubMed Scopus (107) Google Scholar or rat26Chen J. Cooper A.D. Levy-Wilson B. Hepatocyte nuclear factor 1 binds to and transactivates the human but not the rat CYP7A1 promoter.Biochem Biophys Res Commun. 1999; 260: 829-834Crossref PubMed Scopus (38) Google Scholar CYP7A1 transcription. Does the lack of an LXR binding site in the human CYP7A1 promoter affect the suppression of this gene by SHP? The effect of SHP on HNF4α and the CYP7A1 gene could also impact the signaling in humans. Chow et al22Chow E.C.Y. Magomedova L. Quach H.P. et al.Vitamin D receptor activation down-regulates the small heterodimer partner and increases CYP7A1 to lower cholesterol.Gastroenterology. 2014; 146: 1048-1059Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar did show that 1,25(OH)2D3 induced CYP7A1 mRNA in human hepatocytes, although they did not see an effect on SHP expression which was attributed to the short half-life on SHP mRNA in these cells. Promoter studies revealed a conserved VDR binding site (direct repeat 3 element) in both the mouse Cyp7a1 and human CYP7A1 promoters, suggesting that this pathway is also active in humans. Others found that mice lacking VDR had a decrease in Shp RNA and an increase in Cyp7a1 mRNA and that injection of 1,25(OH)2D3 increased expression of Fgf15 mRNA in the intestine with subsequent inhibition of hepatic Cyp7a1 mRNA.29Schmidt D.R. Holmstrom S.R. Fon Tacer K. et al.Regulation of bile acid synthesis by fat-soluble vitamins A and D.J Biol Chem. 2010; 285: 14486-14494Abstract Full Text Full Text PDF PubMed Scopus (165) Google Scholar In contrast with these data, administration of 1,25(OH)2D3 in the present study induced Cyp7a1 mRNA. However, as the authors’ noted, the dose used by Schmidt et al29Schmidt D.R. Holmstrom S.R. Fon Tacer K. et al.Regulation of bile acid synthesis by fat-soluble vitamins A and D.J Biol Chem. 2010; 285: 14486-14494Abstract Full Text Full Text PDF PubMed Scopus (165) Google Scholar was an order of magnitude higher than that employed in the present study although they used a single IP injection and the studies were short term. Thus, it is possible that a dose-dependent effect could characterize the apparent discrepancy in entero-hepatic VDR driven regulation of Cyp7a1 expression. Among the most interesting findings in this paper is that activated VDR repressed expression of the Shp gene. The authors provide evidence using reporter assays in cultured tumor cells and chromatin immunoprecipitation studies in 1,25(OH)2D3-treated mice that VDR binds to an upstream element in the Shp gene. Gene repression by nuclear receptors was found in earlier studies, but the molecular mechanism of repression of transcription had not been flushed out. VDR binds to upstream sites of Cyp24a1 as a heterodimer with the retinoid X receptor (RXR) and activates transcription.30Zierold C. Darwish H.M. DeLuca H.F. Two vitamin D response elements function in the rat 1,25-dihydroxyvitamin D 24-hydroxylase promoter.J Biol Chem. 1995; 270: 1675-1678Abstract Full Text Full Text PDF PubMed Scopus (182) Google Scholar, 31Meyer M.B. Zella L.A. Nerenz R.D. et al.Characterizing early events associated with the activation of target genes by 1,25-dihydroxyvitamin D3 in mouse kidney and intestine in vivo.J Biol Chem. 2007; 282: 22344-22352Abstract Full Text Full Text PDF PubMed Scopus (81) Google Scholar In the current study, VDR/RXR bound to an upstream element of the Shp gene but instead of activating transcription, the heterodimer represses transcription. What element of the Shp promoter mediates this differential response? For nuclear receptors to activate transcription, co-activators must be recruited along with other transcription machinery that modifies chromatin (methylation and acetylation) and attracts RNA polymerase 2. Does binding of VDR/RXR to the Cyp7a1 element recruit a co-repressor instead of a co-activator? Is chromatin modified to a form that promotes transcription or that suppresses transcription after receptor binding? Finally, regulation of the Cyp7a1 gene involves many transcription factors (Figure 1). How does VDR influence the FXR-mediated activation of the Shp gene? In the presence of increased hepatic bile acids, does FXR signaling negate the effects of VDR on Shp? These are questions that can be addressed experimentally. Vitamin D Receptor Activation Down-regulates the Small Heterodimer Partner and Increases CYP7A1 to Lower CholesterolGastroenterologyVol. 146Issue 4PreviewLittle is known about the effects of the vitamin D receptor (VDR) on hepatic activity of human cholesterol 7α-hydroxylase (CYP7A1) and cholesterol metabolism. We studied these processes in mice in vivo and mouse and human hepatocytes. Full-Text PDF