TO901317 regulating apolipoprotein M expression mediates via the farnesoid X receptor pathway in Caco-2 cells
© Zhu et al; licensee BioMed Central Ltd. 2011
Received: 22 September 2011
Accepted: 4 November 2011
Published: 4 November 2011
Apolipoprotein M (apoM) may have potential antiatherosclerotic properties. It has been reported that apoM expression could be regulated by many intracellar and extracellar factors. In the present study we further investigated regulation of apoM expression in Caco-2 cells stimulated by a liver X receptor (LXR) agonist, TO901317.
Materials and methods
Caco-2 cells were cultured in the presence of either TO901317, farnesoid X receptor (FXR) antagonist guggulsterone or TO901317 together with guggulsterone at different concentrations for 24 hrs. The mRNA levels of ATP-binding cassette transporter A1 (ABCA1), apoA1, apoM, liver receptor homologue-1 (LRH-1) and short heterodimer partner 1 (SHP1) were determined by real-time RT-PCR.
When Caco-2 cell cultured with TO901317 alone, the mRNA levels of ABCA1, apoA1, apoM, LRH-1 and SHP1 were significantly increased with dose-dependent manners (p < 0.05), whereas when the cells cultured with guggulsterone alone, the mRNA levels of apoM, SHP1 and LRH-1 (p < 0.05) were strongly inhibited. Moreover, guggulsterone could abolish the TO901317 enhanced mRNA levels of apoA1 apoM, SHP1 and LRH-1.
The present study demonstrated that LXR agonist TO901317 induced apoM expression in Caco-2 cells might be mediated via the LXR/FXR pathway.
With the aging population and changing lifestyles, the incidence of cardiovascular diseases (CVD) has gradually increased . Abnormal lipid metabolism has been considered as one of the major risk factors of CVD . Previously studies have demonstrated that serum concentrations of apolipoprotein (apo) AI and apoB have significantly correlation with the occurrences of CVD [3, 4], and other apolipoproteins may also involve in the initiation and progression of the diseases . ApoM is one of the latest discovered apolipoproteins that is mainly synthesized in the liver, and to a smaller amounts, in the kidney . In human plasma, most apoM are located in the high-density lipoproteins (HDL) and small proportion present also in apoB-containing lipoproteins, i.e. chylomicrons, very low-, and low-density lipoproteins (VLDL and LDL) [6, 7]. Recent investigations have suggested that apoM may participate in the HDL-related biological activities as an important component of HDL particle on the protection of endothelial cells . Wolfrum, et al.,  reported that apoM is required for preβ-HDL formation and cholesterol efflux to HDL particles, which is an initial and crucial stage of reverse cholesterol transport, and subsequently protects against atherosclerosis. In addition, the physiological and patho-physiological roles of apoM may also involve in the inflammatory activities and the potential immuno- and inflamm-reactive properties, and apoM may therefore contribute to the anti-inflammatory function of HDL, being as generally acknowledged as a significant antiatherogenic mechanism [10, 11].
ApoM could be regulated by many factors including leptin, insulin, hyperglycemia and many cytokines in vivo and in vitro . It has been demonstrated that apoM gene expression could be also affected by some nuclear receptors, such as hepatocyte nuclear factor-1α (HNF-1α) , hepatocyte nuclear factor-4α (HNF-4α)  and liver receptor homolog-1 (LRH-1) . Liver X receptor (LXR) is a nuclear receptor, as a lipid sensor, protects cells from lipid overload and directly or indirectly controls apolipoprotein-mediated cholesterol efflux . Our previous studies demonstrated that the synthetic LXR agonist TO901317 could down-regulate hepatic apoM expression in vivo and in vitro . Whereas Calayir., et al. , in recognition of our findings that TO901317 inhibited apoM expression in HepG2 cells, even found that TO901317 could upregulate apoM expression in intestinal cells. In the present study we further revealed the regulative pathway of apoM expression in Caco-2 cells stimulated by TO901317.
Materials and methods
Cells and reagents
Human colorectal adenocarcinoma cell line, Caco-2, was obtained from the American Type Culture Collection (ATCC, Manassas, VA, USA). TO901317 was purchased from the Cayman Chemical Company (Ann Arbor, MI, USA). Guggulsterone was from the Sigma Chemical Co. Ltd. (Shanghai, China). Six-well cell culture clusters and 75 cm2 vented cell culture flasks were purchased from the Nunc (Roskilde, Denmark). Fetal bovine serum (FBS) and Dulbecco's Modified Eagle Medium (DMEM) were obtained from the Invitrogen (Shanghai, China). Total RNA purification kits were purchased from the Shenergy Biocolor BioScience and Technology Company (Shanghai, China). First strand cDNA synthesis kits were obtained from the Fermantas (Vilnius, Lithuania). The LightCycler real-time RT-PCR System was from the Roche Applied Science (Mannheim, Germany).
Caco-2 cells were cultured in DMEM supplemented with 20% FBS in the presence of 100 U/ml penicillin, 100 μg/ml streptomycin and 1% Glutamax at 37°C under 5% CO2 atmosphere. Cells were plated in 6-well cell culture clusters at a density of 3 × 105 cells/dish with DMEM containing 20% FBS. Cell monolayer of approximately 50-70% confluence were grown for 24 hrs in the above media prior to experiments, cells were washed twice with phosphate buffered saline (PBS) and once with serum-free DMEM without antibiotics. Then the experimental medium, containing DMEM with 1% bovine serum albumin (BSA) and different concentrations of TO901317 and guggulsterone, or TO901317 together with guggulsterone were added. As TO901317 must be dissolved in the DMSO and then added into the experimental medium (the final concentration of DMSO was at 1% in the present study), 1% DMSO were always applied in the controls. Unless stated otherwise, cells were incubated at 37°C for 24 hrs.
Total RNA extraction and real time RT-PCR
Primers and fluorescent probes for real-time RT-PCR
Data are expressed as means ± SE. All experiments were repeated twice and the data were represented one of the experiments. Statistical analyses were performed with the software Prism (version 5.0). Multiple comparisons were performed with one-way ANOVA/dunnett-t, and comparisons between two groups were statistically evaluated by the unpaired t-test. Significance was established at a P value less than 0.05.
ApoM has firstly been identified from human postprandial lipoproteins by Xu and Dahlbäck in 1999 . Recent observations strongly suggest that apoM is predominantly confined to the HDL particles in human plasma and it may have antiatherogenic properties involving in the conversion of large HDL to pre-β HDL, the later mainly functions as an acceptor of peripherally deposited cholesterol that is described as the revise cholesterol transportation [9, 17]. Moreover other researches indicated that apoM might be also involved in the immunity, inflammation, and neoplasia [18–20]. However, the physio-pathological functions of apoM are still not fully revealed. Observation of the regulation of apoM expression may reveal clinical importance of apoM in humans.
Previous studies have demonstrated that hepatic apoM expression could be regulated by certain cytokines and nuclear factors. In our previous study , we demonstrated that LXR agonist, TO901317, could significantly downregulate apoM expression in the hepatic cell line, HepG2 cells. Moreover, Calayir., et al.  confirmed our findings that TO901317 did inhibit apoM expression in HepG2 cells, however, they found that TO901317 could significantly upregulate apoM expression in intestinal cell, which suggest that TO901317 regulating apoM expression is cellular dependent. Moreover, Mosialou., et al.  reported that the natural LXR ligand oxysterols could significantly upregulate apoM mRNA level and protein level in HepG2 cells, and LXR could bind to the HRE in the proximal apoM promoter examined by the Chromatin Immunoprecipitation Assays (CHIP). They found that TO901317 caused an increased mRNA level of Short Heterodimer Partner (SHP) in HepG2 cells. In contrast, the SHP mRNA level was not affected by the oxysterols. Enhanced expression of SHP could inhibit LRH-1-mediated trans-activation of apoM promoter in HepG2 cells . SHP is an inhibitory nuclear receptor activated by the FXRs that interacts physically with many nuclear receptors including LRH-1. Furthermore, it has been reported that TO901317 is a dual LXR/FXR agonist that activates FXR more efficiently than its natural ligand, the bile acids . Based on the findings described above, the TO901317 downregulating apoM expression in hepatic cells may be due to the activation of the FXR/SHP pathway that inhibits LRH-1.
In the present studies, we further investigated effects of TO901317 on apoM expression in Caco-2 cells mediated via the FXR/SHP/LRH-1 pathway. TO901317 could significantly upregulate the mRNA levels of ABCA1 that is a classic LXR downstream gene . LXR can bind to the HRE upstream in the proximal apoM promoter to upregulate apoM gene expression. As SHP levels were enhanced simultaneously and SHP is a classic FXR downstream gene, which suggests that FXR has been activated simultaneously , whereas the LRH-1 was not inhibited by the overexpressed SHP. LRH-1 was induced by the SHP and the apoM mRNA levels were upregulated. The similar phenomenon was seen in apoA1 expression , TO901317 could downregulate apoA1 mRNA levels in HepG2 cells but it could be upregulated in the Caco-2 cells. Furthermore we demonstrated that FXR antagonist, guggulsterone, could abolish TO901317 increased expressions of apoA1, apoM, SHP1 and LRH-1 in Caco-2 cells, although guggulsterone alone might also decrease mRNA levels of apoA1, apoM, SHP1 and LRH-1. All these findings strongly suggest that activating FXR may mediate upregulative apoM expression in the Caco-2 cells.
This research project was supported by the national Natural Science Foundation of China (NSFC) (30972955), the research grant of Jiangsu province (BK2008140) and the research grant of the Changzhou Health Bureau (ZD200901).
- Briffa TG, Sanfilippo FM, Hobbs MS, Ridout SC, Katzenellenbogen JM, Thompson PL, Thompson SC: Under-ascertainment of Aboriginality in records of cardiovascular disease in hospital morbidity and mortality data in Western Australia: a record linkage study. BMC Med Res Methodol. 2010, 10: 111-116. 10.1186/1471-2288-10-111PubMed CentralView ArticlePubMedGoogle Scholar
- Vardavas CI, Linardakis MK, Hatzis CM, Saris WH, Kafatos AG: Cardiovascular disease risk factors and dietary habits of farmers from Crete 45 years after the first description of the Mediterranean diet. Eur J Cardiovasc Prev Rehabil. 2010, 17 (4): 440-446. 10.1097/HJR.0b013e32833692eaView ArticlePubMedGoogle Scholar
- Andrikoula M, McDowell IF: The contribution of ApoB and ApoA1 measurements to cardiovascular risk assessment. Diabetes Obes Metab. 2008, 10 (4): 271-278. 10.1111/j.1463-1326.2007.00714.xView ArticlePubMedGoogle Scholar
- Panayiotou A, Griffin M, Georgiou N, Bond D, Tyllis T, Tziakouri-Shiakalli C, Fessas C, Nicolaides A: ApoB/ApoA1 ratio and subclinical atherosclerosis. Int Angiol. 2008, 27 (1): 74-80.PubMedGoogle Scholar
- Arsenault BJ, Boekholdt SM, Kastelein JJ: Lipid parameters for measuring risk of cardiovascular disease. Nat Rev Cardiol. 2011, 8 (4): 197-206. 10.1038/nrcardio.2010.223View ArticlePubMedGoogle Scholar
- Xu N, Dahlback B: A novel human apolipoprotein (apoM). J Biol Chem. 1999, 274 (44): 31286-31290. 10.1074/jbc.274.44.31286View ArticlePubMedGoogle Scholar
- Christoffersen C, Nielsen LB, Axler O, Andersson A, Johnsen AH, Dahlback B: Isolation and characterization of human apolipoprotein M-containing lipoproteins. J Lipid Res. 2006, 47 (8): 1833-1843. 10.1194/jlr.M600055-JLR200View ArticlePubMedGoogle Scholar
- Christoffersen C, Obinata H, Kumaraswamy SB, Galvani S, Ahnstrom J, Sevvana M, Egerer-Sieber C, Muller YA, Hla T, Nielsen LB: Endothelium-protective sphingosine-1-phosphate provided by HDL-associated apolipoprotein M. Proc Natl Acad Sci USA. 2011,Google Scholar
- Wolfrum C, Poy MN, Stoffel M: Apolipoprotein M is required for prebeta-HDL formation and cholesterol efflux to HDL and protects against atherosclerosis. Nat Med. 2005, 11 (4): 418-422. 10.1038/nm1211View ArticlePubMedGoogle Scholar
- Christoffersen C, Jauhiainen M, Moser M, Porse B, Ehnholm C, Boesl M, Dahlback B, Nielsen LB: Effect of apolipoprotein M on high density lipoprotein metabolism and atherosclerosis in low density lipoprotein receptor knock-out mice. J Biol Chem. 2008, 283 (4): 1839-1847.View ArticlePubMedGoogle Scholar
- Venteclef N, Haroniti A, Tousaint JJ, Talianidis I, Delerive P: Regulation of anti-atherogenic apolipoprotein M gene expression by the orphan nuclear receptor LRH-1. J Biol Chem. 2008, 283 (7): 3694-3701.View ArticlePubMedGoogle Scholar
- Mosialou I, Zannis VI, Kardassis D: Regulation of human apolipoprotein m gene expression by orphan and ligand-dependent nuclear receptors. J Biol Chem. 2010, 285 (40): 30719-30730. 10.1074/jbc.M110.131771PubMed CentralView ArticlePubMedGoogle Scholar
- Zhang Y, Chen CJ, Yang QL, Cheng LQ, Wang H, Huang LZ: [Effect of interfering hepatocyte nuclear factor-1 alfa in HepG2 on the expressions of apoM, apoA-I and the correlative key enzyme of cholesterol metabolism.]. Zhonghua Gan Zang Bing Za Zhi. 2011, 19 (2): 121-126.PubMedGoogle Scholar
- Li X, Yeh V, Molteni V: Liver X receptor modulators: a review of recently patented compounds (2007 - 2009). Expert Opin Ther Pat. 2010, 20 (4): 535-562. 10.1517/13543771003621269View ArticlePubMedGoogle Scholar
- Zhang X, Zhu Z, Luo G, Zheng L, Nilsson-Ehle P, Xu N: Liver X receptor agonist downregulates hepatic apoM expression in vivo and in vitro. Biochem Biophys Res Commun. 2008, 371 (1): 114-117. 10.1016/j.bbrc.2008.04.017View ArticlePubMedGoogle Scholar
- Calayir E, Becker TM, Kratzer A, Ebner B, Panzenbock U, Stefujl J, Kostner GM: LXR-agonists regulate ApoM expression differentially in liver and intestine. Curr Pharm Biotechnol. 2008, 9 (6): 516-521. 10.2174/138920108786786376View ArticlePubMedGoogle Scholar
- Huang XS, Zhao SP, Hu M, Luo YP: Apolipoprotein M likely extends its anti-atherogenesis via anti-inflammation. Med Hypotheses. 2007, 69 (1): 136-140. 10.1016/j.mehy.2006.10.052View ArticlePubMedGoogle Scholar
- Feingold KR, Shigenaga JK, Chui LG, Moser A, Khovidhunkit W, Grunfeld C: Infection and inflammation decrease apolipoprotein M expression. Atherosclerosis. 2008, 199 (1): 19-26. 10.1016/j.atherosclerosis.2007.10.007View ArticlePubMedGoogle Scholar
- Luo G, Zhang X, Mu Q, Chen L, Zheng L, Wei J, Berggren-Soderlund M, Nilsson-Ehle P, Xu N: Expression and localization of apolipoprotein M in human colorectal tissues. Lipids Health Dis. 2010, 9: 102- 10.1186/1476-511X-9-102PubMed CentralView ArticlePubMedGoogle Scholar
- Jiang J, Wu C, Luo G, Zheng L, Chen L, Zhang X, Xu N: Expression of apolipoprotein M in human hepatocellular carcinoma tissues. Acta Histochem. 2011, 113 (1): 53-57. 10.1016/j.acthis.2009.08.005View ArticlePubMedGoogle Scholar
- Houck KA, Borchert KM, Hepler CD, Thomas JS, Bramlett KS, Michael LF, Burris TP: T0901317 is a dual LXR/FXR agonist. Mol Genet Metab. 2004, 83 (1-2): 184-187. 10.1016/j.ymgme.2004.07.007View ArticlePubMedGoogle Scholar
- Li G, Thomas AM, Hart SN, Zhong X, Wu D, Guo GL: Farnesoid X receptor activation mediates head-to-tail chromatin looping in the Nr0b2 gene encoding small heterodimer partner. Mol Endocrinol. 2010, 24 (7): 1404-1412. 10.1210/me.2010-0014PubMed CentralView ArticlePubMedGoogle Scholar
- Huuskonen J, Vishnu M, Chau P, Fielding PE, Fielding CJ: Liver X receptor inhibits the synthesis and secretion of apolipoprotein A1 by human liver-derived cells. Biochemistry. 2006, 45 (50): 15068-15074. 10.1021/bi061378yView ArticlePubMedGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.