Liver X receptor agonist inhibits proliferation of ovarian carcinoma cells stimulated by oxidized low density lipoprotein
Introduction
The association between serum cholesterol and cancer risk and survival is unclear. Attempts to correlate risk or outcome of numerous cancers with serum low density lipoprotein (LDL) cholesterol have yielded somewhat controversial results with evidence pointing towards improved outcome in patients with high serum cholesterol [1], [2], [3]. One study also demonstrated that women on a diet high in cholesterol had no increased risk of epithelial ovarian cancer [4]. However, recently it was reported that the levels of oxidized LDL (oxLDL) in the serum of ovarian cancer patients were positively associated with patient outcome [5]. Our group has also recently reported that epithelial ovarian cancer patients on statins had improved survival [6].
Levels of oxLDL are in part regulated by the available oxidizable LDL precursor and may be altered by cholesterol lowering drugs such as statins. Cholesterol homeostatis is maintained in part by cells expressing scavenger receptors (SRA and CD36) that internalize oxLDL which are then converted to oxysterol ligands of the nuclear liver X receptors α and β (LXRα and LXRβ), heterodimers of the retinoid X receptor (RXR) [7]. Activated LXR/RXR heterodimers activate target genes possessing the LXR element (LXRE) including the ATP binding cassette transporters ABCA1 and ABCG1 leading to cholesterol efflux to HDL or cholesterol excretion via bilial and intestinal cells [7]. LXR activation is also associated with increases in the expression of numerous proinflammatory cytokines which also promote cellular proliferation [7]. While statin therapy can lower serum oxLDL [8], [9], [10], [11], statins also have lipid independent functions on oxidative pathways including regulation of scavenger receptors. In monocytes and foam cells, statins can alter scavenger receptor expression via regulation of PPAR, NF-κB, Rho and Ras [12], but these statin functions have not been studied in cancer cells.
One potential means for treating oxLDL related diseases is modulation of LXR by synthetic LXR agonists. Presently three such synthetic LXR ligands have been developed including TO901317, GW3965 and N,N-dimethyl-3β-hydroxycholenamide (DMHCA). LXR agonists upregulate expression of the ATP binding cassette transporters and cholesterol efflux while simultaneously downregulating oxysterol mediated activation of proinflammatory cytokines [13], [14]. In mouse models of atherosclerosis LXR agonists reduce both inflammation in atherosclerotic plaques and serum cholesterol [15]. LXR agonists also inhibit the proliferation of endothelial cells [16]. oxLDL and LXR agonists have been little studied on cells other than endothelial, monocytes and macrophage foam cells in the context of cardiovascular diseases, but one study demonstrated oxLDL is a mitogen to cultured human fibroblasts [17].
In the present study we determined that ovarian carcinoma cells possess CD36 scavenger receptor and are stimulated to proliferate by oxLDL. The LXR agonist TO901317 and fluvastatin reversed oxLDL mediated proliferation. We also demonstrated that oxLDL reduced the sensitivity of ovarian carcinoma cells to cisplatin. Our study demonstrated that increases in oxidized LDL cholesterol may negatively impact ovarian cancer outcome and suggests that LXR ligands and statins may be an effective strategy for treating ovarian cancer patients.
Section snippets
Tissue culture
CAOV3, ES2, OVCAR3, PA1, and SKOV3 were cultured as recommended by American Type Culture Collection. CSOC882 and CSOC909 were cultured as previously described [18]. A2780 and CP70 were cultured in RPMI 1640 and 10% fetal calf serum (FCS) supplemented with 2 mM l-glutamine and 0.2 U/ml insulin. OVCA432 was cultured in modified Eagle's medium (MEM) and 10% FBS + 2 mM l-glutamine. All reagents were purchased from GIBCO.
RNA interference
RNA interference (RNAi) was accomplished by suspension transfection of CAOV3
oxLDL stimulated proliferation of ovarian carcinoma cell lines, but LDL had no effect
We determined the effects of LDL and ox-LDL on proliferation of SKOV3 and CAOV3 ovarian carcinoma cell lines by MTT assays. Increasing doses of LDL did not significantly alter the growth of either of these cell lines (Fig. 1A). However, the proliferation of each of these cell lines was significantly increased upon treatment with oxLDL (Fig. 1B). Increased proliferation was also evident visually by examination of cells treated for 24 h with oxLDL by standard phase microscopy (Fig. 1C).
LXR pathway proteins are expressed in ovarian carcinoma cell lines
We
Discussion
Ovarian cancer outcome may be influenced by genetic, epigenetic, and clinical determinates as well as metabolic conditions that alter cancer proliferation. Recently we demonstrated that ovarian cancer patients taking statins had improved survival [6]. In addition, a recent meta-analysis of 37,248 individuals found reduced cancer incidence for statin users [20]. While not all studies have demonstrated positive associations between serum cholesterol levels and outcome in solid malignancies [1],
Conflict of interest statement
The authors have no conflicts of interest to declare.
Acknowledgments
This work was supported by contributions from the L&S Milken Foundation and an American Cancer Society California Division Early Detection Professorship to BYK.
References (33)
- et al.
Low plasma cholesterol predicts an increased risk of lung cancer in elderly women
Prev. Med.
(1995) - et al.
Oxidized LDL, serum oxidizability and serum lipid levels in patients with breast or ovarian cancer
Clin. Biochem.
(2007) - et al.
Impact of statin therapy on survival in epithelial ovarian cancer
Gynecol. Oncol.
(2008) - et al.
Differential effect of atorvastatin and fenofibrate on plasma oxidized low-density lipoprotein, inflammation markers, and cell adhesion molecules in patients with type 2 diabetes mellitus
Metabolism
(2008) - et al.
Proportion of oxidized LDL relative to plasma apolipoprotein B does not change during statin therapy in patients with heterozygous familial hypercholesterolemia
Atherosclerosis
(2006) - et al.
Statins and foam cell formation: impact on LDL oxidation and uptake of oxidized lipoproteins via scavenger receptors
Biochim. Biophys. Acta
(2007) - et al.
Characterization of CSOC 882, a novel immortalized ovarian cancer cell line expressing EGFR, HER2, and activated AKT
Gynecol. Oncol.
(2007) - et al.
The liver X receptor agonist T0901317 acts as androgen receptor antagonist in human prostate cancer cells
Biochem. Biophys. Res. Commun.
(2007) - et al.
Comparative evaluation of the effects of statins on human stem and cancer cells in vitro
Reprod. Biomed. Online
(2007) - et al.
Statins in tumor suppression
Cancer Lett.
(2008)