Activation of liver X receptors inhibits pancreatic islet beta cell proliferation through cell cycle arrest

Abstract

Aims/hypothesis Liver X receptors (LXRs) are important transcriptional regulators of lipid homeostasis and proliferation in several cell types. However, the roles of LXRs in pancreatic beta cells have not been fully established. The aim of this study was to investigate the effects of LXRs on pancreatic beta cell proliferation. Methods Gene expression was analysed using real-time RT-PCR. Transient transfection and reporter gene assays were used to determine the transcriptional activity of LXRs in pancreatic beta cells. Cell viability and proliferation were analysed using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), DNA fluorometric, BrdU labelling and [3 H]thymidine incorporation assays. Cell cycle distribution was investigated by flow cytometry analysis. Adenovirus-based RNA interference was used to knockdown LXRα, LXRβ and p27 in MIN6 cells and mouse islets.Results We found that both Lxrα (also known as Nr1h3) and Lxrβ (also known as Nr1h2) were expressed and transactivated the LXR response element in HIT-T15 and MIN6 cells. Activation of LXRs dose-dependently inhibited pancreatic beta cell viability and proliferation. This was accompanied by beta cell cycle arrest at the G1 phase. Furthermore, LXR activation increased levels of the p27 protein by inhibiting its degradation. Knockdown of p27 reversed these effects of LXR activation on growth inhibition and cell cycle arrest. Conclusions/interpretation Our observations indicate that LXR activation inhibits pancreatic beta cell proliferation through cell cycle arrest. A well-known regulator of pancreatic beta cell cycle progression, p27, is upregulated and mediates the effects of LXRs on growth inhibition in beta cells. These observations suggest the involvement of aberrant activation of LXR in beta cell mass inadequacy, which is an important step in the development of type 2 diabetes.Keywords Beta cell . Cell cycle . Islet . LXR . p27 . Proliferation Abbreviations CMV cytomegalovirus GFP green fluorescent protein LXR liver X receptor LXRE liver X receptor response element MTT 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide RNAi RNA interference SKP2 S-phase kinase-associated protein 2 siRNA small interfering RNA

The synthetic liver X receptor agonist GW3965 reduces tissue factor production and inflammatory responses in human islets in vitro

“…1a, b). In agreement with our findings, the study by Meng et al also showed a significant upregulation of LXRs by the addition of LXR agonists to mouse beta cells [37]. Furthermore, we showed that GW3965 significantly induced the expression of the typical LXR lipogenic target gene, SREBP-1c, as well as the cholesterol efflux genes ABCA1 and ABCG1 in LPSstimulated islets (Fig.…”

“…Finally, we cannot rule out species-specific differences in the effects of LXRs, as murine islets or murine insulinoma cells were used by Choe et al [30] and Wente et al [40] whereas human pancreatic islets were used in our experiment. Such species differences may apply to pancreatic islets [27,37].…”

Liver X Receptors Inhibit Macrophage Proliferation through Downregulation of Cyclins D1 and B1 and Cyclin-Dependent Kinases 2 and 4

“…Recent work demonstrated that activation of the LXR pathway takes place after phagocytosis of apoptotic bodies (40), and apoptotic uptake is known to inhibit the proliferation of macrophages in response to M-CSF (41), which raises the question of whether this effect is mediated through production of LXR ligands. LXR agonists have been demonstrated to exert antiproliferative actions in other cellular systems as well, including vascular smooth muscle cells (21), prostate (38), and breast (22) cancer cells, T lymphocytes (20), and pancreatic islet b cells (42). While this work was under revision, Kim et al (43) also demonstrated antiproliferative effects of LXR agonists in several other cell lines, including human THP-1 macrophages.…”

Liver X Receptor activation downregulates AKT survival signaling in lipid rafts and induces apoptosis of prostate cancer cells

“…This degradation stimulates the cleavage of mitochondrial BCL family proteins involved in apoptosis through cytoplasmic cytochrome c release (Yang and Sinensky, 2000;Rusinol et al, 2004). Oxysterols have also been identified as inducing apoptosis in a wide range of models such as human leukemia cell lines (Laffitte et al, 2003) or pancreatic b-cells (Choe et al, 2007;Meng et al, 2009). In these cells, the master lipogenic genes SREBP1c, fatty acid synthase and acyl CoA-carboxylase-a drive an increase in intracellular lipid accumulation, which results in cell lipotoxicity.…”