Amplified Inhibition of Stellate Cell Activation Pathways by PPAR-γ, RAR and RXR Agonists

Abstract

Peroxisome proliferator activator receptors (PPAR) ligands such as 15-Δ12,13-prostaglandin L(2) [PJ] and all trans retinoic acid (ATRA) have been shown to inhibit the development of liver fibrosis. The role of ligands of retinoic X receptor (RXR) and its ligand, 9-cis, is less clear. The purpose of this study was to investigate the effects of combined treatment of the three ligends, PJ, ATRA and 9-cis, on key events during liver fibrosis in rat primary hepatic stellate cells (HSCs). We found that the anti-proliferative effect of the combined treatment of PJ, ATRA and 9-cis on HSCs was additive. Further experiments revealed that this inhibition was due to cell cycle arrest at the G0/G1 phase as demonstrated by FACS analysis. In addition, the combined treatment reduced cyclin D1 expression and increased p21 and p27 protein levels. Furthermore, we found that the three ligands down regulated the phosphorylation of mTOR and p70S6K. The activation of HSCs was also inhibited by the three ligands as shown by inhibition of vitamin A lipid droplets depletion from HSCs. Studies using real time PCR and western blot analysis showed marked inhibition of collagen Iα1 and αSMA by the combination of the three ligands. These findings suggest that the combined use of PJ, ATRA and 9-cis causes inhibition of cell proliferation by cell cycle arrest and down-regulation of fibrotic markers to a greater extent compared to each of the ligands alone.

Pathobiology of liver fibrosis: a translational success story

“…Among these many events, intracellular inflammasome activation is increasingly recognised as an important transducer of signals derived from inflammatory cells that is especially relevant to fatty liver disease. 20 Another emerging signalling network is the nuclear receptor family, including farnesoid-X-receptor (FXR), 21 peroxisome proliferator-activated receptors (PPAR), 22 vitamin D receptor (VDR), 23 retinoid receptors, 22 Rev-erbα 24 and liver-X-receptor (LXR). 25 Other transcriptional events contributing to stellate cell activation include Wnt/β-catenin signalling, 26 GATA4 27 and hedgehog signalling.…”

Hepatic stellate cells as key target in liver fibrosis

“…In HSCs, all-trans retinoic acid (ATRA) (an RAR ligand) moderately inhibits expression of procollagen I, III, IV, fibronectin, laminin, αSMA, TGFβ, and IL-6, but with no effect on cell proliferation, whereas 9-cis (RXR ligand) moderately inhibits proliferation without affecting the fibrogenic gene expression in rodents [229, 230]. These receptors, also together with PPARγ signaling, have additive inhibitory effects on HSC activation [231, 232]. RXR can form heterodimer with VDR to transmit signal from p62 in HSCs to promote fibrogenesis and inflammation, and support carcinogenesis [224].…”

Farnesoid X Receptor Agonists and Other Bile Acid Signaling Strategies for Treatment of Liver Disease

“…One study demonstrated the benefits of fibrates in patients with PSC [33]. Activation of RXR and RAR by all-trans retinoic acid (ATRA) has potent anti-fibrotic effects [34,35,36,37,38]. ATRA treatment repressed CYP7A1 promotor activity [39], which was potentiated by UDCA co-treatment [40].…”