Introduction
Almost 6% of the world's adult population now lives with diabetes mellitus. Type II diabetes mellitus (T2DM), i.e. non-insulin-dependent diabetes mellitus, represents over 80% of all diabetics and is dramatically increasing in incidence as a result of changes in human behavior and increased body mass index 1. T2DM are often associated with non-alcoholic steatohepatitis (NASH) 2, 3. NASH is characterized by fat accumulation and inflammation in the liver. Approximate one third of NASH patients develop hepatic fibrosis and even cirrhosis 4. Both T2DM and NASH are most commonly present in obese patients with hypercholesterolemia, i.e. increased levels of plasma low-density lipoprotein (LDL) and oxidized LDL (ox-LDL) 4. However, the role of hypercholesterolemia in hepatic fibrogenesis remains obscure.
LDL becomes the highly reactive form of ox-LDL after lipid peroxidative modification. High concentrations of circulating ox-LDL are associated with high incidences of metabolic syndromes, such as type II diabetes and coronary heart disease 5. Cellular uptake of ox-LDL is mediated by binding to its scavenger receptors, such as lectin-like oxidized LDL receptor-1 (LOX-1), leading to the elevation of intracellular levels of ox-LDL and reactive oxygen species (ROS), as well as to the activation of intracellular signaling 6, 7. LOX-1 was originally identified as a major scavenger receptor for ox-LDL in endothelial cells, and was subsequently detected in many other cell types 7, 8. LOX-1 gene expression is induced by ox-LDL 9 and is up-regulated in obese patients with hyperlipidemia 6.
Hepatic fibrosis is a progressive disorder characterized by accumulation of extracellular matrix (ECM) components 10, 11. Hepatic stellate cells (HSCs) are the major effector cells during hepatic fibrogenesis and are the primary source of ECM production in the liver 10, 11. During liver injury, quiescent HSCs undergo dramatic phenotypic changes from vitamin A, fat-storing cells to proliferative myofibroblast-like cells with acquisition of fibrogenic properties 10, 11. This process is coupled with activation of signaling pathways for pro-fibrogenic transforming growth factor-beta (TGFβ) 12, pro-mitogenic platelet-derived growth factor-beta (PDGF-β) 13 and Wnt signaling 14-16, as well as the depletion of peroxisome proliferator activated receptor-gamma (PPARγ) 17-19. It is important to note that culturing quiescent HSCs on plastic plates causes spontaneous activation, mimicking the process seen in vivo, which provides a good model for elucidating underlying mechanisms of HSC activation and for studying therapeutic intervention of the process 10, 11.
Curcumin (diferuloylmethane), the yellow pigment in curry from turmeric, is one of the best-studied natural compounds. Although the underlying mechanisms remain elusive, curcumin has shown diverse and versatile beneficial effects, including anti-inflammatory, anti-oxidative stress, anti-viral, anti-hypercholesterolemic, anti-infective and anti-carcinogenic effects 20. Curcumin has recently received attention as a promising dietary supplement for liver protection 21. We recently reported that curcumin inhibited HSC activation by inhibiting cell proliferation, inducing apoptosis and attenuating oxidative stress in vitro and in vivo 22-27. In addition, we demonstrated that curcumin dramatically induced gene expression of endogenous PPARγ and stimulated its activity in activated HSCs in vitro and in vivo, which is required for curcumin to inhibit HSC activation 22-24. Furthermore, curcumin suppressed gene expression of LDL receptor in activated HSCs in vitro by activating PPARγ and regulating gene expression of the transcription factors sterol regulatory element binding proteins (SREBPs), leading to the reduction in the level of intracellular cholesterol and to the attenuation of the stimulatory effects of LDL on HSC activation (Kang and Chen, manuscript accepted by British Journal of Pharmacology, in press).
The aims of this study are to evaluate the role of ox-LDL in activating HSCs, to assess the effects of curcumin on eliminating the stimulatory role, and to further explore the underlying mechanisms. Results in the current report supported our initial hypothesis that ox-LDL might stimulate HSC activation, which could be eliminated by curcumin by suppressing gene expression of LOX-1.