Suppression or reversal of stellate cell activation has inherent attractiveness as a therapeutic strategy. Interferons have antifibrotic activity which may be related to down-regulation of stellate cell activation. Interferon-α has been widely used in treatment of viral hepatitis. Its antifibrotic effect may primarily reflect its antiviral activity, leading to reduced inflammation, but a direct effect on stellate cells is possible.166–168 In fact, preliminary data support an antifibrotic effect, even in the patients with HCV who fail to clear virus.169 In contrast to the uncertainty about IFN-α, IFN-γ has been documented to have inhibitory effects on hepatic stellate cell activation.170,171 In culture-activated, hepatic stellate cells, IFN-γ reduces the expression of mRNA of types I and IV collagen as well as fibronectin. In vivo animal models also show that IFN-α inhibits stellate cell proliferation, decreases collagen I mRNA levels and also reduces smooth muscle actin expression.171 In transgenic mice, expressing IFN-γ, chronic active hepatitis can be induced without apparent fibrosis.172 The main obstacle limiting its clinical use is its toxicity and poor tolerance in patients.
Reducing oxidative stress, which an important stimulus to activation, is a relatively practical avenue of intervention. Antioxidants, such as vitamin E, suppress fibrogenesis in some173 but not all studies of experimental fibrogenesis174 and are currently undergoing trials in humans. Recent studies have documented inhibition of stellate cell activation by resveratrol, quercetin and N-acetylcysteine.175 As discussed below, the antifibrotic properties of flavonoid compounds rely heavily on their antioxdative effects.
Silymarin is a natural component of milk thistle that has exhibited promising antifibrotic activity in experimental liver injury. Based on its structure, silymarin belongs to a group of flavonoid compounds, the other members of which include quercetin, baicalin and baicalein. These flavonoids have drawn increasing attention because of their antifibrogenic properties.176,177 Silymarin reduces collagen accumulation by 30% in secondary biliary fibrosis in rats.178 It functions as an antioxidant and may decrease hepatic injury by both cytoprotection and inhibition of Kupffer cell function.179,180 A human trial has reported a slight survival advantage in alcoholic cirrhosis compared with untreated controls.176 More clinical trials are still necessary.
Modulation of cytokine activity represents a relatively feasible and specific approach. Avenues of therapy include receptor antagonists and cytokine antibodies, inhibiting the production or activation of cytokines and using cytokines or proteins which promote extracellular matrix resorption.35
Tranforming growth factor β antagonists are under close investigation, as neutralizing this major fibrogenic cytokine might greatly down-regulate matrix production.181 Several TGFβ antagonists are being developed and tested, including soluble TGFβ type II receptor182,183 and antisense oligonucleotides.184 For example, TGFβ type II receptor inhibits stellate cell activation and fibrogenesis in vivo when administered either before or after the fibrogenic stimulus.181,183 Other strategies to functionally block TGFβ are under study, including TGFβ-sequestering proteins, such as decorin185 or latency associated peptide.186
Endothelin receptor antagonists have also been tested as antifibrotic agents187 and are among the most promising, because agents of this type are already undergoing clinical trials for hyptertensive diseases. Bosentan, an endothelin receptor antagonist, is antifibrotic and reduces stellate cell activation in experimental fibrosis.188
Hepatic growth factor inhibits liver fibrosis and promotes liver regeneration in animal models of liver injury.189–191 A deletion variant of HGF is effective in inhibiting stellate cell activation, down-regulating the mRNA expression of pro-collagens and TGFβ1 and stimulating liver regeneration.192 Pretreatment with this deleted form of HGF also shows strong protective effects against some hepatotoxins.193
Relaxin is a naturally derived peptide which decreases collagen synthesis and increases matrix degradation in vitro and in vivo.194,195 To date, the agent has not be used in models of liver fibrosis, although efficacy has been reported in other tissues, including lung.
Intracellular signalling pathways are potentially important antifibrotic targets. Several signalling inhibitors have been explored in vivo or in cultured stellate cells, including γ-linoleic acid, lipoxygenase inhibitors,196,197 simvastatin (which is an inhibitor of hydroxy-methylglutaryl coenzyme A reductase),198 pentoxyphylline which inhibits PDGF receptor signalling,199–201 and compounds which elevate intracellular cyclic AMP.202
Dilinoleylphosphatidylcholine (DLPC), the active component of polyunsaturated lecithin, has shown protective effects against fibrosis and cirrhosis in alcohol-fed baboons, presumably through a membrane-stabilizing effect.203 Another study shows that polyenphosphatidylcholine (PPC) and its ingredient, DLPC, inhibit PDGF-induced proliferation in rat stellate cells.204 A large multicentre trial is under way and results are expected within the next 2–3 years.
Two other novel antifibrotics, HOE077 and Safironil (Hoechst), have aroused interest as antifibrotic agents.205,206 Originally intended as prolyl hydroxylase inhibitors to reduce collagen cross-linking, their target in intact liver has proven to be stellate cell activation rather than col-lagen synthesis per se. Interestingly, their anti-activating properties appear greater in cells from females than those from males for unclear reasons.205,206 The finding is intriguing because gender related differences in the progression of HCV have been reported in humans.207,208
Halofuginone, a low molecular weight derivative of the anticoccidial quinazolinone, has recently been studied as a potent inhibitor of type I collagen synthesis. In both in vitro and in vivo models of fibrosis affecting several tissues, halofuginone inhibited collagen I synthesis and extracellular matrix deposition.209–212 In normal and simian virus 40-transformed mesangial cells, a small concentration of halofuginone (50 ng/mL) inhibited collagen I expression as well as cell proliferation.209 In dimethylnitrosamine-induced cirrhosis in rats, the dietary addition of halofuginone (5 mg/kg) effectively prevented the occurrence of liver fibrosis and cirrhosis. Based on above data, this compound may become a promising candidate for future treatment of liver fibrosis, although clinical trials are needed.213
In view of the export of retinoids during stellate cell activation, one might assume that restoration of cellular retinoid might reverse or down-regulate activation. However, there is no evidence yet to support this idea and studies even indicate that retinoids may exacerbate fibrosis in animal models.59 The use of retinoids as antifibrotic therapy is still in question because of our limited knowledge about their role in stellate cell activation and due to toxicity concerns.
In Asian countries, such as China, herbal medicines have been used for centuries to treat liver diseases. Recent studies have elucidated the cellular mechanisms of several herbal medicines which have putative activity against liver fibrosis. Prominent among these has been sho-saiko-to (xiao-chaihu-tang), which inhibits stellate cell activation and reduces fibrosis in vitro and in vivo.177,214–216 Administration of sho-saiko-to in experimental liver fibrosis reduces hepatic type I and III collagen expression and hydroxyproline content. It also decreases the number of smooth muscle α-actin- positive stellate cells and increases retinoid concentration in injured liver. The antifibrotic mechanism of sho-saiko-to may include an antioxidative activity216 in which baicalin and baicalein are active components. Another herbal medicine under study is salvia miltiorrhiza (dan-shen) which also inhibits fibrosis in animal models and down-regulates mRNA expression of TGFβ1, pro-collagen I and III.217 Apart from the scientific insight they provide, these studies underscore the potential value of traditional medicine, a system which has been employed for centuries in many parts of the world.177 Traditional therapies could lead to innovative strategies for treating hepatic fibrosis and cirrhosis.