Kuniharu Akita

Chapter 21 – Retinoids in Liver Fibrosis: Induction of Proteolytic Activation of Transforming Growth Factor-β by Retinoic Acid and Its Therapeutic Control by Protease Inhibitors

Publisher Summary Retinoic acid (RA), an active metabolite of vitamin A, has antiinflammatory and scar formation effects in the process of wound healing. These multifunctional actions of RA may be linked to the controversy over its effects on liver fibrosis, depending on the models examined. RA exacerbates liver fibrosis induced by porcine serum that is not accompanied by hepatic necroinflammation. Hepatic stellate cells (HSCs) play central roles in the storage of retinoids in the quiescent phase as well as in fibrogenesis of the liver in the activated phase. During the progression of liver fibrosis, HSCs lose retinoid-containing lipid droplets from the cytoplasm, transform into myofibroblast-like cells, and start to produce a significant amount of extracellular matrices (ECMs). Thus, it is of great interest to understand the relationship between the loss of retinoids and the production of ECM in HSCs. This chapter discusses the ECM-producing effect of RA in HSCs. Generation of a certain isomer of RA may provoke HSC activation in culture and during liver fibrosis in vivo. This RA enhances proteolytic activation of transforming growth factor (TGF)-β, a strong fibrogenic cytokine, by upregulating the plasminogen activator (PA)/plasmin system. Active transforming growth factor (TGF)-β autostimulates its own synthesis by HSCs and enhances ECM production. This sequence of mechanisms of RA may suggest a clue to use protease inhibitors to sever an autoinduction of TGF-β. This chapter presents a study in which a drug that suppresses proteolytic activation of TGF-β and thereby inhibits hepatic fibrosis in rats is investigated.

Retinoid-Induced Apoptosis in Hepatocellular Carcinoma: A Molecular Basis for “Clonal Deletion” Therapy

We propose a new concept of “clonal deletion” therapy for chemoprevention of hepatocellular carcinoma (HCC). We previously showed that administration with acyclic retinoid suppressed alpha-fetoprotein (AFP)-L3-positivity and subsequently reduced the incidence of a second HCC in cirrhotic patients who had undergone curative treatment for previous liver cancers. Such eradication of AFP-L3-producing latent malignant (or premalignant) cells from the liver suggested a new strategy to prevent HCC, which may be identified with cancer chemotherapy. In the present study, we explored the molecular mechanism of clonal deletion and found a novel mechanism of apoptosis induction by retinoid. We demonstrated a modification of a retinoid receptor, RXR-α, by mitogen-activated protein kinasedependent phosphorylation, resulting in the loss of transactivating activity. This change may lead HCC cells to be resistant to natural retinoic acid. However, acyclic retinoid restored the function of phosphorylated RXR-α and induced its downstream gene, including tissue trans glutaminase, an enzyme that has been implicated in apoptosis. Tissue transglutaminase-dependent apoptosis in HCC cells was independent of the activation of caspases. This novel mechanism of retinoid-induced apoptosis may provide a clue for understanding the molecular mechanism of clonal deletion.

Protease Inhibitors: Suppression of Activation of Hepatic Stellate Cells by Inhibiting TGFβ Generation

Transforming growth factor-β (TGFβ) plays a central role in the pathogenesis of liver fibrosis. TGFβ is secreted from hepatic stellate cells (HSCs) in a latent form and must be activated before it exerts biological functions. Several proteases, including plasmin, promote the proteolytic activation of latent TGFβ. Therefore, protease inhibitors that may inhibit TGFβ activation might be useful for therapy against liver fibrosis. We examined this idea employing rat HSC cultures. Among 20 protease inhibitors examined, we found several compounds that strongly suppressed active TGFβ generation induced by plasmin, retinoic acid, or basic fibroblast growth factor. We selected one compound (camostat mesilate) and showed its suppressing effect not only on the activation and generation TGFβ but also on transformation of HSCs. These results suggest that the compound may be useful for therapy against liver fibrosis. Experiments are now under way to examine its effect in vivo employing rat hepatic fibrosis models.