The activin axis in liver biology and disease.

Activins are a closely related subgroup within the TGFbeta superfamily of growth and differentiation factors. They consist of two disulfide-linked beta subunits. Four mammalian activin beta subunits termed beta(A), beta(B), beta(C), and beta(E), respectively, have been identified. Activin A, the homodimer of two beta(A) subunits, has important regulatory functions in reproductive biology, embryonic development, inflammation, and tissue repair. Several intra- and extracellular antagonists, including the activin-binding proteins follistatin and follistatin-related protein, serve to fine-tune activin A activity. In the liver there is compelling evidence that activin A is involved in the regulation of cell number by inhibition of hepatocyte replication and induction of apoptosis. In addition, activin A stimulates extracellular matrix production in hepatic stellate cells and tubulogenesis of sinusoidal endothelial cells, and thus contributes to restoration of tissue architecture during liver regeneration. Accumulating evidence from animal models and from patient data suggests that deregulation of activin A signaling contributes to pathologic conditions such as hepatic inflammation and fibrosis, acute liver failure, and development of liver cancer. Increased production of activin A was suggested to be a contributing factor to impaired hepatocyte regeneration in acute liver failure and to overproduction of extracellular matrix in liver fibrosis. Recent evidence suggests that escape of (pre)neoplastic hepatocytes from growth control by activin A through overexpression of follistatin and reduced activin production contributes to hepatocarcinogenesis. The role of the activin subunits beta(C) and beta(E), which are both highly expressed in hepatocytes, is still quite incompletely understood. Down-regulation in liver tumors and a growth inhibitory function similar to that of beta(A) has been shown for beta(E). Contradictory results with regard to cell proliferation have been reported for beta(C). The profound involvement of the activin axis in liver biology and in the pathogenesis of severe hepatic diseases suggests activin as potential target for therapeutic interventions.

Deregulation of the activin/follistatin system in hepatocarcinogenesis.

BACKGROUND/AIMS: Activins A and E negatively regulate hepatic cell number by inhibiting cell replication and inducing apoptosis. Follistatin and follistatin-like 3 bind activins and antagonise their biological activities. Aim of our study was to investigate, whether activins and follistatins may play a role in hepatocarcinogenesis. METHODS: Expression levels of follistatin, follistatin-like 3, and activin subunits beta(A) as well as beta(E) were investigated in chemically induced rat and human liver tumours by real-time PCR and immunohistochemistry. In addition, the effects of follistatin and activin A on DNA synthesis of normal as well as preneoplastic hepatocytes and hepatoma cells were analysed. RESULTS: Follistatin was overexpressed while both activin subunits were downregulated in the majority of rat and human liver tumours. Follistatin-like 3 expression was low in normal but enhanced in malignant rat liver. In human normal liver, in contrast, it was abundantly expressed but downregulated in liver cancer. Administration of follistatin to normal and preneoplastic hepatocytes stimulated DNA synthesis preferentially in preneoplastic rat hepatocytes, whereas activin A repressed it. CONCLUSIONS: The balanced expression of follistatins and activins becomes deregulated during hepatocarcinogenesis. The sensitivity of preneoplastic hepatocytes to activin signals suggests the activin/follistatin system as promising target for therapeutic intervention.

Cytotoxic effects of novel amphiphilic dimers consisting of 5-fluorodeoxyuridine and arabinofuranosylcytosine in cross-resistant H9 human lymphoma cells.

Various amphiphilic heterodinucleoside phosphates have recently been synthesized in order to overcome drug resistance. These agents contain 5-fluorodeoxyuridine (5-FdUrd) and arabinofuranosylcytosine (Ara-C). We now investigated the action of two of these novel dimers (#2 and #10) in sensitive and 5-FdUrd/Ara-C cross-resistant H9 human lymphoma cells. The dimers were compared with 5-FdUrd and Ara-C for growth inhibition, apoptosis induction, and cell-cycle effects. No significant difference in the cytotoxicity of dimer #2 could be observed between sensitive and 5-FdUrd/Ara-C cross-resistant H9 cells (IC50 values of 220 nM and 200 nM, respectively), indicating that further studies with this compound are warranted.

Expression of activins C and E induces apoptosis in human and rat hepatoma cells.

Activins C and E (homodimers of the betaC and betaE subunits), which are almost exclusively expressed in the liver, are members of the transforming growth factor beta (TGFbeta) superfamily of growth factors. We examined their expression in three different hepatoma cell lines and found that, compared with normal liver or primary hepatocytes, human hepatoblastoma (HepG2), human hepatocellular carcinoma (Hep3B) and rat hepatoma (H4IIEC3) cells have either completely lost or drastically reduced the expression of activins C and E. In order to elucidate the biological function of these proteins we transiently transfected HepG2, Hep3B and H4IIEC3 cell lines with rat activin betaC or betaE cDNA to study the consequences of restoring activin expression in hepatoma cells. Transfection with activin betaA, a known inhibitor of hepatic DNA synthesis and inducer of apoptosis, served as a positive control. We found that transfection of the three cell lines with activin betaC or betaE, as well as with activin betaA, reduced the increase in cell number by up to 40% compared with cells transfected with a control plasmid. Co-culture with a CHO cell clone secreting activin C also inhibited HepG2 cell multiplication. Furthermore, the three hepatoma cell lines studied showed an enhanced rate of apoptosis and elevated levels of active caspases in response to activin transfection. These results indicate that activins C and E share the potential to induce apoptosis in liver derived cell lines with activin A and TGFbeta1.