βA- and βC-activin, follistatin, activin receptor mRNA and βC-activin peptide expression during rat liver regeneration

Abstract

The mRNA expression of two activin growth factor subunits ( A-and C-activin), activin receptor subunits (ActRIIA, ActRIIB) and the activin-binding protein follistatin, and peptide expression of A-activin and C-activin subunits, were examined in regenerating rat liver after partial hepatectomy (PHx). Liver samples were collected from adult, male Sprague-Dawley rats, 12-240 h (n=3-5 rats per time point) after PHx or from sham-operated controls at the same time points. Hepatocyte mitosis and apoptosis were assessed histologically and by in situ cell death detection. RT and PCR were used to assess relative gene expression. A-and C-activin peptide immunoreactivity was assessed in liver and serum samples by western blotting, whereas cellular expression was investigated by immunohistochemistry, using specific monoclonal antibodies. A-and C-activin mRNA dropped to,50% of sham control values 12 h after PHx and remained at this level until 168 h post-PHx, when A-activin expression increased to three times sham control values and C-activin mRNA returned to pre-PHx levels. A peak in follistatin expression was observed 24-48 h post-PHx, coincident with an increase in hepatocyte mitosis. No changes were observed in ActRIIA mRNA, whereas ActRIIB expression paralleled that of A-activin mRNA. C-activin immunoreactive homo-and heterodimers were observed in regenerating liver and serum. Mitotic hepatocytes frequently contained C-activin immunoreactivity, whereas apoptotic hepatocytes were often immunoreactive for A-activin. We conclude that A-and C-activin subunit proteins are autocrine growth regulators in regenerating liver and when expressed independently lead to hepatocyte apoptosis or mitosis in a subset of hepatocytes.

Activins and follistatins: Emerging roles in liver physiology and cancer

“…By immunohistochemistry, significant activin beta C expression has been detected in cells from additional organs, including the prostate, ovary, testes, and pituitary gland [10,77] . After partial hepatectomy, a transient downregulation of activin beta C expression was observed by several studies [42,44,78,79] . We have found reduced activin beta C expression in HepG2 and Hep3B hepatoma cells versus normal liver tissue [80] and a drop of beta C expression was also described in rat hepatocytes during primary culture with and without EGF treatment [44] .…”

“…In contrast, in AML12 cells, an immortalized mouse hepatocyte cell line, and in primary rat hepatocytes activin beta C increased DNA synthesis [84] . Adenovirus-mediated expression of activin beta C accelerated liver regeneration after partial hepatectomy in rats [85] and association of activin beta C immunoreactivity with mitotic hepatocytes was observed in regenerating liver after partial hepatectomy [42] . Activin C does not activate activin A-responsive promoters and it was suggested that the beta C subunit down-regulates the levels of bioactive activin A via the formation of signaling-incompetent activin AC heterodimers in PC3 human prostate cancer cells [9,86] .…”

“…Activin beta A antisense oligonucleotides stimulated cell proliferation in the human hepatoma cell line HLF suggesting a growth inhibitory function of endogenous activin A [41] . In regenerating liver, activin beta A gene expression was reduced at time points when hepatocyte replication took place and was increased at time points when liver regeneration terminated [42] . Other studies, however, have described increased expression of beta A at earlier time points after partial hepatectomy [43,44] .…”

“…Follistatin administration by intraportal infusion or adenovirusmediated overexpression caused DNA synthesis and liver growth in the rat, presumably by antagonizing tonic inhibition of hepatocyte proliferation by activin A [120,121] . Following partial hepatectomy, follistatin expression was up-regulated after 24-48 h, the time period in which hepatocyte replication was increased [42] . Administration of exogenous follistatin after partial hepatectomy accelerated liver regeneration but led to impaired restoration of normal tissue architecture and compromised liver function [122][123][124] .…”

Therapeutic Potential of Follistatin to Promote Tissue Regeneration and Prevent Tissue Fibrosis

“…In carbon tetrachloride (CCl 4 ) treated rat liver, activin A is expressed in hepatocytes and more intensively in hepatic stellate cells (HSC), which secrete excessive extracellular matrix and play a pivotal role in the pathogenesis of liver fibrosis [21][22][23]. In these rats, strong immunoreactivity of activin A is observed in HSC transformed into an activated form of myofibroblasts [21].…”