Niki
et al. were the first to explore the anti-fibrotic effects of HDAC-inhibitors in a model for stellate cell activation. A first
in vitro study showed that both sodium butyrate and TSA could modulate rat stellate cell activation. Collagen 1 and 3 and α-smooth muscle actin (α-SMA) up-regulation was blocked by HDAC inhibition and proliferation was decreased upon treatment, with a pronounced better potential for TSA [
117]. Later studies by the Geerts lab, revealed that TSA treatment led to alterations in actin cytoskeleton forming components. They described how TSA induced a down-regulation of actin related proteins 2 and 3 (Arp2, Arp3) and RhoA, and an up-regulation of two capping proteins: adducing-like protein 70 (ADDL70) and gelsolin. These effects were translated in reduced stellate cell migration following incubation with TSA [
118,
119]. Although, this was a promising kick-off for antifibrotic studies of HDAC inhibitors, information on effects of TSA treatment in
in vivo models of liver injury is limited. As seen by Sirius red staining, TSA administration hampers collagen deposition in CCl
4 treated rats (unpublished data). A more recent study by Zhang
et al. showed the protective effects of TSA on liver injury in a mouse model for sepsis. During sepsis, the liver is not only an important actor in the host defensive response, but it will also suffer from the dysregulation of inflammatory mediators. TSA treatment of mice that underwent cecal ligation and puncture resulted in lower serum levels of transaminases and increased the presence of anti-inflammatory interleukin 10 (IL-10). This suggests that TSA alleviated hepatic injury following sepsis [
120]. In a lipopolysaccharide (LPS) induced model for sepsis, SAHA administration decreased activation of MAP kinases (p38 and ERK)
in vivo. which might explain the described improvement in sepsis-induced liver injury [
121]. These studies all focused on the observed antifibrotic effects upon HDAC inhibition rather than on the role of individual HDACs or mechanisms underlying the potential of the used compounds. In contrast, a recent study by Elsharkawy determined a role for HDAC1 in the NF-κB orchestrated regulation of MMP13 expression. Overexpression of p50 in a human stellate cell line LX2 could suppress MMP13 expression. In addition, the authors show that the presence of p50 is essential for recruitment of HDAC1 to the MMP13 promoter, by performing chromatin immunoprecipitation (ChIP) on freshly isolated HSCs from Nfkb−/− (p50-deficient) and wild type mice. TSA was employed as a tool to show that inhibition of HDAC activity could prevent the p50-induced repression of MMP13 expression. Together, this could explain the overexpression of MMP13 in HSCs from Nfkb−/− compared to wild type animals, but this then seems to be contradictory to the increased susceptibility of these Nfkb−/− mice to CCl
4. While MMP13 is a protease involved in degradation of fibrillar collagen, this matrix remodeling also leads to release of matrix bound inactive profibrogenic cytokines contributing to inflammation and disease progression [
122]. This recent study confirmed earlier data on repression of TNFα by HDAC1 in stellate cells, using the same transgene mouse model [
122,
123]. Other reports emphasizing a role of HDAC enzymes during liver fibrosis used 2’,4’,6’-Tris(methoxymethoxy) chalcone (TMMC), VPA and ectopic HDAC4 expression, respectively [
105,
124,
125]. TMMC reduced the number of α-SMA expressing cells by induction of apoptosis of activated stellate cells at high concentrations [
124]. In the study by Qin, the role of HDAC4, a member of Class II HDACs, was investigated in an
in vitro model. They show that ectopic HDAC4 expression in stellate cells regulates expression of MMP9 and MMP13 following IL-1 stimulation [
125]. A report on the role of HDAC6 in alcohol-induced alterations in Wif-B liver cells, (a hybrid of human fibroblasts and Fao rat hepatoma cells), showed a decreased HDAC6 expression after alcohol or TSA treatment and this resulted in changes in microtubule dynamics. However, the authors did not evaluate the impact of these changes on cell polarity or liver injury [
126]. In the study by Mannaerts
et al., it was shown that VPA administration inhibits stellate cell activation
in vitro and
in vivo. The
in vivo effect was investigated by treating mice with carbontetrachloride and VPA and subsequent isolation of hepatic stellate cells. These cells had lower pro-fibrotic gene expression levels compared to cells isolated from mice treated with CCl
4 alone. The observed effects were partially due to inhibition of Class I HDAC activity, since the VPA effect could be in part mimicked by siRNA mediated knockdown of the Class I HDACs. The knock-down of class I HDACs in contrast to VPA treatment did not affect α-SMA expression, but strongly reduced the expression of matrix remodeling enzyme lysyl oxidase [
105].
In conclusion, most HDAC-inhibitor liver studies focus on the effects on disease development or reversal, without having a closer look at the molecular mechanisms of the inhibition. As a result, the contribution of the individual HDACs to liver disease still remains unclear. A role for Class I HDACs has been described [
105,
122,
123,
127], but also the expression of Class II HDACs was documented in liver biopsies of hepatocellular carcinoma patients. The expression of Class II HDACs (HDAC4, 5, 6, 7, 9 and 10) were gradually elevated from normal to cirrhotic and HCC livers. This trend was closely related to progressive up-regulation of MEF2, suggesting a link among HDAC activity, MEF2 expression, stellate cell activation and the degree of liver disease [
128]. It is clear that HDACs have emerged as interesting targets for anti-fibrotic therapy and that further exploration of their individual function and the possibility for therapeutic intervention is meaningful. In addition, two recent papers [
46,
47] elegantly showed that upon recovery from liver injury, the activated myofibroblasts can be reverted to stellate cells presenting a more quiescent phenotype. Studies by Niki
et al.[
117] and Mannaerts
et al.[
105] have shown that
in vitro this process of HSC reversal can be stimulated by HDAC inhibitor treatment, indicating that the
in vivo process of conversion to stellate cell quiescence could be accelerated by HDAC inhibitory treatment.
The effects of HDAC inhibition on stellate cell activation are not only interesting for the fibrosis field, but also for the development of anti-hepatocellular carcinoma treatment. Hepatocarcinogenesis is modulated by the cross-talk of malignant hepatocytes with surrounding stromal cells.
In vitro and
in vivo studies provide evidence that stellate cells increase hepatocellular growth, EMT, invasiveness and tumor volume [
129‐
133]. Recently, it was shown that treatments have differential effects on the two compartments and targeting of HDACs using TSA can influence this bidirectional cross-talk [
134,
135]. In these studies, an immortalized HSC cell line was used and additional research with primary HSCs could further support this promising therapeutic strategy.