Rho‐kinase inhibition is beneficial in fibrosis

Potential conflict of interest: Nothing to report. Author names in bold designate shared co‐first authorship. To the Editor: In their recent work, Desai et al. discussed the effects of matrix rigidity on the hepatocyte nuclear factor 4 alpha (Hnf4α) transcriptional network and illuminated regulatory effects that maintain hepatocyte function in fibrotic liver tissue.1 This excellent contribution provides insight into the influence of the Rho‐associated kinase (ROCK) pathway on Hnf4α expression in hepatocytes cultured on polyacrylamide gels with matrix rigidity similar to liver fibrosis conditions. Importantly, the authors show that the inhibition of ROCK by Y‐27632 increased Hnf4α expression on a stiff matrix. Of note, RhoA is one of the key upstream regulators of Rho‐kinase, but the present work does not assess the effect of stiffness on RhoA expression in fibrotic liver. Moreover, RhoA expression is down‐regulated in hepatocytes cultured on stiff polyacrylamide gels, as was recently shown by our group.2 Regarding how the RhoA/ROCK pathway is tightly linked to stiffness,3 the present work does not elucidate whether ROCK is up‐regulated or down‐regulated in hepatocytes. Taking into account that RhoA is down‐regulated in hepatocytes, it remains unclear whether further inhibition of RhoA/ROCK may help preserve hepatocyte function. Moreover, the authors suggest a focal adhesion kinase–independent mechanism of RhoA/ROCK pathway regulation through matrix rigidity, possibly propagated through c‐SRC. While c‐SRC may directly decrease Hnf4α expression by phosphorylation,4 in their work the authors speculated that c‐SRC might regulate Hnf4α through RhoA/ROCK inhibition.1 Previously published evidence supports this hypothesis, showing that c‐SRC is up‐regulated and activated in hepatocytes cultured on stiff gels, which leads to RhoA down‐regulation and consecutively ROCK down‐regulation.2 Even though Desai et al. provided strong in vitro evidence using atomic force microscopy and gels with a tunable shear modulus, the study did not sufficiently address the gradient of matrix stiffness across the different zones between portal vein and hepatic vein, as well as the polarity of the hepatocytes. Finally, hepatic stellate cells cultured on stiff polyacrylamide gels induce RhoA expression,2 and RhoA and/or Rho‐kinase inhibition may ameliorate fibrosis and portal hypertension in experimental and human cirrhosis, especially as the overall expression of RhoA/Rho‐kinase expression and activity is up‐regulated in cirrhosis.5 Therefore, the inhibition of Rho‐kinase might be beneficial 2‐fold, as an antifibrotic mechanism and by preserving hepatocyte function; and it should be explored further in the human setting.