Location, location, location: Cell‐level mechanics in liver fibrosis
Authors: Rebecca G. Wells
Journal: Hepatology (2016)

Potential conflict of interest: Nothing to report. See Article on Page 261 Mechanics and patient care are closely intertwined in hepatology. Palpation to assess stiffness (as a surrogate for fibrosis) is one of the first parts of the physical exam taught to students. Radiological methods to assess stiffness quantitatively are widely used in practice, and multiple studies have shown that liver stiffness is clinically relevant: The stiffer the liver, the more likely it is to be cirrhotic and the higher the likelihood of significant hepatocyte dysfunction and hepatocellular carcinoma (HCC).1 In spite of these clinical advances, however, a basic science‐level understanding of the relationship between liver stiffness and pathology is lacking. Whereas increased substrate stiffness is known to drive hepatic stellate cell (HSC) differentiation,3 there is little known about the mechanistic relationship between environmental stiffness and hepatocyte function. The article by Desai et al. in this issue clearly demonstrates that hepatocytes dedifferentiate as a function of matrix stiffness and not matrix concentration, structure, or composition.4 More important, the article fills a critical knowledge gap, showing that increased stiffness leads directly to decreases in the transcription factor, hepatocyte nuclear factor 4 alpha, which is central to hepatocyte differentiation. These findings have important clinical implications, providing an explanation for hepatocyte dedifferentiation in cirrhosis and, potentially, for the link between liver stiffness and HCC. Additionally, data in this article implicate focal adhesion kinase (FAK) and Rho/Rho kinase (ROCK) in hepatocyte mechanotransduction pathways, suggesting that treatment with ROCK or FAK inhibitors could improve hepatocyte function in fibrosis. The work of Desai et al. is also exciting and important for its contributions to understanding liver mechanics. Though radiological studies, as well as a large body of work with explant human and ex vivo rodent livers, have characterized the "bulk" mechanical properties of the liver5—the stiffness of the tissue as a whole—Desai et al. have carried out a meticulous study of mouse liver stiffness at the micron scale (see Fig. 2 in Desai et al.4). Using atomic force microscopy (AFM), they have carefully generated detailed stiffness maps for normal and fibrotic mouse livers, including central and portal regions, as well as tracks both along and orthogonal to fibrous septae. This approach was used productively in the past to demonstrate that fibrotic mouse lungs are stiffer than normal, with localized areas demonstrating particularly dramatic increases.6 Desai et al. show that normal livers are mechanically heterogeneous. The anatomic correlates of this parenchymal heterogeneity are not known; mechanical variations may reflect differences in stiffness among hepatocyte cords, the space of Disse, and endothelial cell monolayers. Not surprisingly, portal tracts are stiffer than pericentral regions. These differences are slight and do not achieve statistical significance, but Desai et al. show that even small variations in stiffness can have outsize implications for cells such as hepatocytes. Cells are the key here—this article is remarkable for defining the stiffness sensed by individual cells and for demonstrating that cells in distinct regions of the liver are exposed to mechanically distinct microenvironments. How mechanical heterogeneity impacts cell behavior in the normal liver is still speculative, but might explain gradients in hepatocyte function, soluble factor expression, and susceptibility to injury. This work suggests that mechanical factors should be incorporated into any effort to understand regional variations in hepatocyte function. The liver parenchyma becomes more mechanically heterogeneous when it fibroses.4 As with normal livers, fibrotic livers will require a combination of AFM and high‐resolution optical microscopy to map the stiffness of specific structures. It would be particularly interesting to know whether capillarization of the sinusoids, an early event in fibrosis, affects local mechanics and whether this alters the function of the hepatocytes and sinusoidal endothelial cells that rest on the neobasement membrane. Livers affected by biliary fibrosis are among the stiffest when measured at the whole tissue level (Corpechot et al.7 and unpublished data). Consistent with these findings, AFM mapping shows that the fibrotic scars of 5‐diethoxycarbonyl‐1,4‐dihydrocollidine‐treated mouse livers, representative of biliary fibrosis, are, on average, more than twice as stiff as CCl4‐treated mouse livers. Surprisingly, however, there is marked variability within fibrous septae in both models. This may have little impact on hepatocytes except in the earliest stages of fibrosis; almost all regions of the scars in both models are above 1 kilopascal, the stiffness at which changes in hepatocyte function level off. HSCs, however, are mechanosensitive over a wider range of stiffnesses than hepatocytes, at least in culture,3 and could demonstrate differences in fibrogenesis, apoptosis, and conceivably protease production in heterogeneous septae. This would have important implications for fibrosis regression and might explain why some septae are acellular and why biliary scars, which are stiffer than nonbiliary scars, are more resistant to degradation. Could septal mechanics define a "point of no return" in fibrosis regression? It will be important in the future to define septal stiffness in human cirrhotic livers and characterize the stiffness range over which myofibroblasts produce proteases and are most sensitive to apoptosis. Some investigators suggest that the collagen content of the liver changes little as fibrosis regresses, but that collagen organization changes and it becomes more fragmented, with large fibers divided into smaller ones by invading hepatocytes.8 How do local mechanics change in this setting, and how do hepatocytes react in response? The AFM studies of Desai et al. have several implications. The first is that fine mechanical mapping of human fibrotic livers would be informative. The dense scars of the rock‐hard, end‐stage cirrhotic liver and the disparate matrix organization observed in viral versus biliary versus fatty fibrosis are poorly modeled in rodents, and regional stiffness maps could provide key information about patterns of fibrosis development, hepatocyte dysfunction, and the potential for regression. The work of Desai et al. also has implications for understanding HCC. If hepatocyte proliferation increases with substrate stiffness, as shown by Desai et al., is it possible that particularly stiff regions of the liver, or particular mechanical/architectural patterns, provide a permissive environment or a first or second "hit" for HCC development? Finally, high‐resolution stiffness mapping at multiple time points could yield insights into the earliest mechanical events in liver repair as well as the mechanical changes associated with architectural rearrangements such as bridging fibrosis. Thus, the work of Desai et al., like all good science, raises more questions than it answers, serving as the opening to an exciting new chapter in the study of liver mechanics. Author names in bold denote shared co‐first authorship.