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1 Institute for Medicine and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
2 600 CRB/6140, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, United States
3 Institute for Medicine and Engineering, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, United States
4 Physiology, University Of Pennsylvania, Philadelphia, Pennsylvania, United States
5 Pathology and Laboratory Medicine, United States; 600 CRB/6140, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, United States
* To whom correspondence should be addressed. E-mail: rgwells{at}mail.med.upenn.edu.
Liver fibrosis, the response to chronic liver injury, results from the activation of mesenchymal cells to fibrogenic myofibroblasts. We have recently shown that two key myofibroblast precursor populations, hepatic stellate cells and portal fibroblasts, undergo activation in culture in response to increasing substrate stiffness. We therefore hypothesized that alterations in liver stiffness precede myofibroblast activation and fibrosis in vivo as well. To test this hypothesis, we induced fibrosis in rats by twice weekly injections of carbon tetrachloride, then sacrificed the animals at various time points ranging from 3 to 70 days after the initiation of injury. The shear storage modulus of the whole liver was measured on fresh tissue; fixed and frozen tissue from the same livers was used to quantify fibrosis. We observed that liver stiffness increased immediately and continued to increase, leveling out by day 28. Fibrosis, measured histologically by trichrome staining as well as by quantitative sirius red staining, increased with time, although these increases were delayed relative to changes in stiffness. There was no direct correlation between stiffness and fibrosis at early or late time points. Treatment of a second cohort of rats with the lysyl oxidase inhibitor 
-aminopropionitrile partially prevented early increases in liver stiffness. We conclude that increases in liver stiffness precede fibrosis and potentially myofibroblast activation. Liver stiffness appears to result from matrix cross-linking and potentially other unknown variables in addition to matrix quantity. We suggest that increased liver stiffness may play an important role in initiating the early stages of fibrosis.
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