Mechanisms of spontaneous resolution of rat liver fibrosis. Hepatic stellate cell apoptosis and reduced hepatic expression of metalloproteinase inhibitors.

TLDR

Liver fibrosis arises from excessive matrix protein secretion by proliferating hepatic stellate cells during injury. The study aims to determine the biological mechanisms mediating spontaneous resolution of liver fibrosis. Livers were harvested from rats at 0, 3, 7, and 28 days of recovery after 4 weeks of CCl₄‑induced fibrosis, and analyses of hydroxyproline, histology, α‑SMA staining, RNAP, and collagenase activity revealed that apoptosis of activated HSC likely drives matrix degradation. Over 28 days, septal fibrosis regressed to near‑control levels, activated HSC numbers fell 12‑fold with apoptosis, TIMP‑1 and TIMP‑2 expression dropped rapidly while collagenase activity increased, supporting matrix breakdown as a key resolution mechanism.

Abstract

Liver fibrosis results from the excessive secretion of matrix proteins by hepatic stellate cells (HSC), which proliferate during fibrotic liver injury. We have studied a model of spontaneous recovery from liver fibrosis to determine the biological mechanisms mediating resolution. Livers were harvested from rats at 0, 3, 7, and 28 d of spontaneous recovery from liver fibrosis induced by 4 wk of twice weekly intraperitoneal injections with CCl4. Hydroxyproline analysis and histology of liver sections indicated that the advanced septal fibrosis observed at time 0 (peak fibrosis) was remodeled over 28 d of recovery to levels close to control (untreated liver). alpha-Smooth muscle actin staining of liver sections demonstrated a 12-fold reduction in the number of activated HSC over the same time period with evidence of HSC apoptosis. Ribonuclease protection analysis of liver RNA extracted at each recovery time point demonstrated a rapid decrease in expression of the collagenase inhibitors TIMP-1 and TIMP-2, whereas collagenase mRNA expression remained at levels comparable to peak fibrosis. Collagenase activity in liver homogenates increased through recovery. We suggest that apoptosis of activated HSC may vitally contribute to resolution of fibrosis by acting as a mechanism for removing the cell population responsible for both producing fibrotic neomatrix and protecting this matrix from degradation via their production of TIMPs.