Autophagy Releases Lipid That Promotes Fibrogenesis by Activated Hepatic Stellate Cells in Mice and in Human Tissues

Background & Aims

The pathogenesis of liver fibrosis involves activation of hepatic stellate cells, which is associated with depletion of intracellular lipid droplets. When hepatocytes undergo autophagy, intracellular lipids are degraded in lysosomes. We investigated whether autophagy also promotes loss of lipids in hepatic stellate cells to provide energy for their activation and extended these findings to other fibrogenic cells.

Methods

We analyzed hepatic stellate cells from C57BL/6 wild-type, Atg7^F/F, and Atg7^F/F-GFAP-Cre mice, as well as the mouse stellate cell line JS1. Fibrosis was induced in mice using CCl₄ or thioacetamide (TAA); liver tissues and stellate cells were analyzed. Autophagy was blocked in fibrogenic cells from liver and other tissues using small interfering RNAs against Atg5 or Atg7 and chemical antagonists. Human pulmonary fibroblasts were isolated from samples of lung tissue from patients with idiopathic pulmonary fibrosis or from healthy donors.

Results

In mice, induction of liver injury with CCl₄ or TAA increased levels of autophagy. We also observed features of autophagy in activated stellate cells within injured human liver tissue. Loss of autophagic function in cultured mouse stellate cells and in mice following injury reduced fibrogenesis and matrix accumulation; this effect was partially overcome by providing oleic acid as an energy substrate. Autophagy also regulated expression of fibrogenic genes in embryonic, lung, and renal fibroblasts.

Conclusions

Autophagy of activated stellate cells is required for hepatic fibrogenesis in mice. Selective reduction of autophagic activity in fibrogenic cells in liver and other tissues might be used to treat patients with fibrotic diseases.

Keywords:  Myofibroblasts , Inflammation , Mouse Model , Energy Depletion

Abbreviations used in this paper:  ADRP, adipocyte differentiation-related protein, AV, autophagic vacuole, BSA, bovine serum albumin, CQ, chloroquine, EM, electron microscopy, FFA, free fatty acid, GAPDH, glyceraldehyde-3-phosphate dehydrogenase, GFAP, glial fibrillary acid protein, LC3, microtubule-associated light chain 3, LD, lipid droplet, 3-MA, 3-methyladenine, MMP2, matrix metalloproteinase 2, OA, oleic acid, ORO, oil red O, PDGFR, platelet-derived growth factor receptor, SMA, smooth muscle actin, TAA, thioacetamide