Peroxisome Proliferator-activated Receptor γ Induces a Phenotypic Switch from Activated to Quiescent Hepatic Stellate Cells

Abstract

Depletion of peroxisome proliferator-activated receptor ␥ (PPAR␥) accompanies myofibroblastic transdifferentiation of hepatic stellate cells (HSC), the primary cellular event underlying liver fibrogenesis. The treatment of activated HSC in vitro or in vivo with synthetic PPAR␥ ligands suppresses the fibrogenic activity of HSC. However, it is uncertain whether PPAR␥ is indeed a molecular target of this effect, because the ligands are also known to have receptor-independent actions. To test this question, the present study examined the effects of forced expression of PPAR␥ via an adenoviral vector on morphologic and biochemical features of culture-activated HSC. The vector-mediated expression of PPAR␥ itself is sufficient to reverse the morphology of activated HSC to the quiescent phenotype with retracted cytoplasm, prominent dendritic processes, reduced stress fibers, and accumulation of retinyl palmitate. These effects are abrogated by concomitant expression of a dominant negative mutant of PPAR␥ that prevents transactivation of but not binding to the PPAR response element. PPAR␥ expression also inhibits the activation markers such as the expression of ␣-smooth muscle actin, type I collagen, and transforming growth factor ␤1; DNA synthesis; and JunD binding to the activator protein-1 (AP-1) site and AP-1 promoter activity. Inhibited JunD activity by PPAR␥ is not due to reduced JunD expression or JNK activity or to a competition for p300. But it is due to a JunD-PPAR␥ interaction as demonstrated by co-immunoprecipitation and glutathione S-transferase pull-down analysis. Further, the use of deletion constructs reveals that the DNA binding region of PPAR␥ is the JunD interaction domain. In summary, our results demonstrate that the restoration of PPAR␥ reverses the activated HSC to the quiescent phenotype and suppresses AP-1 activity via a physical interaction between PPAR␥ and JunD.

JunD is a profibrogenic transcription factor regulated by Jun N-terminal kinase-independent phosphorylation

“…[4][5][6]10,[23][24][25] JunD is also the major Jun family protein expressed in activated pancreatic stellate cells, indicating a generic role for JunD in fibrogenesis. 26 Studies with junD Ϫ/Ϫ mice and cells have revealed new physiological functions of JunD, including regulation of spermatogenesis, control of cell senescence and apoptosis, prevention of renal cell proliferation and hyperplasia, regulation of lymphocyte proliferation and differentiation, inhibition of tumor angiogenesis, and cardiac hypertrophy.…”

Mechanisms of alcohol-induced hepatic fibrosis: A summary of the Ron Thurman Symposium

“…52 On the other hand, treatment of the activated HSCs with an adipocyte differentiation cocktail or ectopic expression of PPAR␥ or SREBP-1c causes their reversal to the quiescent phenotype. 53,54 Of the known adipogenic transcription factors, PPAR␥ has been investigated extensively. The expression of PPAR␥ is reduced in activated HSCs which can be restored with PPAR␥ ligands.…”

“…52 Furthermore, by using adenoviral vector to ectopically express PPAR␥ in cultureactivated HSCs, researchers have demonstrated expression of PPAR␥ can restore the morphological and biochemical characteristics of quiescent HSCs, including accumulation of vitamin A. 53 This reversal was associated with decreased binding of JunD to the AP-1 site. These findings suggest a possibility that PPAR␥ and other adipogenic factors may serve as important therapeutic targets for liver fibrosis.…”

The new‐generation pan‐peroxisome proliferator‐activated receptor agonist IVA337 protects the liver from metabolic disorders and fibrosis

“…The PPARδ isoform (also known as PPARβ) contributes to the regulation of glucose and lipid metabolism while exerting anti‐inflammatory properties in the liver by skewing M2 polarization of Küpffer cells 9, 10, 11. PPARγ and PPARδ are expressed at various levels in hepatic stellate cells (HSCs), a driver of liver fibrosis; PPARγ is key in keeping HSCs in a quiescent nonfibrogenic state 12, 13…”