Differentiation of Human Circulating Fibrocytes as Mediated by Transforming Growth Factor-β and Peroxisome Proliferator-activated Receptor γ

Kurt M Hong,John A Belperio,Michael P Keane,Marie D Burdick,Robert M Strieter

doi:10.1074/jbc.m703597200

Abstract

Fibrocytes are a distinct population of fibroblast-like progenitor cells in peripheral blood that have recently been shown to possess plasticity to differentiate along mesenchymal lineages, including commitment to myofibroblast and adipocyte cells. Here, we demonstrated that transforming growth factor (TGF) beta1 drives fibrocyte-to-myofibroblast differentiation through activating Smad2/3 and SAPK/JNK MAPK pathways, which in turn stimulates alpha-smooth muscle actin expression. We determined that SAPK/JNK signaling acts in a positive feedback loop to modulate Smad2/3 nuclear availability and Smad2/3-dependent transcription. Conversely, fibrocyte-to-adipocyte differentiation is driven by the peroxisome proliferator-activated receptor (PPAR) gamma agonist troglitazone, which is associated with cytoplasmic lipid accumulation and induction of aP2. Treatment with troglitazone also disrupted TGF beta 1-activated SAPK/JNK signaling, leading to decreased Smad2/3 transactivation activity and alpha-smooth muscle actin expression. Interestingly, TGF beta 1 was demonstrated to have reciprocal inhibition on fibrocyte differentiation to adipocytes. By activating SAPK/JNK signaling, which is normally suppressed during adipogenesis, PPARgamma-dependent transactivation activity and induction of aP2 expression were disrupted. Taken together, within the context of the local microenvironmental niche, the delicate balance of PPARgamma and TGF beta 1 activation drives the selection of an adipocyte or myofibroblast differentiation pathway through SAPK/JNK signaling.

Highlights

We recently characterized the plasticity of an adult progenitor cell found in circulation, termed fibrocytes, which can dif
We demonstrated that adult fibrocytes are circulating progenitors that possess the ability to differentiate along different mesenchymal lineages
We showed that transforming growth factor (TGF)␤1 was a key molecule that promotes the differentiation of fibrocyte toward myofibroblast lineage, whereas activation of peroxisome proliferator-activated receptor (PPAR)␥ was critical for fibrocyte-to-adipocyte differentiation

Summary

The abbreviations used are

TGF␤1, transforming growth factor ␤1; ERK, extracellular signal-regulated kinase; SAPK, stress-activated protein kinase; PPAR, peroxisome proliferator-activated receptor; JNK, c-Jun N-terminal kinase; SBE, Smad binding element; TGZ, troglitazone; PPRE, peroxisome proliferator-activated receptor response elements; MAPK, mitogenactivated protein kinase; ␣SMA, smooth muscle actin; RT, reverse transcriptase; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; siRNA, small interfering RNA. We examined the role of TGF␤1 in activating distinct signaling pathways that lead to the stimulation of ␣-smooth muscle actin (SMA) expression during myofibroblast differentiation. Fibrocytes can differentiate into adipocytes, a process that involves activation of the PPAR␥ pathway leading to induction of aP2 expression. TGF␤1-induced SAPK/JNK phosphorylation leads to downstream signaling that negatively affects the transactivation activity of PPAR␥. SAPK/JNK signaling affected the divergence of adipogenic and myofibrogenic differentiation of fibrocytes. This is important because it provides a therapeutic target whereby through the use of a synthetic PPAR␥ agonist we are able to block a key TGF␤1-mediated pro-fibrotic effect, which has exciting implications for therapy of currently untreated fibrotic diseases

EXPERIMENTAL PROCEDURES

Findings

DISCUSSION