Characterization of human fibrocytes as circulating myofibroblast and adipocyte progenitors

There is growing body of evidence which reveal human circulating fibrocytes as an important source of circulating progenitor cells that participate in both physiologic and aberrant fibrotic processes. In this project, we isolated and characterized circulating fibrocytes from peripheral blood and demonstrated its plasticity to commit and differentiate along two mesenchymal lineages. We first demonstrated the ability of fibrocytes to differentiate to alphaSMA expressing myofibroblasts. In the setting of pulmonary fibrosis, TGFbeta in the lung microenvironment promotes maturation of fibrocytes and their differentiation into myofibroblasts. We next demonstrate the ability of fibrocyte to differentiate to adipocytes, a process that is dependent on PPARgamma activation. Adipocytes differentiated from fibrocytes expressed many similar mature adipose markers such as leptin and alphaP2. Genome wide expression profiling revealed activation of mature adipose gene clusters during fibrocyte adipogenesis, such as those involved in lipoprotein metabolism and fatty acid biosynthesis. Using a SCID mouse model, we demonstrated that fibrocyte-derived adipocytes can form adipose tissue in vivo. Lastly, we examined in detail the network of molecular signals that regulates fibrocyte lineage determination. We demonstrated that TGFbeta1-mediated fibrocyte-to-myofibroblast differentiation involves activation of both Smad2/3 and SAPK/JNK MAPK pathways. Specifically, TGFbeta1 induced activation of SAPK/JNK signaling acts in a positive feedback loop to modulate Smad2/3 nuclear availability and Smad2/3-dependent transcription. Conversely, PPARgamma agonist troglitazone drives the differentiation of fibrocytes to adipocytes, a process associated with cytoplasmic lipid accumulation and induction of aP2. Treatment with troglitazone also disrupted TGFbeta1-activated SAPK/JNK signaling, leading to decreased Smad2/3 transactivation activity and alphaSMA expression. Interestingly, TGFbeta1 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 was disrupted. Taken together, within the context of local microenvironmental niche, the delicate balance of PPARgamma and TGFbeta1 activation drives the selection of an adipocyte or myofibroblast differentiation pathway through SAPK/JNK signaling.