Prostacyclin derivatives prevent the fibrotic response to TGFβ2 by inhibiting the Ras/MEK/ERK pathway

Abstract

The SMAD-mediated induction of connective tissue growth factor (CTGF), a fibroproliferative cytokine, by transforming growth factor (TGF)beta is required for the development of sustained fibrosis in humans. Here, we show that in fibroblasts, activation of the Ras/MEK/ERK pathway is required for the SMAD-mediated induction of CTGF by TGFbeta2. We then show that activation of protein kinase A (PKA) in fibroblasts is able to block Ras/MEK/ERK signaling and abolish the fibrotic response. Previously, we found that prostacyclin agonists were able to prevent the induction of CTGF in fibroblasts, and in patients with the fibrotic disease scleroderma. Here, we confirm the in vitro and in vivo antifibrotic effects of prostacyclin derivatives and show that these effects are due to PKA-dependent inhibition of the Ras/MEK/ERK pathway. Ras/MEK/ERK does not directly affect SMAD signaling. The coordinate and varied biological responses to TGFbeta are in part due to the interactions of signaling pathways within target cells. Specific inhibition of fibroblast Ras/MEK/ERK signaling might prevent fibrosis while leaving other physiological effects of TGFbeta unaltered.

Pivotal role of connective tissue growth factor in lung fibrosis: MAPK‐dependent transcriptional activation of type I collagen

“…It has been previously reported that prostacyclininduced, cAMP-mediated activation of PKA leads to inhibition of ERK phosphorylation and CTGF expression in dermal fibroblasts, and that the prostacyclin analog iloprost has antifibrotic effects in patients with SSc (44,45). Indeed, in the bleomycin model of lung fibrosis, as well as in explanted human fibrotic lung fibroblasts, a reduction of the TGF␤-stimulated prostaglandin E 2 results in an increase in collagen production and fibrosis (46).…”

Defective Myofibroblast Formation from Mesenchymal Stem Cells in the Aging Murine Heart

“…Binding of TGF-␤1 to the receptor complex can promote several scenarios. Ligand activation of T␤RII receptor kinase leads to phosphorylation and, thereby, to activation of T␤RI, which then activates and phosphorylates Smad2 and Smad3 proteins, which are subsequently moved into the nucleus, where they associate with other transcription factors and activate transcription of target genes 60,61 (Smad-dependent pathway), and activates Smad-independent signals including GTPase Ras and ERK 62,63 , promotes JNK phosphorylation via FAK and Tak1, 34,35 and p38MAPK phosphorylation mediated by Fyn. 42 Up-regulation of all of these pathways has been associated with fibroblast and myofibroblast activation.…”

“…42 Up-regulation of all of these pathways has been associated with fibroblast and myofibroblast activation. 32,34,35,42,63 There seems to be signal redundancy, in which both Smad-dependent and Smad-independent pathways are involved in TGF-␤1-mediated responses in cardiac fibroblasts; however, the key signal transducers are T␤Rs. As shown in Figure 2E, when compared with fibroblasts isolated from young mice, the fibroblasts generated from aged cardiac MSCs exhibit reduced T␤RI and T␤RII expression and, therefore, demonstrate reduced responsiveness to TGF-␤1, which translates into decreased phosphorylation of 30 minutes of activation with 10 ng/mL TGF-␤1 in cardiac fibroblasts isolated from young (4 months old) and aged (30 months old) mice.…”

FAK Is Required for TGFβ-induced JNK Phosphorylation in Fibroblasts: Implications for Acquisition of a Matrix-remodeling Phenotype

“…An increasing body of evidence supports the role of signaling through MAP kinase cascades in driving tissue repair and fibrogenesis; for example, the ras/MEK/ERK cascade controls the expression of TGF␤ target genes in fibroblasts in a promoter-specific manner (Chen et al, 2002;Stratton et al, 2002;Leask et al, 2003). The JNK cascade has also been appreciated to mediate TGF␤ responses in fibroblasts, yet the overall effect and biological significance of this cascade on TGF␤ signaling in fibroblasts is unclear and in fact controversial.…”